If you were starting with all control surfaces simply hinged at their leading edges and big enough to give all the control authority you wanted, you would find that the rudders and elevators felt fairly nice but that the ailerons felt really heavy. And then we humans are less strong in side-to-side pushing than in pushing or pulling... And then there is that adverse yaw thing going on when you don't apply enough rudder with the ailerons...
So, you start looking for aerobalance on those ailerons to give better control harmony, and maybe balance the drag on the two ailerons too. What do you find? Well, extend the aileron forward of the hinge and the forces to move and hold aileron get smaller.
There is a limit as to how far you can extend the front edge. Airload is roughly triangular on the aft edge of the wing, so the center of the lifting forces about 1/3 back from the leading edge of the aileron. If the hinge approaches that line, the forces can get close to zero, which is not pleasant - we usually like to have some centering forces on our ailerons or the airplane becomes difficult to fly with precision.
Then there is aileron lock. Under some alpha and delta angles, the hinge moment can go through a reversal. Now the aileron wants to go hard over - full deflection. You might have enough strength to hold the stick on center, but frequently, this will be "aileron lock". Never good. So folks do stuff like soften or radius the forward part of the aileron and only move the hinge line to about 25% of the aileron chord. These reduce the rate at which the aero balance and drag build up.
If you want to beat adverse yaw, you can start playing around with putting the hinge above or below the chord line. On some airplanes the hinge is on the top surface. On other airplanes the hinge may actually be below the wing slightly. Configure the portion of the aileron ahead of the hinge properly and it only sticks out into the air for drag when the trailing edge is up.
Now you have a lighter feel on the ailerons, giving much improved control harmony between the axes, you have some restoring moment on the controls giving some feel and a tendency to return to center, and some correction for adverse yaw. But you may want a little more tendency to stay centered, so you may thin the forward part of the aileron a bit, so it does not stick out at the tiniest deflection.
This is basically the history of ailerons on airplanes. Some folks will go add spades, which add aileron area forward of the hinge and are easily adjusted for angle and size and shape to get whatever feel you want. Why do spades instead of adjusting the size of the part of the aileron forward or move the hinges? Because you can easily adjust in small increments if you want to. And they may have less tendency to aileron lock.
Now did we talk anywhere about ailerons being too heavy? Not usually... Most of the time, the manufacturer makes the ailerons lighter until the feel relative to the elevator and rudder is about right, and then they stop adding aerobalance.
Now let's get into the rest of the issues. Wing lift is usually distributed along the span in a nice elliptical shape. High pressure air on the bottom "leaks" around the tip toward the lower pressure air above the wing, giving us that elliptical lift distribution, and this shape does not care much about the planform of the wing. Really. The local lift at the root looks like the lift of a foil in a wind tunnel. But go half way out the wing and the local lift is dropping off. By the very tip there is no local lift at all. And the air being accelerated downward by the wing extends many feet up and down from the wing. If all that air were accelerated uniformly, the air moved would be a cylinder of wingspan in diameter. We are moving a lot of air.
Deflect the flaps, and they are where the local lift (Cl) is strong, and this changes pressure and diverts flows, not just through the part of the span where the flaps are, but out beyond them too, although to a lesser extent, what with the "leak" around the tip. So, flaps work great on the inner half of the span. Put ailerons on what is left, outboard. Now you are trying to make local lift outboard, where the local lift is getting smaller and smaller as we get near the very tip. Hmm, ailerons out at the tip might have longer arms for more leverage, but they also have less and less pressure difference being made out there too. That lowers how effective ailerons can be.
So, we have a gadget that tends to feel heavy and not be as effective as we might like, then we add area to it that increase effectiveness while it reduces the forces to move and hold the surfaces, and also tends to reduce adverse yaw. But does it ever get too light? Most of the time, it does not. Most of the time, we want lighter ailerons than we have. So we design a nice simple aileron with area forward of the hinge that also comes down into the airstream when the trailing edge is raised but we rarely get the forces too light unless we tilt to them going hard over... And if we are close, you can always stick a spade on each aileron and play with its angle, area, and shape.
But make only half the aileron have too much forward and the rest too little? Hmmm. Usually, that gives heavier feeling ailerons, is harder to build and make behave well, is either less stiff torsionally or adds weight than if the whole thing was one shape, and reduces aileron size for less effectiveness. Why would you want to do that? The world has gravitated to the common design.
Now let's look at that spectacular akro bird above. The ailerons there have a great big aero horn at the outer end rather than smaller aero extensions of a Frise, and it is shaped to minimize drag when deflected. Hmm, when you buy an Extra, they expect you to know how to use the rudder to set whatever yaw angle you want.
Billski
![1520076905578[1].jpg](https://cdn2.imagearchive.com/homebuiltairplanes/data/attachments/114/114836-8dd6bc9e67b624bcbfe6ec3be4e69e45.jpg "1520076905578[1].jpg")
