stunthanger.com
Design => Stunt design => Topic started by: Gordon Tarbell on October 25, 2009, 08:48:36 AM
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On a stunt design with a w.s. of around 55" how thick should the high point of the root rib be if the cord length (not including flap measurement) is 10.000"? And same question for tip rib of 8.375" . Also hinge line distance of 17" would be about right? Power supplied by ST 46 with bristunt ABC P&L set and head supplied by same (might try saito 40 also later )
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Great question Gordan.
With the parameters you've established, you may be looking at a 20% thickness at the root. This percentage is governed by the wing loading, and available power.
For the tip, I would use a 21 or 22% thickness, and move the high point forward about 5% further than the root location. This is to help delay the stall at the tip during hard manuevers.
As for the hingeline distance, 17" inches is fine, but it depends on other factors as well. The nose legnth needs to be long enough to mount the engine, and a tank large enough to fly the pattern with. The tail should be long enough to balance the nose without adding weight. Errors should be towards being slightly nose heavy, as it requires less additional weight to balance.
There is a formula we can use to figure the balance, but some weights have to be estimated, such as the total weight of the tail group when finished.
The basic Algebraic formula is; (A times B) minus (C times D) = 0, Where A= the weight of the center of mass of the area in front of the CG. B= the distance from the center of mass, in front of the CG. C = the weight of the center of mass behind the CG. D = the distance from the center of mass, behind the CG.
It seems that there may be a sq function in there, but the answer would include a square root in the solution. I believe that would cancel out.
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OK so if cord length is 10" then the aifoil should be at least 2" thick at the high point. and 1.8425" thick for the high point on the tip rib. Also I am hoping that as long as the flaps are not overly long cord wise they will not dictate a change in wing thickness. I just see so manty ultra fat airfoils at the contests these days and want to be sure I don't go too thin on wing thickness and get a plane that needs to fly 70 mph to work. The flaps will probably be flat 5/16" for quick easy building. I also wondered if it is normal to do the calcs. with or with out the flap size.
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Gordan, the chord legnth includes the flap chord in your figures. For instance, 10 inches from leading edge to flap hingeline, plus, oh, say 3 inches flap chord at the root, equals 13 inches.
So, 20% root chord times legnth, 13 inches, equals 2.6 inches thick at the root. 18% would equal 2.47 inches thick. 16% would equal 2.08 inches thick.
Ty is correct about the entry radii of an airfoil. The blunt NACA 4 diget has the greater radii, but it presents a higher potential for induced drag.
Going to extremes the other way, a sharp pointy airfoil, think of a 1/4 sq set so the point is at the extreme leading edge. Such an airfoil may penetrate turbulence, and groove in level flight, but as soon as the airfoil is rotated so that the angle of attack changes to creat the lift needed to negotiate a turn, the airflow seperates, and the wing begins to enter into a stall.
So often times somewhere between these two examples lies the ideal airfoil for the job at hand.
I've had good success with a particular airfoil known as NAC 63A modified. It is as thick as the 4 digit airfoil, but the entry radius is smaller, while still not being to the point that it stalls as the pointed airfoil does. It penetrates better in wind and turbulence, grooves well, and still doesn't enter into stall territory. at the speeds and angle of attacks we use.
It may be said that the most important part of our airfoils is the entry profile. The part from the leading edge to possibly just aft of the high point. All that's behind it, many believe, does almost nothing for us at our reynolds numbers.
I'm not sure this is always the case, as many feel that the Pathfinder airfoil has some positive characteristics that are attributed to it's particular airfoil. Indeed, many combat ships and several full scale aerobatic designs use about the same airfoil.
Like any exercise, when designing a stunt plane, we need to make some basic choices, and then use what works best around those choices. H^^
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Om the 1948 Aeromodeller Annual there are pages and pages of aerodynamic stuff. The focus is, I think, mostly on free flight. There is a short discussion of airfoil thickness. There is a rough suggestion that for greatest efficiency is that % thickness divided by two gives a minimum chord in inches. So a 20% thick wing should have at least a 10 inch chord, and a 15% thick wing should have at least a 7.5 inch chord. so the smaller the wing, the thinner it should be. George Aldrich once told me that a 1/2A stunt airplane should have a thin wing. i have cheerfully ignored his advice. But maybe this is why we hear of 1/2A's with blank wings flying well. Something to think about anyway.
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In my experience, even at 1/4A level (.020), a moderate built-up airfoil (say, 15%?) flies better than a flat plate.
Re: Pix below. In every case the builtup wing flies better maneuvers.
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OK guys , I think I know where I want to go with this first. With your info I have a starting point . Thanks, Gordon
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A wing profile has a drag coefficient, a typical value for an aerobatic profile we use at a lap speed of 5,2 sec is Cd = 0,014
All calculations in metric system so we can easily compare drag * speed = Watts and Volts * Amps = Watts too so you can easily compare.
The drag of a wing is a function of air density, speed (quadruple), frontal area and this Cd factor.
For an aerobatic wing like the Stiletto of Les McDonald this is:
F = ½ density * speed^2 * Cd * Frontal area
Density of air is 1,2 kg/m^3
Speed is 25 m/s
Cd = 0,014
Frontal area is 1,5 m span * 0,054 m thickness
F = ½ 1,2 25^2 0,014 1,5 0,054 = 0,42 Newton
The amount of Watts that is needed to fly this force around is 0,42 N * 25 m/s = 10,6 Watt. From electrical logging I know that a .46 model drags about 12 Newton in level flight.
Thicker profiles have greater drag, higher alpha’s create higher drag. When using software from Profili diffenrences can be calculated between profiles, there are differences in drag but thick wing profile has approx 50% more drag than a thin profile.
So wing thickness has an influence but not as big as I expected. Thick wings can be very successful, Paul Walker is a good example.
I want to prove this by building an Electric Impact with a wing thickness of 65mm where a Stiletto has a wing thickness of 54mm.
It is for next year so I can only predict the outcome.
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Or you could do it the old fashioned way, choose an airplane of appropriate size with a proven contest track record. Build the wing, change the wingtips to suit, fashion a fuselage styled to your liking but maintaining well proven "numbers" and you have your new world beater! y1
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A wing profile has a drag coefficient, a typical value for an aerobatic profile we use at a lap speed of 5,2 sec is Cd = 0,014
All calculations in metric system so we can easily compare drag * speed = Watts and Volts * Amps = Watts too so you can easily compare.
The drag of a wing is a function of air density, speed (quadruple), frontal area and this Cd factor.
For an aerobatic wing like the Stiletto of Les McDonald this is:
F = ½ density * speed^2 * Cd * Frontal area
Density of air is 1,2 kg/m^3
Speed is 25 m/s
Cd = 0,014
Frontal area is 1,5 m span * 0,054 m thickness
F = ½ 1,2 25^2 0,014 1,5 0,054 = 0,42 Newton
The amount of Watts that is needed to fly this force around is 0,42 N * 25 m/s = 10,6 Watt. From electrical logging I know that a .46 model drags about 12 Newton in level flight.
Thicker profiles have greater drag, higher alpha’s create higher drag. When using software from Profili diffenrences can be calculated between profiles, there are differences in drag but thick wing profile has approx 50% more drag than a thin profile.
So wing thickness has an influence but not as big as I expected. Thick wings can be very successful, Paul Walker is a good example.
I want to prove this by building an Electric Impact with a wing thickness of 65mm where a Stiletto has a wing thickness of 54mm.
It is for next year so I can only predict the outcome.
Erik,
Your numbers seem pretty low to me, in terms of thrust and power. For example, in my measurements with a Nobler and a Vector, I need about ~240 watts (measured) input power to the motor to maintain level flight. If I add motor/ESC efficiencies of ~70-80% to this, plus prop efficiencies of another ~70% (guestimated from PropCalc)==total ~50, that still comes to~120 watts. Also Prop Calc tells me that at 24m/s, this particular prop is making 4.5N of thrust.
These numbers (power and thrust) are about a factor of 10 above your numbers. I am not sure if I have accounted for all the effects except airframe drag.
Anyway if this informations tends to be too esoteric for the main, feel free to post some of your stuff in the engineering section!
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I measured the number of watts needed to fly level in flight, 300 Watts for my .46 converted model. I did not bother about motor/ prop efficiency but wanted to have an idea of how important the drag of the wing is.
The number of Watts is the Force multiplied by the movement per second, so 300 Watts divided by 25 m/s = 12N, due to efficiency it will be lower, I agree.
This is not enough to prevent the model from picking up speed in the loops as the weight of the model (gravity is always 100%) is more than this 12N
I calculated the drag of the wing and was surprised by the low amount of drag Cd 0,014 for a Stiletto and 0,021 for a Walker style profile. 0,42N to 0,63N The 12N drag comes from lines, induced drag, wheels etc.
So my conclusion is that using a thick profile will increase my power consumption very little and I can use the same setup.
Unless I mis interpreted the numbers somewhere, if I missed a factor of 10 somewhere then I might get into trouble. Doing the same will not increase my knowledge about what works best so I will take te risk and build a thick wing model for next year.
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I don't think you missed anything Erik. The drag of a wing is always quite small, at least until you start making lift in a maneuver. As you pointed out, it is really inconsequential compared to the rest of the drag coefficient. The lines are the biggest contributor of drag by far.
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As far as I have understood from other threads... the thick and blunt airfoils tends to work better in corners: they do not stall as easily and carry more speed through a sharp corner, than a slim airfoil. Power for level flight and round maneouvers is not really a problem.
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Erik,
I misunderstood your original post---not realizing you were only talking about the wing.
I claim the total drag is ~4-5 N (in my previous comment), which is in the ballpark of what you are saying if you put in the efficiencies.
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Out of curiosity, I calculated the drag due to a 0.015" diameter line whose tip is moving at 24m/s, and the value comes out near 1N. Given that most of us fly with 2 lines, and ignoring any interference between the two, then the total line drag is ~2N. I grabbed the formula from Martin Hepperle's site
(http://www.mh-aerotools.de/airfoils/control_line_aero_3.htm)
He ignores the curvature of the flying lines, and I ignored some details he mentions, but 2N (about a quarter of a pound) is in the right ballpark. Which is to say the lines are about 4 times as draggy as the wing that Eric calculated. Then from my guestimate of ~5N to fly my Nobler and Vector in level laps, roughly half of the thrust is being taken up by the remainder of the airframe.
Well that's interesting to realize. Thanks Erik! H^^
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For an aerobatic wing like the Stiletto of Les McDonald this is:
F = ½ density * speed^2 * Cd * Frontal area
Density of air is 1,2 kg/m^3
Speed is 25 m/s
Cd = 0,014
Frontal area is 1,5 m span * 0,054 m thickness
F = ½ 1,2 25^2 0,014 1,5 0,054 = 0,42 Newton
Sorry, I did not see this thread earlier, but wonder where it comes from?
By my evidence, the drag of wing of usual srunter is approximately 1.5N.
Erik, where you got that number Cd = 0,014 from? and why you do not use usual formula for calculating of drag? If I use drag of NACA 0018 what is airfoil of thickness very close to out ususal wings, then I see it is cd=0.01. And if I take my wing area 0.45m^2
Then drag of the wing is:
F = ½ density * speed^2 * Cd * WING AREA
F = ½ * 1,2 * 25^2 * 0,01 * 0,45 =~ 1,7 Newton
To the other posts: Yes, the wing drag is relatively low so it really does not matter if we use one or two % more or less, while the lift depends on thickness much more.
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apparently 30 % thick would have good lift, but'd look a bit odd .
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Drag is calculated with frontal area, not surface.
0,01 or 0,014 in in the same range
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The convention for airplanes is to use wing area as the area in the definition of lift and drag coefficients.
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ok,
that helps, so increasing the wing thickness from a Cd of 0,014 to 0,021 will add 50% to the 1,7N
that is great as I want to increase my airplane drag from 7 to 8N, so if this adds 0,85 that is perfect.
thanks
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As usual, I don't know how well my scans will come out, but these from NACA TR 586 of 6/24/36 show that not everything is obvious or simple. Our Reynolds numbers are usually half a million or less. You'll see that profile drag varies with aoa and that the thick sections enjoy not just maximum lift advantages in his area. Look at the behavior of the profile drag coefficient of the NACA 0012 section compared with the 0018 and see whether these profile drag coefficients are proportional to thickness. These measurements were made after NACA discovered that their RN's were faulty and had taken steps to come up with valid "effective" Reynolds Numbers, taking into account tunnel wall effects and turbulence from supports. So this data should be valid. Yeah, I know, "We don't need no steenkin' math!" But I've already posted this material in words in archives that everyone should visit from time to time - 'lots of info there.
SK
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<snip>....
Reynolds Numbers, taking into account tunnel wall effects and turbulence from supports. So this data should be valid. Yeah, I know, "We don't need no steenkin' math!" But I've already posted this material in words in archives that everyone should visit from time to time - 'lots of info there.
SK
Well we are waiting to be banished!
[/quote]
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I found TR 586 in 1962. That first chart won me a lot of combat matches.
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<snip>....
Reynolds Numbers, taking into account tunnel wall effects and turbulence from supports. So this data should be valid. Yeah, I know, "We don't need no steenkin' math!" But I've already posted this material in words in archives that everyone should visit from time to time - 'lots of info there.
SK
Well we are waiting to be banished!
Yeah, it sure seems that way sometimes. Wait! We're already out here in the "south forty"...ever since I posted beginning algebra notation.
Seriously though, the graphs contain particularly pertinent info I've found nowhere else. Three views in 12 1/2 hours...?
SK
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A lot of people have a real aversion to graphs--probably a reaction to some unpleasant exam back in high school. Oh well!
One problem I have with aerodynamics--and the plots--- is the Reynolds number---usually because it is introduced and then the comment about picking the value as a function of the size of the design element you are looking at. Sort of makes it hard to understand exactly since I always wonder what size are they talking about---thickness, chord length,... (of course they are usually within each other by a factor of 10.
