Wider Outboard Flap Design

This is probably a dumb question, but I’ve seen a lot of designs with a wider outboard flap. Why is this? The attached pictures are from D. Rice’s Oriental Plus.

Sorry...forgot to attach the inboard pic.
Stephen

For a fairly complete explanation of this trim device check out my 1977 Mustang article "Evolution...." reprinted in the March/April 2000 Stunt News. (pages 54 & 58)

Asymmetry, or more correctly as we use it, positive asymmetry is the design where the inboard wing and/or flap area is larger than the outboard wing wing and/or flap area. Positive asymmetry tends to roll the airplane away from the flyer in all maneuvers. Negative asymmetry is a design where the inboard wing area is reduced or eliminated and the outboard flap area is increased to the point of rolling the airplane in toward the pilot in all maneuvers.

The point of negative asymmetry is that it allows the carrying of more tipweight. Up to a point, tipweight is more effective in creating line tension than asymmetry, at least, the type of line tension we need as we maneuver.

Prior to the introduction of this Mustang in 1973 nearly all stunt ships used positive asymmetry. Some used a full extra rib bay on the inboard wing with the inboard flap lengthened to match. Some used Palmer's "differential flap" which is an extreme example of positive asymmetry. A few used no asymmetry. The only airplanes that I'm familiar with using no asymmetry were the Sea Furys and Mustunts. I considered the use of tipweight to be more effective in creating useful line tension than positive asymmetry. I wanted tipweight, not positive asymmetry rolling my airplane out in maneuvers to create useful maneuvering line tension. This evolved in the Mustang, first, to using small positive wing area asymmetry with a compensating small negative asymmetry by widening the tip of the outboard flap by 1/16". This farther evolved in that design article to the use of overall negative asymmetry by overcompensating the wing's small positive asymmetry with a larger 1/8" wider outboard flap tip to allow even more tipweight than a no asymmetry configuration would have allowed. This Mustang article is the first published use of negative asymmetry in that wing area differential is reduced and area is added to the outboard flap specifically to enable the carrying of more tipweight. This practice is common now but it began with the Mustang V and this 1977 article regarding its aerodynamic innovations.

There are several reasons for that, but easiest to explain is this: If you perfectly balance the line weight by tip weight (yes, little simplified) in level flight, then you will easily recognize rolling out tendency in tight corner. It is because lines do not follow quickly changing flight direction, so the line mass accelerated by either gravity or lift acceleration has dropouts in corners (it is converted to yaw). In that case the model flies like having too much tip weight and that extended outer flap effectively solve that problem. It is because if you extend flap in level (flap is not deflected) it will change lift of outer wing only minimally. But that elongated flap has huge effect if flap is deflected. Thus it balances that excessive tip weight when lines have drop out.

>This is probably a dumb question, but I’ve seen a lot of
>designs with a wider outboard flap. Why is this?

Because everybody ended up adding "tabs" AKA "warts" to the outboard flap anyway, so might as well build it in. The reason is that the correct tipweight for level flight, round maneuver turns, and square corner turns almost always ends up being 3 different quantities, particularly on airplanes with assymetry. You can't in any practical way shift the tipweight based on the control deflection, so this is an attempt to apply an aerodynamic fix to keep roll angle zero in all three conditions (and presumably/hopefully, all the time if you do it right).

This observation goes back to at least the early 60's, and probably before that, as does the idea of a tab to correct it. It's just a matter of "cut and try" coming up with the right solution without really understanding the underlying cause. Note also that when power was limited and very few people really understood trimming, it was common to induce various forms of intentional roll in the maneuvers by thing like larger *inboard* flaps, more travel on the inboard flap, excess tip weight, etc, which is essentially a matter of intentionally causing the effect the larger outboard flap is correcting!

It's my opinion that the reason is that while we are making the wing assymetrical, as it should be to put the lateral CP of the wing in the right place, we aren't making the *tails* asymmetrical. Effectively, the tail is shifted the wrong direction, and thus causes a rolling moment that gets larger and larger as the turn gets tighter. I finally came to this conclusion in an analysis of the All-American, which is so grossly incorrect in this regard that it can't be trimmed in any sense of the word I understand. Experiments with regular stunt planes have been, so far, inconclusive, but I'm sure enough that it doesn't hurt anything that (like the very slight positive incidence) that I am going to build it in next time.