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General control line discussion => Open Forum => Topic started by: Shorts,David on November 15, 2021, 04:35:51 PM
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Hi, so how blunt is too blunt and what will happen if the LE is too blunt? My dad thinks my LE is too blunt.
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Hi, so how blunt is too blunt and what will happen if the LE is too blunt? My dad thinks my LE is too blunt.
Maybe your Dad is just being Blunt! LL~
A lot depends on power. I would think that any radius over half of the thickness of the wing is approaching too blunt. Just for reference the Thundergazer and MaxBee airfoils have a LE radius about 1/3 the thickness of the wing.
Ken
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Hi, so how blunt is too blunt and what will happen if the LE is too blunt? My dad thinks my LE is too blunt.
It's pretty difficult to make it "too blunt", as long as it is not also "too thick". What airplane are we talking about, with what engine?
Brett
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Dave,
Here is a thread about stab leading edge shape. It points out from test data on top stunt models that sharp leading edges on the stab gives positive results in shapes and control.
https://stunthanger.com/smf/open-forum/stab-aerodynamics-question/msg608996/#msg608996
Best, DennisT
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Hi Ken, I would guess it is approaching half the thickness of the wing. When I made it I thought it'd be great, but now looking at it I'm second guessing.
Hi Dennis, this is the wing LE, my stab is pretty sharp.
Hi Brett, this is a beech staggerwing, I'm planning on using an fp .40. it's about 525-550 squares.
I may have to try it and then modify the LE if it what...maybe can't penetrate on a windy day?
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Hi Ken, I would guess it is approaching half the thickness of the wing. When I made it I thought it'd be great, but now looking at it I'm second guessing.
Hi Dennis, this is the wing LE, my stab is pretty sharp.
Hi Brett, this is a beech staggerwing, I'm planning on using an fp .40. it's about 525-550 squares.
I may have to try it and then modify the LE if it what...maybe can't penetrate on a windy day?
What kit is your Staggerwing?
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Hi Ken, I would guess it is approaching half the thickness of the wing. When I made it I thought it'd be great, but now looking at it I'm second guessing.
Hi Dennis, this is the wing LE, my stab is pretty sharp.
Hi Brett, this is a beech staggerwing, I'm planning on using an fp .40. it's about 525-550 squares.
I may have to try it and then modify the LE if it what...maybe can't penetrate on a windy day?
If it is just a pure radius, that by itself might be OK (albeit not that pretty). It depends on how it fairs into the rest of the airfoil behind it. If it is a nice smooth curve, OK, if the radius has any sudden changes, then, maybe a big problem.
Brett
p.s. how thick is the wing (in percent)? The thicker it is, the more difficult it will be to ensure fair surfaces.
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You can use the wing knock out for the radius or a nickle.
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What kit is your Staggerwing?
Jack Sheeks, 1964 Staggerwing.
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Jack Sheeks, 1964 Staggerwing.
Doc Holiday and a few others have built his Staggerwing Stunt model for VSC. It was a Flying Models Beamer of the Month Sheeks article. It should have a good side view of the airfoil which should be a typical I-Beam wing.
Type at you later,
Dan McEntee
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Basically a Pathfinder/Ice cream cone type of airfoil. Mine flew real good.
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If it is just a pure radius, that by itself might be OK (albeit not that pretty). It depends on how it fairs into the rest of the airfoil behind it. If it is a nice smooth curve, OK, if the radius has any sudden changes, then, maybe a big problem.
Brett
p.s. how thick is the wing (in percent)? The thicker it is, the more difficult it will be to ensure fair surfaces.
I just crudly checked the radius and thickness on the three airfoils I have templates for that are closest to what I have in use on my designs - 7/16". The thickness ranges from 2 1/4" on the Thundergazer to 2 3/8" on the MaxBee Airfoil. I would consider an airfoil that had a definable "radius" to be blunt if the tangent to the forward curve was greater than about 25 degrees. Most airfoils are parabolic and have no definable LE radius, only an approximation. I had a parabolic airfoil on my last Sandpiper design with a 2" thickness and a 40% highpoint. It did not fly exceptionally well under IC power and I was mad at myself for using my classic airfoil on my classic. Then I converted it to Electric and it really performed. I have heard others commenting that electric power does not need the drag of the blunt LE and the more traditional parabolic airfoils are better suited. They may be right. I want to learn a much about them as I can since it is the one thing that cannot be easily changed after the plane is built.
Ken
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Ken,
Seems with electric we get much more motor braking then we thought. If the rpm is set it will just keep backing off current draw and only use what it needs to maintain the set rpm and no more. The smaller radius wing leading edge being a little cleaner can get good penetration with less power. As a bonus we can use more efficient props with a better pitch to diameter ration (more pitch than current trend in IC) and have the same lap time at lower rpm and less current draw another plus. How far we can go with the prop we'll have to see.
Best, DennisT
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Ken,
Seems with electric we get much more motor braking then we thought. If the rpm is set it will just keep backing off current draw and only use what it needs to maintain the set rpm and no more. The smaller radius wing leading edge being a little cleaner can get good penetration with less power. As a bonus we can use more efficient props with a better pitch to diameter ration (more pitch than current trend in IC) and have the same lap time at lower rpm and less current draw another plus. How far we can go with the prop we'll have to see.
Best, DennisT
I have always suspected that it was things other than the airfoil that added to the performance of electric but I think it is what you are pointing to. It is not so much an ability of electric as it is a deficiency in IC that makes the thinner airfoil possible and perhaps even desirable. I hope someone who has seriously looked into this and has some answers chimes in. I have my 2022 ship on he drawing board.
Ken
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What do stunt people mean by “penetration”?
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Show me why you think electric planes can get away with thinner airfoils.
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Penetration in stunt refers to the distance travelled by the prop nut in the time between impact with the earth and the end of forward motion.
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Penetration in stunt refers to the distance travelled by the prop nut in the time between impact with the earth and the end of forward motion.
Chuckle. Thanks for clearing that up.
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The thinner electric airfoil thing has been written about by Bob Hunt in a few columns, others may also have noted it. Electric wants to go to a set point rpm, it doesn't unload as the ship launches and doesn't want to unload as the wind pushes the plane, it holds rpm. It does what the pipe does for IC but better since is looking for a set rpm/voltage from the ESC/Timer. On IC the pipe is holding the IC back by detuning the power point that the engine wants to go to for a given amount of fuel/air entering the engine. To further control the natural unload of the IC low pitch props allow it to spin-up a bit without to much change in airspeed. This control of unload is the same objective of the high rpm/low pitch and 4-2-4 run styles. To aid in controlling airspeed variations designs have evolved that use the thick airfoils to slow the rate that a ship will change speed due to momentary unloading. For electric it naturally reacts to the increase in load very quickly but some improvement in load control with the newer timers give a bit of a power bump as the load comes on but still aims to hold the airspeed pretty constant.
Seems that if you build/finish reasonably light the old Nobler airfoil is a pretty good one for stunt. Not needing to hold the plane back with drag from a 20% thickness gives electrics a benefit of reduced current draw and allows more pitch less rpm better prop efficiency, again a benefit in lower current needs. With the thinner airfoil holding airspeed in higher wind seems to most a bit more consistent. I think what Ken stated is somewhat to the point as not so much that electric by itself creates the benefit as much as allows these features/benefits and IC seems to need some of those features. Not that IC can't compete with electric it just needs different design features that let it perform at its best.
Just my understanding on the how and why for thinner airfoils on electric as told by others.
Best, DennisT
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Penetration in stunt refers to the distance travelled by the prop nut in the time between impact with the earth and the end of forward motion.
LL~
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Hi, so how blunt is too blunt and what will happen if the LE is too blunt? My dad thinks my LE is too blunt.
This set of data from the Illinois University wind tunnel work clearly demonstrates the effect of too small a leading edge radius. The ultimate lift of the smaller leading edge radius airfoil is less than the larger radius.
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Frank,
Looking at the two examples they are not the same airfoil. The Ultra-Sport 1000 is a non-flap, the Trainer 60 has a trailing edge straight flap sections. The Trainer 60 will get the benefit of the fixed flap as it's Angel of Attack (AOA) is increased. On the Barnstormer OTS ship Andrews told Dave Cook that the trailing edge flap should have a straight flat shape to get the most benefit from the flap, it gives the effect of a 10-20% flap deflection.
What is needed is data from a airfoil like the Ultra-Sport 1000 as is then with several radii to see if how just the leading edge radius impacts the lift (Cl) and the stall point. I do agree that a very sharp wing leading edge will contribute to less lift because it will likely stall at a lower AOA. I think the other extreme is the ice-cream cone airfoil with I think would also be a lower performing PACL airfoil. One of the best IMHO would be the airfoil on the Detroit Stunter series, nice smooth and was a proven contest winner.
Best, DennisT
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Frank, I am not advocating a sharper LE at all. A thinner airfoil section with more of an elliptical section but not a small radius LE. Did you have a chance to study any of Bob G's ships over the years. He attributed the improvement in lift to the thicker airfoil but look at the LE on the Gieseke vs the Blue Box. Bob rounded it off nearly double. I never told him but I used his wing in my mostly sky blue Surveyor design that I flew in the late 70's. You may remember it. That plane would turn corners 50% better than I could fly them and didn't wind up in wind. I am hoping to find some data that supports the theory that an 10" airfoil (w/o Flaps) with similar LE "radius" and chord curvature would be better at 2 1/8" or even 2" center thickness tapering to 1 1/4" tip thickness vs 2 1/2" tapering to 1 3/8". My gut says that either will produce more lift at our AOA's than we need but I have absolutely no data to back that up or the resources to test it. The airfoil I envision looks like a slightly thinner version of Al's or Igor's. With an active timer I should get all of the braking when I need it and better penetration when I need it.
Ken
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Dennis
I really don't think that the sheet flap on the Trainer-60 airfoil versus the Ultra-Sport 1000 airfoil, that proceeds on down to a sharp point, makes a whit of difference in the results. The max coefficient of lift (cl) for the Ultra-Sport is about 0.90 versus the max cl for
the Trainer-60 is almost 1.2. I'd bet the farm that changing the rear of the Ultra-Sport to have a finite sheet flap trailing edge would make a big change at all. The stall of the airfoil is pretty well determined by loss of ability of the flow to make it around the leading edge successfully. This effect of the minor alteration of the airfoil trailing edge on the overall flowfield I think is minimal.
Ken
Bob got blunter through the years .... and thicker too. Remember also that his planes were weightless, they were more flexible than II like, but then they were weightless.
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What do stunt people mean by “penetration”?
I always considered Penetration a good thing.
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Ken
Bob got blunter through the years .... and thicker too. Remember also that his planes were weightless, they were more flexible than II like, but then they were weightless.
He once told me that if you could feel the plane in your hand as you carried it to the car it was too heavy!
Ken
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Here is an XFOIL comparison between a standard NACA 0018 airfoil and one with a sharper LE. Re = 400,000.
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What do stunt people mean by “penetration”?
Lack of airspeed regulation. At the Golden State meet, second round, I would guess it took my airplane 50% longer to do the top legs of the square 8, compared to the bottom. That's because it holds the airspeed very steadily, so the ground speed varies all over the place, depending on the vector sum of the wind and the airspeed. Relatively poor pentration. A Nobler, by comparison, holds it's airspeed poorly, so it takes much longer to bleed off momentum, so it will carry itself further before slowing down to the equilibrium speed, which you can take advantage of in some places in the pattern in the wind.
First time I flew the first Thundergazer, it was a bit windy, but it seemed pretty normal compared to my airplane, or the Trivial Pursuit/Star Gazers, until I got to the overhead 8. Up it went, pretty normal until about halfway through the first loop, then, suddenly, it was so far ahead of where I was expecting it, I almost lost sight of it. Turning back into the wind, I was used to the airplane slowing down, and anticipated it, but it barely changed speed at all, so it penetrated much better than our usual "blimp airfoil" airplanes - even with exactly the same engine that David was flying the last Star Gazer with (PA75, same pipe settings, same prop).
Of course that doesn't mean good or bad, necessarily, either way, it just means that you have different problems in different places.
Brett
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Interesting Brett, would you say that the Thundergazer had more line tension overhead then the Trivial Pursuit/Star Gazers in the wind? Seems since groundspeed give line tension the better the wind penetration the more line tension.
Howard, I don't remember enough of my aero courses to have the inputs for XFOIL (and I don't have the app) but it would be interesting if it is possible to take a simple airfoil, like used on the Andrew's Barnstormer, (also used as base for Still Stuka), NACA 00xx of 1.5" high point, thickness with 9" cord (just airfoil section) then add a flat flap, 5/32" thick by 1.5" (makes it like a polliwog) and do the comparison between a sharp and blunt leading edge to see the impact of the flat flap at various AOA's. Could also compare just a straight NACA 00XX 1.5" thick high point with total cord of 10.5" no flap, sharp and blunt. This would answer if the fixed flap has any impact.
Seems that the flat fixed flap should have minimal impact of lift at low AOA (as in rounds) and more impact in high AOA (as in corners) which almost give a variable flap ratio, would need to then determine how much weight the ship should be to get best penetration with best maneuver performance (CG weight box?).
Best, DennisT
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Interesting Brett, would you say that the Thundergazer had more line tension overhead then the Trivial Pursuit/Star Gazers in the wind? Seems since groundspeed give line tension the better the wind penetration the more line tension.
Howard, I don't remember enough of my aero courses to have the inputs for XFOIL (and I don't have the app) but it would be interesting if it is possible to take a simple airfoil, like used on the Andrew's Barnstormer, (also used as base for Still Stuka), NACA 00xx of 1.5" high point, thickness with 9" cord (just airfoil section) then add a flat flap, 5/32" thick by 1.5" (makes it like a polliwog) and do the comparison between a sharp and blunt leading edge to see the impact of the flat flap at various AOA's. Could also compare just a straight NACA 00XX 1.5" thick high point with total cord of 10.5" no flap, sharp and blunt. This would answer if the fixed flap has any impact.
Seems that the flat fixed flap should have minimal impact of lift at low AOA (as in rounds) and more impact in high AOA (as in corners) which almost give a variable flap ratio, would need to then determine how much weight the ship should be to get best penetration with best maneuver performance (CG weight box?).