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I've always had similar feelings about RN, because it's such an imprecise concept - as you hinted. But it's pretty necessary in handling our inability to scale the air to suit our models, because it makes such a big difference. For instance, it reverses the choices of airfoils from from 12% for full-sized aircraft up to 18-24% for our stunt models. The 12% airfoil's maximum-lift curve crosses and dips below those for the 15% and 18% sections as RN decreases; so the Ringmaster must be lighter or fly faster than modern stunters to turn as well. Shapes change too with RN - like leading edges and relative thicknesses for small stabs verses larger stabs or wings. That's what's so nice about graphs; you can look at the lines representing those thicknesses and just follow them to see their relative heights and positions and where they cross. If they jump around or disappear in any range, you know that something unusual or unsteady is happening with the air for that section at that RN. I suspect that just what you said about thickness vs. length affects some of this; in addition to easing contour from l.e. to high-point, the thicker sections cause the air's speed to change as it passes, and this really amounts to a greater local RN for the thicker wing - just a guess.
Another of these reports (at higher RN) covers the effects of turbulent air. When you put these together, you get a hint that my old Skyray would benefit from a turbulator or sharp l.e. on its narrow, thin stab, similarly to something that I think Igor concluded from his CFD work (look way to the left on the graphs). These seemed to help the Skyray and another of my smallish models. I think that you can read some of this in the graphs' shapes, while getting reasonably good numerical data - like the profile drag coefficients of the sections of different thickness. A little arithmetic shows that the drag (based on area) is not a penalty for the thicker wing down where that 10" chord wing operates. Actually, these rare graphs tell us a lot, not just about why things have evolved the way they have in our hobby, but how our own particular models would best fit in. I suppose CFD will completely eclipse things like these graphs, but they contain information that Mr. Jacobs and Mr. Sherman actually found out about our model flying surfaces, something that no one else bothered to do for us.
Well the graphs do bear some study and familiarization. The values can be read easily enough, but there's more yet in the pictures themselves. It would take a lot of words to describe all the information in these graphs, which both picture and quantify it.
SK
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Good point Serge, so it is now clear why my indoors have completaly flat wings and brakes on TE to compensate airfoil drag S?P
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IGOR...WOW! What an interesting design photo!! H^^
Since the first day I saw that indoor video of your's on Flying Lines....I have aways been astounded at your ability to not only create very interesting concept models but also equally astounded at your ability to design-build-n fly them with such expertise.
Do you have any more photos or info on this project of yours...or even maybe a 3 view?
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One problem I have with aerodynamics--and the plots--- is the Reynolds number---usually because it is introduced and then the comment about picking the value as a function of the size of the design element you are looking at. Sort of makes it hard to understand exactly since I always wonder what size are they talking about---thickness, chord length,... (of course they are usually within each other by a factor of 10.
It's another nondimensionalizing convention. For airfoils, chord is the convention. For other stuff, the person citing the Reynolds number should specify the length upon which the Reynolds number is based.
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Thanx :)
There are many tricks used on that model, I know I should write more ... but time is my enemy ... may be "once"
At least I will post some pictures, everything is visible, it just needs "good eye". Brakes are not on pictures from building. I found I need them later. Plan is in corel draw if anyone wants (well - it is not plan, just main parts)
Span is 800mm, weight 160g lines 5m prop 10x4.7 at 4300rpm and lap time ~5s, leadouts are ~20 degrees back from CG, thrust line 2-3 degrees down.
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Your foamie reminded me of these:
http://shop.donuts-models.com/boutique/fiche_produit.cfm?ref=01DM-GEBEER3INDOOR&type=13&code_lg=lg_fr&num=14
http://www.rcgroups.com/forums/attachment.php?attachmentid=2811438&d=1255308105
Is there a leadout guide somewhere on the wing? I cannot see it.
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yes and yes :-)
the shape comes from gb r3 but it was too thinn, I needed more side area so I modified it
LO is visible on the last picture under the wing
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Igor,
We just need to program your controller to apply the dive brakes in the descending leg of the maneuver!
I should get a copy of Don Hutchinson's SBD Dauntless profile plans!
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Igor...
THANKS...GOOD BUD!!! GREAT PHOTOS!!!
(What an interesting full sized full blown stunter project this little electric model of yours might make? H^^
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well ... I am thinking about fuselage like that ... but it will need composite technology ... we will see :-)
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There's an Axi 4130/16 / .61 2-stroke / .91 4-stroke R/C ARF
http://shop.donuts-models.com/boutique/fiche_produit.cfm?ref=01DM-GEEBEER3&type=16&code_lg=lg_fr&num=14
Might take a bit of modding, though.
(The ones I posted are from Mirco Pecorari's concept drawings. I'd give him a shout if I wanted to sell anything based on his work.)
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But that wing is really too thin for such a C/L model
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A wing profile has a drag coefficient, a typical value for an aerobatic profile we use at a lap speed of 5,2 sec is Cd = 0,014
All calculations in metric system so we can easily compare drag * speed = Watts and Volts * Amps = Watts too so you can easily compare.
The drag of a wing is a function of air density, speed (quadruple), frontal area and this Cd factor.
For an aerobatic wing like the Stiletto of Les McDonald this is:
F = ½ density * speed^2 * Cd * Frontal area
Density of air is 1,2 kg/m^3
Speed is 25 m/s
Cd = 0,014
Frontal area is 1,5 m span * 0,054 m thickness
F = ½ 1,2 25^2 0,014 1,5 0,054 = 0,42 Newton
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Aspect Ratio is somewhat relevant, particularly during manouvres and glide.
A (say ) 23 % thick wing of a set area will have the SAME frontal area at any A/R ,Though drag will be less
(at our speeds) on a high A/R (hopefully) This is assumeing similar torsional ridgidity,or we have an ornitopia.
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Matthew, several notes:
1/ It was caculation for wing at 0 lift. Means induced drag is 0 and thus does not make induced drag. Because:
induced drag = something * (lift coefficient ^2) / Aspec ratio
this is on top of arfoil drag, so you are right that drag at low speed (gliding) or maneuverig at higher AoA will be higher at low AR
I note again that the formula used by Erik is incompatible with that Cd=0.014 !!! proper formula uses wing area
It can be calculated by frontal area, but ina that case the drag coefficient vill be different (much higher) number
2/ BUT ... as Serge already wrote, we must calculate with RE number at our speeds and if you will take NACA 0018 as anexample for two wings of the same area (also the same frontal area), you will see that half span will lead to twice RE number and thus lower drag coefficient, so in some cases you can see just opposite result VD~
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apparently 30 % thick would have good lift, but'd look a bit odd .
NACA did further testing with airfoils up to 24%(if I remember right). It appeared that the lift advantages disappeared or went down over 21-22% thick(we're talking non-flapped airfoils here). It is really hard to beat the NACA 0018 for flap-free models in our size ranges. You can graft on the leading edge from a 5 digit section so the wing doesn't look so fat. I've tried that and it seemed to work just fine with no noticeable problems. Making a baseball bat leading edge(just the opposite) also helps improve lift a little, the combat guys have been doing this for eons. Put the high point at 18% or so of the airfoil. Even the big guys are doing it. take a look at the Extra 300: http://www.extraaircraft.co.uk/img/Extra300S_HA-RED.jpg
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Another thing that helps improve Cl max on single-element airfoil sections, although counter-intuitive, is to have a BLUNT trailing edge. While the "linear aero types" might insist that a sharp trailing edge is essential, in reality, Cl max is mostly a function of how long you can keep the airflow attached to the surface. A blunt or squared off shape causes a small vortex to form just aft of the trailing edge. This vortex acts like a "pump" and re-energizes the airflow a bit which keeps it attached to higher angles of attack. The effect can be dramatic. The blunt t.e. also increases the section drag coefficient, but on a stunt plane, that's not too important as it tends to cause the plane to fly at a more constant speed. Not a good idea for a racer, however. A good choice... 18%-20% thick, max thickness at 18-20% chord, a fairly blunt l.e., and a blunt t.e. that is 2-3% of the chord in thickness. Just like the other guys have been recommending.
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I found TR 586 in 1962. That first chart won me a lot of combat matches.
;D ;D ;D ;D ;D
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The Impact I am building has a blunt trailing edge on the plan from 1994. I wondered what the effect would be. Thanks for the explanation.
My current plane has sharp training edges, maybe I will sand the TE down before I retire the model to see what it does.
I had a chance to see the jatsenko models flying, in a square they seem to hit the handbrake, make the turn, come to a dead stop turning and start flying again. A truly impressive flying style. The models are jig built and very light. With a 13cc they are only 1600 gram. Taking into consideration that they are fully detachable this is really impressive.
I found these drawings, on the classic airplane the flaps are part of the airfoil with a sharp trailing edge.
It seems that there are multiple ways to buld a good airplane
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flying, in a square they seem to hit the handbrake, make the turn, come to a dead stop turning and start flying again
A beginner reading this thread must come the conclusion that the best airfoil for c/l stunter has low drag in level flight and high drad (or even brake) in corner, but it is just opposite, proper airfoil is optimized to have low drag at high lift and if possible high drag in level.
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In this post I refer to the package not just the profile, I am convinced that the 13cc motor and the low weight of the model have at least or even bigger influence on the characteristics as the wing profile.
From the desription of the profile you seem to be looking for the CAP21 airfiol like Beringer is using. Big engine and very light model too.
Two totally different models with good flying abilities.
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;D
1/ Yatsenko fly Retro .60 and it is 10ccm motor, not 13ccm
2/ if it is so good setup, then why he must fly 4.8 lap times ? VD~
And I am certainly not looking for airfoil lile CAP (or Beringers wing design) ... if it is so good airfoil, why we do not see it on good places on results? VD~
It is just opposite what I mean.
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;D
1/ Yatsenko fly Retro .60 and it is 10ccm motor, not 13ccm
2/ if it is so good setup, then why he must fly 4.8 lap times ? VD~
And I am certainly not looking for airfoil lile CAP (or Beringers wing design) ... if it is so good airfoil, why we do not see it on good places on results? VD~
It is just opposite what I mean.
Hello Igor,
Interesting observation you've made about Yatsenko's lap times. I got a question: Do you think he flies that fast because he's used to it or just because the plane doesn't perform well at lower speeds, say, 5.1/5.3 laps?
Regarding Beringer's wing designs/airfoils -if memory serves me well- , Remi Beringer won the 2006 World Champs in Valladolid with that design.
Regards,
Claudio.
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Rob Metkemeijer built his own 13cc MB engine and is flying the shark at 5,3 sec/ lap. He flew this model at the Dutch Championships. Although trimming is still in progress I did like the way it turns.
The Beringer profile was developed by Gilbert Beringer and used by his son Remy who became World Champion in 2006. In the St Etienne area there are a lot of Caudron models flying around. I competed at his competition in St Etienne from 1981 to 1988 and only succeeded to beat Gilbert once.
The French pilots claim that the profile can only work on a very light airplane, 1600 grams should be the maximum. This is very light as he is flying a big four stroke.
Common factor on both airplanes is that they are both very light and have a lot of engine up front. With less power, like my Super Tigre .46 I would prefer the NACA 0018 or Eppler 473.
I want less weight for my next model and more drag to reduce speed in down maneuvers. Maybe increasing the wing area is safer than increasing wing thickness. Althoug wing thickness is a lighter solution than area.
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The French pilots claim that the profile can only work on a very light airplane, 1600 grams should be the maximum. This is very light as he is flying a big four stroke.
An that is the point, every airfoil will work at low loading, also flat plate VD~
Good arfoil will carry lot of weight and thus will be good weapon to fly in difficult conditions.
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Interesting observation you've made about Yatsenko's lap times. I got a question: Do you think he flies that fast because he's used to it or just because the plane doesn't perform well at lower speeds, say, 5.1/5.3 laps?
No I do not think, becuse Andrey fly slower laps with basically same aeronamics. In past I wrote about airfoils, I do not know if you saw it. One problem is lift and drag of the airfoil and the second is how "well" it makes the lift.
The amount of lift is important for high wing load as I already wrote in previouse post (look to latest WC and EC what are winning wing loads and airfoils), but very important for good properties of stunter is also how "well" it makes the lift. We see model which will do what we expect every time in every condition same way. Some flapped airfoils does not have linear response lift to AoA because of unstable separating on upper side of airfoil. If it does not happens "repetitively" the model does not have good properties. Aifoiled flaps like used on that modes are such example. So if we want to trim such model, we must push it out of that unstable speed/lift somewhere to stable airflow. And it always means some compromising. I would say that his model just fly well at that speed. I am sure it can fly slower, but I do not thing it will fly that well.
He fly well but if you will look longer, you will see that also after hours and hours of practice, he do not have consistent bottoms. He will do 20 perfect turns and then one bad. And that is sign of that problem.
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It might be interesting to open a thread on "how to fly slowly". Not sure which sub-forum it would make sense in. Maybe the Open forum would be best.
I know I don't feel comfortable with lap times much above 5.1 to 5.2 s--typically I am flying on lines up to 64 feet handle to center of plane. That is with glow or electric. Basically I just don't feel like it will make it over the top.
I am always amazed to see some planes just pull over the top--I wouldn't have thought they would make it.
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I have just received my copy of Profili2 a software where you can compare different profiles. I selected 4 profiles and compared the outcome.
The NACA has the lowest drag, the CAP the lowest increase but a wing load limit.
The GOE has a higher drag in level but seems to match the NACA 0018 in the maneuvers.
This leads to the conclusion that the Eppler 479 works best in the maneuvers and if I want to increase drag in level flight I have to increase wing area.
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I have just received my copy of Profili2 a software where you can compare different profiles. I selected 4 profiles and compared the outcome.
The NACA has the lowest drag, the CAP the lowest increase but a wing load limit.
The GOE has a higher drag in level but seems to match the NACA 0018 in the maneuvers.
This leads to the conclusion that the Eppler 479 works best in the maneuvers and if I want to increase drag in level flight I have to increase wing area.
Erik,
Can you please draw the polars of this two attached airfoils for me ?
This is a .jpeg image. Please let me know if this is OK for Profili or if I have to send you the documents with .dxf or .dwg extensions.
Your help will be much appreciated.
Thanks!
Claudio.
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Profili comes with a database of profiles, I cannot import a drawing. So if you have a name of the profile or something that comes close I can make a comparison
Profili is cheap, for an encouragement fee of 10 euro's you'll get a code to use it. The professional versions are more expensive.
