Stabilizer Twist and Speed Stability

Collar and Grinsted (1942) showed that stabilizer setting can have an effect at high airspeeds on the variation with airspeed of the elevator stick forces required for trim. This variation is called speed stability. Push stick forces should be needed for trim at increasing airspeeds, so that if the stick is released, it will come aft, nosing the airplane up and reducing airspeed.

Airplanes with leading-edge-up rigged stabilizers will require trailing-edge-up elevator angles for trim in cruising flight. The up-elevator angles will put a down load on the stabilizer rear spar, tending to twist the stabilizer further in the leading-edge-up direction. Increasing airspeeds will increase the down load and twist, requiring increasing up-elevator angles and pull stick forces. This amounts to speed instability, pull forces needed for trim at increasing airspeeds rather than push forces. The Douglas A2D-1 Sky Shark, with an adjustable stabilizer, had this problem until its elevator tab was rigged trailing-edge up. This caused the elevator to float trailing-edge down in cruising flight and the stabilizer to be carried more leading-edge down, correcting the problem.

A reverse problem can occur on airplanes whose stabilizers are rigged leading-edge down. A leading-edge-down rig is used on some airplanes to improve nose-wheel liftoff for takeoffs at a forward center of gravity. A leading-edge-down rig can lead to excessive speed stability, requiring large push forces to trim in dives. If an airplane with a leading-edge-down stabilizer rig gets into an inadvertent spiral dive and push forces are not supplied, normal acceleration can exceed the structural limit. This effect is thought to be responsible for some in-flight structural failures, unfairly attributed to pilot inexperience in high-performance airplanes. W. H. Phillips considers this may be the cause for some failures of the Beech Bonanza (Phillips, 1998).

An aeroelastic problem related to stabilizer twist is spurious control inputs that result from airframe distortion under maneuvering loads. Fuselage deflection under positive load factor caused control inputs that increased load factor on the Vought F8U-1 airplane (Phillips, 1998). This was destabilizing in maneuvers. Reversing the position of a link in the elevator control system reversed the effect, providing stability instead.