First, I'll admit that I don't fly conventional stunt ships. This doesn't mean that they lave any lack of performance. They wouldn't be competition stunt ships if they lacked the stability necessary for smooth, groovy flight and smooth and easy round maneuvers which track well. They must also be capable of whatever corner radius the judges seem to require. My airplanes generally have these characteristics but incorporate sharply different design philosophy.
My airplanes have large scale-like fuselages. With large frontal area and lots of wetted area, I have more drag than most any other competition stunters. If you have followed various posts on this forum, you are already aware that to some degree, drag may be beneficial for speed control during maneuvers. Drag is offset by thrust, and I favor very large engines and seldom lack thrust. Drag has never seemed to make my airplanes less competitive.
I build airplanes with the least wing area possible necessary for competitive performance while, at the same time enhancing the appearance of a semi-scale airplane. I believe that a competitive stunter must have enough lift to carry the airplane, buffet free, through the tightest necessary corners. This isn't to say the tightest possible corners which implies an amount of lift exceeding that required to make a well scoring competitive corner. Airplanes with excess lift almost always have unnecessarily large wings and flaps with less area and travel than that which would produce the most lift possible from the available wing area. Airplanes with unnecessarily large wings and "lazy" (small or torsionally weak) flaps also have unnecessary frontal area, wetted area and parasitic drag. Induced drag should be similar to smaller more efficient wings because the lift produced is similar. But, again, drag isn't necessarily a demon. It isn't necessary to build 55 oz airplanes with 700 sq in wings to make competitive corners, and these large "kites" may be at a competitive disadvantage when strong winds make lightly loaded airplanes more likely to be blown out of competitively shaped maneuvers. 55 ozs/700 sq in equal .078 ozs/sq in of wing loading.
I've found that the airfoil tests done to design the Sea Fury has resulted in wings that can be fully competitive at .11 ozs/sq in wing area, although, in all honesty, I shoot for .1 or 550 sq in wing to fly 55 oz airplanes with my combination of wing design features.
First, I use thicker wings. The Sea Fury tests led me, in 1970 to use thicker wings than the then common 18% (less flaps). The Oriental was exceptional, for the time, with 20%. My airfoil tests indicated that 25% should work well, providing high lift with a reasonably decent appearance. I also theorized that an asymmetric rib shape should be beneficial as the corners of the pattern requiring the greatest lift (triangle and hourglass, lower right) were both inside corners. With enough lift to fly those corners, we should be able to get by with less lift from a thinner bottom half of the airfoil for the rest of the pattern. Since then, nearly all of my airplanes have incorporated some asymmetry in their airfoils, and we are talking about three NATs wins, two Walker Trophys and a World 2nd place with asymmetric airfoils. Still, asymmetry aside, I still believe that thicker wings produce more lift and I have proved it to MY satisfaction. I use, typically, 25% airfoil tops and sometimes, but not always less on the bottom.
Second, I have empirical evidence only, but am convinced that moderate leading edge radius airfoils fly better, especially in the wind. I don't agree with the blunt nose designs. They seem to me likely to be less stable in pitch, but that is my opinion.
Third, based on my Sea Fury airfoil tests, I am also convinced that the lifting capability of the wing is improved when the airfoil is "profiled" to reduce the discontinuity at the hinge line. so my airfoils are contoured with a noticeable curve in the ribs ahead of the wing trailing edge. This is certainly less than optimum in level flight where a zero flap discontinuity is noticeable but I really don't care how inefficient the wing may be in level flight as long as I can obtain greater lift and a slight drag reduction from eliminating the discontinuity while maneuvering. Asymmetry, thickness, leading edge radius and rib profile were covered in the Sea Fury and Mustunt articles published in February and March issues of AAM in 1973. I originated the sliding block adjustable leadout and showed the reversed bellcrank in the Bearcat article in 1970 AAM. The Sea Fury article argued against the use of asymmetry in wing length by using equal span wing panels. This idea was updated in l973 in the design of the molded Mustangs (Snaggletooth series) when I wrote that a small amount of wing length asymmetry was usable if the outboard flap was widened to at maintain at least equal flap area. The Elvolution..... article published in Aug 1978 Flying Modeler described the first use of 1/2" of asymmetry and 1/8" wider tip of the outboard flap. Some of these ideas have become commonplace in modern stunt ship design without thought of their orgins.