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Profili comes with a database of profiles, I cannot import a drawing. So if you have a name of the profile or something that comes close I can make a comparison
Profili is cheap, for an encouragement fee of 10 euro's you'll get a code to use it. The professional versions are more expensive.
OK.
Thanks anyway.
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I find the Profili information interesting. I use Compufoil for my wing and airfoil lofting requirements, but the Eppler 479 is not in my database. The Eppler 169 is though, and when the percentage of thickness is set to the same percent, looks almost identical to the NA63A modified I use. I'm beginning to believe that both are very similar in performance as well.
Claudio, your airfoils most resemble the NACA 4 digit airfoils. Possible around 0022 -0024. Performance may wind up being in the same league as the NACA 0018, with the advantages, and drawbacks associated with the thicker high point.
H^^
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I found the E169 in Profili, the NA63A is not in this database. Seems to have even lower drag in alpha.
This should flow easily through the square maneuvers, do you have that experience?
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Erik,
Can you please draw the polars of this two attached airfoils for me ?...
Claudio.
If you have enough of the coordinates to enter by hand, Profili will put your wing section in it's library, draw it, and draw relevant graphs from XFOIL. If these are known, non-proprietary sections, they might already be in the Profili library.
Stephano has been very good about encouraging and helping with its use. For instance, he first sent me a password the day I e-mailed him, allowing me to send the $10.00 later. I don't know whether it's still a $10.00 donation, but it is very inexpensive for what you get. You should just do a Google search for "Profili" and find his site - it comes right up. Then e-mail him and obtain the program. I've used it a lot, even verifying that XFOIL sees the wing section that John Miller and I like as being a bit better than the NACA thick 00xx sections for my particular use. It will modify any profile in several ways and will draw comparisons of profiles and polars by drawing or graphing in different colors on the same grid. Finally, even the basic version will draw up ribs with sheeting and spar holes/recesses for tapered and elliptical wings, even when the wing sections change from root to tip; so it's a great help in building wings. There really isn't any reason not to just go ahead and get Profili, if you like what it does. There will probably be no real waiting, and it's easy to use. If your section is already in its library, you might even get your polars completed in a few minutes by sometime tomorrow.
SK
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Yes Erik, That is my experience. It also does not seem to get upset with turbulence easily, offering good penetration without inducing the stall at our speeds and usage.
Both I, and Serge, have been using the NAC63A and modify it by moving the high point forward to about 25%. When so modified, it seems very similar to the E 169 with it's normal high point at 26% of chord. I have been using a thickness between 18 and 20% on most of the designs I use it on.
You may want to increase the thickness of the E-169 from 14+% to the same range and re-compare.
I hope that Serge will use Profili and compare the E-169 and the NAC63A modified. I'd really like to see where we're going here.
H^^
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Very interesting profile. When I increase the thickness to 20% it shows more drag in level but less in alpha. I kept the highest point at 26% and reduced Re a little to see if that matters a lot.
The drag in level matches the thicker profile, very interesting.
This is a candidate for my next wing.
Have you got experience with the blunt trailing edge?
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Guys, you want fly square figures "easily" at cl=1? VD~
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I cannot follow your remark, can you explain?
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Well guys, I have my Profili installed and a nice set of polar graphics of my airfoil. Now, there's a problem:
I don't know what they mean or how to read them ???!
Can you please help me on this one?
Many thanks and sorry for my ignorance on this matter.
Regards,
Claudio.
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Have you got experience with the blunt trailing edge?
I've been using a 1/8" squared off trailing edge for years. No problems experienced.
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I hope that Serge will use Profili and compare the E-169 and the NAC63A modified. I'd really like to see where we're going here.
H^^
John-
I'm just quickly skimming through here while struggling to get my club newsletter finished today and bowing fiddle parts for orchestra tomorrow. This looks very interesting, and I'll try to do it as soon as I can get free, but I'm swamped for the day. I have posted some low resolution (to meed Leonard's 50Kb restriction) Cl vs alpha graphs comparing it (flapless and with stationary flap) to a comparable NACA 00xx section, but I will have to re-do more legibly and add the new section - not hard, but I have to get some serious effort into this newsletter for now. I'll check back in, when I resurface!
SK
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Well guys, I have my Profili installed and a nice set of polar graphics of my airfoil. Now, there's a problem:
I don't know what they mean or how to read them ???!
Can you please help me on this one?
Many thanks and sorry for my ignorance on this matter.
Regards,
Claudio.
Claudio,
I will attempt to explain, but will probably need to be corrected by others.
So what you are looking at are the coefficient of lift and drag (Cl and Cd) for different angles of attack (alpha). There are mainy ways to plot this stuff. So on the plot of Cl vs Cd, the curve is generated for different values of alpha. SInce this is a symmetrical airfoil, when Cl=0, alpha equals zero. My guess is that the ends of the curves are just the maximum and minimum values of alpha you see on the graphs which show Cl vs alpha directly.
But in the more general case, what does it mean. We have been debating here and there about thick wings, and making sharp corners, and so I think the direction of the argument is to see what airfoil gives the most lift with the least amount of drag at a high angle of attack (alpha), like you would see in a corner. In this case you would want an airfoil that has a high Cl and low Cd value near the ends of the Cl vs Cd plot.
However these plots do NOT include flaps, so maybe they are not completely relevant to the discussion--unless you are flying a non-flapped plane.
Hopefully Serge will fill in the flap question!
Ok, I am ready for the corrections! H^^
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Claudio, (and Erik as well) ... as Alan wrote, those plots are not usefull for maneuvering since they do not iclude flaps. There is no way to make a corner at lift coefficient close to 1 at our wing loads and speed we fly at.
Real analyze must be done with conjuction of elevator. Means 0 flaps and 0 AoA, then it must go by more and more AoA and more and more corresponding flaps deflection. Not so easy task.
That my orriginal statement which started this debate is, that as low as possible drag at high AoA and lot of flaps and as high as possible drag at 0 AoA and no flaps. Then we must include induced drag, prop thrust at actual speed, curvature of airflow etc. It is relatively lot of math.
Then we have to consider influences of changing airfoil moment to the controlling by elevator, bumps on lift polar (cl to alpha) if there are any. It is not so easy to play a numbers game and to say "that is the best airfoil" just by eyeballing of one or two numbers or polars.
However main meaning of those polars was in Alans post, it gives at least some imagination about the airfoil.
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Alan, John, Igor, thanks a lot for the input guys.
I understand now, it's much more complicated than I thought.
With so many variables in the equation, we can use those graphics as a mere starting point and that is it.
Thanks again,
Claudio.
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Yes, in any case you can see what modiffication makes what effect. So just add flaps to your airfoil and play with it, you will see :-)
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Igor has developed an airfoil, using a lot of information, that seems pretty good. Perhaps he'll share it once more.
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this is what I use on latest "MAX"
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SV-11 Rib profiles root tip, I tried many many airfoil shapes..too many planes\wings too little time ::)
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Interesting profiles,
I finally managed to figure out which profile I use. The Sky Writer profile I am using with great pleasure in bad weather conditions is very close to the E169 profile at 17,5% thickness.
My .46 model conversion is 1750 gram and has 40 dm2 wing area, originally it was Super Tigre .46 powered and 1600 gram at the 1984 WCh. I still remember Bob Hunt giving me compliments on the consistency of the bottom maneuver line.
Later I used this profile on a 1660 gram .51 model with a wing area of 44 dm2, this flies more stable and does great hourglasses.
I'll probably keep this profile and change the thickness to 20%
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SV-11 Rib profiles root tip, I tried many many airfoil shapes..too many planes\wings too little time ::)
Randy, you've a good airfoil on your designs. I've studied it in the past, and believe it is very very close to what I've found to be the NAC63A modified, or, now I must add the E-169.
Your tip 'foil shows what the root would look like with the length shortened, high point moved forward a little, and a few percent thicker to control tip stalls.
Good penetration in turbulence, over the blunter 4 digit airfoils, due to the smaller radii at the nose. H^^
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Interesting profiles,
I finally managed to figure out which profile I use. The Sky Writer profile I am using with great pleasure in bad weather conditions is very close to the E169 profile at 17,5% thickness.
My .46 model conversion is 1750 gram and has 40 dm2 wing area, originally it was Super Tigre .46 powered and 1600 gram at the 1984 WCh. I still remember Bob Hunt giving me compliments on the consistency of the bottom maneuver line.
Later I used this profile on a 1660 gram .51 model with a wing area of 44 dm2, this flies more stable and does great hourglasses.
I'll probably keep this profile and change the thickness to 20%
Erik, It looks that you have a good leg up on this aspect of stunt design. Notice Igors airfoil carefully. As I remember, he did a lot of work on it with many plots, and studies. This one had the least problems with the use of flaps. Interestingly, at least to me, there's a passing resemblence to the stock NA63A. No modifications at all. I may be wrong on this, but I seem to have been told one time, that the NAC63A was a laminar flow airfoil.
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Randy, you've a good airfoil on your designs. I've studied it in the past, and believe it is very very close to what I've found to be the NAC63A modified, or, now I must add the E-169.
Your tip 'foil shows what the root would look like with the length shortened, high point moved forward a little, and a few percent thicker to control tip stalls.
Good penetration in turbulence, over the blunter 4 digit airfoils, due to the smaller radii at the nose. H^^
Hi John
Yes that is why the tip rib station look as they do, I have never had any luck with making the root into a farther forward high point, that has hurt, not helped performance, and I have not had any tip stall problems with any of the SV airfoils
Below is the VECTRA airfoil, thinner but never had any problems carrying weight, or any stall problems even in very high altitudes and very hot humid condidtions
This is the airfoil I used in My Vectra and in Bill Werwages P-47, some people call it the Geo Bolt airfoil
Regards
Randy
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These 2 are the NOVAR , and Evolution airfoils, many people have used this on, it is slightly lower aspect ratio with 1\2 inch more chord 30 sq in additional to the large SV ships, and is 1\4 inch thinner, but still retains its roots from the SV design series.
This wing has proven very good in turbulant conditions, This was developed in the early 90s with many many talks with Bill Werwage, I took the SV airfoil shape out very very slightly to the design shape of Bill's USA-1, in hopes of getting a little better wing in high winds and turbulance but still retaing the high performance high lift ability of the SV series
All of the SV series are very good in winds and hi heat\humidity , but we all keep looking for more ;D
The 2 drawings below is not to scale, the tip rib is about 2 inches shorter than the root
Randy
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You can see the family resemblance with all these airfoils. I'll bet that when I take the airfoil I like, which seems almost identical, and alter the percentage of thickness, to match your examples, they will also be close to what you have at the same percentage.
In my experience, these are great airfoils. They work, and I've gotten repeatable results, as you have.
As you said, we always look for "more bettah". H^^
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This is the airfoil I used in My Vectra and in Bill Werwages P-47, some people call it the Geo Bolt airfoil
Possibly the only airfoil to beat Igor's at the last world championships.
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Hmmmm. of course the pilot doesn't matter, right? LL~
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Lots of other stuff matters, particularly team management.
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True enough Howard. I can't agree more, but of course, it's also good to get oneself the best tools as well.
I'm reminded that a flintlock, and a M-60 are both firearms, but what a difference..... LL~
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They are exactly the same, a near hit is a total failure for both. I cannot afford to choose a wing profile that does not work so I decided to stay on the safe side and use the E169 profile at 17,5%. In order to keep future options open I am building the plane without fainings and big flat fuselage sides so any profile will match. The wings bolt on from the side.
The tail section was built in depron, only 14 grams.
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Erok, please keep us posted on your progress. I think you've made a good choice. One question, are you including the chord of the flaps when figuring percentage of thickness? H^^
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I use the profile as specified, so 17,5% is as profili draws the profile.
I start with the root profile for a 295 chord, cut away the last 40mm and add a 10mm TE before sheeting the wing. So effectively the chord then is 295 - 40 + 10 = 265.
For the tip I use 245 - 40 + 10 = 215
I will keep you posted on the progress.
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On a stunt design with a w.s. of around 55" how thick should the high point of the root rib be if the cord length (not including flap measurement) is 10.000"? And same question for tip rib of 8.375" . Also hinge line distance of 17" would be about right? Power supplied by ST 46 with bristunt ABC P&L set and head supplied by same (might try saito 40 also later )
I think you guys have covered most of the airfoil possibilities as far as thickness goes. But based on some recent experience, I think that it makes almost no difference if you don't get the first 3/4" or so right. The "as delivered" ARF Strega is has a hugely thick wing, far more than even I had considered using, OK wing loading (about the same as my NATs airplane) and relatively large flaps. But the leading edge is pointy and it won't fly even mild corners, even at sea level and reasonable temperatures, without being on the edge of stalling. Blunt it a little, and no problem. As described here:
http://www.brodak.com/files/file/Strega_Building_Wing_Instructions.pdf
It's essentially as Al described in his 1973 AAM stunt column - if it comes to a point (say, less than 1/8" LE radius) it "buffets" (i.e. stalls) in the corners, and you can fix it by sanding back through the sheeting to the underlying LE wood and rounding it off.
I also think the other issue is the "departure angle" of the leading edge shape. The ARF Strega airfoil also seems to be designed to fair smoothly into the 1/4 square at about a 45 degree angle, essentially going straight back at 45 degrees until it needs to start turning to fair in with the high point. Sort of like conformally mapping the Nobler airfoil with the x ordinates not scaled, and the Y ordinates scaled by 150%.
Point being, no matter what you do to the rest of it, if the LE radius isn't sufficiently blunt, you're going to have problems.
And before anyone jumps on me for jumping on ARFs again, or Cardinal/Tsunami/Stregas, I would note that the Vector 40 and the T-Rex "Bradleymobile" ARFs don't have the same issue at all - Brad's being particularly blunt, not quite to "Intimidation"/"Excitation" standards, but noticeably blunter than most.
Brett
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" I think you guys have covered most of the airfoil possibilities as far as thickness goes. But based on some recent experience, I think that it makes almost no difference if you don't get the first 3/4" or so right. The "as delivered" ARF Strega is has a hugely thick wing, far more than even I had considered using, OK wing loading (about the same as my NATs airplane) and relatively large flaps. But the leading edge is pointy and it won't fly even mild corners, even at sea level and reasonable temperatures, without being on the edge of stalling. Blunt it a little, and no problem. As described here:
"
Brett
You are Exactly correct , the LE is very important, as is the hingelines, that is why the SV airfoil look like this...
