CH-46E helicopter automatic flight control system evaluation

This thesis seeks to validate that the proposed production Automatic Flight Control System (AFCS) computers installed in the CH-46E helicopter reduce pitch extensible link actuator oscillations and provide the same aircraft handling qualities as the current AFCS computers. The testing was conducted in two phases at Naval Air Station Patuxent River, MD.

The first phase consisted of ground testing during September 1995. The ground testing included electric-hammer rap tests on the stiffened AFCS computer-mounting shelf to document the vibratory characteristics, and frequency response testing of the pitch and roll extensible link actuators to document the frequency response differences between the test aircraft and the baseline aircraft. The second phase consisted of flight tests that were conducted with the proposed production -5 AFCS computers and the current -4 AFCS computers installed. Flight tests were conducted from November 1995 to April 1996 in a fleet representative CH-46E helicopter with the exception of the installed instrumentation package. Flight-testing included classic helicopter flying qualities testing techniques to include static and dynamic control response testing and fleet representative mission maneuvers.

The stiffened AFCS shelf increased the m1mmum resonant frequencies to a minimum of 39 Hz; well above the aircraft's resonant frequencies of 13.2 and 26.4 Hz. During flight conditions that had been determined to cause pitch chatter and during other mission representative flight maneuvers, the -5 AFCS computers reduced pitch extensible link actuator (PELA) oscillations and attenuated high frequency amplitudes from the levels exhibited by the test aircraft configured with -4 AFCS computers. Though not proved conclusively, the aircraft's weight appeared to have an influence on PELA response, a heavier aircraft causing less PELA frequency content. The flying qualities of the aircraft with the -5 AFCS computers installed were the same as those exhibited by the current -4 AFCS computers.

A recommendation was made to replace the -4 AFCS computers with the -5 AFCS computers. Additionally, recommended that further testing be conducted with -5 AFCS computers:

I) at differing aircraft gross weights to determine the extent that aircraft weight influences PELA frequency responses for all mission maneuvers, and

2) on an aircraft with a known history of pitch chatter while in the -4 AFCS computer configuration to confirm pitch chatter elimination.