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Design => Stunt design => Topic started by: Dan Bregar on August 04, 2011, 04:40:16 PM
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Having been away from C/L for a long time I seem to notice that very many of today's Stunt airplanes have what I would call very thick wing tips. I realize that today we use adjustable lead out guides and inherently with this feature the inboard tip must be a certain minimum thickness. But from what I've seen in general it seems the wing tips are much thicker than in the good ol days. So I ask all you Guru's, (and you know who you are don't you) what's up with the fat tips. A performance advantage perhaps, or what ? I'm all ears guys. Lay it on me. ::)
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I think, but don't know, that the thicker tips resist tip stalling. I have seen some arguments against that. By thicker, I mean percent thickness as compared to the inner parts of the wing.
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Its not only the tips that are thick , older airfoils tended to be reworked naca .0018 , 18 % thick .
Typically around 22-23% these days . pushes a bit more air so gets a bit more grip , fo the bigger
heavier planes . With more power to match the increased adhesion of the air .
The old cica 1960 unsilenced Fox .35 things could be down around one kilo ( 36 Ounces) , flown on
60 ft. lines , they flew like a bird on the wing .
The muffler knocked the power back and kept the heat in more , starting a vicious circle .
Next stage was maybe the Super Tigre .46 generation , up to 60 Oz and 60 in span , in the early seventies.
Then ST .60s in ships about this size , on long lines maybe, for better power reserves .This is about where the
thicker airfoils came along , to carry the weight . Jim Greenaway being one of the development pioneers in this respect.
There were Schneurle ported .40 planes in all shapes and sizes , Bob Hunt and collegues being predominant in these developments and the piped arrangement .
Nowadays theres all sorts of dedicated aerobatic engines produced utiliseing the tecnology learnt in these efforts .
The Stalker series being SuperTigre inspired , with PA & Ro Jett more tecnologically inclined , and the YatsenkoTractor Engines . n~ :## :! LL~ <=
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Jim
It would seem to me that they create more drag and are possibly less efficient than say a Flite Streak style tip. I'd have to check that out on my wind tunnel though. ;)
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Jim
It would seem to me that they create more drag and are possibly less efficient than say a Flite Streak style tip. I'd have to check that out on my wind tunnel though. ;)
Probably is less efficient, but we have almost unlimited power.
Brett
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With electron power, there are good reasons to design a low drag stunter. Should allow use of a smaller & lighter battery, or pulling less out of the same battery, which makes the LiPo batteries happier. With IC power, drag helps regulate speed, but it seems to me that a plane that has cleaner tips should make for less wake turbulence. That is scary stuff...like suddenly flying into a vacuum...no lift at all. :o Steve
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A number of years ago I had a running conversation with "The Bear" on how to build a nobler. He INSISTED on 45 degree wing tips for handling in the wind. That is, you attach your 1/8th (or so) wing tip down the centerline of the tip rib with a couple of gussets to hold its position. The LO adjuster goes beneath the sheet tip producing better centerline balance.
Since then I've built all my airplanes that way. Ya'know, I think the man was right!
W.
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Steve
I'm with you 100%. Makes sense to me. That's kinda what I was thinking and the reason I brought up the topic in the first place. Just wanted to hear a few more educated opinions than my own. ;)
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Great questions Dan. Were this 1999 through 2003, fFat tips of sorts, which was the norm for a few years might have been standard but I know that for me aft tips and foils are a thing of the past. Now I just have to prove it! ;D
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The Pathfinder LE has tapered wing tips. More so than most modern designs. the caveat is one you already touched on; the adjustable leadouts. In George Delany's design the adjuster is buried quite aways in from the edge of the wing tip. This treatment requires the use of tubing to create a no snag, no rub exit from the lead out tunnel for the leadout cable. (the adjuster is almost 2 inches behind the wing tip edge). The design and fabrication of such a setup is a bit more challenging, and does incur a increased weight penalty so in the fine tradition of KISS, most designers will opt for the blunt design so long as performance is not effected.
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I realize that today we use adjustable lead out guides and inherently with this feature the inboard tip must be a certain minimum thickness.
Well, there are other kinds of adjustable guides besides the sliding type and the push in plug type with multiple holes comes to mind that need nothing more than a sheet of ply's thickness to accommodate it.
But it does seem that in some cases fashion dictates form and the sliding guide seems the most popular - and its not that a design strictly needs an adjustable guide anyway. :)
A Thunderbird 'over and under' adjustable lead-out system would indeed be in need of thick tips though!
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I realize that today we use adjustable lead out guides and inherently with this feature the inboard tip must be a certain minimum thickness.
But Dan, with this system that I think was first developed by Tom Morris, the tip only needs to be 1/4" thick.
