Roll is the angular motion of an airplane about its centerline, a line of equal distance between the wings through the fuselage. Pitch is the angular motion of an airplane about a line from wingtip to wingtip perpendicular to its centerline.
The primary airplane controls that generate roll are the ailerons, which are located on each side of the wing and are identified as the left and right ailerons. Both ailerons are of the same size and are located at the same distance from the centerline of the airplane. These controls are essentially the same in both expensive, high-performance general aviation jets and low-performance training airplanes. In both, each aileron is attached to its corresponding wing by a hinge. The ailerons deflect upward and downward about the hinge line. When one aileron deflects upward, the other aileron deflects downward. The pilot deflects the ailerons by moving the control wheel. If the control wheel is rotated counter-clockwise, the left aileron moves upward and the right aileron moves downward.
This movement is in contrast to that of the elevator; both right and left elevators move together. The elevator is the primary control for changing the pitch angle or the angle that the centerline makes with the horizontal. The pilot deflects the elevator by moving the control wheel or stick backward and forward. Rearward movement of the wheel raises the elevator and therefore the nose of the airplane.
Even though the ailerons are the primary roll control, the rudder is often moved with the ailerons in making turns. The rudder moves the nose of the airplane in the direction of the lower wing. However, movement of the rudder also can affect the airplane roll. In a turn (to the left, say), the aileron on the left wing is raised and the aileron on the right wing is lowered. In this aileron position, the rudder is moved to the left. This rudder movement pushes the tail to the right and therefore the nose to the left. The force on the tail due to the rudder deflection is shown pointed to the right. A force at the tail to the right will push the nose of the airplane to the left. Since this force due to the rudder is to the right and above the centerline of the airplane, the result is an initial rolling action opposite to that resulting from the aileron movement. Therefore even though the rudder is required to move left to help turn the airplane to the left, there is a secondary effect as soon as the rudder is applied, which detracts from the rolling motion of the ailerons.
However, because the rudder forces the nose to the left, in spite of its contrary rolling effect, the nose is moved to the left of the direction of the oncoming air. This deflection produces a cross-flow coming from the right to the left for the left turn.
Airplane wings are designed with a slight upward bend. This bend is called a wing dihedral, and the angle that the wing makes with the horizontal is called the dihedral angle. As a result of both dihedral angle and cross-flow, the right wing has a slight increase in upward flow, and the left wing has a slight decrease in upward flow. The result is that the lift on the right wing is increased and the lift on the left wing is decreased. The result is a roll angle, left wing down and right wing up, that is in the same direction as that caused by the deflection of the ailerons. The rudder initially causes the airplane to roll in a direction opposite to that of the ailerons; however, the yawing motion of the rudder causes a cross-flow to develop, and that flow, along with the built-in dihedral angle, causes the airplane to roll in the proper direction for the turn: left wing down for turn to the left, right wing down for turn to the right.
An airplane can, however, have an excessive amount of dihedral, the result of which would be that wind gusts from the left or right would cause a rolling motion that would increase with the dihedral angle. The airplane would have an unpleasant rocking motion in response to even small gusts.
As has been pointed out, the rudder can cause the airplane to roll, but the ailerons can cause the airplane to yaw. The aileron can cause the airplane to yaw because there is always a drag associated with lift. If lift is increased, then the drag is increased. When the right aileron is deflected downward, the lift increases on the right wing. At the same time, the left aileron is moved upward, decreasing the lift on the left wing. Thus, there is an increase in drag on the right wing and a decrease in drag on the left wing, which will cause the nose of the airplane to swing to the right. Because the ailerons are moved to turn to the left, the yaw that results from aileron deflection is called adverse yaw. The main purpose of the rudder is to counteract this adverse yaw. When the airplane rolls to the left, the nose-right adverse yaw of the ailerons is countered by moving the rudder left.
The primary roll control on an airplane is managed by the ailerons, one on each wing. The pilot controls airplane roll by rotating the control wheel in the direction of the desired roll. Aileron deflection produces an adverse yaw, which is countered by the rudder. Pitch is controlled by movement of the elevator, moved in turn by the pilot by a backward, or nose-up, and forward, or nose-down, movement of the control wheel.
Raymer, Daniel P. Aircraft Design: A Conceptual Approach. 3d ed. Reston, Va.: American Institute of Aeronautics and Astronautics, 1999. A highly recommended, comprehensive, and up-to-date book on airplane design, directed at the engineering student, but featuring many sections requiring little more than high school algebra. Stinton, Darrel. The Design of the Airplane. New York: Van Nostrand-Reinhold, 1985. An excellent introduction to airplane design. Taylor, John W. R. The Lore of Flight. New York: Crescent Books, 1974. A massive, well-illustrated, oversized book featuring nontechnical descriptions of airplanes and spacecraft, and covering controls and cockpit instruments.
Ailerons and flaps
Flight control systems
Forces of flight
The F-15B is equipped with nozzles that allow it to control both pitch and yaw.