Roll Control

Roll Control is provided by the ailerons. 

When the ailerons are deflected the down going aileron increases the camber of one wing. The up-going aileron decreases camber on the other wing. The result is an asymmetric lift between the wings. This causes the roll rate to increase away from the wing with the greater lift.

It is important to note that as long as a net moment (lift times distance) exists between the two wings the aircraft will roll faster and faster. 

Use this movie to experiment until you understand why the roll rate does not continue to increase, but rather becomes constant quite quickly after the ailerons are deflected.

Roll Damping

Roll damping refers to the fact that the down-going wing experiences and increase in angle of attack (due to the change in relative wind) and the up-going wing experiences a reduction in angle of attack. The quickly eliminates the lift differences caused by the camber change due to the deflection of the ailerons. As a result the aircraft quickly settles in to a constant roll rate in which there is not net moment.

It is a curiosity of aerodynamics to note that the down aileron on the up-going wing gives that wing a longer arm. But, since the net moment must be zero the down-going wing actually produces slightly more lift than the up-going wing. (But, both wings produce equal but opposites moment, for a net moment of zero.)

Why an Airplane Turns

We know that an airplane turns when place in an angle of bank. But, why?

Yaw (Directional) Control

Directional or yaw control is provided by the vertical fin and the rudder. This is probably the easiest of the three axis to visualize. Still people often fail to realize that in a slip the fin produces no net force (yaw moment equals zero.)

Use the movie below to experiment with yaw control until you understand how the rudder produces a constant yaw rate when deflected.

Yaw Damping

Yaw damping is just like roll damping discussed above. 

When the rudder is first deflected the fin produces a net force, due to the camber created. This force, acting at an arm from the center of gravity creates a moment which starts the aircraft yawing, faster and faster.

But, as the aircraft yaws the angle of attack on the fin changes until the fin is at its zero lift angle of attack. Once this happens there is no longer a moment. From this point on the aircraft yaws at a constant rate.

It is worth noting that as soon as the pilot releases the rudder pedal the fin is no longer cambered. Therefore, it is not at the zero lift angle of attack anymore. A yawing moment will therefore, be created which will yaw the aircraft back in the opposite direction.

Of course since the fin is a symmetric airfoil the yaw rate will damp out at zero degrees of slip.

Pitch Control

Pitch control is provided by the elevators. Just as with roll and yaw described above a pitch damping effect quickly develops once the elevators are deflected. As a result the aircraft soon establishes a constant pitch rate once the elevators are deflected. Use the movie below to explore this effect.

Net Lift and Pitch Control

In order to fly the total pitch moment must be zero almost all the time. Momentary nose up and nose down moments are required to get the aircraft attitude changing. But, then a quick return to zero moment must be initiated.

Most pilots are aware that the main wing produces positive lift while the tail produces negative lift. The movie below shows how these are added to create a "net lift" which opposes weight.