Stuka Stunt Main Forum · 23 of 23 known posts recovered
That doesn't answer your question, but I had some spare time at the computer..
Floyd
Don't ask me why, but "traditionally" the flaps are left off when talking about wing percent thickness. Say you had a 2.25" maximum thickness on a wing with a 10" root chord and 3" flap root chord. That would be considered a 22.5% airfoil.
Other mysteries include measurement of the "nose moment" from back of spinner to LE of wing and "tail moment" wing TE to stab TE.. 
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"I never said all actors are cattle; what I said was all actors should be treated like cattle." -Alfred Hitchcock
For reference, all my measurements are based on the entire wing as built, including flaps.
Brett
As Brett says, the plane flys by the whole wing area, including flaps. You also need to know the root and tip chords of the wing and stab, the half span measurement(for both wings if they are different) and the leading edge sweep( or some other measurement that allows you to do an accurate layout of the wing) If the owner will let you pick it up and roughly locate the CG. so much the better.
Wing root and tip thickness measurements, however they are referenced, are likely to differ for most modern wings.
If you're seriously into CL stunt wing airfoil design, there are certainly more parameters to consider than simply percentage thickness. I'm not sure how CG has anything to do with it, but the position of maximum thickness is important.
Here's an example of Igor's stunt airfoils; he's a man who has done the analysis and ought to know:
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(Notice how this airfoil differs from the "classical" stunt airfoil.)
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I agree that Igor's airfoils are certainly different from "classic" airfoils. I have been using this type of airfoil for more than 30 years. I wrote in the Mustunt article published in 1973, "Third, the wing of a Stunt ship loafs in level flight and lifts significantly only when the airplane corners and the flaps are deflected. To make the wing work better with a deflected flap, the aft portion of the wing rib was PROFILED for smooth transition into a deflected flap. This reduces or eliminates the discontinuity at the hinge line."..."The angle between the deflected flap and aft wing surface on a conventional stunt wing tends to promote separation of airflow and premature loss of lift due to flap stall."
I thought, at the time, the idea of fattening the aft wing surface (I called it profiling airfoils)to reduce the discontinuity at the flap hings was novel and original. I have used airfoils of this family since in all my designs.
There are two schools of thought on describing airfoil thickness. Many of us use the ratio of the thickness to the length of the airfoil without flaps. It strictly more correct to define airfoil thickness including flap. This is typically used on the West coast. I am not persuaded this is the best way to describe the thickness of a control line stunt wing. I intend to stick with thickness descriptions which omit flap. In the early 70"s most builders used airfoil thicknesses typical of the Nobler's 18%. Don still and Dee rice used "radical" 20%. Based on airfoil tests, I began to use 25% profiled airfoils, the first of the really fat airfoils.
Al
Also for the record, measured "with flap", mine is 20.6%, and measured Al's way, mine is 26.25%.
Brett
Hi! ...been a while...
I had a sharply tapered orig a number of years back with same percentage airfoil root to tip. It had more tendency to buck in roll than I like. The model was a computer clone of an earlier model I'd done (Larry C, these are the Atavists, you should recall them...)Otherwise they weighed and flew as nearly identically as any models I'd ever done. The high tapered A-2 was a handful in choppy wind, tho.
Ever since, I've split the THICKNESS difference between root and tip, and not had the problem. Example: 10" root chord -- 20% airfoil is 2" thick; if tip is 7" chord, same % would be 1.4" thick. I've been splitting the difference, which here is 0.6" and the tip thickness gets hald of that added and goes to 1.7".
And yes, the airfoil involves the entire chord. I usually plot an NACA 4-digit form so that the max thickness fits the full chord, which means the main panel is a thicker NACA 00XX section.
\BEST\LOU
Al
I remember the Atavists, with the long, hand tuned wooden props, that I managed to break two of trying to launch you at Tucson! 
I even remember heading for the dictionary to look up "Atavist"..
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I was looking at Igor's fattened airfoil, and with just a little patience I think I could define it mathematically with 4 or 5 arcs/circles.
If we can say "computer-generated", I know that will impress some folks, but mostly I'm just too lazy to do them myself manually.
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"Sects, sects, sects. Is that all you monks ever think about?"
So I keep the aft section curved as per Al and the high point forward with a blunt LE especially nearer to the root and MAC as per Ted. Oddly enough, they do both agree that a thick wing is best and of moderate aspect ratio and span...about a 5:1. The biggest difference in my opinion is that Ted uses a 17 to 20 % flap while Al uses more flap for his nearly scale creations that need for an efficient, nearly scale wing that can handle the weight. Either way...its a fast wing.
Now they both will start shooting at me for this finding of common ground....but its ok, I like and respect em both and I am younger so I can run a little faster! If only I could dodge bullets!
Air has to move faster over a thick wing section than a thin one even if both are traveling at the same speed. Thats why you get more lift on less square inches with a fatty such as mine or Teds or Pauls and it makes weight a bit less critical.
