I’ve noticed that on a lot of WWII aircraft, the bottom surface of the aileron is larger than the top, i.e. on the P-36 of D3A. In other words, the bottom surface extends farther forward than the top. Why is this, and how does the aileron go up if there’s a part of it that can’t extend fully? My other question is about the elevators on the P-36/P-40. do the small tabs stick up or down when the elevator moves, or is that where they are hinged?
They’re known as “Frise” ailerons, and the idea is that, as soon as they’re deflected, airflow pushes against the protruding part, and flows through the gap, thereby assisting with the movement. It only assists, since there is a greater area behind the hinge than in front, so the pilot still has to exert some effort. Rudders, and elevators, often had a “mass balance” horn at the top (rudder,) or end (Spitfire elevators,) which did the same thing. Getting the balance right could be difficult, and it varies, with airspeed, which is why you need trim-tabs, at the rear of the surface, the position of which needs constant adjustment, as speed changes. Movement of the trim-tabs is not automatic, but set by the pilot; usually they’re neutral, at rest. All of this became redundant, (except the trim-tabs) once powered controls arrived. At times, you will read of a rudder “overbalancing,” which is the result of the horn(s) having too much effect.
Edgar
Jaypack, this discussion could get hairy, I’m not too good at keeping things simple. For the elevator question, some of this was covered in a thread jininth started a few days ago:
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Here is a drawing of a Frise type aileron, which also has differential movement, meaning the aileron which moves up into the airstream moves more than the other aileron moves down.
This is the way that most ailerons have been designed after the late 1920’s. It is not typical of high performance jets, we won’t go there.
You asked why the aileron extends further on the lower surface of the wing than on the top, and how that could allow the aileron to move. The answer lies in the curve of the leading edge of the aileron, and where the aileron pivots when it rotates. By moving the hinge point down and aft, and by shaping the aileron’s leading edge so that it always the same distance from the aileron hinge (pivot) point, the aileron can then move freely and always remain the same distance from the trailing edge of the wing. The shape of the trailing edge of the wing is also important, as it is cut out to allow the aileron to nest underneath it.
This drawing is excerpted from of a set of plans I drew for an R/C J-3 Cub, and is based on the 1:1 airplane’s design.
The reason for designing the aileron so that the gap stays constant is to improve the efficiency of the aileron. By keeping this gap small less air can spill from the lower surface of the wing to the top. This is similar to the design of a slotted flap.
The Frise design most strictly defined is an aileron whose leading edge moves down into the airstream under the wing as the aileron’s trailing edge is deflected upward. This is accomplished by having the aileron’s hinge point moved back from the trailing edge. To see an aileron that is not a Frise type, look at any WW I biplane. Frise type ailerons make it possible for the pilot to bank the airplane without having to use as much rudder as he would need to if the airplane did not have this type of aileron. By also creating a differential in the movement between the aileron that moves up and the one that moves down, even less rudder is required. In some aircraft, like the Van’s RV-4 or the Beechcraft Bonanza you can roll the airplane without the use of the rudder at all in cruise flight.
The square shape near the outer tip of the elevator on the P-40 is a counterweight which contains lead to balance the elevator statically. Lead is inside it. It does move with the elevator. The P-51 had the same design. On the Corsair there is also a counterweight (see the thread above by jininth) but it is at the tip and is a combination static and aerodynamic counterbalance.
to add to what Jeaton has said (excellant description and illustration) it’s called ‘adverse yaw’. Some planes have more than others, gliders being prime among the worst due to their very long wingspans. The part that drops down increases drag on that wing, reducing the need for rudder. Some trainers don’t require much rudder at all (like a cessna 150) which doesn’t really make them good trainers for when you graduate to ‘real’ planes. Other factors that go into how a plane handles and how much rudder it requires are things like amount of dihedral (the angle between the wings when viewed from the front)
-Bret
Thanks guys, that’s all I needed. [:D]