Best, DennisT
The impact of flaps on the LE section of the airfoil is negligible. XFOIL and other airfoil models struggle with determining the effect of flat plate sections. The funny looking TEs that Frank showed are the result of the digitization of the sections. We take the validity of these models much more seriously than they truly deserve. They are a tool that is good at gaining an understanding of the dependencies and the answer changes with the model assumptions input. Take the same dat Howard used in XFOIL and put in Martin Heberles Java Foil or Hanley's Visual Foil and you get a different answer for the magnitude of the Cl albeit similar profiles. Changing the radius impacts the curve in all of them.
How the LE behaves is dependent on the smoothness of the contour. Contour drives the pressure variation and steep pressure gradients are what cause separation. Small radii cause steep pressure gradients. Here are three pressure plots. What is desired in the pressure curve is smoothness and low slopes. In the smallest radius notice how the pressure curve goes up steeply then back down and then back up again. That rapid change in slope is what causes the separation. Imagine a marble rolling on the curve if the curve were turned upside down which is basically what they are. The Cp is actually presented as -Cp vs T/c. The steeper the curve coming out of the well the more difficult the marble has getting back up.
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How the LE behaves is dependent on the smoothness of the contour.
If I am following this properly you are saying that a fairly "blunt" parabolic curve up to the highpoint with a gradual curve to the TE (less flaps) is where we are headed. This leaves thickness not only at the highpoint but for the aft part of the wing. Right now I am torn between the Rabe Mustang/MaxBee style and the Thunderbolt style. Not a good place to be when you are getting ready to order the wood. It is the same issue as always, corner vs rounds.
Ken
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With too blunt l.e. it’ll be more difficult to glide the 1 full lap from 1,5m height to touchdown, as required in FAI-rules. L
I don't think David is particularly concerned with that problem.
Brett
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With too blunt l.e. it’ll be more difficult to glide the 1 full lap from 1,5m height to touchdown, as required in FAI-rules. L
Not a problem. Most of us here could easily whip a dead horse. LL~
Ken
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If I am following this properly you are saying that a fairly "blunt" parabolic curve up to the highpoint with a gradual curve to the TE (less flaps) is where we are headed. This leaves thickness not only at the highpoint but for the aft part of the wing. Right now I am torn between the Rabe Mustang/MaxBee style and the Thunderbolt style. Not a good place to be when you are getting ready to order the wood. It is the same issue as always, corner vs rounds.
Ken
Not necessarily. Look at the E472 section. Round and blunt and not desirable due to the sharp stall break. To properly design an airfoil you need to be somewhat versed in the pressure distribution behavior. From that you can "message" the pressure distribution. This a kind of inverse design methodology.
Here's an example of the airfoil I've been using. It has a fairly "blunt" LE but not overly. And it features a smooth transition and pressure recovery ramp. My flight testing does not confirm the Cl max and as we know I don't use predicted Cl max as more than just something interesting. More important Cl/Cd vs Cl polar. From that you can see at higher Cl's the NACE is better than the 0018. That is discernable in flight. Here's the reality of Cl max of a symmetrical section on an airplane using flaps. We never operate at that angle of attack therefore Cl max is not a design parameter but just a curiosity. We operate at some intermediate AOA and create the required Cl by changing the contour of the airfoil by using flaps.
The NACE airfoil originate 15 years ago when I was doing a Stolp Acroduster II wing redesign. When that airplane was designed, they wanted an airplane that would "snap good" so they took a 63-0012 section and sharpened the LE. The result was much improved snap characteristics. Much too improved. So, I spent a bunch of time to make a better airfoil. The variation for the model is different than the one for the biplane and different than the one for the Frankenlaser. Each has some tuning for its mission. The models airfoil is the only one fully symmetrical and the biplane has two different sections designed for top and bottom wings.
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Not necessarily. Look at the E472 section. Round and blunt and not desirable due to the sharp stall break. To properly design an airfoil you need to be somewhat versed in the pressure distribution behavior. From that you can "message" the pressure distribution. This a kind of inverse design methodology.
Why do you care about how sharply it stalls?
Brett
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I have a couple planes equipped with the Selig-Donovan SD8020 airfoil scaled to 22% minus flaps. Works really well. Norm Whittle used this airfoil on his Sultan-e scaled at like 17 or 18% minus flaps. Just one of many good choices out there. Looks a lot like most other good modern stunt airfoils.
Dr. Selig designed the 8020 for good performance at low Reynolds numbers. I would be interested to hear details about how this section compares against other known, good sections.
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Why do you care about how sharply it stalls?
Brett
That's a good question Brett. When there is a sharp stall with AOA as in the E472 there is a significant difference in Cl. An airplane in a skid will have one wing with a higher AOA than the other typically. When a section that has a sharp stall is stalled like this, one wing stalls much greater than the other causing a snap roll or spin. I have seen some wierdness hinging "events" which after a while thinking about it decided what is occurring in essence an attempt for the airplane to do a snap roll ona string. It's highly likely that some of the difficult hinging cases can be traced to this kind of stall characteristic. Wing sweep can exasperate this behavior as well. The basics are in my spin course presentation.
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In every airfoil debate that I've read here, we keep referencing polars for fully symmetrical airfoils and looking at high AoA region, whereas polars for undercambered foils are more appropriate, as 99.999% of stunt planes enter the corner with a flap deflected.
We also fail to mention wing loading and air conditions. A 15 oz/ft^2 plane flying in hot, humid, still air (summer morning in Muncie?) is much more likely to stall than a 12oz/ft^2 plane on a cool windy afternoon.
Reynolds number includes viscous properties of the media, if I understand correctly, but we somehow keep assuming that air never changes.
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In every airfoil debate that I've read here, we keep referencing polars for fully symmetrical airfoils and looking at high AoA region, whereas polars for undercambered foils are more appropriate, as 99.999% of stunt planes enter the corner with a flap deflected.
We also fail to mention wing loading and air conditions. A 15 oz/ft^2 plane flying in hot, humid, still air (summer morning in Muncie?) is much more likely to stall than a 12oz/ft^2 plane on a cool windy afternoon.
Reynolds number includes viscous properties of the media, if I understand correctly, but we somehow keep assuming that air never changes.
Exactly, sorta. The common Cl v AOA plot isn't actually a polar since the Cl is dependent on AOA. The polar is the CL v Cd both of which depend on AOA and not each other directly. Actually the flapped section instead of an underchambered airfoil although the analogy is correct. This is what I've said many times that there is an over attention to the Cl max of the symmetrical airfoil, at least when it comes to a PA airplane with flaps. Wing loading is important but doesn't change the L/D only the speed at which the airplane will fly at a given L/D. Reynolds number does change ever so slightly for a PA model which works in the roughly 500,000 zone. Taper ratio can have a much larger effect on Rn than does the in flight velocity variation. If you look at the chart on page 8 of my presentation, therein is a diagram showing lift curve as a variation of flap angle.
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That's a good question Brett. When there is a sharp stall with AOA as in the E472 there is a significant difference in Cl. An airplane in a skid will have one wing with a higher AOA than the other typically. When a section that has a sharp stall is stalled like this, one wing stalls much greater than the other causing a snap roll or spin. I have seen some wierdness hinging "events" which after a while thinking about it decided what is occurring in essence an attempt for the airplane to do a snap roll ona string. It's highly likely that some of the difficult hinging cases can be traced to this kind of stall characteristic. Wing sweep can exasperate this behavior as well. The basics are in my spin course presentation.
All that is correct, however, you can never let the wing stall in the first place, if you hope to be successful. So what matters is not what it does afterward, it's (at most) how much Cl you get before it stalls, or alternately, how much AoA you can get at the point of a stall.
Same reasoning applies to the various theories about increasing the section percent thickness as you to toward the tip, to avoid the dreaded "tip stall". Stall the root first, you lose anyway. Make sense, maybe, for full-scale, but not for stunt. There may be *other* reasons to alter the section thickness from root to tip, but not stall characteristics.
BTW, we have long suspected/known that some of the very blunt/forward high point airfoils used in some of our designs have violent stall characteristics. Some of the most successful stunt airfoils are one short of putting a beer can on the paper and tracing around it to get the leading edge. The idea, correct or not, being to get enough Cl to avoid stalling in the first place. The only downside I have see with it is that they tend to be ugly.
Brett
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What do stunt people mean by “penetration”?
Probably different for everyone, because of equipment differences and personal preferences? LL~ Steve
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All that is correct, however, you can never let the wing stall in the first place, if you hope to be successful. So what matters is not what it does afterward, it's (at most) how much Cl you get before it stalls, or alternately, how much AoA you can get at the point of a stall.
Same reasoning applies to the various theories about increasing the section percent thickness as you to toward the tip, to avoid the dreaded "tip stall". Stall the root first, you lose anyway. Make sense, maybe, for full-scale, but not for stunt. There may be *other* reasons to alter the section thickness from root to tip, but not stall characteristics.
BTW, we have long suspected/known that some of the very blunt/forward high point airfoils used in some of our designs have violent stall characteristics. Some of the most successful stunt airfoils are one short of putting a beer can on the paper and tracing around it to get the leading edge. The idea, correct or not, being to get enough Cl to avoid stalling in the first place. The only downside I have see with it is that they tend to be ugly.
Brett
Brett speaks wisely, as usual. I used to fly a little combat. The airfoil I used (more on that later, maybe) had a high Clmax, but a violent stall. Airplanes using it had to be built straight. I put stops on my bellcrank to limit stabilator (hinged at the LE without flutter) travel to keep the wing just below stall. I would test airplanes on a windy day so the right wing would stall first until I got the airplane trimmed. I could have used a sharper LE to give a gentler stall. It would have also saved me the bother of fixing string slits in my wing leading edge.
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Wing loading is important but doesn't change the L/D only the speed at which the airplane will fly at a given L/D.
Pertinent to Lauri's issue, but not to the part of the stunt pattern when the engine is running.
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Bob got blunter through the years .... and thicker too.
Doesn't this describe most of us...
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Seems I'm at the end of what I have to contribute.
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We also fail to mention wing loading and air conditions. A 15 oz/ft^2 plane flying in hot, humid, still air (summer morning in Muncie?) is much more likely to stall than a 12oz/ft^2 plane on a cool windy afternoon.
Reynolds number includes viscous properties of the media, if I understand correctly, but we somehow keep assuming that air never changes.
Reynolds number is proportional to air density / viscosity. As temperature goes up, density goes down and viscosity goes up, hence Reynolds number goes down. Lift capability varies more with density than with Reynolds number. I found this chart showing how temperature affects various things at Muncie's elevation compared to their values at a sea level standard day (59F, pressure 29.92 in Hg, density .002377 slugs / cubic ft.).
Definitions:
SLSD is sea level standard day.
Sigma is the ratio of air density / SLSD density.
Re / Re0 is Reynolds number / SLSD Reynolds number for a given true airspeed and wing chord.
0018 is NACA 0018 airfoil
Cl is lift coefficient, useful for comparing airfoils and airplanes. It's nondimensionalized lift, lift / (dynamic pressure * wing area)
Dynamic pressure is air density * true airspeed2
Details:
Impact data are for the Impact mean aerodynamic chord with flaps 20 or 30 degrees (I forget which) from Javafoil.
NACA 0018 data are from NACA TR 586, which was such an influence on me as a kid.
Humidity doesn't have much of an effect. I forgot what I assumed for humidity. You can see its effect and other cool stuff at https://wahiduddin.net/calc/calc_da_rh.htm .
Moral:
Reynolds number changes a lot with temperature.
Maximum Cl doesn't change much with temperature, hence with Reynolds number over the speed range stunt planes fly.
Lift capability changes with air density. Minimum loop size in inversely proportional to density.
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But like many things, a good landing glide has more variables than just that, like how engine quits (fits, tank..).
I've noticed that European engines have a significant burst of speed when they run out of fuel. I'm counting on electrickery.
And Ken, the more you can manage without whipping, the better. L
For one thing, you might wander out of the center circle and hit the fence. Fortunately it was my old airplane.
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Not necessarily. Look at the E472 section. Round and blunt and not desirable due to the sharp stall break. To properly design an airfoil you need to be somewhat versed in the pressure distribution behavior. From that you can "message" the pressure distribution. This a kind of inverse design methodology.
Here's an example of the airfoil I've been using. It has a fairly "blunt" LE but not overly. And it features a smooth transition and pressure recovery ramp. My flight testing does not confirm the Cl max and as we know I don't use predicted Cl max as more than just something interesting. More important Cl/Cd vs Cl polar. From that you can see at higher Cl's the NACE is better than the 0018. That is discernable in flight. Here's the reality of Cl max of a symmetrical section on an airplane using flaps. We never operate at that angle of attack therefore Cl max is not a design parameter but just a curiosity. We operate at some intermediate AOA and create the required Cl by changing the contour of the airfoil by using flaps.
The NACE airfoil originate 15 years ago when I was doing a Stolp Acroduster II wing redesign. When that airplane was designed, they wanted an airplane that would "snap good" so they took a 63-0012 section and sharpened the LE. The result was much improved snap characteristics. Much too improved. So, I spent a bunch of time to make a better airfoil. The variation for the model is different than the one for the biplane and different than the one for the Frankenlaser. Each has some tuning for its mission. The models airfoil is the only one fully symmetrical and the biplane has two different sections designed for top and bottom wings.
NACE looks dandy. I don't agree that Cl/Cd is important, but you might fix the scale on your polar so we can see what it is.
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Howard, I don't remember enough of my aero courses to have the inputs for XFOIL (and I don't have the app) but it would be interesting if it is possible to take a simple airfoil, like used on the Andrew's Barnstormer, (also used as base for Still Stuka), NACA 00xx of 1.5" high point, thickness with 9" cord (just airfoil section) then add a flat flap, 5/32" thick by 1.5" (makes it like a polliwog) and do the comparison between a sharp and blunt leading edge to see the impact of the flat flap at various AOA's. Could also compare just a straight NACA 00XX 1.5" thick high point with total cord of 10.5" no flap, sharp and blunt. This would answer if the fixed flap has any impact.
Seems that the flat fixed flap should have minimal impact of lift at low AOA (as in rounds) and more impact in high AOA (as in corners) which almost give a variable flap ratio, would need to then determine how much weight the ship should be to get best penetration with best maneuver performance (CG weight box?).