, this size wing should not be any sharper than about the radius as a dime, or depending on the thickness maybe a little blunter
I have a guide printed on most of my plans that can be cutout and used as a template for getting the LE radius correct. It also needs to be consistant across both halves of the wing panel LEs
Regards
Randy
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Randy, is that the center and tip ribs?
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I feel I need to apologise to Howard for my playful, flip, answers to his recent posts. It was jousting in fun. H^^
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Randy, is that the center and tip ribs?
Hi
Yep that is the center and tip for 1 of the SV wings
Randy
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I feel I need to apologise to Howard for my playful, flip, answers to his recent posts. It was jousting in fun. H^^
Aw, John. I can't expect to have the monopoly on playful, flip answers.
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I would note that the Vector 40 and the T-Rex "Bradleymobile" ARFs don't have the same issue at all - Brad's being particularly blunt, not quite to "Intimidation"/"Excitation" standards, but noticeably blunter than most.
The Bradleymobile uses the Saturn airfoil. Which is what I call my "thin standard" airfoil. I believe it is the perfect compromise in the slightly lower aspect ratio T-Rex (I mean Bradleymobile). I was particularly adamant with the factory that the front of the airfoil must perfect match the plans. They did not disappoint.
Viewed in CAD all of these airfoils start looking alike, at least from the Hunt and SV series. Randy's SV airfoils are about 3/32" thicker top and bottom than the Saturn airfoil (if blown up the same chord). The Saturn airfoil was supposedly derived from Billy Werwage. The Saturn airfoil looks suspiciously like a Genesi/Legacy/Buccaneer airfoil. As Brett said, these are all not way off the Imitation airfoil (which is a favorite of mine also).
I would not go thicker than Randy's airfoil. There is no need. Just adds weight, drag, etc.
I would suspect the Saturn airfoil is similar to PT airfoil also.
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""Randy's SV airfoils are about 3/32" thicker top and bottom than the Saturn airfoil (if blown up the same chord). The Saturn airfoil was supposedly derived from Billy Werwage. The Saturn airfoil looks suspiciously like a Genesi/Legacy/Buccaneer airfoil. As Brett said, these are all not way off the Imitation airfoil (which is a favorite of mine also).""
Hi Brad
To be accurate..one of the SV airfoils is 3/32 thicker, The SV airfoil system lets me plot many adjustable sizes of the STuntcraft airfoils, by sliding up and down the scale of SV rib stations R-2 - R-16, there are 18 avalible Stations for the SV foils, more if I want
So there are SV wings that are thinner, the same, and thicker than the Saturn.
Regards
Randy
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I would not go thicker than Randy's airfoil. There is no need. Just adds weight, drag, etc.
How do you figure a thicker airfoil adds weight?
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looks like the air in the wing weights 1oz VD~
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How do you figure a thicker airfoil adds weight?
More wood...
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I'll pass that along to the structures guys at work. They had it backwards.
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More wood...
Hey, crazy and maybe impossible idea - if you cut away the wood in the middle, it doesn't weigh anything. And the wood at the edges might not have to be as strong, either, so maybe make it thinner or softer. Why, it's almost as if the thicker you make the airfoil (or the cross-section any other part), the lighter it is for a given stiffness, or, can be stiffer for the same weight. If only we had known that back in the 80s, we could have started making our wings thicker than aerodynamically necessary just to gain rigidity. And what great luck, in a startling coincidence, it just so happens that we have such powerful engines the associated drag could easily be overcome. My God, we could have even told people about all this and why in print since 1979 and on the internet dating back to 1995 on the old Compuserve and RCO C/L forums and published plans in magazine utilizing that effect.
Alas, if we had just thought of this, it would have been possible to win innumerable big contests against guys trying to build the same old 60-70's design ideas with 50's engine and prop technology lighter and lighter and with shinier and shiner finishes. They may never have realized that our airplanes always seemed in trim and were more controllable because the added stiffness permitted far more consistent control responses, even without 3000 flights per year of practice. They might not have even figured out what we were doing, or didn't believe us, so they couldn't take advantage of it. They might even come to the conclusion that the whole thing was rigged!
I am sure glad none of that ever happened.
Brett
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Touchy touchy...
I thought you used foam wings... The thicker the wing the more sheeting required and the heavier the core.
The thicker the wing they higher the surface area. Even with an I-beam the middle beams and the capstrips would all be longer.
I never said that thicker wings would not be stronger...but to what end? How strong do they need to be?
PS: Every time you go on one of your rants about how the design of stunt airplanes has reached some consensus and everyone has conceded that you are correct, I am reminded of the great Al Gore. I am also reminded that you keep getting beat by guys doing stuff you don't do, in fact they are beating with stuff you say *will not work*... Since your method works for you, it does not make everyone else an idiot.
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...Alas, if we had just thought of this, it would have been possible to win innumerable big contests against guys trying to build the same old 60-70's design ideas with 50's engine and prop technology lighter and lighter and with shinier and shiner finishes. They may never have realized that our airplanes always seemed in trim and were more controllable because the added stiffness permitted far more consistent control responses, even without 3000 flights per year of practice. They might not have even figured out what we were doing, or didn't believe us, so they couldn't take advantage of it. They might even come to the conclusion that the whole thing was rigged!
I am sure glad none of that ever happened.
Brett
At first, I was going to ask how we got from airfoil thickness to sarcastic conspiracy theories, but, I've decided to make some popcorn and sit back and enjoy it instead. These are rising(sinking?) to the level of some of the Ted Fancher/Al Rabe "debates" S?P
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Looks like the administrator is being too lax. Some people just know what button to push.
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Guys, what's with this acrimony?
Brad made a mistake with his weight comment.
A perfectly good teaching momment was presented that could have educated a lot of potential designer-builders about how to go to thicker wing structures without a large weight gain, as well as going into the added strength issues that could be realised. The thread could also have gone into the desirablilty, use of, and options between thin and thick airfoils.
I know that the above has been discussed before on these forums, but It seems that about every two or three years, the subjects get re-run as new members are added to the forums
"Can't we all just get along?"
I know it's sometimes hard to keep onself civil, when the buttons are pushed. I've had to wait a while, or apoligise for resposes made in haste.
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When a guy whose aeronautical knowledge is magazine-article deep continually claims technical superiority over the rest of us because we "don't understand" the concept of the gyroscopic effect on a propeller being a function of it's distance from the airplane's cg, the stupidity of full-span flaps because of their excessive wing tip "vortexes", or the special aerodynamics of weather vanes and lawn darts not taught in school, it's pretty hard to keep from picking on him.
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Brad made a mistake with his weight comment.
A perfectly good teaching momment was presented that could have educated a lot of potential designer-builders about how to go to thicker wing structures without a large weight gain,
No, I did not make a mistake. You just agreed with my original assertion.
All things being equal a fatter airfoiled wing has more surface area, and if all the structure and sheeted areas are the same it will be heavier than a wing with a thinner airfoil. That is a plain and simple fact, comparing apples to apples.
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When a guy whose aeronautical knowledge is magazine-article deep continually claims technical superiority over the rest of us
BS. Those are the low self esteem voices in your head, Howard. I never claimed any such thing. I just don't agree with everything you and your substantial group of self proclaimed "definers of the everything stunt that is right" say.
By not agreeing with you (in any case) it means that means I am "claiming superiority" over you.
In fact, I have never claimed to be a member of the SACRED BRAIN trust that you, Buck, and all the other aerodynamic supra geniuses belong too. I have gone out of my way in my articles to say that I make no claim whatsoever to being an aerodynamic engineer.
Now go find ManBearPig... I am cereal.
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No, I did not make a mistake. You just agreed with my original assertion.
All things being equal a fatter airfoiled wing has more surface area, and if all the structure and sheeted areas are the same it will be heavier than a wing with a thinner airfoil. That is a plain and simple fact, comparing apples to apples.
That's one of the main problems with forums. Not being extreemly clear on your meaniing.
The mistake I'm refering to is not so much that you were wrong in your assertion, rather in your blanket statement regarding weight increase due to more balsa. As I'm sure you are aware, careful design practises can create a thicker wing, of the same area, with minimal increase in weight. Look at Gordy's Tony, over 700 sq inches area, and a thick wing, yet it is very close, in weight, to a thinner wing of the same area.
Without making some design decisions, to save weight, a thicker wing can gain weight. But, with the added strenght available due to the thicker structure, the parts can be made a little lighter, and still have adequaqte strength where needed.
So, the mistake was not that what you said was totally wrong, rather as a blanket statement, IMHO, needed clarification, and thus the fact that with my next statement, we agree. H^^
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I think Sparky should create a new forum, called the "EGO parking lot", and that it should be the first place we all go.
You know what it is for.
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That's one of the main problems with forums. Not being extreemly clear on your meaniing.
The mistake I'm refering to is not so much that you were wrong in your assertion, rather in your blanket statement regarding weight increase due to more balsa. As I'm sure you are aware, careful design practises can create a thicker wing, of the same area, with minimal increase in weight. Look at Gordy's Tony, over 700 sq inches area, and a thick wing, yet it is very close, in weight, to a thinner wing of the same area.
Sure. There is no argument whatsoever that some people build lighter than others, but surface area is surface area, and the growth of surface area is not linear to the scale of the plane. All things being equal, a thicker winged plane IS HARDER to builder as light as a thinner winged plane.
PS: I did not realize the tremendous scrutiny I was under at the time, I suppose. I did not realize we would be tested later... n1
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What's missing is how beams work. The max stress in beams, as I recall, is proportional to the moment of inertia of the cross section, so, for a given strength, the thicker the wing, the lighter. Similarly, for a given torsional stiffness, the thicker the wing, the lighter. Brett's story was about how he was able to exploit these properties when better engines reduced the drag penalty for thickness. Indeed there's stuff like paint that is proportional to surface area. You could trade these and see how they come out. By my ciphering-- and you guys can check--, an NACA 0018 has 0.094% more surface area than an NACA 0015, and the square of thickness ratio is 1.44. So the weight that's in an 0018 wing for bending strength would be 69% as heavy as for an 0015 wing, and the stuff that's proportional to surface area in an 0018 wing would be .094% heavier than that of a 0015.
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Whats missing here is some hard data, not just the math. Randy, where are you? I know you have weight data for sheeted wings of all your stuff, 'cause you told me how much overweight the wing panels were on my Dreadnought when I asked you how much they should weigh. :D You could tell us how much more a larger SV type wing weighs vs a smaller one.
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I miscalculated on my last Impact and used the sheeted airfoil as the template for the unsheeted root rib, so the root was .15" thicker than that of the previous Impact. The new dog weighs 63 oz., and the old dog weighed 66 oz. There you have it.
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Brad, I apologize for being mean to you. People asked why I was being a jerk, and I just blurted it out, more to explain it to myself, than to others, I think, because I don't understand why you get me so upset. You are a proud papa and you get a little enthusiastic about promoting your product, as salesmen will, but It doesn't make sense for me to be so offended by your incredible technical statements. Looking back at them, I do think you characterized me as being a bit thick about understanding the gyro thing, but you didn't come out and say I was stupid for using full-span flaps, as I remembered. Maybe I was offended because I thought you were belittling my professional ability, or even my profession, but it was probably just my ego, as Alan said. It was wrong for me to have said what I did. I might still challenge you to prove your more outrageous statements, but I'll try to maintain some humor. Please try to contain the bluster, though. I'd rather think of you as the guy who fixed Jimby's fuel tank problem at the Nats than as the guy who made fun of me when I tried to answer his question on flap efficiency.
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Now listen up all science people...
Stop using the word *proved*.
Little if ANYTHING in stunt has been *PROVEN* as there has been very little (if any) *real* SCIENTIFIC METHOD applied to the application. There are rare examples of real science, and little if any *real* modern aerodynamic testing or data collection has taken place to date when it comes to our stunt models that fly about 55 MPH held by a string in cross winds that can reach nearly half that of the forward air speed. Even in the case of wind tunnel testing I REALLY DOUBT the conditions and speeds that we fly at have been tested by Boeing, NASA, Lockheed, etc.
When it comes to stunt airplane designs, there are "theories", "observations", "opinions" (both informed and uniformed), consensus (be it large or small), a whole crapload lot of conjecture, and finally a huge dose of political posturing from the brass collectors to mix it all up (you know...so and so's design is "the best" because he won XXX number of contests with it----never mind he mind have superior hand eye coordination and simply be a superior pilot or a frothing at the mouth completely obsessed type A perfectionist winaholic).
Aside from the extremely few times that someone actually made apparatus, collected data, or performed calculations or simulations (as in the case of Frank Williams experiments, Line 2, and now the ESC feedback in flight load data from electric) very little data has been collected, or analysis performed to conduct real SCIENCE. No real conclusive *PROOF* of superiority is evident in any of the stunt designs that are in use today.
To mock the simple methods of observation used typically be just about everyone in this sport with pseudo scientific claims of "consensus" claimed as "absolute proof" is an insult to every stunt flier that does not have a formal education, and relies on simple observation, simple experimentation, sweat, and open mindedness to try new things to achieve better performance.
All this "show me your data" stuff is just elitist bull. Nobody has data. If I say full span flaps cause drag... it is because that is what I saw in the models that I built. It is not my responsibility to "prove" that I am right, maybe you should PROVE that I am wrong!!! You can't!!! You can give your "opinion", and throw out your pedigree but I doubt you have the facilities or willingness to do any real testing to show one way or the other (and frankly, I don't care what flaps you use---go with God my son...).
To throw the word "proven" around simply shows how little science is actually being discussed. REAL scientists take this word *proven* very seriously...
What you have here people is consensus science (Al Gore), not proof, and usually the consensus groups are very small. Try to get a consensus among the world's stunt fliers on anything!!! I dare you!!!
Finally, I tell you what, I will show my doctoral thesis on stunt design when you show me yours. Even if you had one, it would just be one....waiting to be disproved by the next.
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Now THERE'S a good idea! A doctoral thesis on stunt design! 8)
Heck, I would settle for a master's thesis on stunt design..
(Well, I take that back, I've read some thesi (sp?) that were pure crap..)
