Several discrepancies were encountered during the airborne analyzer check of the MH-96 flight control system. Most reflected errors in the reaction controls and the auto pilot system and were not of concern for this flight. Another test which the analyzer failed to pass was the transient response of the model output. Since the steady state model output checked satisfactorily, it was decided to continue with the flight. No surface drift was reported from the time of engaging the MH-96 flight control system until drop 3 minutes later by either the chase or a telemetry ground monitoring personnel. At launch, all 3 axes of the flight control system disengaged and a severe right roll occurred with accompanying yaw and pitch excursions. After the engine was lighted and pullup initiated, one yaw pulse was performed on the basic airplane, after which the flight control system was reengaged in each axis without transients.

After establishing the climb angle, yaw pulses and some small roll steps were performed. The yaw damping was deadbeat and the roll steps showed little or no overshoot. During the climb, at approximately 500 knots indicated airspeed, a moderate amplitude limit cycle revealed itself in the yaw axis at about 10 cycles per second. The fixed gain mode of the yaw channel system was selected and the limit cycle disappeared. The limit cycle manifested itself at the cockpit by structural vibrations and feedback through both the stick and rudder pedals.

Since X-15 radio reception was extremely poor, the pushover was initiated by the pilot on his own clock. The aircraft came level at about 80,000 feet indicated inertial altitude and engine shutdown was performed at 105 seconds at 3600 fps indicated by the pilot and his own clock. Immediately after shutdown, the yaw axis was re-engaged in the adaptive mode without transients and without limit cycle. A yaw pulse was performed at about 3.7 Mach number with a deadbeat damping. A pitch pulse damped to 1/10 amplitude in 1 - 1 1/2 cycles. Roll steps from O to 45° left bank angle were performed at 6° angle of attack and about 40° per second with excellent roll damping as evidenced by no noticeable roll overshoot. A roll from 45° left to approximately 30° right wing down bank angle was performed at l0° angle of attack with similar damping. Auto trim operation was particularly pleasing being almost unnoticeable except for stick motion.

The angle of attack hold mode was energized but the aircraft did not maintain the selected angle of attack. Bank angle hold was selected and the switch remained engaged but the airplane maintained a steady slow left roll rate as if no reference were available. At this time it was noted that the roll trim knob had moved from its initial wings-level position to full left roll. Pitch attitude hold mode was attempted but the appropriate solenoid-held switch would not remain engaged.

The gains in each axis were then reduced to minimum by energizing the fixed gain selector switches on the straddle panel. Pitch and yaw pulses and roll steps were performed on fixed gains at a Mach number of approximately 2. The dynamics of these maneuvers showed the damping to be approximately that predicted by simulator results. The pitch axis could not be re-engaged in the adaptive mode, but the adaptive roll and yaw channels were re engaged without transient.

While decelerating through approximately 250-300 knots, a limit cycle, apparently the same as the one previously encountered, reappeared in the yaw channel. The yaw axis was disengaged for the remainder of the flight to prevent any re-occurrence of limit cycle during the landing pattern. The airplane was slowed to approximately 200 knots with the flaps up and several small pitch steps and yaw pulses were recorded to evaluate the response of the airplane in the approach configuration. The handling qualities were satisfactory and this configuration (roll-adaptive, pitch-fixed gain, yaw-disengaged) was retained for the approach and touchdown. Although it was expected, the lack of trim change, associated with flap and gear extension, was surprising, as was the amount of corresponding stick motion in the cockpit resulting from auto trim operation. Although skid contact seemed quite gentle, nose wheel touchdown and impact seemed to be somewhat harder than that of previous flights in #1 airplane. It was attempted to keep the runout along the painted right hand stripe of the runway. With a light to moderate left cross wind, the airplane finally veered to the right of the painted stripe and could not be returned.
 
 

(Signed)

Neil A. Armstrong

Aeronautical Research Pilot

NAA:bjc