I can't even say "flat fixed flap".
XFOIL comes with Profili, https://www.profili2.com/ . There are some YouTube tutorials on it, which I oughta watch. Mark mentions some other programs that might be more recent. I had trouble getting XFOIL to accept flaps the way we use them on stunt planes, so I'm a little suspicious of the results. I did compare an Impact airfoils with flaps at zero (a flat fixed flap) with some good combat airfoils. XFOIL liked the Impact airfoil better, which was embarrassing. You may have a point. Flite Streaks fly well.
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Reynolds number is proportional to air density / viscosity. As temperature goes up, density goes down and viscosity goes up, hence Reynolds number goes down. Lift capability varies more with density than with Reynolds number. I found this chart showing how temperature affects various things at Muncie's elevation compared to their values at a sea level standard day (59F, pressure 29.92 in Hg, density .002377 slugs / cubic ft.).
Definitions:
SLSD is sea level standard day.
Sigma is the ratio of air density / SLSD density.
Re / Re0 is Reynolds number / SLSD Reynolds number for a given true airspeed and wing chord.
0018 is NACA 0018 airfoil
Cl is lift coefficient, useful for comparing airfoils and airplanes. It's nondimensionalized lift, lift / (dynamic pressure * wing area)
Dynamic pressure is air density * true airspeed2
Details:
Impact data are for the Impact mean aerodynamic chord with flaps 20 or 30 degrees (I forget which) from Javafoil.
NACA 0018 data are from NACA TR 586, which was such an influence on me as a kid.
Humidity doesn't have much of an effect. I forgot what I assumed for humidity. You can see its effect and other cool stuff at https://wahiduddin.net/calc/calc_da_rh.htm .
Moral:
Reynolds number changes a lot with temperature.
Maximum Cl doesn't change much with temperature, hence with Reynolds number over the speed range stunt planes fly.
Lift capability changes with air density. Minimum loop size in inversely proportional to density.
So, here's a question (and this is a real question, not an "assertion in the form of a question") - why is it that we have so much more problem with marginal airplanes stalling and losing engine power in Muncie, rather than Davis or Tucson, even though the nominal air density is the same? It gets pretty darn hot in Davis, and even at 85 degrees the air is pretty thin in 2200 Ft Tucson.
We talked about this before, and you suggested the viscosity difference, but this suggests it is a nearly negligible effect. I had previously speculated about humidity, that makes the air less dense (and I think it causes the "vacuum air" spots you sometimes get in calm afternoons in the Midwest), but it seems like a pretty weak effect.
Brett
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So, here's a question (and this is a real question, not an "assertion in the form of a question") - why is it that we have so much more problem with marginal airplanes stalling and losing engine power in Muncie, rather than Davis or Tucson, even though the nominal air density is the same? It gets pretty darn hot in Davis, and even at 85 degrees the air is pretty thin in 2200 Ft Tucson.
We talked about this before, and you suggested the viscosity difference, but this suggests it is a nearly negligible effect. I had previously speculated about humidity, that makes the air less dense (and I think it causes the "vacuum air" spots you sometimes get in calm afternoons in the Midwest), but it seems like a pretty weak effect.
Brett
I probably made that plot after our conversation. I remember one Nats when they'd recently repaved the L-pad. It was black and really hot. I suspect that's what caused Doug Moon to crash. That's all I can think of, and it wouldn't explain other years.
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NACE looks dandy. I don't agree that Cl/Cd is important, but you might fix the scale on your polar so we can see what it is.
There's a scroll bar on he side of the picture window.
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I think this thread has established that smooth radius leading edge airfoils can have a higher angle of attack before stalling. It also seems to confirm that thinnish to medium thickness (15% - 18%) can have a slightly better wind penetration when matched with the right weight ship.
The open question is what is the minimum LE radius as say a % of airfoil thickness were we see benefit for a PACL ship? For example could the older designs were the LE is a 1/4" sq. with the 1/16" sheeting attached be rounded to a 1/4" radius be enough or should that be a 3/8" sq. or larger?
Best, DennisT
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So, here's a question (and this is a real question, not an "assertion in the form of a question") - why is it that we have so much more problem with marginal airplanes stalling and losing engine power in Muncie, rather than Davis or Tucson, even though the nominal air density is the same? It gets pretty darn hot in Davis, and even at 85 degrees the air is pretty thin in 2200 Ft Tucson.
Brett
It's the humidity which has a significant impact on density altitude. Tucson is very dry and Muncie isn't. An H2O molecule is lighter than either the N2 or O2 molecule is displaces.
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So, here's a question (and this is a real question, not an "assertion in the form of a question") - why is it that we have so much more problem with marginal airplanes stalling and losing engine power in Muncie, rather than Davis or Tucson, even though the nominal air density is the same? It gets pretty darn hot in Davis, and even at 85 degrees the air is pretty thin in 2200 Ft Tucson.
We talked about this before, and you suggested the viscosity difference, but this suggests it is a nearly negligible effect. I had previously speculated about humidity, that makes the air less dense (and I think it causes the "vacuum air" spots you sometimes get in calm afternoons in the Midwest), but it seems like a pretty weak effect.
Brett
The best example I have ever seen of this question, I think, is a model that Mark Hughes built years ago for VSC. Basically a jet looking model based on the Jetco Shark 45 that was a copy of a model Chris McMillin's Dad flew back in the late 60's. Mark had a warp creep into the wind during construction that seemed to get worse as the finish went on, but he was determined to fly it any way. It was a nice, light model, probably in the high 50 ounce range, powered by the then new ST G.51. We got there on Friday afternoon (this was when it was still a two day weekend contest, 1994 I think) and put in the maiden flight. Not good as it was flying in a steep bank with the outboard wing way high. Bob Whitely and Jim Armor were there, grabbed him, the model and a tool box and went over to the grass circles and started cutting and gluing. About an hour later, then came back to the main circle and Mark started to really burn flights in and everyone stopped to watch him, wondering who he was!! It was an amazing transformation, and Mark went on to place 2nd or 3rd against some pretty heavy competition. He really surprised everyone, as no one knew who he was. It had only been a few years since he fist called me to help him get a good needle on a Fox .35 and this new model was only about the 3rd one he had built to that time. That was the last time that model ever flew well! When flying season opened up here in the Midwest, the airplane flew like crap! As far as we could tell, nothing had changed on the airplane and our attempts to cure it were fruitless. Mark eventually hung the model on the wall and retired it. Some where along the line it fell and it broke up, then Mark finished the job! It was the damnedest thing I had ever seen. The only thing I could think of was that the warmer, dryer high altitude air was just perfect for the wing and it's acquired tweaks and tabs that it needed to fly level. The adjustments may have been over done for the air here at lower altitudes, but even with trying to undo them we never got anything like the performance at Tucson.
Type at you later,
Dan McEntee
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What's one of the first things a pilot adjusts when he's in the cockpit? Full size aircraft.
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What's one of the first things a pilot adjusts when he's in the cockpit? Full size aircraft.
Trick question? Seat and seat belts.
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It's the humidity which has a significant impact on density altitude. Tucson is very dry and Muncie isn't. An H2O molecule is lighter than either the N2 or O2 molecule is displaces.
Everything I have read says the opposite: humidity has little effect on DA.
https://www.boldmethod.com/learn-to-fly/performance/density-altitude/
https://www.flyingmag.com/pilot-technique-tip-week-correcting-density-altitude-humidity/
An interesting document, (Page 20+ is especially interesting):
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjPu9Ghz6T0AhUAmGoFHRVADhsQFnoECCgQAQ&url=https%3A%2F%2Fcommons.erau.edu%2Fcgi%2Fviewcontent.cgi%3Farticle%3D1124%26context%3Dijaaa&usg=AOvVaw1pMy02CmT8uPbfku2tZ6uC
"While the role of humidity in density altitude calculations is indeed
secondary to the role of temperature, the effects can be potentially significant, when
the dew-point temperatures are high (e.g., errors over 400 feet)....."
"... a very simple-to-use ROT; that is, we can find the correction in feet due to dew-point temperature by
simply doubling the dew-point temperature and multiplying by ten or “doubling
and adding a 0.”..."
Many other citations available via Google search.
Bottom line as I read it: "some" effect, but "significant" is an exaggeration.
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Bill ....
Do you experience the very noticeable atmospheric anomalies at the racing circles in Muncie that we see at the L-pad. About 11am the bottom seems to fall out. Muncie is a flying site that is surrounded by heavy crops. I still think that the layer of vapor that you see across the fields melts away around noon and things seem to change rapidly. Stunt Physics?
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It's the humidity which has a significant impact on density altitude. Tucson is very dry and Muncie isn't. An H2O molecule is lighter than either the N2 or O2 molecule is displaces.
That's the conclusion I came to, but everyone keeps telling me otherwise. I don't dispute Bill's data, but there is definitely *something* going on, and humidity is the most obvious difference.
Interestingly, it (whatever "it" is) has more effect on the power than it does aerodynamics. After all this time, I know what trim changes I/we need to make for different conditions, it's the mechanism underlying it that seems to still be in question. Howard had previously suggested it was changing viscosity, but as above, it makes very little difference.
Howard likes to tell Paul and I we are exhibiting "princess and the pea" syndrome. In my case, I can't rule that out, but in this case, it must be by proxy, since over the years Ted and David found the same things I did, whether we understand the mechanism or not.
Brett
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That's the conclusion I came to, but everyone keeps telling me otherwise. I don't dispute Bill's data, but there is definitely *something* going on, and humidity is the most obvious difference.
Interestingly, it (whatever "it" is) has more effect on the power than it does aerodynamics. After all this time, I know what trim changes I/we need to make for different conditions, it's the mechanism underlying it that seems to still be in question. Howard had previously suggested it was changing viscosity, but as above, it makes very little difference.
Howard likes to tell Paul and I we are exhibiting "princess and the pea" syndrome. In my case, I can't rule that out, but in this case, it must be by proxy, since over the years Ted and David found the same things I did, whether we understand the mechanism or not.
Brett
You and David fly at sea level correct? Could it just be the change in altitude? I haven’t really noticed a difference in the last few years going to Muncie regarding changes in the motor run or trim compared to practice in Dallas or Tulsa
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You and David fly at sea level correct? Could it just be the change in altitude? I haven’t really noticed a difference in the last few years going to Muncie regarding changes in the motor run or trim compared to practice in Dallas or Tulsa
Certainly the altitude and air density difference requires re-trimming. But it's not the same at different places at the same apparent air density. Muncie is right at 1000 feet, and Tucson 2200 feet, the Napa and Davis sites are around 20 feet. But a hot day in Davis (say, 105 degrees) gets us about the same apparent air density as a typical day in Muncie. 55 degrees in the morning to 105 at noon requires no more than screwing in the needle very slightly. Going to Muncie other requires several pretty drastic changes.
I guess to state it a different way, I have an air density meter, and if it simply followed air density, you would think the same trim would be good at a given air density everywhere. It seems to be drastically different, so it must be something else. The one hallmark difference is, as Mark notes, the humidity. Again, not disputing Bill's (or previously, Howard told me the same thing) data, but it has to be *something*.
Brett
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Bill ....
Do you experience the very noticeable atmospheric anomalies at the racing circles in Muncie that we see at the L-pad. About 11am the bottom seems to fall out. Muncie is a flying site that is surrounded by heavy crops. I still think that the layer of vapor that you see across the fields melts away around noon and things seem to change rapidly. Stunt Physics?
"Melt's away" or "gets blown away?" On the topic of "vacuum air" spots, we used to notice when I flew in the Kentucky summer, what appeared to be "dead spots" in the air at semi-random on very hot, humid, calm days. This is not flying through wake, it would happen even when you hadn't flown through the same spot before, and it always acted as if there were pockets of thin air, where you just suddenly lost all the lift and the airplane appeared to go in a straight line until you flew out the other side of it. If it were thermals, you would expect it to move you in either direction, in these, it was *always* as if the lift went away suddenly. We always attributed it to local spots of high humidity. It never happened if there was even the slightest breeze, theory being that the wind was sufficient to mix it all so it was homogeneous.
I have only occasionally gotten that at Muncie, but it may be that I never flew in the right conditions enough.
The effect you are talking it about might be due to the thermal gradient. That is inevitable, near the nice hot blacktop, the air is naturally much hotter, get up in the air, and it is the free-air temperature. Usually this screws up needle settings, the engine speeds up more than normal once you get it off the ground. It doesn't happen over grass.
Brett
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...
"While the role of humidity in density altitude calculations is indeed
secondary to the role of temperature, the effects can be potentially significant, when
the dew-point temperatures are high (e.g., errors over 400 feet)....."
...
Bottom line as I read it: "some" effect, but "significant" is an exaggeration.
400 feet is not insignificant, it's 40% of the change from sea level to Muncie. The Northwest Regionals site is 500ish feet, and we have to retrim for that.
I would agree, however, that the effect we are talking about acts like a lot more than a 400 foot altitude change.
Brett
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I know about the vacuum spots in Muncie. Luca lost his plane in one of those on the morning of Advanced finals. It was during practice over grass (!) and at temps in the low 60s . But the humidity was unbelievable.
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"Melt's away" or "gets blown away?" On the topic of "vacuum air" spots, we used to notice when I flew in the Kentucky summer, what appeared to be "dead spots" in the air at semi-random on very hot, humid, calm days. This is not flying through wake, it would happen even when you hadn't flown through the same spot before, and it always acted as if there were pockets of thin air, where you just suddenly lost all the lift and the airplane appeared to go in a straight line until you flew out the other side of it. If it were thermals, you would expect it to move you in either direction, in these, it was *always* as if the lift went away suddenly. We always attributed it to local spots of high humidity. It never happened if there was even the slightest breeze, theory being that the wind was sufficient to mix it all so it was homogeneous.
I have only occasionally gotten that at Muncie, but it may be that I never flew in the right conditions enough.
The effect you are talking it about might be due to the thermal gradient. That is inevitable, near the nice hot blacktop, the air is naturally much hotter, get up in the air, and it is the free-air temperature. Usually this screws up needle settings, the engine speeds up more than normal once you get it off the ground. It doesn't happen over grass.
