What does the rudder in an airplane do?

Answer 1

The rudder is used to control the airplane on the yaw axis (left and right.) It is most commonly used to keep an aircraft "coordinated" through a turn.

During a turn, the pilot first uses the ailerons to roll the aircraft towards the direction he wishes to turn. Once the desired bank-angle is achieved, the pilot will use the elevators to maintain altitude, and make minor adjustments with the ailerons to maintain the bank angle. During the turn the aircraft may have a tendency to "skid" meaning the tail of the aircraft does not follow directly behind the nose of the aircraft, just like a car skidding around an icy corner. There are many reasons that this is undesirable, not the least of which is the comfort of the passengers. The pilot will use the rudder to keep the aircraft "coordinated" through the turn. To the passengers in back, a properly coordinated turn will feel no different than straight and level flight, whereas a skidding turn will feel like zipping around a corner in a car.

The rudder is also used to counteract "adverse yaw" which is the tendency of the aircraft to yaw in the opposite direction when using the ailerons to adjust the bank angle. The reason for this is because the ailerons produce additional drag when they are being deployed, the additional drag produced is not left/right symmetrical and thus the aircraft will yaw towards the side where there is more drag.

P-factor also plays an important role with propeller-driven aircraft. When a propeller-driven aircraft is at a high power setting and in a climbing configuration (such as just after takeoff) the angle of airflow onto the prop produces thrust stronger on one side of the prop arc than on the other. The spiraling wake of the spinning propeller also contributes to P-factor. This makes the aircraft tend to yaw left requiring the pilot to activate some right-rudder to counteract it.

When a multi-engined aircraft loses one engine, the thrust of the remaining engine will of course be off-centered in relation to the aircraft. This can lead to a very dangerous situation if the pilot does not counter-act this off-center thrust with the rudder. For this reason, multi-engined aircraft almost always have a rudder trim-tab which allows the pilot to set the rudder to whatever position is needed, but without having to exhert constant pressure on the rudder pedals.

Aircraft also make use of the rudder during cross-wind landings. During a cross-wind landing, the aircraft obviously must point the nose to the upwind side slightly in order to maintain alignment with the runway. Think of rowing a canoe across a fast-moving stream. You would need to point the canoe up-stream in order to cross the stream directly, otherwise you will wind up downstream. In a cross-wind landing, the pilot must maintain this up-wind pointed angle (crab angle) all the way down the glideslope until immediately before touchdown. Right before touchdown, the pilot must use the rudder to align the aircraft heading with the runway heading.