Designer's Notebook

An airplane's horizontal tail is critical to stabilizing the airplane in pitch, trimming out pitching moments caused by the wing and providing control power for maneuverability.

Multiple engines complicate design, because not only must the airplane be able to maintain flight on one engine, but also the pilot must be able to control it. Lateral/directional stability issues are key;

In order to bank the airplane and execute a turn properly, adverse yaw must be countered or eliminate altogether. The discussion of how to do this includes rudder control, aileron-rudder interconnects, directional stability, differential ailerons, aileron drag and the use of spoilers;

The aerodynamics of an airplane sometimes don't cooperate with the pilots desire to roll, and instead produce both yaw and roll. Contributing factors are aileron parasite drag, induced drag between the wings and changes in the roll rate itself;

Yaw rarely occurs without rolling, and roll rarely occurs without yawing. This month Barnaby Wainfan discusses coupled motion, including adverse yaw, the spiral mode and Dutch roll.

This month columnist Barnaby Wainfan takes a look at how the airplane responds in roll when the pilot is maneuvering. Two key factors are roll acceleration and the steady-state roll rate.

The discussion of lateral/directional stability continues with an examination of dihedral effect and the effect of roll;

Pitch is not the only axis involved in making an airplane fly well. Yaw and roll are also important, and this month Barnaby Wainfan turns his attention to lateral/directional stability and its effect on flying qualities.

When is an airplane in trim? At least to the pilot, the airplane is trimmed when no stick force is required to maintain equilibrium. So how do we achieve this? Barnaby Wainfan explains any number of ways to go about it, including the use of trimtabs, spring systems, sparrow strainers and variable incidence tails.

This month Barnaby Wainfan turns his attention to the control system and how various aspects such as linkages, the elevator planform and tail aspect ratio, and trailing-edge treatments affect pitch control.