Brett
Dead spots are an interesting description. It would be interesting to take some cat tails with you to events or even just sport flying and when you encounter conditions having dead spots cast the cat tails in to the air. My guess is what you are seeing or noticing is a thermal about to break or is breaking. The HLG guys in the FF arena look for this kind of tell tell. The cat tails will swirl about. The same thing can be done with a bubble machine. The conditions of a light wind and black top transition are a perfect thermal generator and can be quite vigorous. Easily enough to bump a PA airplane around.
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My guess is what you are seeing or noticing is a thermal about to break or is breaking.
This should almost be it's own thread. I flew HLG and Nordic with some of the best and learned to "smell" thermals 100 yards away. Knowing what they can do to you has saved several ships for me over the years.
Ken
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It's true that for the majority of flight planning we do not take humidity in to account. The reason is that the temperature has more of an impact than humidity. That does not negate the impact of humidity only leaves unaccounted for in the performance calculation. The assumption here is what is the difference between to location with similar temperature conditions. Humidity is a factor. Perhaps significant is a bit strong in terms of all of the element but insignificant it isn't.
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.... the older designs were the LE is a 1/4" sq. with the 1/16" sheeting attached be rounded to a 1/4" radius be enough or should that be a 3/8" sq. or larger?
Best, DennisT
Sorry I didn't see this before the thread went barometric. There were two ways that LE's were done back then. Sheeting up to the 1/4" LE and sheeting over the 1/4" LE. You could get a 1/4" radius sheeting over but you couldn't if you were sheeted up to. I mentioned earlier that the Gieseke Nobler (at least Bob's own ships) had a much more rounded LE than the Blue Box. This is why it could be rounded more. Still, 1/4" is pretty pointy by todays standards.
Ken
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Ken,
I check the plans I have for the Gieseke Nobler, it indicated that you trace the Nobler ribs with a FAT pen and cut out on the outer edge of the line then use 3/32" planking for the leading edge sheeting and cap strips. This gives a bit more meat to work with on the GN, question is what is the minimum that one can get away with for these classic ships?
Best, DennisT
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Bill ....
Do you experience the very noticeable atmospheric anomalies at the racing circles in Muncie that we see at the L-pad. About 11am the bottom seems to fall out. Muncie is a flying site that is surrounded by heavy crops. I still think that the layer of vapor that you see across the fields melts away around noon and things seem to change rapidly. Stunt Physics?
I have been flying Free Flight at the nats since 2005.
Early on I was warned about the 'doldrums' that arrive. From 11:00 to 1:00 there are a lot of guys who simply will not fly. The air gets VERY squirrelly. You can use every thermal detecting trick in the book, decide "there it is" , launch and be in big downer.
Until 9:30 or 10:00 the air is very bouyant. You get a short bit when thermals come through and stay together, then the doldrums hit. There are thermals but it's tricky. I have wondered if it's because we are surrounded by green.
About five-six years ago we were there for a record heat wave. It was just miserable although nothing new to guys who fly in Texas, Oklahoma and Florida. That was the year that I( learned about air density affecting the engine's power. Probably cost me a first place. If my planes aren't up to speed they don't pattern and transition correctly.
I didn't know that it affected our Ukie planes to such a degree. I've learned that engine speed and needs change as the day goes from 45 degrees to 80 degrees but learning that the Muncie doldrums bother Control Liners is news. I've never flown FF at another field where the 'doldrums' exist.
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Ken,
I check the plans I have for the Gieseke Nobler, it indicated that you trace the Nobler ribs with a FAT pen and cut out on the outer edge of the line then use 3/32" planking for the leading edge sheeting and cap strips. This gives a bit more meat to work with on the GN, question is what is the minimum that one can get away with for these classic ships?
Best, DennisT
Interesting question. LE radius is not something I would think is possible under the rules to enforce. Bob got his rounded to just over 1/4 which is fine. The greenbox is much smaller. Probably around 1/8" LE radius was not a hot topic in the 70's. Small parabolic was the norm. Some were really sharp, well pointed might be a better word but remember, most of us had to deal with the same winds we have today with only a single Fox 35 and a couple of Budwisers. I think you still got 5 points for starting your engine back then and you had to wave to the judges that you thought it would keep running! :)
Ken
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If you are inclined to compare airfoils, check the plans for Dick Mathis' original Excalibur and Coyote. Thin airfoils and a rather pointed leading edge. Then look at the Excalibur II. Does Excalibur II fly better than the original due to a blunter leading edge? I can't say as I have never built Excalibur II.
Way back then, I built both the Excalibur and Coyote. Both flew well enough for me but that was then. Maybe now, if I built one of those, I might have enough experience to make a comparison between them and a modern design.
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If you are inclined to compare airfoils, check the plans for Dick Mathis' original Excalibur and Coyote. Thin airfoils and a rather pointed leading edge. Then look at the Excalibur II. Does Excalibur II fly better than the original due to a blunter leading edge? I can't say as I have never built Excalibur II.
Way back then, I built both the Excalibur and Coyote. Both flew well enough for me but that was then. Maybe now, if I built one of those, I might have enough experience to make a comparison between them and a modern design.
I can't speak to the Excalibur other than I sure wouldn't build one with that sharp leading edge radius. I would say look at Ted Fancher and Brett Buck's work with the Strega ARF. It has a very thick wing and a pretty standard forward highpoint airfoil for stunt. However, as delivered the Strega ARF has an extremely sharp leading edge. So, large flaps, thick airfoil, forward highpoint, PIPED PA or RO-JETT...and it still stalled all over the pattern. Peeling back the China-kote and blunting the leading edge solved most or all of the stalling problems that could be easily addressed.
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I think, the best and most correct way is to build a perfect half-ellipse, as front part of the airfoil, (beginning by the thickest point).
Well, let us name the bigger "radius" of this ellipse (in other words: the distance from the LE to the position of thickest point) as "a", and the smaller radius (in other words: halfth of the maximal thickness) as "b".
The touching circle's radius of this half ellipse, (in other words: our airfoil) will be
r= b^2 / a
It is SO simple.
In a simple example: the wing is 2" thick, and the thickest point lays 4" from the LE. r= (2/2)^2 / 4 = 1/4" (.25").
If somebody is not certain, shall make a template: drill a 1/2" dia. hole in a piece of 1/16 plywood, and continue it making a nice half-ellipse.
Istvan
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I can't speak to the Excalibur other than I sure wouldn't build one with that sharp leading edge radius. I would say look at Ted Fancher and Brett Buck's work with the Strega ARF. It has a very thick wing and a pretty standard forward highpoint airfoil for stunt. However, as delivered the Strega ARF has an extremely sharp leading edge. So, large flaps, thick airfoil, forward highpoint, PIPED PA or RO-JETT...and it still stalled all over the pattern. Peeling back the China-kote and blunting the leading edge solved most or all of the stalling problems that could be easily addressed.
Absolutely spot on about the basic problem.
In point of fact, the airplane had a Rustler-Merco 60 "Metamorph" that Ian Russell very kindly gave Ted (and I got one, too...), as it was the closest thing either one of us had to an ST60. It runs *a lot* better than most ST60s, but in the same way. I think it was a 13-6 Rev-up or something like that. A PA/RO-Jett running our usual setup or something close may have made a difference in the answer, but given a lot of other examples, not very much.
The airfoil is more-or-less straight off the Patternmaster plans I saw, near as I could tell. This airfoil appears to be a Nobler airfoil, scaled up in the Y ordinates to get the desired thickness. Thickness-wise it is nearly as thick (in terms of percentage) as the Infinity. The high point is not as far forward, and not as far forward as the Imitation and its more extreme descendants, like the Excitation. When the Nobler scale-up was done, the curve of the front of the airfoil faired very neatly into the 1/4" square leading edge, unlike the Nobler where, until your sheet and sand it, has a noticeable edge right at the junction. On the Strega, this is something like Red Oak instead of balsa. They then sheeted over it as shown, but just left the sharp 90 degree angle at the LE, then just covered over it. The only radius seems to be however much the covering iron round it off as they ironed it down.
Big Jim certainly knew better than to do that, Windy certainly never left it that way (if nothing else, it looks bad), Baron and Mike Rogers flew Patternmasters, and theirs certainly were not like that. But that is how the ARF came (comes?). We started getting inklings of trouble during the morning test flights (we had never flown it before the day of the contest). Ted was the first to encounter a stall, 3rd corner of the triangle - unfortunately, it was only 68 degrees at our sea-level site, and it was only 8:00 in the morning. Ted, as befits his superior talent, was much better about babying it in the weak areas, I pretty much can only do one thing, so by the afternoon, it's in the mid-90's, and he is backing off to make it fly, I am continuing to hammer it and figure I might crash it, outboard-wingtip-first. The kicker was on the last flight of the day, where I got through, am doing the clover, and the damn thing stalls right at the entry to the last loop , inverted, but positive G, and now rapidly heading toward the ground with about a 45 degree roll. It recovered, I got it turned around without causing a second stall trying to pull out, with well under a foot of clearance.
I also note that in addition to the aerodynamics, and in contrast to my general low-key demeanor during contests, when I get a handle in my hand I am just as big of a hyper-competitive maniac as the rest of you guys. I was determined to win, and to win, it had to go where I wanted it to, and damn the consequences.
At any rate, the obvious solution was described in Al Rabe's old AAM Stunt column in about 1973 - that as long as you sand back through the sheeting to the LE stock, and round it off, you don't have a problem. I did that to Ted's airplane, out at the field, with a Perma-Grit file and a devil-may-care attitude. I flattened the entire leading edge off completely blunt and about 1/16" or so into the Red Oak, then rounded it off, and then covered over it. Ted flew it, no problem.
Later, Gerry Arana posted this sequence of events on SSW, and someone from Brodak found out, and gave some supplemental instructions along the same lines. So, this more-or-less took care of the problem for everyone. I also think it might explain more of the thought process that led them to the gigantic flaps, but Big Jim is no longer around to ask.
Brett
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Yup. That is what a sharp break in the lift curve of an airfoil will do to you. ;)
Look at the Cl v AOA graph. This airfoil is a bit different but it is an example of how airfoils behave and an example of the reply to your question of why I won't use an airfoil with a sharp break. With a little yaw the wings can see a couple degrees different AOA. In this case the result, besides creaming an airplane from it, it that one wing sees a Cl of 1.35 and the other 0.9 a 40% difference which causes a tremendous rolling moment. This is what I call the snap roll ona string hinger.
This is exactly your example.
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Yup. That is what a sharp break in the lift curve of an airfoil will do to you. ;)
Yes, I agree on the "45 degree roll angle" part. If the stall characteristics were more gentle, it might have only been 30 degrees. But to *my* point, either way is completely unacceptable. I won this contest only because Ted had the same problem I did, choosing to give up corner (and although I didn't mention this part, my willingness to put the bottoms at 4 feet - or lower). Had he been flying an airplane that gave a good corner but did not stall, like the excellent Vector 40, I would have gotten my ass kicked.
The real fix (which may have made the stall break worse...) is to prevent it from stalling in the first place - meaning I don't care how sharply it might stall, just that I can avoid it without compromising the rest of the flight. So, as earlier - delaying the stall to a higher AoA or at least getting a higher CL before then.
BTW, I don't think we are really arguing the point, I think we are stating it differently.
Brett
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Yes, I agree on the "45 degree roll angle" part. If the stall characteristics were more gentle, it might have only been 30 degrees. But to *my* point, either way is completely unacceptable. I won this contest only because Ted had the same problem I did, choosing to give up corner (and although I didn't mention this part, my willingness to put the bottoms at 4 feet - or lower). Had he been flying an airplane that gave a good corner but did not stall, like the excellent Vector 40, I would have gotten my ass kicked.
The real fix (which may have made the stall break worse...) is to prevent it from stalling in the first place - meaning I don't care how sharply it might stall, just that I can avoid it without compromising the rest of the flight. So, as earlier - delaying the stall to a higher AoA or at least getting a higher CL before then.
BTW, I don't think we are really arguing the point, I think we are stating it differently.
Brett
Communication style can lead to the impression of arguing. "My point" is there isn't a reason to compromise. Case in point, my NACE kicks ass in the performance arena and doesn't do bad stuff. The one thing I'm not worrying about is if it's gonna bite me. Take one more worry off my list. Gives me more margin. The crash videos are full of airplanes crashing 80% plus could have survived if they had a better airfoil. I have an ARC Strega on the shelf and it will likely remain there. Reason being is it represents a risk and frustration I don't need. Yes, you can fly around its bad habits but why would you if you didn't have too. To me that's silliness but again, I have the tools in my pocket and not everyone does. I do things like buy and fly some ARFs because they are a fast path to gaining knowledge. I reserve my major efforts to the development of better. In that end I most likely wouldn't build a Strega or many others but I surely would watch and listen to what Windy has to say, which I have. Trust me, I pay attention.
There's no question you guys can fly better than I can. And that you have more CLPA experience than I do. The reality is that it isn't likely two things will happen. One is that I would win a major title or even participate. That just isn't my thing. I don't have the wherewithal to spend the time to get there, I have other sit I wanna play with. Two is that what I present will make it very far as it takes the first get peoples attention. So, I just do my thing. That thing is, I make cool sit. I am good at it. But most often it just gets put in the corner to collect dust. And that's ok it's what I do for fun.
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I probably made that plot after our conversation. I remember one Nats when they'd recently repaved the L-pad. It was black and really hot. I suspect that's what caused Doug Moon to crash. That's all I can think of, and it wouldn't explain other years.
I was the next up after that, and my opinion was that it was pretty much the usual dead-air turbulence. We were really hoping he didn't take an attempt, and he certainly wasn't leaving any margin (this was Top 20 day, after all). Afterwards, we "cleared the circle" for about 20 minutes, to the point we were picking old glow plugs out of the sealer from 10 years earlier. Of course we were super-concerned about debris, but, the fact that it was dead calm didn't suggest a speedy operation. There was only a breath of air when I flew, not enough to really matter, I just got the luck of the draw and had no big hits at critical moments.
Brett
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I found some of my old data and diagrams concerning leading edges and flaps from several years ago, and they fit somewhat with the original question and the flap issue that came up. While I certainly agree with Mike and others who have pointed to changes in Reynolds Number with atmospheric conditions, the relevance of changing camber with flap deflection, second derivative items, etc. and long ago wrote a little on those myself, I have been more interested in simpler things. So here - again FWIW - are a few items I explored and found interesting for my own purposes. Unfortunately, some of my old scans were done at low resolution for SSW forum and aren’t as clear as I’d have liked. I hope the wing section comparisons are visible.
