British Young Pilots · Principles of Flight · Module POF-7.3
The ailerons are little flaps at the back of each wing, right out near the tips. They always move in opposite directions. Push the stick left and the left aileron goes up while the right one goes down. Hold your arm flat out of a car window and tilt your hand so the front edge points up: the air shoves your hand upwards. A down-going aileron does that to its wing, so that wing rises, the other wing drops, and the aeroplane rolls.
Source: Pooleys Vol 4 (10th ed.) p.91–92.
The ailerons are the primary control in roll. They sit on the outboard trailing edge of each wing and always move in opposite directions: one goes up and reduces the lift of its wing, the other goes down and increases the lift of its wing. The pilot moves them with sideways control column or by rotating the control wheel.
The unequal lift creates a rolling moment. Its size depends on the lift difference and on the moment arm to the CG, which is why the ailerons are placed outboard. The rule: the down-going aileron is on the up-going wing, and the up-going aileron is on the down-going wing.
Source: Pooleys Vol 4 (10th ed.) p.91–92.
Back to the car window. When your tilted hand gets shoved up, it also gets shoved back. More lift always brings more drag. So the wing with the down-going aileron rises, but it is also held back, like rowing harder with one oar. Pull harder on the left oar and the boat swings right. The rising wing is the hard-working oar: the nose swings towards it, which is away from the turn you wanted. Pilots call this adverse yaw. Roll left, nose sneaks right.
Source: Pooleys Vol 4 (10th ed.) p.92.
A down-going aileron effectively increases its wing's camber and angle of attack, so that wing makes more lift and more drag. The up-going aileron does the reverse: less lift and less drag. The lift difference rolls the aeroplane, but the drag difference (differential aileron drag) holds the rising wing back and yaws the nose away from the direction of roll. This is adverse aileron yaw, also called aileron drag. It is hard to spot unless you look for it, and it is mainly a low airspeed problem, most obvious in a slow turn.
Source: Pooleys Vol 4 (10th ed.) p.92.
Two helpers stop the nose sneaking the wrong way. First, the designers: on most modern trainers the up-going aileron moves through a bigger angle than the down-going one, adding a bit of drag to the dropping wing so the two wings pull back about the same. Second, you: press the rudder pedal on the same side as the roll. Stick left, foot left. Try it now: arms out, roll left by dropping your left arm, and turn your nose left at the same time. That is a tidy turn: ailerons and rudder together.
Source: Pooleys Vol 4 (10th ed.) p.92–93.
Good design has largely removed aileron drag from modern trainers, using one or more of three ideas:
Whatever remains, the pilot removes with rudder. Rudder's primary effect is yaw, its secondary effect roll; the ailerons' primary effect is roll, their secondary effect yaw. Using rudder to cancel adverse yaw while the aeroplane is rolling is one of the most important elements of aeroplane control.
Source: Pooleys Vol 4 (10th ed.) p.92–93.
Once banked, the lift is tilted and its sideways component has nothing to balance it, so the aeroplane slips towards the lower wing. That sideways airflow hits the fuselage and the big keel surfaces such as the fin, mostly behind the CG, and the nose yaws progressively towards the bank. With the ailerons back to neutral the yaw continues, and the aeroplane will drift into a spiral descent and lose height unless the pilot levels the wings or applies back pressure.
STRETCHTwo yaws point opposite ways during a turn entry: adverse yaw (away from the roll, while the ailerons are deflected) and the secondary-effect yaw (towards the bank, once banked). Which one is the rudder mainly fighting on the way in?
Source: Pooleys Vol 4 (10th ed.) p.94.
Where the adverse drag comes from. The rising wing's extra drag is the cost of its extra lift: Pooleys' figure labels it high induced drag. The design fixes do not remove it. Differential and Frise ailerons instead add parasite drag to the descending wing (a bigger upward deflection, or an aileron nose protruding into the airflow beneath the wing) so the two wings' drags more nearly balance and the yawing moment shrinks.
Coupled controls. Linking the rudder to the ailerons puts an aerodynamic sideforce on the fin as the ailerons deflect, producing a yawing moment into the bank that opposes the adverse yaw directly.
Source: Pooleys Vol 4 (10th ed.) p.92–94.
Tap a card to flip it.
One statement at a time: is it true of the wing with the DOWN-going aileron or the wing with the UP-going aileron? Tap to sort it.