Two pieces of 1/16" ply separated by 1/8" balsa with a slider. I use 1/8" pop rivets in a 1/8" ply slider because I have more than enough rivets to last me a lifetime. You don't have to fit wear faces in the slot either. The example in the pic is a bit wide in the critical area so it requires a cap head screw. A narrower support permits the use of a button head screw which is barely visible.
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Let me define what (I mean) by thick wing tips. I don't just mean the edge of the tip, like where the lead outs exit, but the last 3" or so of span. Like almost from the last rib bay to the edge of the tip. So I'm talking how much taper from the last rib to the tip edge where the lead outs exit. I seriously doubt there is much significance whether the tip edge thickness is 1/8" or 1/4" at the edge. I wish I had a good picture to post to show what I mean. Heck, this is academic anyway and probably not worth the discussion. Amen ;)
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A lot of guys use the type of wing tip you are talking about because it can be built so it flows into the main wing, with no kink at the last rib. It looks better and will get some appearance points.
Another reason is that several good designs use a Flite Streak shaped wing tip. Filling this type of tip out and getting a smooth transition to the main wing can significantly improve overall lift because it separates the tip vortices more. It's like adding a couple of inches to each wing.
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Exactly ! :)
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A lot of guys use the type of wing tip you are talking about because it can be built so it flows into the main wing, with no kink at the last rib. It looks better and will get some appearance points.
Another reason is that several good designs use a Flite Streak shaped wing tip. Filling this type of tip out and getting a smooth transition to the main wing can significantly improve overall lift because it separates the tip vortices more. It's like adding a couple of inches to each wing.
The Pathfinder LE wing tip. A slightly modified Flight Streak shape
(http://stunthanger.com/smf/index.php?action=dlattach;topic=22328.0;attach=87351;image)
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Dan, I have had several people tell me that they think Pat Johnston's P-40 wing is the best 40-sized wing going around.
This wing has uniform taper from the root to the second last rib. When drawn in elevation, the tip rib is smaller that the straight-line taper so that the tip starts 'flowing' down from the second last rib.
Certainly, the model that I'm flying that uses that wing does a pretty good job of cutting through in gusty conditions.
As an aside, I'm about to draw my first wing using that approach.
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Filling this type of tip out and getting a smooth transition to the main wing can significantly improve overall lift because it separates the tip vortices more. It's like adding a couple of inches to each wing.
Hi Phil,
Do you mean that the Flite Streak tip 'postpones' the vortex? (I don't understand what separating a vortex is.)
I have always held that a curved back tip such as the Streak uses the span wise air flow of the wing to follow the tips edge and in doing so forces the vortex beyond the edge of the physical wing tip and so it is indeed like 'adding a couple of inches to each wing.'
Cheers.
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Not a bad way to do it them and so on is , USE the Front of the Tip Rib shape , pivoted @ the spar , round to there , Max. Thickness posn. At least its consistant .
Wether a block , shaped with a template to match the Front Wing Section , or a sunburst run of five or six ribs , radial about the spar, with 1 or 2 running aft as per
rear rib . Like a Nobler . Indeed .
The smooth flowing bit from a few ribs in is necesary to get the odd acale tip shape transposed .
Opposite theory is all the ribs hanging out like a scrawny cow , so the flow is guided by the hollows , sorta vortex generator ish / like and so on . Grain direction of tissue on the Nobler Tip
if wrong ( should be acroos ribs , not along , really scarey people do Ea. Bay on tip L.E. , one after the other , individually , for optimum smoothness. there all in a hurry these days though .
And . . . the wake turbulance is the thing pulling the plane all over the place on windy days , if youve made a blue on the tips , or left a QUATER INCH canyon for the adjustable lead outs .
Can check that one by a piece of sellotape sealing the crevass . . .
AND , any irregularity , is a irregularity . n1
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Holy Mackerel Andy ! :D We could be on to something here ! #^
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Hi Phil,
Do you mean that the Flite Streak tip 'postpones' the vortex? (I don't understand what separating a vortex is.)
I have always held that a curved back tip such as the Streak uses the span wise air flow of the wing to follow the tips edge and in doing so forces the vortex beyond the edge of the physical wing tip and so it is indeed like 'adding a couple of inches to each wing.'
Cheers.
I meant just what you said. The FS tip tends to move the tip vortices further apart spanwise. Makes the wing effectively a bit longer, which is almost always a good thing to do.
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Makes the wing effectively a bit longer, which is almost always a good thing to do.
Except maybe on a stunt plane.