First, my answer to a question above: for me, "penetration" is just a measure of how far a wing or aircraft flies into suddenly increased headwind from gusts or entering the upwind part of a lap before regaining its normal air speed. The better the penetration, the less quickly a wing’s airspeed and groundspeed slows, when encountering a headwind. While a less “draggy” wing is less desirable when accelerating downward in a maneuver, it has its advantages elsewhere.
Of all NACA reports I’ve found, NACA TN 763 relates most closely to the “Stationary flaps” that interest me. Seeing it again, I see that I did not remember what they call a “Guinn” flap quite accurately, thinking it to have a more abrupt hinge line at neutral and to be flatter. So it is thin, but very tapered. Anyway, the first two pictures below, from TN 763, do concern wings for flapless planes. Figures 1 and 2 (first picture) show the wing sections compared in the report. They are not symmetrical, but rather the most common of the 5-digit sections that they liked at that time. Their aft camber lines were straight from the “high point” aft, giving them low c.p. movement. I think such sections were used on the “Corsair” and later the “Bonanza,” among others. Aspect ratios are 6.0 for both models at Re = 609,000 (we fly at about 400,000-500,000; you can calculate yours here:
https://www.omnicalculator.com/physics/reynolds-number
NACA compared 18% and 15% sections, because the actual percent thicknesses of the 23018 with the Guinn flap was close to 15% at14.4% of the total chord.
The second picture is figure 8 from TN 763. From it you see that the “Guinn” Flap gives more maximum lift when not deflected than the plain flap (yellow line). NACA preferred the plain flap, due to lower drag and higher L/D, considerations for cruise range and speed. Other graphs in TN 763 show Cl’s for all angles of attack for each flap deflection, where data can be compared for our maneuvering CL's. ...and yes, I have always been aware that we try not to fly at high lift coefficients near the stall!
I played around in Profili 15 years ago to create a wing section to suit an original flapless plane. Like John Miller, I modified one from the NACA “laminar” series. Liking its leading edge, but wanting a “friendly” flapless version, I moved the “high point” of the 63a010 forward to .24C and doubled its thickness. I got a section I thought might have less drag than the NACA 0020 (as for other thicknesses). The next picture shows it compared to the NACA 0020 section, both stock and with its maximum thickness moved forward to the same point. The leading edge “radius”, if it has one (elliptical, since thickness has been multiplied over the entire chord), is smaller than the 0020’s, but the transition curve seemed nice. So, I thought this less blunt section might have better “penetration.”
I wanted to see how a flat stationary flap would affect it and drew a version of the modified 63a010 with such a flap of only 10% chord. It is shown in the next picture. Unlike NASA in TN 763, I was more interested in preserving the shape of my wing section. So rather than making it thicker still, so that with flap, it would still make a 20% section, I just added the flap, making the flapped version into an 18.8% section. Since NASA and X-Foil had shown thicker sections to have a greater maximum lift coefficient in this range, I thought that any improvement in XFoil would be on the conservative side. XFoil’s comparative results are shown in the next picture, where curves 2 and 3 are for the stationary-flapped versions, while 1 and 4 are the unflapped version (1) compared to the NACA 0020 (4).
The last picture is of the LA .25-powered plane I built incorporating this modified airfoil. It had a D-tube Morris “New Millenium” wing that preserved the leading-edge shape, with a straight spar through the 25%-chord points of all ribs. Wanting to try a higher aspect ratio, I tapered the wing more than usual to ease the effects of gusts. Its MAC is at 45.8% of the half span, somewhere between the more ideal elliptical-wing’s (42.4%) and usual tapered wings' (47% - 49%). It was intended to get the stationary flap, after I’d flown it without, but unfortunately, falling victim to its pilot, it never got to the flapped stage. However, it flew well, even in moderate gusts, and it glided really well.
Addendum - just noticed this (new last pictures), a 23% wing section that is elliptical to 30% of the chord and then tapers along a NACA profile to 3.18% thickness to allow for a flap. Obviously, I hadn't used Excel for this one! Anyway, the profili comparison shows this fat section to be a bit less blunt than the corresponding 00xx section.
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Thanks for re-posting to this forum. I believe there's good information in this work an d I'm glad to see it is still available.
John M.
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I found some of my old data and diagrams concerning leading edges and flaps from several years ago, and they fit somewhat with the original question and the flap issue that came up. While I certainly agree with Mike and others who have pointed to changes in Reynolds Number with atmospheric conditions, the relevance of changing camber with flap deflection, second derivative items, etc. and long ago wrote a little on those myself, I have been more interested in simpler things. So here - again FWIW - are a few items I explored and found interesting for my own purposes. Unfortunately, some of my old scans were done at low resolution for SSW forum and aren’t as clear as I’d have liked. I hope the wing section comparisons are visible.
Addendum - just noticed this (new last pictures), a 23% wing section that is elliptical to 30% of the chord and then tapers along a NACA profile to 3.18% thickness to allow for a flap. Obviously, I hadn't used Excel for this one! Anyway, the profili comparison shows this fat section to be a bit less blunt than the corresponding 00xx section.
Very nice Serge. I read the report which has an interesting conclusion that the plane flap was more desirable. They cite the speed range being the reason from which I conclude the lower drag of the Gwinn flap at higher deflections is the reason. Otherwise the Gwinn flaps have lower drag in almost all tests. There isn't anything generally wrong with using an ellipse to generate an easy airfoil. It's tough to make a better section which requires lots of messing around. The NACE section I designed which honestly has many variants tuned for specific purposes once had some ellipse built in. However, during a couple projects I participated in at Scaled Composites, I had the opportunity to chat with Jon Ronz and learn how he goes about airfoil design.
Jon isn't an aerospace engineer, he's a graphic designer. His method is to look at the pressure data and conceptualize what he would have to do with the profile to adjust the pressures. Make it a little thicker here and little thinner there... When I sat down wit him, he was doing it Matrix style by just looking at the number on big print outs of the dot matrix printers of the day. The tool we use today are spiffier but not necessarily better.
BTW I am a NACA report junky. I love reading those things.
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However, during a couple projects I participated in at Scaled Composites, I had the opportunity to chat with Jon Ronz and learn how he goes about airfoil design.
...BTW I am a NACA report junky. I love reading those things.
Mark -
How fascinating that you have worked at Scaled Composites, where so much exciting work has been done. I talked with John Roncz on occasion, mostly via e-mail, but a couple times at Oshkosh. He's a cool thinker and greatly self taught in his area of expertise. He also worked with Jim Marske on the Genesis project and I guess they didn't quite agree on the need for a "trimmer." Is he still creating America's Cup sails? Those were exciting times for me too, although I watched at a distance and just helped Jim a bit. On the NACA reports, one thing I have noticed is that they were so well conceived and written, often at times when new solutions to old problems (some theretofore unrealized) required re-editing and processing earlier data. Sometimes you can even recognize styles in the reports. I have always liked the TN's because so many were translations or real finds from the literature. Munk, Jones, Diehl,...what a treasure trove.
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Mark -
How fascinating that you have worked at Scaled Composites, where so much exciting work has been done. I talked with John Roncz on occasion, mostly via e-mail, but a couple times at Oshkosh. He's a cool thinker and greatly self taught in his area of expertise. He also worked with Jim Marske on the Genesis project and I guess they didn't quite agree on the need for a "trimmer." Is he still creating America's Cup sails? Those were exciting times for me too, although I watched at a distance and just helped Jim a bit. On the NACA reports, one thing I have noticed is that they were so well conceived and written, often at times when new solutions to old problems (some theretofore unrealized) required re-editing and processing earlier data. Sometimes you can even recognize styles in the reports. I have always liked the TN's because so many were translations or real finds from the literature. Munk, Jones, Diehl,...what a treasure trove.
I did not for work for Scaled Composites directly, I worked on a project that Scaled did the airframe which took me to Scaled for a while. It is a very interesting place and many things go on there. This was in the early middle nineties when I was there. In many ways, I am allot like John in being self trained. While I have aero in my education and some limited professional experience, I have learned much more on my own. My first job out of college was in the rotor and drive systems at McDonnel Douglass Helicopters which the job I did using my education was a rotor blade improvement. That was on the NOTAR helicopter which is a fascinating aerodynamic machine. From there I went in to powerplants and helped put engines in various aircraft. The one that took me to Scaled was a UAV tilt rotor Eagle Eye. That was an aircraft that should have won the contract but you know how that goes. We demonstrated we could fly the mission but a paper vehicle by Boeing won the contract. Wonder how that works?
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Thanks for re-posting to this forum. I believe there's good information in this work an d I'm glad to see it is still available. John M.
Thanks, John! Unfortunately, the numbers of views for the graphs pretty much tell the story. I hope I've provided some perspective. I'm tempted to finish the plane (half built) for which I created the elliptical-leading edged section or to re-build the blue and silver one. The "New Millennium" wing was stiff, but easily damaged.
SK
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Thanks, John! Unfortunately, the numbers of views for the graphs pretty much tell the story. I hope I've provided some perspective. I'm tempted to finish the plane (half built) for which I created the elliptical-leading edged section or to re-build the blue and silver one. The "New Millennium" wing was stiff, but easily damaged.
SK
Well, people generally don't really comprehend what graphs tell them unfortunately. I think you should finish the airplane unless there is a significant problem you spotted which can't be fixed. There's always something to learn from finishing and flying even it is simply confirmation of what you think you know.
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Well, I tend to agree with Mark, the work has merit and is useful. I hope you will finish up your project, and let us know the results. I am particularly interested in your tests with the fixed flap work you have done, as it compares to my own.
I have been working on a design that uses the fixed flap. I hope to start building this season.
John M. H^^
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It was intended to get the stationary flap, after I’d flown it without, but unfortunately, falling victim to its pilot, it never got to the flapped stage.
Serge, i strongly suspect that it was a victim of an unreliable powertrain setup rather than the pilot, based on what I`ve observed over the years. Maybe the next plane can get an electric heart? They dont stall overhead.
Which, by the way, is another aspect of plane performance that wasnt really discussed here. With elevators and flaps deflected to enter a corner, the plane presents a much higher load for the propulsion source to overcome due to drag. How quickly and powerfully your powertrain responds will determine how much speed is lost, and also whether the wing needs a higher AoA to maintain lift. So, a Fox35 powered plane would warrant a blunter airfoil than a 6S active electric system.
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Which, by the way, is another aspect of plane performance that wasnt really discussed here. With elevators and flaps deflected to enter a corner, the plane presents a much higher load for the propulsion source to overcome due to drag. How quickly and powerfully your powertrain responds will determine how much speed is lost, and also whether the wing needs a higher AoA to maintain lift. So, a Fox35 powered plane would warrant a blunter airfoil than a 6S active electric system.
Or, looking at it from the other end, you might be able to get away with a thinner airfoil on a piped airplane, or more so, an electric. Fox-powered airplanes couldn't really take advantage of it because of the overall lack of power.
This is the other reason why all the ST60 airplanes had hugely thick airfoils - to create parasitic drag to control the speed (by decreasing the ratio of induced to parasitic drag). Of course it also was only possible to that degree because it was an ST60 and had tons of power. Same with the ST46, the airplanes got progressively smaller and the wings thicker. The last ST46 airplanes were hardly any bigger than Fox-powered airplanes, but had 25% airfoils to keep the whip-up under control. Not to beat a dead horse, but - *like the Imitation*.
I would note that at least our (Ted and I) first 40/46VF airplanes were essentially blown-up versions of the same (late ST46 airplane) idea, very intentionally to do the same thing with a more powerful engine. What we didn't fully appreciate at the time was the degree to which piped engines could control the speed without resorting to the parasitic drag. I think we rather overdid it, and although the airplanes have done pretty well (2 NATS for Ted, 6 or so for David, one for some random slug, and a Junior WC), they are probably not optimal, lacking penetration in certain places in the pattern, and certainly is not optimal for electric, just because it sucks up so much power for the advantage gained. David actually did something about it, going to the much thinner Thundergazer airfoil with small flaps that is very similar in most respects to the Imitation. I would also note that Paul had it about right all along!
For fear of further inflaming the sensitive, and with absolutely no malice - these sort of considerations are not new, it goes all the way back to GMA himself, making thin wings to get decent wind penetration with his Fox 35 (and living with the consequences). It was a very hot topic throughout the 80s and 90s (when the engines got so much better). It hasn't been discussed a lot in recent years for various reasons, but my electric design certainly doesn't use an Infinity airfoil for exactly the reasons we are discussing here.
Brett
p.s. to address Serge's apparent disappointment - yes, you are giving good information, and a lot of people did not look at it or pay any attention. Welcome to my world. But I would also add that the theoretical differences you are exploring are so small compared to the "noise" that aside from a very narrow group of people, it is either not understood at all, or swamped by orders of magnitude by far more practical issues with trim and power. The vast majority of the people who might be able to take advantage of it *already know*.
That is the problem I have had over the years, if you assume everyone has the necessary background, they already know what you are telling them, or, if you assume they don't, you have to start from scratch every time. What you (and Mark, and everyone else) are doing is very worthy, however, you have to understand that your potential audience is extremely narrow range of people who have enough background but haven't already studied the problem in depth before. That does not mean to stop doing it, by all means, continue. But don't be too disappointed when it can only be appreciated by a few.
The other issue, is, of course, that some people will take this very narrow and specific bit of information, and act like *it is the only thing* and fixate on it, while ignoring the many other far more obvious issues like trim and power. The reality is that just about any airfoil that does not come to a point or have sharp ridges running down it is going to be OK, if you get everything else right.
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The other issue, is, of course, that some people will take this very narrow and specific bit of information, and act like *it is the only thing* and fixate on it, while ignoring the many other far more obvious issues like trim and power.
The junior engineer syndrome
The reality is that just about any airfoil that does not come to a point or have sharp ridges running down it is going to be OK, if you get everything else right.
I got interested in airfoils as a kid because I figured they're free: a good one doesn't cost you any more than a poor one. This isn't quite true. There are structural and being-able-to-wipe-off-your-wing considerations, but I think it's worth pursuing optimal airfoils.
