CH-46C Sea Knight

In theory, there could have been a CH-46B and C, but, as was not uncommon, changes could be proposed, and a designation assigned and then no aircraft of that designation, or possibly only a test aircraft, ever built. The CH-46B was never built, and only a single CH-46C was built for NASA. Some military purchasers might accept a modification while others did not. Hence, helicopters in the Marine Corps did not always have their letter designations in strict sequence, and the CH-46 jumped from an "A" version directly to the "D".

By several accounts, HC-1A was the initial designation for the CH-46C, and the HC-1B is the Chinook. NASA published a series of reports in 1970 addressing a flight control software package for digital flight control and landing system of a CH-46C helicopter. This Boeing Vertol 107 helicopter was used by NASA Langley in flight tests at Wallops Island through 1972.

Although flight directors had come into widespread use by the late 1960s for performing instrument approaches with conventional aircraft, their application to VTOL aircraft was very limited. Early attempts at adapting the flight-director concept to VTOL aircraft were hampered by the poor low-speed handling characteristics of the vehicles employed. Without adequate stabilization, the vehicle motions that were reflected back into the flight-director commands produced high-frequency commands with which the pilot could not cope.

With the advent of high-gain control augmentation systems, VTOL aircraft can be provided with virtually any level of stabilization required for a given task. It appears desirable, therefore, to reevaluate the application of flight directors to VTOL aircraft in which some form of stabilization is used to lower the frequency content of the required control inputs. A flight investigation was therefore initiated by NASA to determine the potential instrument-approach capability afforded by a VTOL-oriented flight director used in conjunction with a high-gain control augmentation system.

The tests were conducted with a CH-46C research helicopter employing a "fly-by-wire" attitude command system designed to provide inner-loop stabilization which allows the pilot to concentrate more fully on following the guidance information. The helicopter was equipped with a control augmentation system which utilizes a central analog computer to process the pilot's control input signals and various sensor signals. The computer outputs position the control surfaces of the helicopter by means of electrohydraulic actuators. The control augmentation system provides attitude stabilization in pitch and roll and an automatic turn-following system in yaw. The system also suppresses the response of the vehicle to external disturbances with a level of resistance which is essentially independent of the vehicle response characteristics to pilot control inputs.

Power (collective), roll, and pitch flight-director commands were presented for the control of glide slope, localizer, and speed, respectively. The tests involved only the constant-speed approach capabilities afforded by the system, although the pitch command could be easily programed to direct a decelerating approach to a hover. The approaches were flown under simulated instrument (hooded) conditions at a constant ground speed of 45 knots along a straight-in 6O glide path to an altitude of 50 ft (15 m).