Fourth, To obtain the maximum lift from a given amount of wing area, large flaps are required. My designs usually use flaps which are 25% of the wing chord. My base rib is 10" root chord with 3.3" flap chord. My typical tip chord is 6" with 2" flap chords (+ - 1/16" for the elimination of asymmetry). If maximum lift is needed, it isn't enough to simply build large flaps. To be optimally effective they must also be torsionally stiff. A large flap which "washes out" will produce no more lift that a smaller stiff flap. To this end, I build very stiff flaps. the original Mustang of the Snaggletooth series,E2-S, had 3/8" flaps with 2 layers of fiberglass and epoxy glue. For the time this was considered typical or a bit more. The Mustang VII which I built for the World champs (but crashed) had 3/8" flaps with 5 layers of glass cloth, each layer cured and sanded individually. This made for an unusually stiff flap. This airplane had 530 sq in wing, weighed 51 ozs and had a home made .60 engine which I called a Rabe 10. (10cc) The wing loading was .096 ozs/sq in.
Finally, It does no good to build large, stiff flaps and fail to deflect them optimally in maneuvers to obtain maximum performance. from my airfoil tests, I determined that measured lift of my stunt sections peaked at a flap deflection of 30 degrees. My airplanes are designed to use 30 degrees of control deflection in hard corners. To obtain a 30 degree of control deflection in corners without uncomfortable handle deflections, requires a larger than normal throw on the bellcrank. I use 4" bellcranks with a 1 1/8" - 1 1/4" operating arms. This requirement for large control (not handle) deflections means that, generally, I fly airplanes which are nose heavier than most. I view this as serendipitous as nose heavy airplanes are more stable and tend to fly particularly nice tracking rounds. This is very similar to earlier stunt configurations which used Top Flite 3" bellcranks with 15/16" throws, forward CG and were flown with 6" EZ-Just handles. I don't agree with the modern trend toward aft CG's. (Or, "There is more than one way to skin a cat".)
The process of building an updated Snaggletooth which had a bit larger wing than the later Mustang VII. It preceded the Mustang VII and had a wing which, at the time, I considered too big at 550 sq/in and 51 ozs for a loading of .092 oz/sq in. The BBQB Bearcat and Millennium ran nearly .11 oz/sq in wing loading and turn well. My problem now is to try to build a very competitive airplane with a new, modern engine which is heavier, together with its muffler and larger adjustable tank and the use of heavier glue (CA instead of water thinned Tightbond).
My goal is a wing loading of .1 for a flying weight of 55 ozs. I won't make it. I figure I can get by with 1.1 oz/sq in loading which would allow a weight of 60.5 for a small .65 powered airplane. I am sure that I will beat that figure, but just to be sure, I am going to build the stiffest flaps I have ever built to squeeze the most possible lift from the available wing area. The new flaps are built with a .312" (5/16") carbon tube (.022 wall) embedded in the length of the flap leading edge. These were very difficult to build, but are somewhat stiffer than the Mustang VII flaps and will weigh about the same. The problem with this flap is the relatively large leading edge radius of the flap. Installing the hinges on the trailing edge of the wing and leading edge of the flap gives about 1/16" gap between the wing and the flap. Aerodynamically, this isn't a problem as the gap closes as the flap is deflected until at about 35 degrees where the flap gap closes completely. the fact that the gap closes completely means there is no advantage to sealing the flap with tape. It just doesn't look as good as the flaps of the Mustang VII which had a very close fit. I almost built Mustang VII flaps because they were almost as stiff and the gap looked better.
This is a pretty sophisticated and labor intensive flap design. That being the case, it is hard to justify unless you need that last bit of lift possible from a small wing.
Yes, for what its worth, I think that nearly all classic stunt ships have too large wings and too small, springy flaps. But that is just my opinion.
Al
The photos below show the tooling required to build the flaps, and (1) an inboard flap from the long gone E2-S, two layers of glass cloth, and (2) an inboard flap from the Mustang VII which is also long gone and has 5 layers of glass cloth, and (3) the new inboard flap for the Snaggletooth 2 which has the carbon leading edge and one layer of glass cloth. This flap will get somewhat stiffer as epoxy paint is added in the finishing process.