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Exactly ! :)
;D
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I've always assumed the thicker % wing tip was used to delay wing tip stall. And, as pointed out, modern IC powered ships have an abundance of power, and drag is even desirable. Large thrust and drag vectors tend to stabilize airspeed, and a good CL stunter can fly much the same anywhere on the hemisphere as a result.
However, with electric power it would seem that reduced drag is in order, which would imply not only a thinner airfoil, but perhaps more thickness taper between root and tip. I'll be interested in seeing where electric power takes the evolution of modern stunt, aerodynamically.
The trend has been toward a blunter leading edge, say starting with something like NACA0023 and moving the high point forward a bit, ending with something more like Ted Fancher's airfoils. I suspect this might change for electric models, leaving leading edge less blunt, with high point more toward the 30% of chord NACA00nn, again in the interest of efficiency and reduced drag.
We might end up with electric CL stunters looking more akin to semi-scale models of modern full sized aerobatic ships, which could be a refreshing change, although our beautiful classic models will never be out of style. Electrifying a classic like a Cobra or Ares could be easier than expected.
L.
"I have great faith in fools - self-confidence my friends call it." -Edgar Allen Poe
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Here is my last experiment in the Profile category, the Mako Jet:
The root airfoil was 18% thick overall with the tips at 16%. The high point was set at 20% back from the root to the tip. These figures include the flaps, which some people forget to include on this sort of discussion. The wing plan mimmicked the Shark 35/45 with an area of 600 squares. The tips were fairly small which makes them rather thin compared to many of my previous stunters. The large tips allowed them to very gracefully taper to a nice thin section towards the very edge of the tip. The front of the airfoil is an elliptical section whose maximum thickness is at the previously mentioned 20% back. The trailing edge shoots a straight line tangent to the ellipse. Overall this makes designing the ribs this way very easy with ACAD.
I wanted to check out the (Bob Hunt's) thought that thinner tips were more efficient in bucking wind induced turbulence. I have not flown this plane much, but it does fly beautufully. Power is the LA46 and weight is 45 ounces. Pretty light overall for a 600 inch plane.
On its eighth flight I managed to trail Norm Whittle by less than 10 points and actually was a few points ahead of my friend Howard Rush. Both were flying Norm's cool Twin engine machine which all love after experiencing it in flight.
The more I play with the elliptical arifoils, the better I think they are. Thin tips don't seem to hurt the overall effectiveness.
Pat Johnston
Skunk Works
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'just reaffirming that slider type adjustable leadout units do not thicken tips more that to about 1/4" total thickness at the tips - they do not matter in that vein. Dan, I'll send personal e-mail. - SK
Edit - I think Pat's photo pretty well illustrates the point (I had this thread up a long time doing other internet things and missed his post).
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Pat
From your picture the tip looks really good to me. Your verbal description of the airfoil sounds good, but I wish I could see a sketch. Are there many stunters these days with 18% sections ? I think I like your 18% root & 16% tip numbers. The proof is in the flying of course. :)
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Serge
Thanks for the e-mail.
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Are there many stunters these days with 18% sections ?
The flapless P-Force is supposed to have an 18% (NACA 0018) section, although it measured out closer to 17% for me. OTOH Wild Bill's unconventional "Doodlebug" series seem to be at or below 16%. My flapless design had 20% sections (my own), but if I had gotten the chance to add the stationary flaps, that would have reduced to about 18%+. Most people thought that this thickness would give it a steep glide, but, perhaps because of the high aspect ratio (6.3), it glided best of all my models. Flapped stunters employing 22%+ sections (not including flap chord) and 20%-chord flaps actually are about 18% in thickness. Many of the flapless designs seen at our field are old designs from the 1950's - 1970's and were designed with thinner sections. They need to be flown lighter or faster to match the 18% wings's lift coefficient potential. Ditto for the flapped classics. The "Teosawki" has pretty thin wings, but it is extremely light - like combat wings. Just some thoughts
SK
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Here is a drawing showing the Ringmaster +P and Warbird +P airfoils and the Mako Jet airfoils. Pretty similar.
Pat Johnston
Airfoil Testing Lab
Skunk Works
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Thanks Pat. The Mako Jet airfoil reminds me of one I used back in the day on a couple of my combat designs. :) Worked very well if I do say so myself.
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An odd 'practical' anecdote?
In the late-'80s I designed and built a .40/.46 size stunter with a quite slight taper - tip chord 0.8+ of root chord. Came out about 49 oz, and was the most successful model I'd flown to that time. After 600 or so flights, stupidity caught up with it, or with me. Decided to do it again, but with a sharper tapered planform. Same everything except for the taper - tip chord a bit under 0.7 of root chord. Also made the root chord larger to preserve the original's wing area. Line through Aerodynamic centers was strictly spanwise and in the same location on both
Came out about 49 oz, and flew as well as the original, except for one thing. In choppy crosswind conditions, the model was much "looser" in roll than the first. Didn't deviate in altitude or pitch attitude, just rolled a lot...