Larry,
You are right. The airfoil in reality consists of circular leading edge, hingeline matched flap in its deflection in corner to have smooth surface as Al wrote, and ONE vectorized curve defined by its end points and high point at maximal thickness in Corel Draw. I wanted easy to define shape just because I needed about 20 trials until I got wanted numbers. After the testing, I know that while the leading edge diameter is VERY important, the thickness and position of high point is very insensitive (until the LE and hingeline is the same). Basically – while speaking about our flapped airfoils - the leading edge radius is related to angle of attack where the separation propagates from TE to LE (thicker LE – later separation – at higher AoA) while the curvature from high point to TE gives its progress (thicker airfoil means slowly progressing separation, flat airfoil means dramatic propagating of separation from TE to the LE).
My airfoil has LE giving safe separation free airflow till ~15 deg AoA with 30 deg flap deflection. It is angle, which can be reached by INSTANT handle deflection to full control deflection. I note we do not do any instant deflection, and thanx to long tail, our models fly up to half that AoA in corner. It is typically at 5 to 7 degrees. It means the stall cannot happen also at low speeds or landing.
The hingeline is designed for flap deflection in corner at 30 degrees (it is necessary to have proper flap area and control ratios to match it). More deflection can cause separation on TE and its quick propagating to the hingeline where is that “discontinuity” as Al stated. It makes drop in lift. The airflow is relatively stable front of that corner or Al’s “discontinuity”, so it does not propagate to LE as the AoA grows more. It makes that unwanted “bump of polar”. My main idea while designing that airfoil was to suppress those bumps on polar. I remember thread about bumps several times, so I do now want to open it again, - if anyone wants I have short Word document …
The feeling that the high point position IS important comes probably from fact that if you change it, you change also leading edge and angle at hingeline. In this case you get different airfoil properties, but the reason is different LE and hingeline.
igor
Igor-
I would like to read your paper. Would it be possible for you to e-mail me your word document? Thanks much. My address:
>The feeling that the high point position IS important comes probably from fact that if you
>change it, you change also leading edge and angle at hingeline. In this case you get different
>airfoil properties, but the reason is different LE and hingeline.
Thanks also for giving me an "authority" to cite next time we argue this at the field. From earlier discussions and field observations, I have felt that l.e. radius and (surface) transitions were the important things, including managing flow over the flap hinge as you and Al have done. Would you also agree though that, for a flapless wing, a straighter (flatter, less convex) rearward slope toward the trailing edge would then be better? The reason I ask is that it seems to me that flow separation might then vary less in the chordwise direction with aoa changes, making aircraft pitching moment variation more predictable (I know that theoretically, it's zero about 1/4-c for the symmetrical-section wing alone). Additionally, I've been thinking that reversing surface curvature near the trailing edge, as in using a flat stationary "flap", might increase L/D for "flapless" wings at aoa's > 0, through better exit flow management. Is there any validity to either of these thoughts? Always pondering...
SK
Serge Krauss
You know, the question is what the “better” means. We know that there are tons of different airfoils, but no one can be pointed as “BEST” at all, and also not “BEST” for given application. If we speak about stalling ONLY. I can make better example, you will understand. Let’s say we have limited power for 18% airfoil (for given weight, thus known area). Now comes question. If we go for maximal lift, the trivial solution can be very blunt nose, and it all means flat surface toward the TE. Such an airfoil will carry lot, but if go to its maximum, the separation will probably quickly propagate to separation at LE and thus losing of half of its maximal lift. If you will want be warned before, you will probably go to sharper LE and move the high point little back. The curved surface will be more kind, but maximal lift will be limited. Question is what you prefer. In absolute numbers of lift, you can get better result with blunt airfoil. Such an airfoil is used also on full-scale stunters. Some of them are also little modified to for “programmed” separation at LE in some maneuvers (we call it “lomcovak”) and than quick sticking back.
>>>The reason I ask is that it seems to me that flow separation might then vary less in the chordwise direction with aoa changes, making aircraft pitching moment variation more predictable (I know that theoretically, it's zero about 1/4-c for the symmetrical-section wing alone).<<<
Yes, every separation changes the moment. Thus the moment is also very good indicator of changes in airflow quality. I wrote about bump o polar. If you look at the moment polar, you see clear stair responding to the flap area. The moment IS CLOSE TO ZERO for symmetric airfoil AT REGULAR conditions, it means not stalled. If you look at to stalled airfoil, you will see the moment can go up to –0.5. Its value is very liar to amount of stall in chord direction.
>>>Additionally, I've been thinking that reversing surface curvature near the trailing edge, as in using a flat stationary "flap", might increase L/D for "flapless" wings at aoa's > 0, through better exit flow management.<<<
I do not think you can get some benefit on drag. But that curvature can do one simple thing. It keeps little higher pressure close to TE on upper side. It is because the air accelerated toward the airfoil surface is slowed down by opposite angle. It causes more stable airflow at TE and so later propagating of separation to the high point. The lift polar will be little modified. The usual smooth round hill will have more linear (straight) “uphill” section between zero and maximal lift. Than comes corner when separation point over jumps that “hingeline” and than it goes on usual way, it means top at maximal lift and down the hill to half of maximal lift, where the separation starts at LE. But do not think of some significant differences.
igor
You can see that the first airfoil separates earlier, at 10 deg AoA, while the second has little advantage thanx of upper curvature. But the difference is really minimal, it is only two degrees in this case.