L.
"Keep a stiff upper chin." -Samuel Goldwyn
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I don't think I'd settle for less than a "Triple Dog" dare, but I'd concur anyway on the unlikelihood of any sort of universal agreement.
>Even in the case of wind tunnel testing I REALLY DOUBT the conditions and speeds that
>we fly at have been tested by Boeing, NASA, Lockheed, etc.
Here I'll disagree some. While I don't think that exact conditions or any sort of comprehensive set of conditions for hemispherical flight can be duplicated and definitively handled, useful data for stunt-model design has been found and presented. I think the NACA researchers of the past have been underrated, along with current NASA folks too. In particular, on this thread is evidence that NACA did try to duplicate our conditions in a limited but very useful way. If there is a problem in disemination of information, it may well lie at least as much in reluctance to examine what is presented as in a shortage of research. Data on page one of this thread is an example. It answers the original query and then some, but has just sat there relatively unheeded, while people continued to discuss the topic.
There were some fine minds conscientiously working on problems we have, and while their data may be modest, it is also good. So I'll just say for this thread that NACA TR 586 ('Airfoil Section Characteristics as Affected by Variations of the Reynolds Number') and NACA TN 364 (deals with scale (RN) and turbulence affects on the actual NACA sections found in a large number of stunt models today) are relevant, although TN 364 was probably done before the full effects of tunnel wall interference were known. Even then, these guys tried their best to solve that problem and presented revised data later. Having read through a lot of this NACA/NASA literature, I have high respect for the people who researched and wrote it. One has to look for the material, but I have found it to explain a lot about the evolution of stunt and why certain things work the way they do so many years after the research was done. Newton's Laws need to be understood too.
"Pseudo science" is not what is practiced by serious science and engineering types; it is jargon thrown around by people who don't know the basics. The tools and language of science help handle complex ideas. They do not create the ideas, but rather make progress possible. Fortunately, we can enjoy this hobby with a pretty good margin of success, without much scientific background. Knowing real causes and dynamics helps make things right at the top end. I only see a problem with the spreading of misinformation, some of which is not even a matter of opinion; I lost count long ago of the mistaken attributions of causes to effects. The problem here, as elsewhere, though is intolerance.
So, before anyone gets paranoid about anything further here, this has not been aimed at any particular individual, even though it began with a response to Brad's quote. My "rules of engagement" from some years ago on the "other" forum included the caveat that whenever a disagreement includes any statement about another party involved, there is a liklihood of unnecessary damage being done. I think we should lighten up a bit. There is really nothing wrong with kindness, regardless of how ridiculous you feel another person's behavior. Well, that's my "opinion" anyway. FWIW.
SK
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Serge, good to see you back. Have you had time to compare NA63A and E-169? I've been waiting to see your findings. %^@
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There's lots of pertinent science extant. It merely takes some effort. Much of it is now available free on the Web, so it's easier than ever to acquire. Serge's mention of page 1 of this report is a good example. When I was in high school, my family moved to a town in Indiana where there wasn't much to do. I was into combat, and I figured that if I could get a killer airfoil, I could do me some winning. My reasoning was that good airfoils and bad airfoils weighed the same and took about the same effort to build, so a good airfoil would be "free". There was a good library in town (Serge knows it well), so I went there and looked up aerodynamics. That led me to the stacks in the back room where they kept the old NACA reports. I found the airfoil plot from NACA TR 586, which Serge posted here on page 1. That, plus one other NACA paper gave me the killer airfoil. Without anything I learned in college, that airfoil gave me, a mediocre flier, the advantage I needed to win the Nats and make the US team twice.
As for control line airplanes in wind, we discussed that on SSW. I added a little to the explanation of flying in sideslip last night, including my guess at how Brad thinks of the effect of wind on stunt planes. All you need (except for a couple of definitions) is in NACA Report 1098, http://naca.central.cranfield.ac.uk/reports/1952/naca-report-1098.pdf . The ironic thing is that, rather than claim that science cannot explain the forces on a stunt plane flying in wind, a guy with no formal training could spend a couple of hours looking at that report and have a better understanding of the forces on a stunt plane flying in wind than 80% of aeronautical engineers.
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"All this "show me your data" stuff is just elitist bull. Nobody has data."
There are the NACA report examples above. Here's another example just above. Folks were discussing wing weight. I wondered how wing surface area varied with thickness, so I calculated it. With Excel, it took me 15 minutes, using no elitest math. The answer I got was that it's a trivial amount. In fact, I hardly believed how little it is. I figured that somebody, particularly somebody who asserts that fatter wings are heavier, would spend a few minutes of his own doing a calculation to refute mine. Nope, too much work.
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"All this "show me your data" stuff is just elitist bull. Nobody has data."
There are the NACA report examples above. Here's another example just above. Folks were discussing wing weight. I wondered how wing surface area varied with thickness, so I calculated it. With Excel, it took me 15 minutes, using no elitest math. The answer I got was that it's a trivial amount. In fact, I hardly believed how little it is. I figured that somebody, particularly somebody who asserts that fatter wings are heavier, would spend a few minutes of his own doing a calculation to refute mine. Nope, too much work.
Did you just "prove" what I said? I think so... thanks.
You said the answer was trivial in the case of the surface area...comparing what two airfoils? A Strega and a Smoothie? Did you count the added finish weight which also a function of surface area? Fully sheeted? Partially sheeted?
Of course, surface area is only one consideration in the discussion. Yes, indeed one can *compensate* in the construction of fat winged models so that the weights are comparable to thinner winged models, but it inherently obvious that a larger structure, all things being equal, will weigh more than a smaller structure. Heck the music wire that suspends the bell crank is longer on a fatter wing... there is one weight gain right there.
Then their are the other compensations that must be made in the model to accommodate a big fat airfoil. Typically, the fuselage has to be taller to accommodate the fatter wing (this is doubly true in the case of requiring clearance for a tuned pipe--which when I believe all that *beneficial side area* came into vogue).
If you feel these are all acceptable weight gains, then fine. It does not change the facts that in every apples to apples comparison larger=heavier.
I cannot even believe this is even up for debate.
PS: My "elitist" comment was made in regards to the attitude that there are some who feel their logic is just a little bit better than everyone else's logic. I have no problem with using science as much as possible.
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John-
I do plan to do that comparison, but I'm just finishing a little personal investigation into elliptical wings that I started on SSWF. I've done all the derivations, but am trying to put them into a nice reference document using Microsoft Word's equation editor. Like the word processor itself, this is a great tool with one big disadvantage: it is extremely labor intensive for heavily formatted documents. IOW, I'm taking a long time to do something that can be done quickly with a pencil. This and some violin work are holding me up a bit, and I want to post the final results, when I'm done. I'm intrigued by what XFOIL might think about the airfoil comparison - with flaps - too, and I'd like to include my elliptical-leading edge/NACA aft section in the comparison. It'll be a while, but I'll post the results - probably here - unless I can get in under the SSWF's 50-Kb limit with reasonable resolution.
>"but it inherently obvious that a larger structure, all things being equal, will weigh more than a smaller structure."
The problem is that all things are not equal. There is a wealth of material out there about thicker wings allowing lighter structure to accomodate equal loads. Now in deference to the scaling back of standard-sized modeling materials, this is compromized, but the structural design of wings can be tailored to these advantages. Among the 6000+ items listed in a bibliography I compiled of material on tailless and related aircraft, there are numerous weight analyses for full-sized aircraft having wings using airfoils comparable to our thickest stunt airfoils. This was necessary in the case of "flying-wing" aircraft, even at very high RN's, to accomodate aircraft loads carried within the wings themselves. This continues to be an important consideration in design for range and energy consumption. I believe Howard's analysis already takes into account any finishing weight.
Certainly fuselage weight is relevant, but I believe that it has been argued and demonstrated effectively that the lightest weight is not necessarily the best weight, just as high aspect ratio, with all its advantages in lift and induced drag reduction, has its limits in CLPA. Wing plan form also affects how heavy the structure needs to be, and this too is a more mathematically perceived separate topic.
SK
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There is a wealth of material out there about thicker wings allowing lighter structure to accomodate equal loads.
I never said otherwise.
Of course, what "loads" are required?
In the case of the foam wing, very little cane be done to reduce the weight of the wing. Most foam cores are cored to the same thickness, and the sheeting is the same. I would not hazard a guess at how strong a foam wing really is compared to in flight loads that are actually required. Probably serious over kill.
I never said that thicker wings cannot be built light.
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Brad-
I was editing my last post as you responded. It is worded slightly differently.
SK
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Assuming that it applies to model airplanes, one could calculate the incremental weight of that bellcrank axle. Steel is about .283 lb./cubic inch. For a 12" chord, the difference between the 15% and the 18% wing, assuming the bellcrank axle is at the max thickness, is .03 * 12 inches * Pi * .0625^2 sq. inches * .283 lb./cubic inch. * 454 gm. / lb. = .57 gram. You might want to go to a thicker axle because of the increased stress, but the lighter, thicker wing might let you get away with a thinner one. Which would it be?
The .094% increase in weight for the surface of the 18%-thick wing relative to the 15% wing still looks suspicious to me, but I'm safe, because nobody will ever check it.
Ironically, my structures advisor is flabbergasted that I don't weigh every component of my airplane as I'm building it. Heck, it weighs what it weighs. The last one came out OK. I did break down and weigh before and after the fancy clearcoat to see if the plane could tolerate another toot of it.
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"Did you just "prove" what I said? I think so... thanks."
You missed the nontrivial beam part.
"I cannot even believe this is even up for debate."
How'd that statement hold up in your strength of materials class?
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Geez, I just finally tossed my ancient CE233 "Strength and Materials" text.. And to think, there was a probably a wealth of good information pertinent to model airplane structure in there!
I kept my Statics and Dynamics texts (also ~40 years old), however, just in case I had to discuss distributed loads on a beam or design a bridge or something. I remember those (sophomore ME) classes, they were easy and fun.
(I also tossed my Thermo book - I hated that class, and despised steam tables or anything related to them. Only the Carnot Cycle seems to have any lasting value to me..)
((Yes, most of that NACA material is approaching 60 years old, but that doesn't mean it has no value..))
Steer me to a thesis on CL Stunt airfoil design.
L.
"My experience is that as soon as people are old enough to know better, they don't
know anything at all." -Oscar Wilde
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Steer me to a thesis on CL Stunt airfoil design.
http://www.profili2.com/
http://www.netax.sk/hexoft/stunt/notes.htm
http://www.mh-aerotools.de/airfoils/
http://www.ae.illinois.edu/m-selig/pd.html
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>((Yes, most of that NACA material is approaching 60 years old, but that doesn't mean it has no value..))
Ah, old air too? Sometimes I think our (sometimes) wry humor bemuses even ourselves!
A bit of whimsy suggested by Larry's observation... I remember reading in John Paulos' book, Innumeracy - Mathematical Illiteracy and Its Consequences , his simplified computation of the probability that one is presently inhaling an air molecule exhaled by Julius Caesar with his dying breath. His conclusion, based on thorough mixing of all air molecules over the intervening millenia, was that the probability exceeded 99%.
...So a couple of these molecules, long ago expelled from the Langley wind tunnel, meet up somewhere over Muncie some summer, and one says to the other, "Oh-oh, here we go again! 'another of those d**ned NACA 0018's headed this way - all painted up and shiny, with strings attached!"
Hmmmmf. 'just a ...thought.
SK
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""...So a couple of these molecules, long ago expelled from the Langley wind tunnel, meet up somewhere over Muncie some summer, and one says to the other, "Oh-oh, here we go again! 'another of those d**ned NACA 0018's headed this way - all painted up and shiny, with strings attached!"
"
Hmm I think I like that !! y1
Randy
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Just an additional note to Howard's posted URL's:
The index on Martin Hepperle's site includes control-line topics not otherwise listed, including a derivation for approximate wing asymmetry due to air-speed variance across the span, as well as prop moments of inertia and gyroscopic precession, among various interesting technical topics. I think all his low-RN airfoils are for soaring, FW's, and racing though.
SK
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Is there a formula for calculating the total surface area of a wing in there somewhere? I was looking over on Nasa.gov today and didn't find anything...
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Is there a formula for calculating the total surface area of a wing in there somewhere? I was looking over on Nasa.gov today and didn't find anything...
Steve-
I haven't looked for one, but I think you'd come close enough if you just measured along the outline of your preferred wing section with a flexible rule or marked strip of paper, took the ratio of that measurement to the chord, and multiplied it by the computed wing area.
Otherwise, I think it would be kind of a complicated calculus problem or perhaps something for a CAD program to do, since wing sections differ and have lengthy mathematical definitions. Whatever you might find would be limited to specific sections or "How-to's". Maybe there's a formularized approximation, but there really can't be a generalized exact formula.
SK
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What I did was to take the NACA formula, calculate the ordinate for a thousand points with Excel, then add up the straight line distances between each of the points. Pretty crude.
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What I did was to take the NACA formula, calculate the ordinate for a thousand points with Excel, then add up the straight line distances between each of the points. Pretty crude.
Pretty neat! Sort of the poor man's integration again.
SK
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Pretty neat! Sort of the poor man's integration again.
SK
Howard, I normally use my ancient Stuntrib program to calculate a NACA00nn airfoil (you can
specify a zero height TE to be "pure"), and open the DXF file it produces in AutoCAD. AutoCAD
and most CAD programs will report the length of a polyline instantly. You can use the root
and tip airfoil surface lengths and wing panel length and get a surface area calculation of a trapezoid
very easily.
Also, the CAD programs will report a cross sectional area of a closed airfoil shape very accurately,
and a similar approach can calculate volume of a wing panel.
More able 3D software will do more serious analysis, finite elements, of course, if you have access to
it and can do the effort to get your model data entered accurately. If you're going to do the wings, you
might as well do the entire model, and all component parts, and have it find centers of mass, moments
of inertia, and other data that will need analyzed! ;->
Of course, if you're simply handy with a spreadsheet, you can get it all pretty easily, as you have.
L.
PS - thx kindly for the links..
"One of the symptoms of an approaching nervous breakdown is the believe that one's
work is terribly important." -Bertrand Rusell (1872-1970) English mathematician and
philosopher
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AAAGGGGHHHH.....
