MISCELLANEOUS STABILITY PROBLEMS

There are several general problems of flying which involve certain principles of stability as well as specific areas of longitudinal, direc­tional and lateral stability. Various condi­tions of flight will exist in which certain problems of stability (or instability) are un­avoidable for some reason or another. Many of the following items deserve consideration because of the possible unsafe condition of flight and the contribution to an aircraft accident.

LANDING GEAR CONFIGURATIONS

There are three general configurations for the aircraft landing gear; the tricycle, bicycle, and “conventional” tail wheel arrangement. At low rolling speeds where the airplane aerody­namic forces are negligible, the “control-fixed” static stability of each of these configurations is determined by the side force characteristics of the tires and is not a significant problem.

The instability which allows ground loops in an aircraft with a conventional tail wheel landing gear is quite basic and can be appre­ciated from the illustration of figure 4-31. Cen­trifugal force produced by a turn must be balanced and the aircraft placed in equilibrium. The greatest side force is produced at the main wheels but to achieve equilibrium with the

center of gravity aft of the main wheels a bal­ancing load on the tail wheel must be produced toward the center of turn. When the tail wheel is free to swivel, the equilibrium of the turn requires a control force opposite to the direction of turn—i. e,, control force insta­bility. The inherent stability problem exists because the center of gravity is aft of the point where the main side forces are developed. This condition is analogous to the case of static longitudinal stability with the center of gravity aft of the neutral point.

The conventional tail wheel configuration has this basic instability or ground loop tend­ency which must be stabilized by the pilot. At high rolling speeds where aerodynamic forces are significant, the aerodynamic direc­tional stability of the airplane resists the ground looping tendency. The most likely times for a ground loop exist when rolling speeds are not high enough to provide a con­tribution of the aerodynamic forces. When the tail wheel is free to swivel or when the normal force on the tail wheel is small, lack of pilot attention can allow the ground loop to take place.

The tricycle landing gear configuration has an inherent stability due to the relative posi­tion of the main wheels and the center of gravity. Centrifugal force produced by a turn is balanced by the side force on the main wheels and a side force on the nose wheel in the direction of turn. Note that the freeing the nose wheel to swivel produces moments which bring the aircraft out of the turn. Thus, the tricycle configuration has a basic stability which is given evidence by control displace­ment and a wheel side force in the direction of turn. Because of the contrast in stability, the tricycle configuration is much less difficult to maneuver than the tail wheel configuration and does not provide an inherent ground loop tendency. However, a steerable nose wheel is usually necessary to provide satisfactory maneuvering capabilities.

The bicycle configuration of landing gear has stability characteristics more like the automobile. If directional control is ac­complished with the front wheels operated by power controls, no stability problem exists at low speeds. A problem can exist when the airplane is at high speeds because of a distribu­tion of normal force being different from the ordinary static weight distribution. If the airplane is held onto the runway at speeds well above the normal takeoff and landing speeds, the front wheels carry a greater than ordinary amount of normal force and a tend­ency for instability exists. However, at these same high speeds the rudder is quite powerful and the condition is usually well within control.

The basically stable nature of the tricycle and bicycle landing gear configurations is best appreciated by the ease of control and ground maneuvering of the airplane. Operation of a conventional tail wheel configuration after considerable experience with tricycle configu­rations requires careful consideration of the stability that must be furnished by the pilot during ground maneuvering.