Designer's Notebook

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.

The diagnosis of pitch sensitivity problems boils down to one of two things: too little static margin or control system problems. This month we discuss how to modify the airplane to fix them;

This month we use the accumulated flight-test data to determine what exactly is causing the unacceptable flying qualities of the airplane so that the best way to fix the problem can be identified;

Flight testing for longitudinal stability problems continues this month as columnist Barnaby Wainfan details tests for stick force as related to trim and stick-free stability of an aircraft. The object is to gather data and analyze it to see if the hypotheses about the nature of the problem hold up under scrutiny.