This is from Ted Fancher and does at least address some of the questions here:
As I mentioned in the Doctor article in Stunt News, flaps pretty much only do a few things. First and most importantly, by changing the airfoil from an uncambered section to a cambered one they inherently increase the lift per unit of total area (assuming the gap between the main plane and the flap isn't so excessive as to allow the value of the flaps to be largely defeated); second, they increase the induced drag and, therefore, the total drag (induced plus all others ); third, they produce a negative pitching moment (the cambered wing which results from deflected flaps has a natural desire to rotate about itself in the direction opposite to the camber).
With the above understanding and the realization that turning high g maneuvers in the pitch axis requires abundant lift, it becomes obvious that if we eliminate the flaps we need to achieve the necessary lift in another manner. Lift produced is the result of air density (we've no control over that), surface area (we can make a wing any size we wish), the effectiveness of the airfoil (co-efficient of lift), angle of attack and the wing planform (aspect ratio), and, importantly, the square of the airspeed.
Thus, if we eliminate the use of an airfoil which can be variously cambered (via flap deflection) we must re-manage the other aspects to regain the necessary lift. We can; A. increase the area thus reducing the wing loading; B. devise a more efficient airfoil for the unflapped configuration; C. utilize a more efficient planform (a higher aspect ratio develops more lift at a lower angle of attack and less drag than does a lower aspect ratio of the same area. Both can produce the same approximate maximum lift but the lower aspect ratio will require a higher angle of attack and produce more drag in doing so.); D. fly faster.
Thus, as has been suggested, it is factually obvious that a very lightly loaded, high aspect ratio flapless airplane flown at very high speeds and with sufficient tail authority can indeed turn in the pitch axis at very high rates…certainly higher than a slower, higher loaded, low aspect ratio ship encumbered by an airfoil which fights the very thought of a tight pitching maneuver in opposition to its natural desire. So, “Yup” a well-designed combat ship can turn on a dime…and does every day.
Despite the suggestion in the rule book that we are supposed to be flying five foot radius corners, the fact are that we both don’t and (before the nay-sayers jump on the apparent dichotomy) can’t do so in a competitive fashion in a stunt pattern. The reason is simple. We simply don’t have the reflexes to do so in a precise fashion. Thus, the event has evolved into the development of aircraft which can turn reasonably “tightly” but can do so with precision and consistency. Heck, even nose heavy toads have won the Nats by flying “reasonably” tightly and very consistently.
Now, where does the foregoing leave us in terms of designing a flapless ship competitive enough to compete at the highest levels?
At first blush it would seem that we need only make the wing big enough to produce the lift we need and forget about the flaps. This isn’t far wrong, actually. Put a decent airfoil on a large enough wing (keep the leading edge reasonably blunt and the high point fairly well forward), provide decent tail authority and an outstanding engine run (<80% percent of ANY otherwise competitive package>) and you can fly pretty dang good patterns. The basic DOCTOR can fly 500 points plus flight after flight in the hands of a good pilot when the engine runs right (sometimes a problem).
However, conditions are seldom perfect and it is under less than perfect conditions that the shortcoming of the flapless ship begin to manifest themselves. Once again, we have to look at what exactly flaps do which we will regret not having under those conditions. In addition, what negative things happen when we pursue high lift through simply enlarging the area of the wing?
Two of the answers go hand in hand. One of the predictable bad conditions we encounter is high winds. High winds make the airplane accelerate in consecutive maneuvers. The greater the area (lighter the wing loading) the greater the acceleration. Thus our flapped ship with its greater drag for a given amount of lift will resist acceleration better. The unflapped ship will, conversely, make matters worse because of its larger “sail” area. It will accelerate at a much greater rate. In addition, pitch rate for a given control deflection will vary more since the lift produced by the tail increases as the square of speed also, ergo, the tail gets more effective as the ship accelerates.
There is, however, a flip side to this coin. In calm air (another dreaded competition condition) the unflapped ship has it all over the flapped one in that wake turbulence is significantly less without flaps (especially flaps deflected large amounts for unnecessary reasons) thus reducing the hazard of turbulence encounters and simplifying the flyer’s task by minimizing the need to maneuver in and out to try to avoid the wake. This advantage comes very near worth putting up with the high wind disadvantage above.
It should be noted that other means are now available to help control the wind-up in the wind more effectively than just a few short years ago. High RPM low pitch systems are natural speed controllers and go a long way toward harnessing the wind up and making it controllable. Tuned pipes are one way to go but are handicapped by the high weight involved. I think the current development of four strokes for our purposes should be high on the “must check out” list for anyone seriously contemplating a flapless competition ship. FYI, the DOCTOR is now flying with an OS Surpass .40 and as the system is refined is producing the best combination yet utilized…including a very good Aero Tiger .36. So far this combination includes a 10 X 4 Tornado three blade, conventional 10 percent Sig fuel, and a slightly choked down venturi. Delightful to fly.
Some have postulated that flapless ships by definition have to turn body angle “beyond” the desired pitch track because the symmetrical airfoil must have a positive angle of attack to produce lift. This is technically correct but my experience has been that given adequately low wing loading and enough speed the phenomenon is unobservable.
The center of Gravity is pretty much cast in concrete. Flapless ships pretty much end up at 15% of the average chord. (15% MAC for the technically oriented). With the Aerodynamic Center of a symmetrical wing pretty much fixed at 25% MAC this provides a modest “tension” between the lift and the CG which provides the feedback necessary to the controls for precision changes of direction. The CG can be moved further aft but begins to feel sensitive and tracking in maneuvers may suffer. Further forward and the the arm between lift and weight makes for excessive differences in required control input uphill versus downhill.
Tails needn’t be as large as for flapped ships since they don’t need to overcome the negative pitching moment of the cambered wing (deflected flaps) in maneuvers. Fifteen percent of the wing’s area is probably a minimum and anything over 20% probably more than necessary. Either will provide adequate stability margin for an easy handling ship. I think the current practice of low aspect ratio tails for stability in turns is probably still valid. I’d start with about 4.5 to one.
Modestly higher aspect ratios (for the wing) are probably desirable, say 5.2 to 5.5 to one. The greater the aspect ratio (span/average chord) the less the angle of attack necessary for a given lift requirement. This would mean; 1. a smaller wing area is necessary for a given weight, and; 2. the reduced necessary angle of attack required in hard corners would result in less of the theoretical need for body angles in excess of the track change desired, i.e. it wouldn’t seem to have turned “inside” the desired track.
The wing should be tapered so as to approximate the “ideal” elliptical lift distribution. A simple taper should suffice with probably a minimum of a 75% tip/root ratio and a minimum of 65-70%. Chords at the tip of less than nine inches or so are likely to be less effective than they should be. I’m guessing that the taper should be distributed between the leading and trailing edges so as to result in a quarter chord line perpendicular to the fuse. I’m really open on this and would be interested in divergent points of view. I doubt very much that a swept forward quarter chord is desirable but a slightly swept one could be stabilizing in roll.
The resulting airplane should be flown slightly faster than an equivalent flapped ship, using a power train which produces airspeeds as consistent as possible in all attitudes. The compromise of a slightly higher speed and the therefore slightly smaller useable wing area should produce a very flyable airplane even under adverse conditions.
Anything else….hmmm? I’ll have to think about it.
Ted