Hi Guys,
In any discussion of flaps on stunt models it is essential to remember that flaps don't "make an airplane turn".
Flaps serve only a single function. They allow the wing to be reconfigured so as to produce a higher coefficient of lift when required out of the same wing area. Yes, they do increase drag a certain amount when doing so, but that shouldn't be a huge consideration since you only need to produce enough lift to fly a corner as tight as you're able to fly it repeatedly and well. Most stunt ships (especially those whose design goal is to come as close to a zero wing loading as possible) could fly perfectly acceptable patterns with no flap movement whatsoever. A 720 square inch, 50 oz P-47 being a classic case.
Flaps, when deflected, will actually impede the rate of turn of which a given amount of elevator deflection is capable of producing if they were fixed. This is the result of the negative pitching moment they produce (this is easy to visualize. Simply think of the airplane with no tailplane attached so that when the flaps are deflected they are "acting as elevators". Flaps go up, the airplane goes up ... not down; and vice versa). Such a tailess plane is often referred to as a "flying wing". I guess we all know what happens when you deflect the flipper on a flying wing, right?
The tail must overcome this adverse pitching moment first in order to ultimately produce the pitch change in the desired direction. This is no small part of the reason why flapped ships typically required bigger tails and usually more elevator deflection (see a modern combat ship which maneuvers like crazy with only a few degrees of up and down control). It is also part of the reason they almost always prefer a more aft CG.
An earlier poster (John McFeydan, I believe) referred to some notes I made about reducing the chord on Paul Ferrel's profile Cardinals. We literally removed a ful 3/4" from the trailing edge of the flaps and the results were exactly as described, much improved pitch response and reduced control loads. By the way, we not only reduced the area but also reduced the amount of movement to the minimum available using the stock control horns. The reason was simple, the wing was producing way more lift than necessary to support the airplane in the corners. Even worse, deflecting those controls with the available line tension required "massive" yanks on the control system which resulted in, frankly, terrible maneuvers.
This improvement (please note that removing that much area and movement from the flaps had "zero" effect on any visible tendency to "stall", especially when combined with blunting up the leading edge of the wing a bit) was the result of two things pertinent to this discussion. First of all, reducing the area of the flaps reduced the pitching moment from deflecting them, allowing teh airplane to respond to elevator inputs in a more timely fashion; second, reducing the chord of the flaps reduced the force required to deflect them ... the control load.
Think of this experiment. The lower the aspect ratio of a movable surface, the harder it is to deflect it into the moving air stream. Say you've got a sheet of plywood two feet by four feet (total area eight square feet) you can stick out the side of your pick-up (like a "flap") while driving 55MPH down the highway. You can stick it out with the "hinged" portion either on a two foot edge or a four foot edge.
When extended with the four foot "hinged" section it simulates a high aspect ratio surface and, when deflected only two feet of surface extend beyond the hinge. The force required to deflect the eight square feet will be large but doable. Now extend it with the two foot hinged edge and the same eight square feet will be essentially impossible to deflect because the air forces (which are pretty much the same) are centered twice as far from the hinge.
This is what Wild Bill termed hinge load in his classic articles back in the '60s. Must reading for those who find this thread of interest, by the way.
When you combine the negative pitching moments of large area flaps plus the increased hinge loads of low aspect ratio to produce that area you get the kind of uncomfortable and maneuver debililtating responses described in the earlier post and in the example of Paul's Cardinal in this post.
The bottom line of all of this is that there is "no good reason" to handicap a stunt ship with this sort of penalty unless, (and only unless) the wing loading is so high that that much flap must be deployed to produce the lift required to perform maneuvers of the desired size and corner radius.
If you believe that producing "more lift" is inherently to your benefit, I 'm sorry, but that just isn't true. The closer you can "balance the lift produced to the lift required" the better the airplane will follow the desired path while "drawing" the geometry of the stunt pattern; and the less effort will be required of the pilot to place the airplane in those paths.
Ted Fancher