Both models had NACA 0018 root airfoil, almost exactly. Flaps were a bit, not much, thinner than would actually meet that profile. The percentage thickness was the same from root to tip, so the more tapered wing had noticeably thinner tips. Stongest suspicion: - the change in chord lengths did harmful things to Reynolds Number effects. ...And, that something was needed to recover some of that. Tried, and found it worked, to 'split the difference' between actual max rib heights at root and tip.
Whadayamean? Example, if the root chord was 15" an 18% section max depth would be 2.4"; a 10" tip chord at 18% would be, well duhh, 1.8". Splitting the difference is fairly simple: the average between 2.4 and 1.8 is 2.1. These are not the actual numbers, but show what I mean. Used the idea on my next similar model and the sensitivity to roll in choppy air appeared to be much less.
Some used another approach: same actual thickness at tip and root, even with different chords. Doesn't look right... Looks like the tips are thicker than the root. .. Same % thickness at both ends looks good. Splitting the % thickness change as I mentioned still looks about "right."
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Both Walker and Fitzgeralds latest models have thin tips.
At the very tip same as an Ares' perhaps not the chord but certainly at the tip they are.
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Pat,
The more I play with the elliptical arifoils, the better I think they are.
I viewed your elliptical airfoils? What makes them an elliptical wing airfoil? Better for what, for elliptical wings only?
How are you testing your airfoils? In an actual flying model?
I'm in the process of researching elliptical wing construction. That's why the questions.
I do hope you don't mind. I need all the input I can get. ;D
CB
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CB-
I think Pat is referring to the wing section's leading edge portion. I may have sent you this one below. If not, it's an wing section (technically an 'airfoil' is a wing) whose contour to the 30%-chord point is half of an ellipse, and whose remaining contour to the trailing edge is an NACA 0023 section truncated at the place where it would match my flap in thickness (3.18% chord) for the size plane I designed. The NACA section coordinates are mathematically defined by a polynomial; so the fore and aft sections can be joined smoothly at their points of maximum thickness. They are not tailored to any particular shape of wing.
You can tell in the comparison figure approximately how the elliptical compares to the NACA - slightly less blunt for the same thickness, although Profili seems to have left off some of the curve's data points and segmented it. The thing about the mathematical definitions is that they allow any thickness and "high point" location. Also the elliptical section with any kind of aft taper can be "squashed" in Profili and still be exactly elliptical, whereas, if you compress sections in some families whose l.e.'s are defined in terms of circle radii, you do not get precisely the corresponding family member, since the leading edge is no longer circular. While you might get a "better" or "worse" section, it won't be defined by the same perameters.
Edit: 'forgot to mention that wheras my ellipse goes only to the "high point", Pat's goes slightly further to where straight line segments to the t.e. are tangent to the ellipse, a great way to make ribs easy to cut and probably aa good a flapless shape as you can get, past making the far aft concave or adding stationary flaps as on the "Flite Streak".
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Serge,
That is a handsom airfoil. I'll download that! ;D
CB
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Sorry for the delay on getting back to this thread. Serge is right, the term "elliptical" refers to the front portion of the airfoil which is very easy to do in ACAD using the (odd how this is...) Ellipse function. Also, exactly right that the ellipse is intersected at the tangent point with a straight line from the trailing edge. I have heard some express that this may not be the best for flapped planes, but I am not sure that I believe that so much. Possibly as long as the flaps are not deployed more than a modest amount, the effect is still very good. Gospel according to me...
Setting this up for laser cutting using ACAD is really nice. Lots of ease in altering the percentages from the high point location to the overall thickness. As an example a person could make a root with a highpoint 30% back and a tip with the high point 20% back. At the same time, the percentage thickness could start out at 20% thick at the root and end up at 12% thick at the tip. This is an extreme example, but would be just as easy to do as drawing any other regular set of rib patterns.
Pat Johnston
Skunk Works
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Pat
That is way cool to be able to do that. If you had an airplane with a bolt on wing, you could conduct some neat experiments by changing just the wing, and see what the different high points and percentage thickness variables do to the flying qualities. Lots of work making the different wings but I'll bet a lot could be learned ! :)
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Or you could use one of the free or cheap airfoil analysis programs.
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Nothing like "hands on" experience so to speak ! ;D
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Or you could use one of the free or cheap airfoil analysis programs.
After fiddling with Profili for awhile, I see that it requires knowing something to specify boundary layer and other stuff I don't understand. I got some results, but I don't believe them. Smart people have been consulted.