LATERAL DIRECTIONAL STABILITY AND THE MH-96 FLIGHT CONTROL SYSTEM

As expected on the basis of experience with the earlier supersonic X-airplanes, the lateral – directional stability of the X-15 decreased as the Mach number rose to supersonic and hypersonic
speeds. Honeywell’s adaptive control system automatically compensated for the aircraft’s unstable lateral-directional behavior in various flight regimes, and it utilized the combined operation of the aerodynamic control surfaces and the rocket reaction controls in their respective regions of flight.

Originally, the vertical tail sections above and below the airplane were large. That section, located below the airplane, is called the ventral tail. Wind tunnel data showed a need for a large ventral tail, so large that it would hit the ground first before the landing skids. This necessitated designing the bottom part of the ventral to be ejected prior to landing. The flight data showed a lesser need for the large area of the ventral tail, and in subsequent flights the bottom half was left off.

A relationship between the wind tunnel data and the flight data was thus established. The Honeywell MH-96 adaptive control system allowed the airplane, unstable in certain regions of flight, to be operated in a conventional manner throughout. Moreover, it provided an automatic transition from the conventional aerodynamic control system (rudder, elevator, etc.) used within the sensible atmosphere to the reaction control system for high- altitude flight, where the aerodynamic forces were too weak. This relieved the pilot from manually making this change, both on ascent to high altitudes and back again for descent.