Originally posted by locharrow
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Well…..I shall give my perspective from flying planes with shorter wing spans and my 80+ ft long wing ASH-25E sailplane which has tremendous adverse yaw. Also, knowledge from several instructors and text books. Take it for what it is worth. Apply it as you will.
First the aerodynamics and physics. When wings are level, the lift vector is straight up. This vector counters the weight vector. In a bank, assume no pitch change, the lift vector now points sideways at an angle. Since the lift vector hasn’t changed in magnitude, if you resolve the vector into a right triangle, you will see the vertical component is less. Therefore, the aircraft will descend in a turn. This has nothing to do with CG. It has everything to do with vectors. To offset the loss of effective lift and to counter the weight vector so you do not descend, the pilot must apply up elevator when banked.
Now for the rudder. The ONLY purpose of the rudder (outside of aerobatics) is to counter the adverse yaw generated by the ailerons. So, a proper/coordinated turn will require application of the rudder while the ailerons are deflected at the start of the roll/bank. Once the bank is established, the rudder is moved to the center because the ailerons are no longer deflected. To keep the “ball” centered or the yaw string centered, rolling to the right one MUST apply right rudder…and rolling left, one must apply left rudder. All the while applying an amount of up elevator. Coordinated turns look good, and feel good if you are in the plane.
In the ASH-25E sailplane, a nearly full rudder is required to keep the yaw string centered while rolling into the bank. The rudder is re-centered once the bank is established. And, a small amount of up elevator is required. The ailerons are WAY OUT THERE, so they generate a huge amount of adverse yaw.
Now let’s move to short wing aircraft (say a Cessna) or RC planes. The ailerons are so close to the fuselage, very little adverse yaw is generated when the ailerons are deflected. Not much rudder is needed when a roll is initiated. Technically speaking, the yaw moment of ailerons close to the fuselage is minimal.
Some long wing RC planes AND lllooonnnggg wing RC sailplanes benefit from the application of rudder in the turn (or maybe they don’t even have ailerons) to counter adverse yaw. For RC planes without aileron, the skidding by the rudder application causes a differential lift vector on each wing, and the plane banks. For these planes, the rudder banks them. You can do this in full size planes, too….but your passengers will throw up. It feels terribly uncoordinated because it is.
For the F-4 or MiG-29, or P-38 or AL37, I never touch the rudder unless I am on the ground, or doing some aggressive aerobatics. The ailerons are so close in and/or tiny that their adverse yaw is negligible. For my LONG wing RC sailplane, I apply enough rudder to make it look coordinated. Right bank = right rudder at the start of the bank and vice versa.
However, when I bank….up elevator is applied to counter the effective loss of the vertical lift vector. The AL37 needs a lot of up elevator in a bank.
If rudder is used to raise the nose, you are no longer in a coordinated turn, but instead slipping sideways.
Bottom line, in short wing RC aircraft, using rudder in a turn is a waste of effort. Use it if you want to, but it isn’t needed. And using it to raise the nose in a bank will mean you are in an uncoordinated slip.
And my full scale fighter jet pilot friends tell me they leave their feet on the floor most of the time (figuratively speaking). Why? The ailerons of these jets (or tailerons) produce very little adverse yaw. The rudder isn’t needed when turning. In my long wing sailplane, by golly, a lot of rudder is needed…but needed to keep the turn coordinated…not to raise the nose.
-GG
PS Answer this question on any FAA exam other than “to counter adverse yaw” about the purpose of the rudder, and you will miss that question. Why? There are many dead pilots who attempted to turn their aircraft from base to final using rudder….skidding as it were. This set-up can induce a stall of the wing on the inside of the turn (among other things the pilot may do), and the plane fatally spins in.
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