Don't even ask me why but....My favorite airfoil is 20-21% with about 1/2 to 5/8" leading edge radius.
It just seems to me that the overall performance is good enough in all conditions. Kinda like what Dave
Fitz was looking for.........out!
Jim Pollock
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including a derivation for approximate wing asymmetry due to air-speed variance across the span, a
A long time ago, I was curious and did that calculation for a 48" span, .6 taper ratio combat plane. It turned out that for that particular planform, the lateral center of lift was .625 inches outboard of the geometric center line, So, being a combat pilot, I just took the easy way out and wacked off 5/8" inch from the outboard wing HB~> However, It would probably be a good idea to take a more exact approach for a stunt plane due to the more critical nature of their flying qualities. Or you could just adjust things with weight so that the lateral position of the c.g. is at the same butt line as the lateral position of the a.c. That's easier.
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Stunt guys have ballast boxes at the right wingtip. They pick an asymmetry, add a bunch of ballast, then remove ballast until it comes out right.
Did you notice the Son of Snort wings? They're a result of that kind of calculation. The left tip chord is 3/4" more than the right tip chord. That and the 2-degree skew in the leading edge kinda cause vertigo when you look at it.
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To all:
I would be more than happy to do Xfoil runs on your favorite stunt airfoil. Since I don't fly stunt, you are in no danger of my "giving away your secret weapon". I can do them with or without plain flaps and at the appropriate Reynolds number and ambient free-stream turbulence. All I would need is a non-dimensional list of coordinates, OR a CAD drawing that I can import and then non-dimensionalize. If you want it done with deflected flaps, I need to know how big the flap is and how much you deflect it (in degrees). I can make "pretty pictures" comparing the results of various sections and post them here. (or not if you don't want the results posted)
This is what the results might look like.
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That is a generous offer, up upon which I have taken Gary. I asked him to run a plethora of flap positions. Although I have a history of abusing Gary's generosity, I didn't want him to spend a lot of time at work with his laptop screen up in front of him, so I wrote an Excel spreadsheet that calculates upper and lower surfaces as a function of flap deflection. If that format works for Gary, I can send the Excel file as a template to anybody wanting Gary to analyze his airfoil. I'll also explain the laptop joke and include a Reynolds number calculator.
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Just my opinion, based upon my experience with trying to come up with the perfect airfoil, but you can get to blunt and to thick at the high point. With the improvements in power train over the last 15 years or so, what worked well on a conventionally powered 4-2-4 type run may create to much lift and have more blanketing effect on the tail with a piped ship, creates an over rotation/ dragging of the tail after a hard corner. D>K
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You sound like Ted Fancher. What is "too much lift"?
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Whats missing here is some hard data, not just the math. Randy, where are you? I know you have weight data for sheeted wings of all your stuff, 'cause you told me how much overweight the wing panels were on my Dreadnought when I asked you how much they should weigh. :D You could tell us how much more a larger SV type wing weighs vs a smaller one.
HI Steve
My SV wings of the large size SV weigh about 4 to 4.5 ounces per panel, keep in mind this is sheeted with LE and TE caps installed and done with epoxy and has a full strenght coat of dope, In my airplanes this makes a total weight of the wing ready in install in the airplane of 675 to 694 sq in of 12.5 to 14 ounces.
This weight ready to install includes the tips, LE guide, Bellcrank, tip weight box, flaps, hinges, horns, and is glassed on the center with the entire wing covered with paper and doped.
If my wings were thicker the weight would be just a very small amount more.
My airfoils are thin for my use...they are thicker than a Nobler, but much thinner than a PM or an Impact.
This is what works best in all of my years of testing. The leading edge radius is very important to me because of the way it affects tracking and the way the wing reacts in the whigher winds
The radius in my SV foils are about the shape of a dime or nickle..depending on which stations you use for the wing.
If you go thinner it will stall in hard corner, as will most all airfoils, if you go very blunt, the plan doesn't track very well, especially in winds.
I don't use very thick airfoils because in my opinion, it buys you nothing, and it hurts performace, too thick of a wing is not bad because of weight, it is not acceptable for me, because of the added power it takes to manouver a really thick wing with all of the extra drag.
This is exactly why for so many years Billy W stayed away from really thick airfoils..drag.. He didn't have the power to overcome it in his ships to the extent he wanted.
So bottom line I would not worry about the tiny amount of extra weight in a really thick wing but the added drag would be my major concern.
I will also say the addition of the new motors with so much power availible has made much of this not nearly as big of an issue as it was.
Regards
Randy
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So bottom line I would not worry about the tiny amount of extra weight in a really thick wing but the added drag would be my major concern.
Back to the weight issue, why would a thick wing be heavier than a thin one?
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I never said otherwise.
In the case of the foam wing, very little cane be done to reduce the weight of the wing. Most foam cores are cored to the same thickness, and the sheeting is the same. I would not hazard a guess at how strong a foam wing really is compared to in flight loads that are actually required. Probably serious over kill.
Actually a LOT can be done to lighten foam wings, have you followed Dee Rice's buildup of a wing over a foam core? using carbon tow for ribs, he then removes virtually all the foam from in side the wing, it is significantly lighter than conventional foam construction. Most foam wings are overbuilt anyway. Of note with a foam wing, the potential exists to get a significantly lighter finish on it versus an open bay wing FWIW
I never said that thicker wings cannot be built light.
Actually, you did say that all things being equal( which they never are) a thick wing is heavier.
From my stock car days, a race car chassis can be built significantly lighter by using bigger circumference tubing with thinner wall AND it will be stronger, more rigid, and lighter.
If a person uses the same materials on a thicker wing then he is not DESIGNING the wing. If you DESIGN a wing to take the loads, the spars will taper thinner towards the tip, the sheeting will get thinner, because the loads drop off significantly towards the tip. However, in our little world, the means to make this transition , while available, is somewhat intense for the minor gains aquired. Now when I designed the spars and ribs for the "Avenger" ( shultzies classic legal one)( I did in fact taper the spar because its an I beam type wing so I have graduated laminations . The ribs also have larger lightening holes towards the tips. Probably saved 1/5 oz,, Do you take advantage of that design perameter when you are laying up a new ship, if not, then you arent comparing apples to apples
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Back to the weight issue, why would a thick wing be heavier than a thin one?
Hi Howard
You will notice I said a tiny amount of weight differance...however if i build a wing say 1 inch thick and the same size wing 3 inches thick I willl need a longer metal rod for my BC mount, this will add weight, I also will need more material to glass and epoxy the center section, this will weigh more etc..
However as I stated the tiny amount of weight is not worth talking about
Regards
Randy
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Well-----,
A 630 Sq In plane with a 20 or so % airfoil using a PA 75 not breaking anywhere in the pattern did win the last World Championship. I guess drag was absolutely no problem for that setup, right Randy?
Jim Pollock :o
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You sound like Ted Fancher. What is "too much lift"?
While I am sure Ted said it, look at the wing David is now using combined with absurd power. For years we have had tons of adequate lift with coupled flaps....and not enough controlled power despite some feather weights on the flight line. "To much lift" is the opposite of "not enough"! I used all Teds published data, Al Rabes stuff, Gialdini etc. and turned up with an airfoil that was just a dandy drag wagon for a conventional 60 but was hard to get to track with a piped 61. Sure, smaller flaps/throw, but the problem pointed to excess lift and inability to lock in during hard corners and oddly round shapes suffered as the CG moved back despite a good sized stab. The proof resides at David's house.
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You and Ted may well be describing a real phenomenon, but it's difficult to figure out what it is. Best I can tell, you guys have your own definition(s) of "lift" and are describing a reduction in pitch stability with increasing lift. I usually read Ted's stuff pretty carefully, because he certainly knows how to design, build, trim, and fly stunt planes, but the "excess lift" stuff has me puzzled.
Modelers use a number of technical terms differently than do disciplines from which the terms were taken. Other examples are "cavitate", "moment", "inboard", and "power". A definition is neither right, nor wrong, but workers in a field generally settle on unique meanings for the technical terms they use. The standard definition for lift is the component of aerodynamic force on the airplane perpendicular to the direction the airplane is going relative to the air. Thus the more lift, the more the airplane will accelerate perpendicular to the direction it's going relative to the air. Although you perceive me to be a dummy because I don't understand the simple concept that "'To much lift' is the opposite of 'not enough'!", your superiority might be because I don't understand your private slang.
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Hmmm ... my native language is Slovak, but I would say that opposite of "not enough lift" is "enough lift" and that is what I think is happening here. I did excel analyze of model and what I found, is, that what we need is the airfoil which gives "enough" lift. Anything more than that "enough" is not useful (means it cannot bring anything positive, because we cannot employ it).
I remember this thing years ago on compuserve.
Centrifugal force (quadrate of speed) needs some amount of lift coefficient (also related to quadrate of speed).
Both thin airfoil and also thick airfoil (to some extent) gives lift coefficient which grows 0.11 per 1 deg AoA. The difference is where this linear part of polar stops (where is critical AoA). So what we really need is the airfoil which will cover all our needs in that linear part of polar. Anything over is not important.
So what we really need is simple. The airfoil must:
- make enough lift in linear part of polar
- that part of the polar must be smooth in drag and in moment polars (without bumps, stairs etc)
- and that all must be true in all used angles of flaps deflection
I would say that any statement “too much lift” is related to improper flap/elevator ratio or too low wing load leading to improper (too low) AoA during the corner.
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to excess lift and inability to lock in during hard corners
You and Ted may well be describing a real phenomenon, but it's difficult to figure out what it is. Best I can tell, you guys have your own definition(s) of "lift" and are describing a reduction in pitch stability with increasing lift.
So, is this something like the increased wing lift in hard corners moving the model's neutral point forward and decreasing the static margin so that the built-in tendency to return to neutral is decreased? A divergence? 'still trying to get comfortable with these stability concepts.
SK
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Yes, that's my guess. I think the plots I sent you show that. Igor has some tricks to avoid it.
I think a stunt wing should have as much lift coefficient as possible without doing perverted things. I suspect what John and Ted are seeing is separation near the TE at high flap deflection. For a given loop radius, I can't see how having a thinner wing than that with the highest Clmax would help.
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Or as John said in the thread below, "Hey man....its all in how you make it stall!"
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You and Ted may well be describing a real phenomenon, but it's difficult to figure out what it is. Best I can tell, you guys have your own definition(s) of "lift" and are describing a reduction in pitch stability with increasing lift. I usually read Ted's stuff pretty carefully, because he certainly knows how to design, build, trim, and fly stunt planes, but the "excess lift" stuff has me puzzled.
Modelers use a number of technical terms differently than do disciplines from which the terms were taken. Other examples are "cavitate", "moment", "inboard", and "power". A definition is neither right, nor wrong, but workers in a field generally settle on unique meanings for the technical terms they use. The standard definition for lift is the component of aerodynamic force on the airplane perpendicular to the direction the airplane is going relative to the air. Thus the more lift, the more the airplane will accelerate perpendicular to the direction it's going relative to the air. Although you perceive me to be a dummy because I don't understand the simple concept that "'To much lift' is the opposite of 'not enough'!", your superiority might be because I don't understand your private slang.
Uh, I am superior to none and whatever expertise I have is trial and error. Wasn't taking a shot at you Howard. I went all out to design my own airplane, noticed differences from one to the next and discussed it with those more knowlegable than me. My fat blunt airfoil under the best available power was not settling in after abrupt change of direction and did accelerate out of corners and round shapes suffered. Going backwards a bit with a semi-blunt, thinner section, cured the problem in combination with airfoiled stab/elevator. It took about a year or so for me to let Teds/Davids works, ideals and long explanations sink in. Tried it and it worked! Voila, my scientific method. The Geobolt foil was used in my best effort to date. It had issues too, but corners were hard and flat and it locked in immediately upon return to neutral handle deflection. It had a higher aspect ratio than my nominal fat 58" span self designed fast foil and was now 63" with my tips on it. My current build will employ the same airfoil with 60" span and 630 to 640 sq" area and electric power. We will see next season. For me, I do not have to completely understand a concept in order to accept it and use it. As stunt power moves forward, the aerodynamics, however minimal, is bound to change some also.
Having stepped back and observing more than competing has given me some perspective on CLPA design and less reliance on text book aerodynamics understanding and terminology. I am no one to argue those points with. Proof is in results and I may be wrong here, but hey, I'm back in the game. H^^
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YES!.... Serge and Howard! #^
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Yes, that's my guess. I think the plots I sent you show that. Igor has some tricks to avoid it.
I think a stunt wing should have as much lift coefficient as possible without doing perverted things. I suspect what John and Ted are seeing is separation near the TE at high flap deflection. For a given loop radius, I can't see how having a thinner wing than that with the highest Clmax would help.
It is necessary to look to moment polar of the airfoil. I attached polars of 3 airfoils with the same amount of flaps (in area and deflection). The first is thin and sharp, second is thicker, and the last is “that my” .
Moment polar of such flapped airfoil has always a place, where the moment starts to change. This change of moment makes unwanted input to pitching rate of model in maneuver, also means change of neutral point, and center of pressure. And since it is abrupt, pilot cannot control it well. Result is difficult to make round loop, or corners with unpredictable angle, or height. The only way how to fly such a model is to trim it that model fly outside of that area (AoA) or goes over them quickly. Such model is sensitive to CG position, or flap/elevator ratio.
Moment changes as the separation point on TE goes more and more forward. The sharp and thin airfoil has very early separation and that separation is abrupt and happens on whole area. Thicker airfoil can abruptly separate from TE to hingeline, so it visibly keeps the lift, but moment polar shows problems. And blunt airfoil with smooth surface at hingeline can have late and smooth change of moment slowly with higher and higher AoA.
It means the best will be thick airfoil like that last one and model designed to fly inside its “safe” AoA. All my models are designed to fly at max 8 deg AoA in corner. It has still some reserve and in reality (circular flow) it has even safer properties. Result is that it allows almost any trim and it is usually also very insensitive to tip weight. That wing is used by me with extremely back CG, or Alex who placed 4th on WC with moderate CG location and also Richie K. current EC champion with extremely nose heavy setup.
