NCH-46A Sea Knight

The NCH-46A was a single CH-46A permanently converted for test duties. Of the two US military test pilot schools, only the Naval school at Patuxent River, Md., possessed a rotary wing test pilot curriculum. A variable stability helicopter was therefore an essential training vehicle at the Naval Test Pilot School to supplement flights in the normal inventory of military helicopters. In 1972 the Hover Augmentation System (HAS) of a Navy CH-46A helicopter was modified by Sperry to create a variable stability capability. The modified NCH-46A helicopter control system used limited authority series servos in the longitudinal and lateral axes. This allowed variations in control sensitivity, rotational damping, control lag, attitude feedback, control cross coupling, and attitude rate cross coupling in both the pitch and roll axes. The NCH-46A did not possess a variable feel system.

In 1981, the VSS cmputer and the method of controlling the system in flight were modified by Calspan. These improvements increased the reliability of the system and eliminated the requirement for an on-board engineer to operate the variable stability system. The in-flight simulation capability permitted a wide variety of flight experiments that ranged from investigations in support of flying qualities specification to the evaluation of advanced multi-input, multi-output control laws.