However, the first one looks it can produce more lift, but I hote it is not usefull lift. If you look at the lift/drag chart, you see excessive drag, which cause stall in both versions. The only advantage of the first one is its stalling character. It look it does not have any hysterezy, and thus gives chance to stick back easily what can be important, or even rescuing .
igor
Very interesting airfoil data, but how many airplanes have you built with it and what practical results have you experienced? Have you compared it with other sections, both related and unrelated? I am of the opinion that my sharper leading edges fly better in the wind. Dose your theoritical data support or refute this?
Al
1/ >>>How many airplanes have you built with it and what practical results have you experienced? Have you compared it with other sections, both related and unrelated?<<<
I know for sure about minimally 50 wings done with that airfoil (which vent via my hands). I do not know how many are really used and in air, because I do not have feedback. I only know about some 5, which are still not used. However I personally own 6 such models from .15 till .61 engines. One is already lost due to broken line. I know for sure there are another may be 10 models used by friends around me with success. All others are away, but I never heard unsatisfying result.
If I can judge it, I do not think there is some significant improvement in flying ability like more lift or better resistance to stalling. I think this all is question of proper design, arms, wing loads and so, because we do not use wing to extreme we fly AoA around 7 degrees – it is far under its maximal capability. If I see significant advantage, than it is repeatability and easy trimming. I remember past, if I tried to build two equivalent models, I got two different models. Now if I build new model, and if I follow my spreadsheet, I am sure that if I come to field with new model and if my engine runs well, I go home with acceptable flying model after first session. It is also typical that one flies the same way as any other. The reason is that there are no “black spots” where the model is difficult to trim.
Whole Slovak team now uses this airfoil. We placed on 4th place in teams on last WC 2002 and I personally on 10th place with !!!4 weeks!!! old model with less than 50 flights. I note we have only 5 flyers in Slovakia, so we do not need any team trials to find those three stars. Another flyer usage it was also German junior who placed 3rd in juniors. All our models performed well in that washmachine type of air.
I know only one flyer that tried it and told me it is not what he wants. The reason was too little feedback. He is used for model heavily loading hand. Now he fly Cardinal based model with modified wing with round leading edge. As far as I know he is happy with this modification and he prefers it to the original sharp airfoil.
So far for “how many” and how great it flies it is from my subjective point of view. If I should compare it to different models of different designers, I wanted to know them and I did many of them. I did many clones of local star Supermaster designed by Jozef Gabris as well as US designs Juno and Stiletto. I also ordered, build and fly kits by Windy U. or Randy S. I do not think there is any significant difference in AIRFOIL (may be beside the Juno which I did wrong – my mistake). But if I compare a Cardinal (sharpest airfoil on LE) and my model, and especially if I switch them one day, I see clearly it does not make enough lift. But it can bee still question of wing load. The real difference was time necessary for trimming. The best is clearly my Max (the first), which has little more wing load and little more sweep back and which was very unsensitive to trimming and thus very good for beginners (or lazy experts ). The German junior flew this type modified for ST 60. It was really very good choice for him.
2/ >>>I am of the opinion that my sharper leading edges fly better in the wind. Dose your theoretical data support or refute this?<<<
Sharp and blunt airfoil in linear numbers has equivalent lift production capability. It is 0.11 per 1 deg. I do not speak about extremes, and my airfoil with 3% LE radius is really not any extreme. In reality it is on down side of acceptable range. I tried to keep it on minimal value just over problems with bumps and separation.
However there are another 2 significant differences. First of all sharp leading edge can bring a benefit in low RE mode. It is not sharp enough to keep the airflow turbulent, but thinner and sharper airfoil has lower critical RE, so it can be an issue. But I note we fly at RE ~400 000 and it is FAR over any critical numbers, so I do not think it is important if we use it on our large models – may be on small model up to .15.
The second think is limited lift in extremes close to stall. It acts on two ways. First is, that it can slow down in corners in wind. This can support your opinion. I am pipe believer and think well working powertrain is better solution. Another think is higher angle of attack necessary for the same corner because of limited lift production. That higher AoA gives more time for controlling in corners. It is exactly what is happening with Suchoj by Beringer. A judge watching flight see heading much more than real path of CG. If your model needs lot of AoA in corner, you can do very quick 90 deg turn around pitch axis, but in reality you continue on circular flow and you slowly straighten the deflection on elevator while the fuselage angle is constant – so you have more time to hit the level properly. But again, good power train keeping speed safely, model which does not surprise you in any condition, which makes round loop if you ask, and cuts exactly 90 degrees every time your handle asks for 90 degrees not needs such dirty tricks, which are explorable by experienced judge. You can cut corners also with quickly and clean flying model which tracks well and safe.
But if you think the sharp is better in wind, can you somehow describe WHAT you think is giving you that feeling?
igor