But those thin and sharp wings can also fly. You can see that the separation happens already at 0 AoA, so it happen “for sure”. It had limited lift, but if wing load allows it, it can fly also well. It is something what we know from sharp versus blunt stab. Both works well … while moderate radius makes troubles … the same reason is why I used flat airfoil on my indoors, thicker airfoil does not work well.
And one small note – my flap is flat, it also gives some chance for abrupt separation even with flat hingeline at 30 deg deflection. I think airfoiled flap (I mean the flap making his full airfoil, not integrated to airfoil of the wing) can help to some extent. If I remember well, Juno has such flaps.
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One note to drag of thick wings.
I do not think it is reason of airfoil itself. Thicker airfoil has more drag, but the difference is only small and its drag in maneuvers (with flaps and at high AoA) is sometimes lower compared to drag of thinner airfoil. But anyway, it is known fact that thicker wings show signes of higa drag in corners. And not only that, it is usualy also visible on 4-2-4 run of wet engines as a boost after every corner as model accelerats to its previouse speed and thus makes motor leaner.
The reason is capability of such wing to carry more weigh. Loaded wing with thick airfoil will easily make high lift coefficient, but induced drag of such wing increases with square of the lift coefficient. Simply if wing exceeds some lift coefficient, then the induced drag is higher that airfoil drag. So if we push such a thick wing to high lifts, it brakes, while thin airfoil exceeds critical AoA, its airfoil drag can be higher, but since lift is limited, also induced drag does not increase so much.
However thick wing with lower wing load will not brake as much and there no need to make stress from high drag of thick wing.
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Wow. That is very impressive, Igor. Thank you. I am glad that I have not cut the ribs for my new airplane yet.
For a given airplane weight, wingspan, and turn radius, wouldn't induced drag be the same, regardless of Cl or Clmax?
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I got JavaFoil and tried a couple of airfoils at different flap deflections. It looks like the flow stays stuck on the flap even at high flap deflections. Does JavaFoil model separation on the flap accurately?
While I was fiddling with JavaFoil, I found that it can make cool plots of the flow field around the airfoil. Here are two views of an Impact MAC airfoil with 20 degrees flap deflection at an angle of attack of 10 degrees. The black lines are streamlines. The color is pressure. It doesn't give the whole story, because it doesn't have the finite wing effect, but you can see flow angle and "wake" position behind an infinite-aspect-ratio wing.
JavaFoilshows that the Impact leading edge literally sucks at high angles of attack.
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For a given airplane weight, wingspan, and turn radius, wouldn't induced drag be the same, regardless of Cl or Clmax?
induced drag = something * cl ^2 / AR
so if you extend the chord only (lower cl and lower AR), the induced drag will be linearly lower, but as the area linearly grows, the final drag will be the same
so lower drag needs either increades span instead of chord (like gliders) or even better lighter model :-)
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Does JavaFoil model separation on the flap accurately?
I do not know how much acurately, but does. However it is not visible on flow field analyze, you must look to boundary layer folder. You can see where is transition and separation points on both sides.
If you want play with AR, you can set it in options folder.
Regarding accuracy - I do not know how accurate could such analyze be if we cannot specify surface, ribs, prop disturbance etc, but at least it gives feeling what is happening there.
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Hi Howard
You will notice I said a tiny amount of weight differance...however if i build a wing say 1 inch thick and the same size wing 3 inches thick I willl need a longer metal rod for my BC mount, this will add weight, I also will need more material to glass and epoxy the center section, this will weigh more etc..
However as I stated the tiny amount of weight is not worth talking about
Regards
Randy
Isn't that just darn near exactly what Brad said?
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Wow, I'm impressed with the serious analysis you guys are doing with
Javafoil. Awesome.
I need to watch these threads more closely, interesting and useful work
is going on.
Like everyone else, I'll be happy to absorb the fruits of your labors! 8)
Best,
L.
"NO KILL I" -Horta Mother
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Isn't that just darn near exactly what Brad said?
Yes, and both disregarded the beam part. The point was that the increase of weight with thickness due to a bigger surface area, hence more foam, sheet, and paint is trivial, the bellcrank axle weight growth is also pretty small, but the structure weight needed for a given wing bending moment goes down pretty fast as thickness increases. Something else could be sizing the spar, of course. If the thickness of wood on a foam wing suffices as a spar on the thinner wing, and is picked not for strength, but as a barrier to the solvents (or the Monokote iron) getting to the foam, then the thicker wing would be-- what was it?-- .09% + delta bellcrank axle heavier. Also, if a thicker wing has more lift, it would require some more spar. If you say that weight increases with wing thickness, which is the opposite of most airplane design cases, you might back up your assertion with calculation.
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Yes, and both disregarded the beam part. The point was that the increase of weight with thickness due to a bigger surface area, hence more foam, sheet, and paint is trivial, the bellcrank axle weight growth is also pretty small, but the structure weight needed for a given wing bending moment goes down pretty fast as thickness increases. Something else could be sizing the spar, of course. If the thickness of wood on a foam wing suffices as a spar on the thinner wing, and is picked not for strength, but as a barrier to the solvents (or the Monokote iron) getting to the foam, then the thicker wing would be-- what was it?-- .09% + delta bellcrank axle heavier. Also, if a thicker wing has more lift, it would require some more spar. If you say that weight increases with wing thickness, which is the opposite of most airplane design cases, you might back up your assertion with calculation.
Howard
The extra amount of weight could very easlily be calculated, and as you mentioned the ply spar that most use in a foam wing would also add a little more weight, as It would be thicker, thus heavier, and I for one use 1/16th balsa and would never think of going thinner, anyone doing so would be asking for problems.
However as i stated the weight is too small to argue over and would not be the determining factor in me building a thinner or thicker wing, I stated above why I don't use very thick wings, and weight has little to nothing to do with it.
I did not disregard "the beam" part. I didn't argue that point because there was no need for me to do so. My statement was for practical purposes in building a foam, ply and balsa wood toy airplane wing, which I can't buy decimal sized wood, it would serve no purpose for me to sand the sheetining down a few thou of an inch for use in a thicker wing, nor to sand down the ply spars a few thou thinner to save weight in a thicker wing. This does make sense in full size ships since we would not be talking a matter of a grams.
Regards
Randy
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If you say that weight increases with wing thickness, which is the opposite of most airplane design cases, you might back up your assertion with calculation.
"which is the opposite of most airplane cases"... like give some examples????? Who are you talking about?
Randy Smith = most prolific designer of the modern era (designed and tested dozens of new designs) + most popular supplier of pre built components stunter in the US for the last two decades (a man who has probably built literally hundreds of wings of all different sizes)
Howard Rush = never designed his own stunter (as far as I know) + has been flying the same plane for 8 years and does not like to build so may never build another at the rate he is going---no offense
Howard has taken what I said and twisted it to be something I never said. I never said that thicker wings cannot be built lighter, I said they are heavier than thinner winged planes... all things being equal (which I believe Randy pretty much backed up exactly). No one has seemed to argue any different. In fact, everyone is eager to say I was right, however it was "trivial" to what degree.
Also, Howard seems to believe there is some magic to the ultimate strength that can be gained from fatter airfoils. Within reason, I do not see it. I agree a thicker beam will be stronger... but to what end? Remi Berringer out flew all of those fat winged stunters in Muncie in the 2004 WC. His airplane did not seem to know it could not possibly be better. Compared to an American stunter his plane had more in common with a free flight model than an Impact.
Lastly, what is thick? What is thin? No one has really defined what examples were used for the supposed "calculations". This isn't a very well defined discussion at all. It is just people throwing crap out that, as far as I can see is baseless. At least *my baseless crap* was based on my personal building experience.... the wall of my shop is covered in weights of every single component from planes I have built. I also have numersous spread sheets *calculating* what all the components *SHOULD* weigh *BEFORE* I built them. I did this with several planes.... and that is number is *trivial* compared to Randy...
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Remi Berringer out flew all of those fat winged stunters in Muncie in the 2004 WC.
No, he - and it - did not.
To us all:
This is a stunt design board, and AGAIN we find discussions of each other - and semantics. ALL of these ideas can be addressed for their own merit and specific content without ever mentioning or obliquely referencing another personality. Regardless of what one feels about another's presentation, personality, or qualifications - in fact, regardless of how grievous one sees any given post to be - there is absolutely no compelling reason to address anything but its design content and/or specific assertions. So let's all cut the crap.
Edited to add a comma. Oh, yeah, and - sincerely! - Merry Christmas, happy holidays, and a fulfilling, long, and happy life to all...or else!
SK
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No, he - and it - did not.
I was there... he did.... by a lot.
He damn sure did in 2006 in Spain... even the judges saw that.
He was third in 2004 behind the Chinese guy (not flying a fat airfoil that I could see) and Billy which uses the same airfoil I use in the T-Rex plus the thickness of the sheeting top and bottom (if you want to consider his "fat" and mine "thin").
Didn't David Fitz abandon the Coke bottle airfoil for a much thinner airfoil to gain "penetration"? Didn't he win the WC's after that?
Serge, it makes perfect sense. Stunt is more about personalities than designs anyway. People defend *whatever* because of the guy using *whatever* more than they defend the *whatever* because they can *prove* it works better.
This whole argument is stupid... the guys who use fat wings will say they are the best for all kinds of reasons. The guys who don't... won't. End of discussion. Oh, and yes there will be some belittling and down talking along the way, just to be sure.
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I just picked on you because you made the blanket statement that thick wings are heavier than thin wings. Being a nobody, I had to do some calculation to support my assertion that it's more complicated than that. It's above somewhere. I'm sure you reviewed it for correctness. Examples? Here's some beam stuff. I didn't spend much time checking it for relevance: http://www.engineersedge.com/beam_bending/beam_bending8.htm , http://courses.washington.edu/mengr354/jenkins/notes/chap3.pdf . I don't do wing design myself, but I've sat through meetings on the subject. Aerodynamics for those airplanes favored thinner-than-optimal wings; structures and fuel capacity favored thicker-than-optimal wings. Sometimes the optimal wings turn out to be amazingly thick: the wing root thickness of the B-17 and the DC-8 is 18%, of the B-24 and B-29 is 22%.
As I said above, and in the case Randy cited, something else could be sizing the spar. If the thickness of wood on a foam wing suffices as a spar on the thinner wing, and is picked not for strength, but as a barrier to the solvents (or the Monokote iron) getting to the foam, then the thicker wing would be .09% (as I remember) of the surface stuff + delta bellcrank axle heavier. Given such a wing with 1/16" sheeting, as you make it thinner, eventually the 1/16" sheeting + shear web will no longer suffice, and the weight of the requisite beam will make the wing weight go up. As Randy also said (I think), weight doesn't matter much in picking the thickness of a stunt wing.
Like you, I have selected my stunt wing design based on experience and lots of experimentation. In my case, as you note, somebody else did the work. I may try tweezing the airfoil some, so I am hoping to get some analysis (also outsourced to somebody who knows what he's doing) to reduce the risk of going to a lot of effort (and time, as you correctly observe) and coming up with a dog. So far, the old Impact wing looks pretty good. I know that any slight deviation I've made to it by hunch has come to naught.
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Howard Rush = never designed his own stunter (as far as I know) + has been flying the same plane for 8 years and does not like to build so may never build another at the rate he is going---no offense[/b]
You are pretty much correct. I last "designed" a stunter in 1961, but it didn't amount to much. I have been flying my current plane for two years; I flew its predecessor for ten. You are not alone among Walkers at critiquing about my building rate. No, I'm not at all offended by that.
Serge, I don't think Brad is making a personal attack here. I think he's just parodying what he feels is an unjust attack on his work that somebody made challenging his qualifications.
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OK Guys,
Let's keep it nice now... O.K.? Since this is my board, I hereby declare Bradley Walker as good a designer as anyone else who has ever posted anything to this board!
Done!
Jim Pollock, Moderator :o
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Serge, I don't think Brad is making a personal attack here. I think he's just parodying what he feels is an unjust attack on his work that somebody made challenging his qualifications.
That is correct. I like Howard a lot. He cracks me up! Obviously he is quite brilliant. Smarter than me.
I do, however, think these discussions get rather silly.
(http://images2.fanpop.com/image/photos/8900000/Silly-idiots-monty-python-8932359-460-288.jpg)
A lot of dancing on the head of a pin, and posturing to protect ones turf. Not always (I am not saying that is what I going on here), but it is quite common. Like disagreeing with someone insults their intelligence, or worse yet, their legacy.
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I was there... he did.... by a lot.
As before, we will have to continue to disagree on this - "a lot." I too was there in 2004 and watched the tail of that plane wag all over the place in yaw and pitch. I actually like some of that plane's design and have some similar things in my sketch pads, not copied, but started from scratch. So I am not necessarily critcizing the design or plane itself, but certainly its trim. 'might have overloaded the wing (aoa) and tail (size, c.g.)? I also carefully noted sizes, shapes, and intersections. So you saw what you saw, and I saw what I saw. I am as amazed by your conclusions as you apparently are by mine. There doesn't seem to be a lot more to be said.
SK
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As before, we will have to continue to disagree on this - "a lot." I too was there in 2004 and watched the tail of that plane wag all over the place in yaw and pitch. I actually like some of that plane's design and have some similar things in my sketch pads, not copied, but started from scratch. So I am not necessarily critcizing the design or plane itself, but certainly its trim. 'might have overloaded the wing (aoa) and tail (size, c.g.)? I also carefully noted sizes, shapes, and intersections. So you saw what you saw, and I saw what I saw. I am as amazed by your conclusions as you apparently are by mine. There doesn't seem to be a lot more to be said.
SK
Planes have a tendency to waller around some when they are flown at the correct size, at slow speeds, with the correct amount of corner.
Of course, you will not see that when everyone else is wizzing around at blistering fast lap times flying 60 degree maneuvers with swooping corners (as was most of the field---including Werwage).
I believe Remi would have won in 2004 (as he did in 2006) if he had not been forced to fly in the rain for his final flight, and the contest was immediately stopped for Werwage halfway through his flight in the rain (which was not a good one), and then restarted again in stunt heaven air...
The point is that Remi has whipped a bunch of fat winged, US style stunt planes (with pipes and all), with his skinny winged 4 stroke plane that looks like a free flight...