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NACA compared 18% and 15% sections, because the actual percent thicknesses of the 23018 with the Guinn flap was close to 15% at14.4% of the total chord.
My guess is that they wanted to compare the Guinn flap to something and picked models they had around to compare. They had some with "plain" flaps and they appended the Guinn flap on another. Do you reckon they picked the 15% foil based on thickness ratio, or did they run the Guinn flap case and then pick the plain-flap model that came close to the Guinn-flap model's lift curve. They probably already had data on a range of plain-flap airfoils.
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Unfortunately, the numbers of views for the graphs pretty much tell the story. I hope I've provided some perspective.
Guys:
I appreaciate all the graphs even though I don't always fully understand them. Keep on posting them and I'll keep looking.
Thanks!
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Back to density. We had this conversation before: https://stunthanger.com/smf/open-forum/air-density/50/ , where the concept of "oxygen density altitude" was introduced, but maybe not accurately calculated. I just now wondered if 15% relative humidity was a reasonable number, so I asked Alexa what the relative humidity is in Death Valley and Tucson is now. It's 21% and 8%, respectfully.
This reminds me of another story. An aero friend just returned from swimming with dolphins and manatees in Florida. I wondered what the Reynolds number of a dolphin is, so I asked Alexa. She instantly had numbers, but didn't know how to read scientific notation. Ask her; the answer is interesting.
Correction: "It's 21% and 8%, respectfully." That is backwards, so I guess it's disrespectfully, to extrapolate the malapropism.
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Also, did they test the Guinn flap because someone at NACA saw a Nobler fly? ;)
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Also, did they test the Guinn flap because someone at NACA saw a Nobler fly? ;)
I think it was pre-Nobler. There was this guy Guinn, who made and seemed to have hyped his airplane with Guinn flaps: https://en.wikipedia.org/wiki/Gwinn_Aircar. NACA probaly felt obligated to check it out.
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I appreaciate all the graphs even though I don't always fully understand them.
Me neither
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Guys:
I appreaciate all the graphs even though I don't always fully understand them. Keep on posting them and I'll keep looking.
Thanks!
I prefer pie charts. Pie are good! Pecan is my favorite. D>K Steve
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Me neither
Sometimes that's because the person making them didn't either.
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a " BLUNT " leading edge , in the extreme , would be flat , vertical , the depth of the wing ! S?P
(https://cdn.shopify.com/s/files/1/0014/4578/9798/products/36_Inches_Rectangular_Canopy_Black_600x.jpg?v=1564762130)
This ones got a blunt trailing edge too . S?P
My thinkng is the ' way round ' full size plane aerobatic airfoil , is in part , to enhance the rate of roll .
As in , a sharper L E is gunna need greater forward tavel per degree of roll .
(https://i.stack.imgur.com/ZGUS6.jpg)
If in ' the stall ' , a extreme sharp L E , the breakaway / seperation of flow is more pronounced , premature .
But the yellow Folkerts with a 3/8 Sq. L E @ 45 degrees , no radiusing bar the covering - would rotate ,
slaming on full control .
Though the flow would detach over the top , the air caught under & held by the flaps , would be sufficient to support the plane . And made a neat ' woosh ' noise .
Was set up hair trigger , aft C G , No slack at control T E's . Was very good in 15 Knot plus winds , IF you had your undivided attention on it .
As in was extremely responsive , not unstable .
WHICH BRINGS US TO . . . Gust Response .DRAG . If the ' flow seperation ' drag figure is of lesser variation , when wind speed is erratic & unstable ,
the Inirtia of the plane ( on any steady course ) is more a STABILISING influance .
My view is a rather blunt L E , say 50 % depth diameter , would have the airspeed more influanced by gusts . TED should be the man to ask here ,
Note refering to Gust Responce rather than steadyish high wind . Two differant tho often simultaeneous conditions .
As for a 1/4 sq 9 or whadeva ) L E member & 1/16 sheet , SHARP would be shaped to / from the very front edge of the 1/4 Sq , For LIGHT plane . Light Loading .
Wereas whacking it back till the depth between the sheeting is 1/16 to 1/8 , tho Id say 2 m.m. is near the limit , you can get a fair sort of radius ,
if youre looking for a ROUND seperation point at the front . Which tecnically near no part is vertical . But a lot more is pretty close to it .
(https://stunthanger.com/smf/index.php?action=dlattach;topic=15886.0;attach=316371;image)
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(https://sun9-4.userapi.com/impg/AtxIj4vYVPmvkBoEFKlzvTaBEyvrE44CvB-WCw/NNduod_D95o.jpg?size=800x600&quality=96&sign=10e78bf6ef65a055e0ffd53e08817c44&type=album)
Tripped overthis so Why Not . Eastern Block job
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a " BLUNT " leading edge , in the extreme , would be flat , vertical , the depth of the wing ! S?P
This ones got a blunt trailing edge too . S?P
My thinkng is the ' way round ' full size plane aerobatic airfoil , is in part , to enhance the rate of roll .
As in , a sharper L E is gunna need greater forward tavel per degree of roll .
The Eppler section E472 used on the Extra would be a bad choice for a model. It was designed to be able to Snap Roll and in models it will cause quite a bit of bad things to happen. Further, it really doesn't have good drag and Cl numbers. Even the full size extra will "tell you" about pulling too hard. There are much better selection for the models than this one.
The rolling rate isn't so much the airfoil but the size of the aileron. Notice how far back on the aileron the hinge is. That's what I have been talking about with offset hinges. The hinge on the Extra is set back to about the 20% chord point.
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The Eppler section E472 used on the Extra would be a bad choice for a model. It was designed to be able to Snap Roll and in models it will cause quite a bit of bad things to happen. Further, it really doesn't have good drag and Cl numbers. Even the full size extra will "tell you" about pulling too hard. There are much better selection for the models than this one.
Agreed, they make them like this precisely because they have the characteristics we we arguing about before - they want it to snap roll abruptly, unlike conventional full-scale aircraft.
Brett
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My guess is that they wanted to compare the Guinn flap to something and picked models they had around to compare. They had some with "plain" flaps and they appended the Guinn flap on another. Do you reckon they picked the 15% foil based on thickness ratio, or did they run the Guinn flap case and then pick the plain-flap model that came close to the Guinn-flap model's lift curve. They probably already had data on a range of plain-flap airfoils.
That was pretty much my guess, although I THINK I remember seeing a lot of 23012's in that period's researches. I should have noted (for Scott now) that the date of the TN is 1940. I also remember the best L/D numbers falling in the 15%-thickness area for RN's around 3 million. So , yeah, I think they picked the models they had. I don't remember any other reports at this low Reynolds number though - except that one about RN effects, which indicated two reversals of which thicknesses were "best" as RN's decreased below 1 M. I think they had to already have much data on plain flaps in the five-digit and other series by the time of this report, but there's only one way to find out, and that's not for me at the moment. Now, I just need to read the third-page posts.
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I found the Extra stall to be very benign. Much better than a Mooney for example. The wing does give a good amount of feedback and will protest, but has no surprises if coordinated. Anything else would be treacherous at contest or Airshow altitudes.
The Extra does snap well inside but has to be forced to do outsides. Spins are opposite. Inverted it is effortless but upright has do be set up just right or the airplane develops a high rate of sink and ignores the rudder. C/G with me is close to aft limit. The Extra 330SC solved the snap and spin issue with a much improved elevator, same airfoil.
Attached is my Extra at a reasonably high AOA
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Not all gusts are the same .
2.2.1 Sharp-edge gust
The concept of sharp-edge gust was reported in 1931(Reference Rhode and Lundquist17). This simplest gust shape proposed by albeit a century ago, is still used for response analysis to date. The gust shape is of a step type, as shown in Fig. 3. The gust velocity profile can be expressed by Equation (6).
Figure 3. Illustration of sharp-edge gust velocity distribution.
(6)
where U is the gust maximum velocity value, u(s) is the gust velocity at any penetration distance, s.
The U.S. Bureau of Air Commerce regulated the first U.S. civil requirements related to gust loads in the “Airworthiness Requirements for Aircraft Components and Accessories” issued in 1933(18). These requirements were merely based on the sharp-edge gust concept. The design gust velocity is 30 fps for aircraft at cruise speeds and 15 fps at dive speeds.
2.2.2 Linear-ramp gust
The linear-ramp gust concept had become apparent by the late 1930s. This concept was used to account for the differences in airplane motion due to gust encounter from one airplane to another. The linear-ramp gust shape means the gust velocity increases linearly with the gust gradient distance, as shown in Fig. 4. The gust velocity profile can be expressed by Equation (7),
Figure 4. Illustration of linear-ramp gust velocity distribution.
(7)
where, H is the gust gradient distance (horizontal distance from zero to maximum gust velocity, usually in wing chord length).
The regulations that resulted were contained in the 1941 issue of the Civil Aeronautics Manual (CAM 04)(19), where the equivalent gust velocity was specified as 40, 30 and 15 fps for three different forward air speeds.
2.2.3 One-minus-cosine gust
The current certification regulations utilise theoretical work undertaken by the NACA where the concept of one-minus-cosine gust was reported in 1953(Reference Pratt14). The gust shape is shown in Fig. 5 and is mathematically defined as
Figure 5. Illustration of one-minus-cosine gust velocity distribution.
(8)
where the symbols are identical to those for sharp-edge gust. In the whole history, the gust gradient distance H was designated to different values successively, such as 12.5 and 25 chords; however, the latest regulations of the gust gradient distance in FAR-25(20), CS-25(Reference Easa15) and JAR 25(21) require a sufficient number of gust gradient distance in the range of 30 feet to 350 feet to be investigated in order to find the critical response for each load quantity.
https://www.cambridge.org/core/journals/aeronautical-journal/article/gust-loads-on-aircraft/C37F9E9233E81557F8435CAA406DABB6
Some diagams there ,
Gusts effects begin to act in advance of the aircraft’s encounter and last sometime even when the gust is located far away from the aircraft. This result is particularly important to flight dynamic performance of very flexible and light aircraft.
Was intresting the low darg tips on the Phantom stopped the yaw & roll in high wind through smaller tip vortices under high load / squares .
essentially tip rib at 45 deg. to vertical , canted . with top sheet running outward aft , about 15 m.m. wider . A straight line in planform from preceeding the max depth .
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Tis with added swept top sheet , about the same as the tip flap chord ( 1 1/4 " ) wider . So as the sweep is OUT to + 5/8 at the rear . sanded and contured but just a 1/8 " sheet tacked on , sanded faired in finished .
(https://stunthanger.com/smf/index.php?action=dlattach;topic=41658.0;attach=250101;image)
Flying in 20 & 30 mph winds , with ' trees in the valley ' , the rolling air would pull it around , with a fair bit of control & load to counter . Slamming & heaving the handle helped it get turning ,
or it'd tend to wait for a break . Even if you got the nose up the breeze would hold it flattened level .
55 Ft. of .018 7 the little FSR 25 clone fighting away . Figured it matched Gieske's 58 ft. ( to center ? ) G Nobler .
Tered tend to be a ' ground flow ' of hard air till about 10 Ft. altitude . 50 / 80 Ft trees up & downwind would have you wander to find the least disrupted area ,
Downwind big bushy tree with branhes even from 8 or 10 Ft. As in a 8 / 10 foot clear under them had a big sucktion / venturie effect 60 ft. upwind , starting around 80 . Close Itd get pulled near into the ground .
The bigger tree upwind had massive turbulace within 100 / 150 foot .
Had a typal noblerish airfoil & construction , with only about 1/8 dia. front edge .
Would stagger out off hard bottoms nose up at times , but not as bad as P J's
large noseweight version . More it had to old nose up to counter the sink it the air
which'd smatter about anything . Needed that weight of line in that air to steer .
The tips made it a bit less volitile but perhaps more effort to ' ungroove ' it to hit the turns .
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O.K. call it 1/8 " Rad. , say 1/4 Dia. ' entry ' .
(https://stunthanger.com/smf/index.php?action=dlattach;topic=41658.0;attach=250091;image)
Seeing it again reminds us the anheadral Stab. , had sections NOT in the wing / flap wake . You could still feel it at times ,
but it didnt get the dull spot & thud or a tailplan working trough or out of the wing wash , when everythings earning its keep .
Will putta picha of the Strega LE ere , as its the 1/4 Sq. drg . and has very good bottoms at 5 ton / Sq Ft loading .
( With Very STIFF Flaps , Stab, Elevators & no control flex . Not to mention a bit of line tension , wearyingly so .
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So BLUNT SHARP LEADING EDGE . 1/4 Sq with 1/16 + sheet stock ARC Strega . plus 80 Wt sandpaper & sanding block .
Tip .
(https://stunthanger.com/smf/index.php?action=dlattach;topic=15886.0;attach=316377;image)
Tip .
(https://stunthanger.com/smf/index.php?action=dlattach;topic=15886.0;attach=316375;image)
Root .
(https://stunthanger.com/smf/index.php?action=dlattach;topic=15886.0;attach=316373;image)
Whichifyoulookatit , is COMPLETELY DIFFERANT than to one at the Top of This Page .
and brings us to , nomatterwhatyouvegot , Zero Flex , smooth controls and extreme torsional stiffness
are the priority , as nomatterwhatyouvegot as far as airfoils , if the rest is as soggey as eggs you shoulda hadit
as stiff as the egg shell . Edibility notwithstanding .
The 3/4 deep tailplane . NOT the kit one . Or Controlls , custom steel bushed 1/8 & 5/32 wire .
Initially hot weather got the vertical bellcrank pin binding , end float gallng . Cut & lube 7 a few blows with the 16 lb. hammer ( Taps with a drift ) has it smooth & no binding .
The tear your arm out line tension overcame the friction initially though . So it stll grooved and hit turns . But the longer in the sun the stiffer & louder the binding was . :P
Be intresed in Ted's comments on His ships of the big round front style airfoil , in conditions where youre hanging on like grim death , 25 Knots with a FSR , Ro Jett or P A - piped
and Working .
Mush , overheads & wakes .
Surpriseingly the 6 ft. Mewgull with a Merco 61 on a 12 x 6 was real good in 20 knot smooth wind . But crosswindat 50 foot would have it roll/ snap 45 degrees for a moment in the outside of the o. h. 8 .
On the Top three list to get onto & done . But again , required your full attention . Like Mototrcyles anything less is inadviseable usually anyway . Bout the same actually .Particularly in the city .
