Flight Control Coupling

Flight control coupling is a slow loss of control of an airplane because of a unique combination of static stability and control effectiveness. Day described control coupling—the second mode of dynamic coupling—as " a coupling of static yaw and roll stability and control moments which can produce untrimmability, control reversal, or pilot-induced oscilla­tion (PIO).”[742] So-called "adverse yaw” is a common phenomenon associ­ated with control of an aircraft equipped with ailerons. The down-going aileron creates an increase in lift and drag for one wing, while the up – going aileron creates a decrease in lift and drag for the opposite wing. The change in lift causes the airplane to roll toward the up-going aile­ron. The change in drag, however, results in the nose of the airplane swinging away from the direction of the roll (adverse yaw). If the air­plane exhibits strong dihedral effect (roll produced by sideslip, a quality more pronounced in a swept wing design), the sideslip produced by the aileron deflections will tend to detract from the commanded roll. In the extreme case, with high dihedral effect and strong adverse yaw, the roll can actually reverse, and the airplane will roll in the opposite direction to that commanded by the pilot—as sometimes happened with the Boeing

B-47, though by aeroelastic twisting of a wing because of air loads. If the pilot responds by adding more aileron deflection, the roll reversal and sideslip will increase, and the airplane could go out of control.

As discussed previously, the most dramatic incident of control cou­pling occurred during the last flight of the X-2 rocket-powered research airplane in September 1956. The dihedral effect for the X-2 was quite strong because of the influence of wing sweep rather than the existence of actual wing dihedral. Dihedral effect because of wing sweep is non­existent at zero-lift but increases proportionally as the angle of attack of the wing increases. After the rocket burned out, which occurred at the end of a ballistic, zero-lift trajectory, the pilot started a gradual turn by applying aileron. He also increased the angle of attack slightly to facili­tate the turn, and the airplane entered a region of roll reversal. The side­slip increased until the airplane went out of control, tumbling violently. The data from this accident were fully recovered, and the maneuver was analyzed extensively by the NACA, resulting in a better understanding of the control-coupling phenomenon. The concept of a control parame­ter was subsequently created by the NACA and introduced to the indus­try. This was a simple equation that predicted the boundary conditions for aileron reversal based on four stability derivatives. When the yaw­ing moment due to sideslip divided by the yawing moment due to aile­ron is equal to the rolling moment due to sideslip divided by the rolling moment due to aileron, the airplane remains in balance and aileron deflection will not cause the airplane to roll in either direction.[743]