PS: Shhhhh... don't tell anyone but that Sportster is not really very "strong" either (at least by foam winged jumbo airfoil US standards). In fact it flexes a lot.
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A couple observations-
1) Having watched Bill fly locally and heard his comments about F2B results, I am convinced that he deliberately flies differently in F-2B than otherwise. Here, his corners and bottoms are quicker and very clean, perhaps sometimes on the low side. There, he flew higher, did larger maneuvers and rounded out the corners. He speaks diplomatically, never to me disparaging the preferred corners, but leaving no doubt that he prefers the cleaner - more pristine appearing ones that also leave slightly longer sides in square maneuvers. It seems from internet comments over the past few years that F-2B corners might have tightened some, but nothing I saw there was like the (bobble-free) sharp corners I've seen him do around here. I did not think his WC flights were as "good" as what I was used to, but I believe he deserved the win.
2) My comments on Berringer's performance came from watching only two (three?) last-day flights, one of which was not from directly behind the judges, but rather close and downwind.
3) I think Berringer is quite good and flew quite well. His intersections were about the same as Bill's (not perfect that day), but on all the flights I watched, the plane wiggled after corners, and the repeated figures and adjacent figures were not as close to the same size. I felt that Bill's rounds were more circular, but then as a certain multi-time oriental champ showed, that was of little concern to anyone but perhaps me. That particular pilot flew diameters and tops that differed by as much as 4-7 feet on his worst scored flight (hard to judge higher in the hemisphere).
If I were to guess, I'd say the plane's wing with it's small flaps and apparently forward c.g. needed a high aoa, perhaps near its stall, and the small stab wasn't as stabilizing, even on the longish arm. 'could of course be wrong, but that would explain what I saw. It's a neat design, but I'm not convinced that it is what a guy of Berringer's talent needs to do his best. FWIW, the most impressive single flight I saw the last day was done by the other Mr. Walker.
As usual, FWIW...
SK
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Yes, and both disregarded the beam part. The point was that the increase of weight with thickness due to a bigger surface area, hence more foam, sheet, and paint is trivial, the bellcrank axle weight growth is also pretty small, but the structure weight needed for a given wing bending moment goes down pretty fast as thickness increases.
If I'm not mistook, the beam stiffness varies with the 3rd power of the thickness. The extra lift from a thicker section is only a small fraction, say from 1.6 to 1.8 coefficient of lift going from 15% thickness to 18%. So even small increases in the wing thickness make it much stiffer. With some careful design you could probably drastically reduce the wing skin thickness and other material sizes, but as Randy points out, there are some practical limits. Unless you go to match molded carbon fiber skins with balsa cores and minimal internal structure.
I hear a CNC mold this size only costs $5K or so.
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If I'm not mistook, the beam stiffness varies with the 3rd power of the thickness. The extra lift from a thicker section is only a small fraction, say from 1.6 to 1.8 coefficient of lift going from 15% thickness to 18%. So even small increases in the wing thickness make it much stiffer. With some careful design you could probably drastically reduce the wing skin thickness and other material sizes, but as Randy points out, there are some practical limits. Unless you go to match molded carbon fiber skins with balsa cores and minimal internal structure.
I hear a CNC mold this size only costs $5K or so.
Do carbon fiber wonder planes fly better?
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Stiffness is only better with the 3rd power of thickness if the top and bottom of the I beam cannot shift their position, if they do the stiffness is the cumulative of the top spar and bottom spar alone without the multilication factor due to wing thickness.
So it is no use making a very strong top and bottom of the spar if you separate it by just foam or a 2mm balsa horizontal webbing. I prefer a 4mm vertical webbing between spars. I prefer spar against carbon as it is easier to glue to balsa.
I once flew a detroit wing and my fellow competitors informed me that the wing was flexing more than 4 inches. I wondered where that came from and found out it was held by the silk covering alone. I decided that it was probably broken the week before at another competition and as I came this far it would survive the final flight. It did although accuracy was gone but good enough to secure 3rd place.
At home I took off the covering and found out that the main spar was broken in pieces on both sides of the fuselage and repaired the model.
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Not sure if I'm reading this right. The value of thin webbing between top and bottom spars is that it converts two very small section independant spars into one integral I section spar. Stiffness is proportional to "D" squared?
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True, but if you use 2mm balsa lengthwise you will see that it is not enough to fix the position of the upper spar to the lower spar. My Flying Dutchman design from 1980 had this type of webbing and a very flexible wing.
Vertical webbing is a lot better, 2mm is already an improvement but I feel 4mm is needed to really fix the movement, once fixed you get the strength.
I believe that stiffer wings fly more accurate although my Detroit wings with an "I" beam from 8x3 spruce top and bottom separated by a 8mm vertical webbing of 30mm height worked well on my .46 airplanes.
The D tube on my .51 airplane with 4mm vertical webbing feels a lot more accurate to fly.
Foam wings without spars feel very rigid but I have not enough experience with this material to build them light enough.
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Hi Igor,
I was wondering while re reading all of this about the effects that Al Rabe came up with on his Sea Fury. He actually went to a slightly asymmetrical airfoil on his Sea Fury to help in certain areas of the schedule. I would have to get that 1973 issue of American Modeler back out to verify the numbers he used, but he did specifically point out a change to the airfoil, and that it was not fully symmetrical. I know he was working with very scale numbers, sticking to his Scale Stunt regime, but I have always found it interesting as to his thoughts. The plane did very well!
Any thoughts?
Big bear
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Hi Bill,
Yes I know about that asymetry. Al wrote that reason for that is extra lift necessary for corners close to ground in triangles and hourglass.
Well ... I see some reasons for that, but I mean that if the airfoil works well at all used AoA then we not need such a trick. The airfoil makes the same amount of lift does not matter if we co corner 10 degrees, 90 degrees or 120 degrees. Important is radius and radius is the same. The only difference between 90 and 120 degrees is speed of the model which slows down more after 120 degrees than 90 degrees.
It is not big difference, because lift necessary for centrifugal force in that radius is of the same second power of speed as the lift of wing. So the wing needs still the same AoA and the same lift coefficient does not matter what is the speed. However gravity has stronger effect if we do it close to the ground at lower speed, so may be there is really some good reason to do something for lower corners of triangles. But I think it is much better to keep some reserve in calculation and not to allow airfoil go to such extremes. For example little lower wing load, or little stronger power train or bigger flaps or so. I wrote somewhere upper, that good airfoil should keep its linear part of lift polar in all used AoA, so we can do it that it will last little more it is possible. I mean that there are airfoils good enough and those others. I do not think that we can find something even better and even better. It is either good enough or not. :-)
And why NOT do do it? just because symetric airfoil is easy to calculate, easier for bench trimming and predictable in positive / negative G in symetric figures (rounds and squares) ... I hate if my model fly well either positives, but not negatives or negatitives and not positives .. but never both n~
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Yes I know about that asymmetry. Al wrote that reason for that is extra lift necessary for corners close to ground in triangles and hourglass.
....However gravity has stronger effect if we do it close to the ground at lower speed, so may be there is really some good reason to do something for lower corners of triangles. But I think it is much better to keep some reserve in calculation and not to allow airfoil go to such extremes.
And why NOT do do it? just because symetric airfoil is easy to calculate, easier for bench trimming and predictable in positive / negative G in symetric figures (rounds and squares) ... I hate if my model fly well either positives, but not negatives or negatitives and not positives .. but never both n~
I suspect the reason for the cambered airfoil that helped most was to get more lift early in the control movement. Al did not have much room for getting lighter weight, or bigger flaps, given what he was trying to do. The hourglass especially can get very difficult in a bit of wind. The plane accelerates towards the ground from gravity, plus the normal speed pickup coming out of a corner, plus the help from the wind. If the controls generate too much hinge torque you simply can't move them fast enough against the extra control loads in the lower right corner. An airfoil that generates lift quicker with less control movement could help a lot.
Doing things to reduce the control loads are a good idea too, as long as it doesn't affect controllability more than suits you.
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hmmm ... I do not know, but just as a quick look, I would say that if the airfoil is in its linear segment of polar, and if model is trimmed to be neutral, means the airfoil is either at its negative AoA of flaps are little up, then the same amount of controll defelction will do the same change of lift. The shape of airfoil will take effect only if it reaches its extremes. Means if the lift must be so high, that goes to stall, then it can be. ... However I wrote it in previous post - IF the airfoil is in linear segment .... etc
Regarding the handle load - I would say that if model has some weight and wing has some flaps to area ratio, then the hinge moment will be the same, does not matter what is the airfoil - again - IF the airfoil is not stalled ( at least partially). If yes, then it is another storyy and it can be so (for example separated on flaps) but I think we cannot design model to reach its stall AoA regularly in flight.
I agree with one - as Al wrote, that asymetry of airfoil will give higher maximal lift (critical AoA) even if flaps are little up in neutral. (flaps will limit lift in level, but will not limit maximal lift when it is needed). I mean that if the fixed part has say 1% chamber, and flaps needs say -2 deg deflection for neutral, then maximal lift at -3 deg flaps (flaps down, positive G) can be higher in absolute value than maximal negative lift at +7 deg (flaps up, negative G)
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By the way Igor, mw~ I think you explained all of this well...one decade ago or there about, you did. The graphics over time are much more defined!
I like playing with flyable designs based on my WAG! I accept the best available info and move on...got it the first time...still like Bill's foil.
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http://www.iroquois.free-online.co.uk/rabefury.htm
The article in case you did not want to go through your archive.
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hmmm ... I do not know, but just as a quick look, I would say that if the airfoil is in its linear segment of polar.......
The problem here is that a stunt plane(wing) does not have a "polar" in the sense used in the airfoil analysis graphs. In level flight the stunt wing is pretty much at 0 aoa, with a very slight bias from either the flaps or stab to generate enough lift to support the plane. When the pilot moves the controls the flaps move and the stab pushes the tail down. The airfoil develops an angle of attack(between the chord line drawn from the LE to the TE of the flaps) due both to the movement of the flaps, and the pitch up of the fuselage. At the maximum control movement used in a maneuver this might be 6-7 deg. from pitch up and 6-7 deg from the flap movement. None of the conventional airfoil analysis accounts for the change in shape of the airfoil with angle of attack. The only way to get close is to select the right numbers from a series of plots where the true polar is generated using the actual(airfoil+ pitch) angle. So, run the program with zero flap deflection. Then run it with say 5 deg. of flap deflection and select the number from where the AOA is about 10 deg. Then run with 10 deg. of flap and select the number from ~20 degrees, etc. and plot the result at lift(and/or drag and/or L/D) vs. the true angle of attack. You kind of have to guess at the appropriate true angle of attack for each specific degree of flap angle, since nobody has any measurements of airplane pitch vs. loop radius and control movement, except for a few examples of a tight loop.
The other thing to keep in mind that it is very possible to move the controls faster than the air can react. The plane may fly one or more lengths before the airflow settles down. The racers do this all the time, wagging the tail to slow a plane down to hit a particular pit position. You can do the same thing with a stunter. So I can see plenty of instances where hitting the controls too fast or hard might cause all or part of the flaps or elevator to stall briefly. This could cause all sorts of transient control issues. You can see this if someone biases the loops to far into the wind. As the plane comes around and goes up past vertical(the left hand side of a round loop) it will jerk with no imput from the pilot as the plane goes from flying into the wind to suddenly flying down wind and one wing loses a significant amount of lift.
I suspect this kind of transient behavior is what Al was trying to take advantage of by biasing the airfoil like he did. When the plane is coming downhill in the third leg of an hourglass the wing has to be producing virtually no lift in either direction and the flaps probably are biased up a little. When you start to move the controls it takes very little effort to apply 5 deg. or so of flap and generate a lot of lift easily, compared to the amount of control force to actually get the controls to their maximum movement. You may remember somes post from downunder where he built a plane with fully adjustable flap travel. He ended up with only 5 deg. of flap needed to keep the plane flying in hard maneuvers, making the difference between a stally turn and a smooth one.
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Phil, I agree in most points. That transient AoA is known effect. I tried to estimate it some years ago. You can see data fro my old “Next” on my old pages. It is still alive. Look here:
http://www.netax.sk/hexoft/stunt/thenext.htm
You can see it in right column almost on end of page, value called “angle of attack wing before corner” what is angle where elevator can push the fuselage at its maximal deflection before wing starts to make lift. It is very pessimistic value. Then you can look at data of the airfoil on:
http://www.netax.sk/hexoft/stunt/notes.htm
The table under the airfoil and you can see that it will still work well (still before critical AoA, means on end of linear segment) at that angle and deflected flap.
I cannot speak for Al’s airfoil, but I expect that it will be also close to that value.
Since it is still not stalled, I expect that the airfoil IS in its linear segment.
You are also right that we have moving flaps, but does not matter what, the single points of polar with defined flap and elevator deflection are still on some place of polar, I think well designed model should keep them on that linear part and they usually ARE on linear part. And yes, the polar changes with moved elevator, so it is another curve, but that point can be still on its linear part. Once again here, I cannot speak for Al, but I do not think that he got it outside. May be I am not right, and his model stalls somewhere during normal flight, but if not, then the airfoil will be sensitive symmetrically.
The only difference I can see is that such asymmetric airfoil will have MAXIMAL lift in positive G’s better. And that is what I have seen several times written in his posts.
Where did you see him to speak about such sensitivity in transient?
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What's the matter with trial and error and doin' what the other guys do?
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What's the matter with trial and error and doin' what the other guys do?
There's nothing the matter with doing whatever you want to do. 'also nothing the matter with seeking to understand.
SK
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i was joking of course. But there is something to be said, I think, for evolutions of traditional designs that turn out to be an effective compromise. Evolutions that might be arrived at via dumb luck. The Gieseke Nobler, for instance. If the story told about it's derivation from George Aldrich's original is accurate.
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Where did you see him to speak about such sensitivity in transient?..........
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I never talked to Al about this. The transient response is based on my own experience, where getting into the third corner of the hourglass can be a very exhilerating experience because the plane takes its time starting the turn and tends to run wide.
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Phil,
The second turn of the hourglass can be very exciting as well, especially when you have too much weight near the outboard tip!! Ask anyone that was on circle 4 on Thursday at the '04 Nats....
Jim Pollock :o H^^