Not trying to say this is the only way to do it. Just jumped in and threw it on . Respect others evaluations and approaches . Suspect anything over 30 knot wind will have anything
not entirely under full exact control no matter HOW you go about it at either end of the lines .
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I found the Extra stall to be very benign. Much better than a Mooney for example. The wing does give a good amount of feedback and will protest, but has no surprises if coordinated. Anything else would be treacherous at contest or Airshow altitudes.
The Extra does snap well inside but has to be forced to do outsides. Spins are opposite. Inverted it is effortless but upright has do be set up just right or the airplane develops a high rate of sink and ignores the rudder. C/G with me is close to aft limit. The Extra 330SC solved the snap and spin issue with a much improved elevator, same airfoil.
Attached is my Extra at a reasonably high AOA
Very cool photo... The key words here are "if coordinated" which means the airplane is pointed straight ahead with no yaw / skid when the stall occurs. With a pilot onboard this is fairly straight forward. On a model that generally doesn't happen and there is some yaw when the airplane stalls. The Eppler stall characteristic shape is "peaky" which is to say it has a continuous Cl curve which is sharp, bends quickly, but has no discontinuity. This is desirable for the 4/4 aerobatic airplane because it leads to a controllable stall which can be "commanded" to autorotate (snap / spin). An undesirable characteristic of the shape is when there is a step change in CL which occurs over a small angle of attack as airfoils like the 23012. The yaw during stall creates a different angle of attack between the two wings and if the section has a discontinuity as in my graph in a previous post then huge rolling and yawing moments result. That accounts for the Mooney v Extra as I think the Mooney uses the 23012 which does have a significant discontinuity in the Cl curve. It's misunderstanding about aerobatic airplanes thinking they are tougher to fly, they aren't, if anything they are easier by design.
Why the E472 isn't a good option for the model lies in the way the airfoil behaves as the Rn is reduced. For the operating range Rn of the 4/4 Extra the E472 has a fairly smooth CL curve at stall although is bends quickly. At lower Rn numbers the sharpness turns in to abrupt separation creating the Cl curve discontinuity which makes for a bad behaving model. To exacerbate this problem it is common practice in the control line arena to cause the airplane to yaw outward which is the perfect recipe to generate the "snap roll ona string" hinging. The characteristic of an airfoil for a well behaving model has a smooth Cl curve around the stall Cl.
Unfortunately the understanding of this involves all of those graphs and such which aren't very popular to fully understand. Words simply won't fully convey the concept. Having said all of the above, the E47x series airfoils are good coordinate donors for modification in which to tune for application. Primarily for the lower Rn reducing the nose roundness "radius" would help and adding some thickness behind the max thickness.
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At lower Rn numbers the sharpness turns in to abrupt separation creating the Cl curve discontinuity which makes for a bad behaving model.
To exacerbate this problem it is common practice in the control line arena to cause the airplane to yaw outward which is the perfect recipe to generate the "snap roll ona string" hinging. The characteristic of an airfoil for a well behaving model has a smooth Cl curve around the stall Cl.
Your comments may apply to your application, but, as previously explained, not to control line airplanes optimized for either stunt or combat competition, where we want more Clmax. The "snap roll on a string" is benign: you won't crash. I don't remember ever seeing it on my stunt planes, and, as previously explained, we trim it out of combat planes. You can still get it with a full load of fuel in combat if you overcontrol the airplane, but it won't cost you the match.
Unfortunately the understanding of this involves all of those graphs and such which aren't very popular to fully understand. Words simply won't fully convey the concept.
I don't know about being popular to fully understand, but I can't understand a graph that was autoscaled with too few decimal places.
Having said all of the above, the E47x series airfoils are good coordinate donors for modification in which to tune for application. Primarily for the lower Rn reducing the nose roundness "radius" would help and adding some thickness behind the max thickness.
Although it's been more than 30 years since I used an Eppler 47x airfoil on airplanes at a world championships, I remember it as working OK. Gary James has showed us that airfoils modified opposite to your advice (increased LE radius, fatter in front of the max thickness) work even better.
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Your comments may apply to your application, but, as previously explained, not to control line airplanes optimized for either stunt or combat competition, where we want more Clmax. The "snap roll on a string" is benign: you won't crash. I don't remember ever seeing it on my stunt planes, and, as previously explained, we trim it out of combat planes. You can still get it with a full load of fuel in combat if you overcontrol the airplane, but it won't cost you the match.
I don't know about being popular to fully understand, but I can't understand a graph that was autoscaled with too few decimal places.
Although it's been more than 30 years since I used an Eppler 47x airfoil on airplanes at a world championships, I remember it as working OK. Gary James has showed us that airfoils modified opposite to your advice (increased LE radius, fatter in front of the max thickness) work even better.
The snap roll ona string is a description of a kind of hinging I see occasionally when the airplane stalls during some maneuvers. A normally good flying plane with limited hinging will, at times produce this. Note to self, don't pull too hard as Brett would say. My note to self, next airplane fix that. I don't disagree with Brett except that I won't tolerate it if I don't have to. I have seen a very interesting video of a CL biplane doing and absolutely wonderful snap roll to destruction.
Yes, it is true that in general moving the max thickness forward can improve the Cl max but too far forward requires moving it rearward again. The E47x are in this state. What happens is the rapid velocity increase around the big round nose creates low pressure which then faces a long trip back to the high pressure zone. You can see this on the E472 graph as the reynolds numbers drop in the Summary of Low Reynolds Number Report . The result of this is separation towards the front first creating the undesirable stall characteristic.
For the stunt model the non flapped airfoils Clmax is pretty much irrelevant as the flap changes that significantly. The forward profile has some impact though and its character plays roll in the flapped performance.
I'm going to guess the graph I put up which doesn't make sense is the Cd v Cl polar. Which I didn't save. In general, I don't take the model as a gospel but rather a tool to understand dependencies. I don't have the desire to spend the time and resources to verify the models and as such any confirmation I may make is qualitative. I'm assuming that your testing is quantitative and confirm the numbers directly via test. So any graph I generate shows the difference in characteristic or a characteristic of interest or how it may change. Overlaying different airfoils on the Cd v Cl polar is one of the methods of comparison. It is the speed range and maneuvering performance.
In general though my comment was more about that not many people really get graphs. I guess my humor was missed yet agin.
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This one is sad but it is and example of a poorly behaving airfoil causing the snap roll ona string. In this case more like a spin but same difference a snap roll is just a spin going in a different direction. Probably from yawing outward. The airplane was flying slow and the pull to recover pushed the airplane to stall. The inside wing stalled first. Once started autorotation is self progressing and in this case unrecoverable. You can see the inside wing dip at 5:14. That was the wing stalling. Some call it tip stall. But it is that discontinuity of the lift curve at stall that drives it. The recovery is to release the back pressure but sometimes the reflexes won't allow it as in this case. The nose is down and the desire is to pull harder which only makes things worse.
https://www.youtube.com/watch?v=7e8mMSZmJAA
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I've watched that video before. Looked like an official flight in a scale contest. After flying his twelve laps, high flight and a touch and go, I wonder if he didn't have some sort of control failure in the elevator? He was flying 7 or 8 second laps, so I don't think he tried a loop, but may have been an attempt at a wingover? It was such a shallow climb that I'm thinking it wasn't and he just lost elevator control, it climbed up and got even slower and into the stall realm of things. That snap to the left , though was just as you call it, and really surprised me the first time I saw it. A few more RPM and a few less pounces of weight would have helped a lot. The trees seemed still enough, so don't know if a gust of wind caused anything. Just ran out of air speed, altitude and ideas all at the same time!
Type at you later,
Dan McEntee
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I've watched that video before. Looked like an official flight in a scale contest. After flying his twelve laps, high flight and a touch and go, I wonder if he didn't have some sort of control failure in the elevator? He was flying 7 or 8 second laps, so I don't think he tried a loop, but may have been an attempt at a wingover? It was such a shallow climb that I'm thinking it wasn't and he just lost elevator control, it climbed up and got even slower and into the stall realm of things. That snap to the left , though was just as you call it, and really surprised me the first time I saw it. A few more RPM and a few less pounces of weight would have helped a lot. The trees seemed still enough, so don't know if a gust of wind caused anything. Just ran out of air speed, altitude and ideas all at the same time!
Type at you later,
Dan McEntee
I don't know but could be. I watched in slow mo to see what was going on and I don't think I saw any elevator movement at the stall which is a typical reflex. Usually what happens in this scenario is the airplane pitches down and the pilot pulls back. I think he was just putting around trying to be "scale like" and the climb sucked the last bit of energy out, the airplane stalled and spun to the ground. It probably shocked him and he didn't have a chance to respond which probably wouldn't have done any good.
Actually, I just watched it again and could see an intentional pull up to what might have been a wing over of sorts. The airplane "told him" it didn't want to do it. Watch carefully the first part of the climb. The inside wing dipped twice and the second time probably made the lines slack and they just came back when the airplane snapped probably from up elevator effort.
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In low Reynolds number airfoil analysis, there is one airfoil that is always included in any work that is done. The Wortmann FX63-137 has shown remarkable performance through the years. The maximum lift coefficient is above 1.5 generally, and the stall is pretty benign. It has as high an L/D as any airfoil around. I think this airfoil has been used for some of the human powered airplanes attempts.
I know I poo pooed the small sheet flap extension on the symmetrical airfoil from a few posts back as not being a driver in the flow conditions and here I am touting a radical under cambered airfoil. Granted the FX 63-137 is a highly under-cambered airfoil and not a symmetrical airfoil like used for pa airplanes, but I’ve always thought that the upper surface curvature might be of special importance. It might be magic. This upper surface seems to hold onto the flow better than most airfoils. I wonder what would happen if I took this upper surface and flipped it over to make a symmetrical airfoil. Would the surface be as “sticky” when applied as a symmetrical foil.
I built a mold to form the surface skins because I didn’t think that a rib and fabric construction would accurately enough replicate the surface. It makes a pretty thick section. It’s not too blunt or too sharp. The fully sheeted wing makes for a bit heaver airplane. The half wing and flap in the photo is 11.7 ounces. Does it fly better and stall later? So far I haven’t had a stall in the 3rd corner of the hourglass or triangle and it s a 70+ oz. ship. But I must admit that I haven’t actually had opportunity to fly it a whole bunch.
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This, of course, reminds me of a story. One can make a Flite Streak spin. Not a whole Flite Streak— first you have to remove the outside wing. This can be done by flying combat with it. Fly the remaining Flite Streak high and give it up elevator abruptly. It will spin and chew up a set of lines. You should get out of the way, because it will land of the center of the circle. This can be done remarkably consistently.
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This, of course, reminds me of a story. One can make a Flite Streak spin. Not a whole Flite Streak— first you have to remove the outside wing. This can be done by flying combat with it. Fly the remaining Flite Streak high and give it up elevator abruptly. It will spin and chew up a set of lines. You should get out of the way, because it will land of the center of the circle. This can be done remarkably consistently.
I'm spittin snot..... Thanks...
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In low Reynolds number airfoil analysis, there is one airfoil that is always included in any work that is done. The Wortmann FX63-137 has shown remarkable performance through the years. The maximum lift coefficient is above 1.5 generally, and the stall is pretty benign. It has as high an L/D as any airfoil around. I think this airfoil has been used for some of the human powered airplanes attempts.
I know I poo pooed the small sheet flap extension on the symmetrical airfoil from a few posts back as not being a driver in the flow conditions and here I am touting a radical under cambered airfoil. Granted the FX 63-137 is a highly under-cambered airfoil and not a symmetrical airfoil like used for pa airplanes, but I’ve always thought that the upper surface curvature might be of special importance. It might be magic. This upper surface seems to hold onto the flow better than most airfoils. I wonder what would happen if I took this upper surface and flipped it over to make a symmetrical airfoil. Would the surface be as “sticky” when applied as a symmetrical foil.
I built a mold to form the surface skins because I didn’t think that a rib and fabric construction would accurately enough replicate the surface. It makes a pretty thick section. It’s not too blunt or too sharp. The fully sheeted wing makes for a bit heaver airplane. The half wing and flap in the photo is 11.7 ounces. Does it fly better and stall later? So far I haven’t had a stall in the 3rd corner of the hourglass or triangle and it s a 70+ oz. ship. But I must admit that I haven’t actually had opportunity to fly it a whole bunch.
I built some FF models utilizing some of the Wortman sections in my younger years. I'm going to say my intuition is that you are correct in your assumption. I didn't take that you were poo pooing the thin flap. Personally I have always thought of the flat plate Gwinn flap, didn't know it had a name until recently, worked much like the concave (?) r portion of the laminar sections in that it helped reduce the adverse pressure gradient which TN 763 confirmed for me. That confirmation comes from the Cl v Cd polar where the Gwinn performs better at higher Cls for drag. It's unfortunate they didn't generate Cp plots in their testing. Oh well. I do have the Wortman in my arsenal of tools and I'll make a run at the Symmetrical version. I think it's worth pursuit.
You may have diverged me once again from my current wing project. The larger flap area ratio of that wing messes up the section in more ways than just the aerodynamics. When scaled a "normal" airfoil ends up with the spar much forward of the wing MAC Cg which means a section redesign would be good.
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I don't know but could be. I watched in slow mo to see what was going on and I don't think I saw any elevator movement at the stall which is a typical reflex. Usually what happens in this scenario is the airplane pitches down and the pilot pulls back. I think he was just putting around trying to be "scale like" and the climb sucked the last bit of energy out, the airplane stalled and spun to the ground. It probably shocked him and he didn't have a chance to respond which probably wouldn't have done any good.
Actually, I just watched it again and could see an intentional pull up to what might have been a wing over of sorts. The airplane "told him" it didn't want to do it. Watch carefully the first part of the climb. The inside wing dipped twice and the second time probably made the lines slack and they just came back when the airplane snapped probably from up elevator effort.
Next time freeze it just after impact, and look where the pilot is standing. He's almost on the circle, no where near the center pad. I think he lost it in the climb, and prop pitch and torque started bringing it in on him so he headed out to try to get tension back. I don't think it was under control at all once the nose when up and it got to higher than 45 degrees and it free flighted the rest of the way. That is what let it roll as violently as it did, no tension at all. Whatever it was, it's a shame and the guy lost a nice model. If things aren't optimal, it's best to just fly it out safe and get it down in one piece.
