Chapter 6: stability and control

The aircraft has six degrees of freedom, six ways it can move. It has three degrees of freedom in translation (linear motion) which are orthogonal to each other. Components of its velocity along the x, y, and z axes are labeled u, v, and w. Note that lower case is used to avoid confusion with V , which typically has both u and w components. The aircraft also has three degrees of freedom in rotation, also orthogonal to each other.

Figure 6.2 shows the three degrees of freedom in rotation, and the control surfaces which typically produce the moments which cause those rotations. Figure 6.2 (a) shows rotation about the aircraft’s longitudinal (x) axis. This motion is called rolling and the maneuver is called a roll. Control surfaces on the aircraft’s wings called ailerons deflect differentially (one trailing edge up and one trailing edge down) to create more lift on one wing, less on the other, and therefore a net rolling moment.

Figure 6.2 (b) shows the aircraft in a pitch-up maneuver. Rotation of the aircraft about the lateral axis is called pitching. A control surface near the rear of the aircraft called an elevator or stabilator is deflected so that it generates a lift force which, due to its moment arm from the aircraft center of gravity also creates a pitching moment. An elevator is a moveable surface attached to a fixed (immovable) horizontal stabilizer, a small horizontal surface near the tail of the aircraft which acts like the feathers of an arrow to help keep the aircraft pointed in the right direction. A stabilator combines the functions of the horizontal stabilizer and the elevator. The stabilator does not have a fixed portion. It is said to be all-moving.

A number of unique aircraft configurations have given rise to additional types of control surfaces. These often combine the functions of two surfaces in one, and their names are created by combining the names of the two surfaces, just as the name “stabilator” was created by combining “stabilizer” and “elevator.” For example, the surface on the F-16 in Figure 6.2 labeled “aileron” is actually a “flaperon,” because it combines the functions of an aileron and a plain flap (for greater lift) in a single surface. Figure 6.3 (a) shows the French Rafale multi-role fighter aircraft. Pitch control for this aircraft is provided by canards, stabilators placed forward of rather than behind the wings, and elevons, control surfaces at the rear of the wings. Elevons move together to function as elevators and also move differentially like ailerons to provide roll control. Flying wing aircraft, including delta-wing jet fighters such as the Mirage 2000 and Convair F-106 use elevons alone for pitch and roll control. It is interesting to note that the Vought F7U Cutlass twin-jet flying-wing fighter of the 1950’s and 60’s used control surfaces exactly like elevons, but the manufacturer called them “ailerators!” The name did not find as widespread acceptance as “elevons.” Figure 6.3 (b) shows the Beechcraft Bonanza which, unlike most aircraft with separate vertical and horizontal tail surfaces has a V-tail. The moveable control surfaces attached to the fixed surfaces of the V-tail are called “ruddervators,” because they function as elevators when moving together and rudders when moving differentially.