HAPPY THANKSGIVING!
Dan McEntee
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Back in the days of the "traveling Nats" you could see lots of different events. One of my favorites was Scale. A common characteristic of the scale guys is that they would spend all their time building, almost none flying. Many times the first time a scale plane would fly was at the Nats. It could be very exciting and often gruesome since they may not have been trimmed at all. Nothing like flying tail-heavy! I recall a beautiful WWII warbird at the Chicopee Nats (don't remember any more if it was a Hurricane or a Zero or...) being released for take-off and almost immediately going into a wing-over ending in a resounding "Splat!" into the asphalt on the opposite side of the circle. Yikes! Alnost as much carnage at the scale circles sometimes as at the combat circles.
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Next time freeze it just after impact, and look where the pilot is standing. He's almost on the circle, no where near the center pad. I think he lost it in the climb, and prop pitch and torque started bringing it in on him so he headed out to try to get tension back. I don't think it was under control at all once the nose when up and it got to higher than 45 degrees and it free flighted the rest of the way. That is what let it roll as violently as it did, no tension at all. Whatever it was, it's a shame and the guy lost a nice model. If things aren't optimal, it's best to just fly it out safe and get it down in one piece.
HAPPY THANKSGIVING!
Dan McEntee
I did. He had full up elevator in it. The second snap ona string caused it to get loose. The catching up and getting line tension with full up elevator finished it. I prolly woulda done the same. Not really, I wouldn't have flown it with half throttle. I'd have been blasting full bore tearing up the sky. In my version it'd have exploded in flight sheading debris in to the wind. It would have been epic... Just sayin..
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I did. He had full up elevator in it. The second snap ona string caused it to get loose. The catching up and getting line tension with full up elevator finished it. I prolly woulda done the same. Not really, I wouldn't have flown it with half throttle. I'd have been blasting full bore tearing up the sky. In my version it'd have exploded in flight sheading debris in to the wind. It would have been epic... Just sayin..
I expanded the screen this time to watch it again. It was a nice looking Neuport 11 with Lafayette Escadrille markings. It's kind of a mascot of the club I belong to, the Lafayette, Esquadrile. The different spelling is along story. The model looked to be pretty well trimmed. It didn't act like it was tail heavy, properly balanced and was only rocking a bit in some sort of breeze. The pilot looked to be signaling before each maneuver. It looked in decent shape while flying at 45 degrees for several laps. Then he signaled again for some climbs and dives, and then again for a touch and go. No apparent problems there. He signaled one more time and a lap later pulled up. At about 45 degrees or higher is when I think it initially stalled and broke left and the rest was history. I'm changing my assessment to attempting a wingover of some sort, and then he who stalleth, falleth. Like Scott surmised, I have watched a lot of scale pilots struggle with flying their models, over weight and not balanced and agree with his view. This guy looked to be in pretty good control until that one point. Just too slow Should have put the throttle forward through the stops! ( or at least pulled the trigger back through the stops! Still sad. I'll bet he wished he would have waved it off and got a better engine run.
Type at you later,
HAPPY THANKSGIVING!
Dan McEntee
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In low Reynolds number airfoil analysis, there is one airfoil that is always included in any work that is done. The Wortmann FX63-137 has shown remarkable performance through the years. The maximum lift coefficient is above 1.5 generally, and the stall is pretty benign. It has as high an L/D as any airfoil around. I think this airfoil has been used for some of the human powered airplanes attempts.
I know I poo pooed the small sheet flap extension on the symmetrical airfoil from a few posts back as not being a driver in the flow conditions and here I am touting a radical under cambered airfoil. Granted the FX 63-137 is a highly under-cambered airfoil and not a symmetrical airfoil like used for pa airplanes, but I’ve always thought that the upper surface curvature might be of special importance. It might be magic. This upper surface seems to hold onto the flow better than most airfoils. I wonder what would happen if I took this upper surface and flipped it over to make a symmetrical airfoil. Would the surface be as “sticky” when applied as a symmetrical foil.
I built a mold to form the surface skins because I didn’t think that a rib and fabric construction would accurately enough replicate the surface. It makes a pretty thick section. It’s not too blunt or too sharp. The fully sheeted wing makes for a bit heaver airplane. The half wing and flap in the photo is 11.7 ounces. Does it fly better and stall later? So far I haven’t had a stall in the 3rd corner of the hourglass or triangle and it s a 70+ oz. ship. But I must admit that I haven’t actually had opportunity to fly it a whole bunch.
Here is an example of the upper section of the Wortman FX63-137 taken from Profili, then flipped and stacked.
Looks quite a bit like an Al Rabe or Igor Burger type section this way.
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I expanded the screen this time to watch it again. It was a nice looking Neuport 11 with Lafayette Escadrille markings. It's kind of a mascot of the club I belong to, the Lafayette, Esquadrile. The different spelling is along story. The model looked to be pretty well trimmed. It didn't act like it was tail heavy, properly balanced and was only rocking a bit in some sort of breeze. The pilot looked to be signaling before each maneuver. It looked in decent shape while flying at 45 degrees for several laps. Then he signaled again for some climbs and dives, and then again for a touch and go. No apparent problems there. He signaled one more time and a lap later pulled up. At about 45 degrees or higher is when I think it initially stalled and broke left and the rest was history. I'm changing my assessment to attempting a wingover of some sort, and then he who stalleth, falleth. Like Scott surmised, I have watched a lot of scale pilots struggle with flying their models, over weight and not balanced and agree with his view. This guy looked to be in pretty good control until that one point. Just too slow Should have put the throttle forward through the stops! ( or at least pulled the trigger back through the stops! Still sad. I'll bet he wished he would have waved it off and got a better engine run.
Type at you later,
HAPPY THANKSGIVING!
Dan McEntee
Pretty much my assessment as well.
Happy Thanksgiving.
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Unfortunately, my Profili is no longer licensed (should I trust PayPal?); so to see Frank's Wortman idea, I had to convert to JPEG, edit to create halves, insert into Word, arrange the halves, and photograph my monitor....not pretty, but I like the idea. I'm wondering whether I would ruin the whole idea by thinning it and stretching to move the point of maximum thickness forward from that 37%(?) point for a flapless plane. If I just added a flap, as Frank apparently did, or stationary "flap," the "high point" would move from that .37 Chord point to .31C - .32C and reduce the thickness to around 20%. I wonder though whether that might raise pressure near the flap and defeat the purpose - 'just playing with ideas. I can't use this picture though. I presume this one undercambered section is in the Profili library now.
SK
Edit: Sorry! I somehow missed Brent's post above. Questions are the same though.
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Unfortunately, my Profili is no longer licensed (should I trust PayPal?); so to see Frank's Wortman idea, I had to convert to JPEG, edit to create halves, insert into Word, arrange the halves, and photograph my monitor....not pretty, but I like the idea. I'm wondering whether I would ruin the whole idea by thinning it and stretching to move the point of maximum thickness forward from that 37%(?) point for a flapless plane. If I just added a flap, as Frank apparently did, or stationary "flap," the "high point" would move from that .37 Chord point to .31C - .32C and reduce the thickness to around 20%. I wonder though whether that might raise pressure near the flap and defeat the purpose - 'just playing with ideas. I can't use this picture though. I presume this one undercambered section is in the Profili library now.
SK
Edit: Sorry! I somehow missed Brent's post above. Questions are the same though.
No, DO NOT trust PayPal. They decided our business did not fit within their moral standards and cleaned out our account. Many thousands of dollars. They call a fee for violating their policies which you cannot find in detail.
Having said that, excel is the easiest way to do the coordinate task. Just take them massage. I you wish, I'll provide them to you. I'm way past you on the evaluation. It was the first thing I did when you mentioned it. As is it turns in to a 20% ish section has a decent Cl curve a sharpish stall and decent Cl v Cd polar. Not an airplane killer like the above biplane. I'd use it. I also did a couple thickness variations to see the impacts but unfortunately my program crashed and I had to head to bed. I'll see what I can produce. Any specific requests?
Attached coordinates
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Thanks, Mark. I guess I'm a dunce when it comes to my current Microsoft stuff. Sometimes, as has just happened, I try to open a file by clicking on a link and it asks what program I want to use to open it. I try a couple, and it then fixates on the last unsuccessful attempt and from then on gives an error message saying that last program cannot open the file, no longer giving me a choice. That's where I am now, unable to open your coordinate file. Maybe I should have used Excel. Anyway, I've never before had it not renew, when I re-entered the site. - SK
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Thanks, Mark. I guess I'm a dunce when it comes to my current Microsoft stuff. Sometimes, as has just happened, I try to open a file by clicking on a link and it asks what program I want to use to open it. I try a couple, and it then fixates on the last unsuccessful attempt and from then on gives an error message saying that last program cannot open the file, no longer giving me a choice. That's where I am now, unable to open your coordinate file. Maybe I should have used Excel. Anyway, I've never before had it not renew, when I re-entered the site. - SK
right click on the file, choose properties, select note pad or word pad. Or ask me to rename the file. They're just text files but they are in the data folder for the application I use which requires the *.dat extension.
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right click on the file, choose properties, select note pad or word pad. Or ask me to rename the file. They're just text files but they are in the data folder for the application I use which requires the *.dat extension.
The attached file is the NACE section not the Wortman. I encountered a file corruption issue when moving data back and forth. I'm going to redo the whole effort and repost once I figure out what what its actually what.
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cant view the graphs ... that disappeared ....
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cant view the graphs ... that disappeared ....
I made a grand mistake during the movement of coordinate data and I have to completely redo the analysis. There is a completely different result. Not bad but different. The resulting Wortman modification isn't as good of performance as the initial results. It isn't a lost effort as I think the impact of adding flaps will compensate. The NACE section isn't well suited for large plate Gwinn style flaps, I don't think. It will take me a couple hours to repeat the analysis and it is Thanksgiving so it will be tomorrow.
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cant view the graphs ... that disappeared ....
BTW, very nice way of building the wings. That's how I built my F1C models. Always consistent results.
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EGAR .
If you put a line at 45 Deg. ea. way , intersecting the front at 45 deg. %^@ you wouldnt miss much , or add much infill to fill the point in front .
A folded cardboard or styrene taped on for air tests'd prove something . One way or another . VD~ Just dont deck it .
(https://stunthanger.com/smf/index.php?action=dlattach;topic=60560.0;attach=329156;image)
Looks like the C G'd be back amost near the 3rrd mark . 30% . BUT , Flaps'd make it about 2 wider , plus the bit where the backs cut so its not as wide . Unless the flaps are as thin as paper .
3/12ths then is 25 % . But we said ALMOST . so 22 or 23 % . Or 20 - 25 % . Fairly Normal . THOUGH any area FORWARD of the C.G. , one would think , Is force Fwd of inertia , so enhances
the turn . Whereas if its aft its countering it and is counter countered :-X by the elevators . Unless its a Canard .
SORRY .
But we think the aftish high point may / can give more stable center of lift , resitance ( drag Cr. in turn ) and low adhesion .
The Folkerts uses a Werwage airfoil . Said to be a wind man . Larger version with rounded L E needed larger empenage ,
To get ' The Groove ' , and spo turns clean . Eliminate the corrections for track .
AIRFOIL .
(https://outerzone.co.uk/images/_thumbs/plans/10342.jpg)
Airfoil , Yellow one , pirated from here Via Xerox . Miller said He borrowed the Arees section . HOWEVER -> to The Eye its % percentages , poportionately , look comparable to yours .
As in ones say 10% ( of size ) fatter or thinnner , tho the entry - front - L E might not be identical .
Is the assumption that the resistance at the front edge of a blunt nose section requires a C G more forward . TEST a foam or shaped thin to tack on fwd to evaluate turn & exit . With Std. (tail) weight variation .
say 4 or 5 ,
To find where it gets dull & where it gets snappy . General Parameters NOT fine tuning . As in , start 0 rear weight -> as there added if its gotten ugly you woulnt try for more ugly -> C G rear Max . H^^
This had the thrift airfoil . Told the yatsenkos are set up hair trigger to turn . This'd handle a gale . But flew daily NZ around 2002 , so the driver wasnt all dull & awkward . One learns to fly, in wind, in N Z .
(https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTpUuWYLU_ZCaOkRpyDEberd5nkQqfbiYvcNA&usqp=CAU)
SNAARRRL.
The L E was just smoothed 1/2 or 3/8 Sq , on edge . As we said , It'd ROTATE on full handle , Triangle corners . No sink & exit clean ( for theconditions . one doest see EXACT laser acurateness . But Close )
The flow might break away , but still supported under . Woosh Noise ! . Still going Fwd so result is tight turn rather than changeing direction in three inchs .
Pity I didnt build it 76 or so , as planned . Will give a clean patter in smooth 20 - 30 knot . And flown in 40 . Not for amatures . A good combat pilot'd like it .
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Having said that, excel is the easiest way to do the coordinate task. Just take them massage.
When I had an issue with a Microsoft product I would go at lunchtime to an Indian restaurant near the Redmond campus and ask the cutest woman in the buffet line. Mark takes this to a whole 'nother level.
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When I had an issue with a Microsoft product I would go at lunchtime to an Indian restaurant near the Redmond campus and ask the cutest woman in the buffet line. Mark takes this to a whole 'nother level.
"in deep foreign accent"....
Ha ha. You be funny man Mr Rush...
<=
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So, I spent some time once again using lessons learned and made absolute certain that I had the correct profile loaded. I am struggling with the software crashing when I create the flat plate Gwinn flap. It probably my method of coordinate messaging, Mr Rush (deep foreign accent) and the curve fit is choking on the transition. The other part of the Hanley's analysis software I have does multi surface analysis but it is not real easy to do a fast analysis with and it isn't coefficientized for individual elements. I have some ideas how to get around this divergence problem but they'll take some time to do.
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I have trouble with the flap on XFOIL, too. It doesn’t like the dent between the wing and flap. I draw a straight line across the dent and Profili fairs it even more.
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When I entered the "stationary flapped" section, I did it with closely spaced coordinates. It printed that out as intended (its in my post on p. 2) and then processed with X-Foil.
SK
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Yeah I know. XFOIL handles that a little better but it isn't as good at mass comparison.