Plane: X-15 No. 670 Date: August 15, 1960
Flight: 1-11-21 Launch: 0835
Pilot: J.A. Walker Land: 0844
This flight was accomplished as planned. At 50,000 ft., during the climb to altitude, when the angle of attack was reduced at minimum indicated airspeed, a tendency for lateral wallowing was noted, coincident with the reduction of angle of attack. The same impression had been received previously at higher altitudes during pushovers. However it is not difficult to correct this condition.
The large amount of stabilizer required for trim at 8° angle of attack was especially noticeable in the climb between 60,000 and 70,000 ft. The trim appeared to respond to indicated airspeed, which was nearly constant, rather than to the gradually increasing Mach number.
At 72,000 ft. and Mach No. 2.1, pushover to zero g was accomplished. In the course of performing this pushover it was noted that the angle of attack crosspointer sensing was reversed from that desired in that when attempting to fly the airplane to the needle, the needle departed further so that the required correction was to fly away from the needle in order to bring it down to the reference aircraft. This put a kind of a bob in the pushover at the top. It is not significant performance-wise however. As .soon as indicated Mach No. 2.95 was reached, a level pullup to 6 1/2° angle of attack was performed, and held steady. The burning time of the engine appeared to be extending further than we had allowed in flight planning so initiation of bank to go into a left turn was accomplished at which time the engine shutdown with a residual fuel of about .06 fuel remaining indicated. After making certain that the turn was steady at about 6 1/2° angle of attack, the roll and yaw dampers were turned off and an attempted steady turn at this angle of attack was continued. However, a lateral directional oscillation began which gradually built up to an estimated bank angle change from trim of about 20° each side at which time the angle of attack was reduced in order to effect a recovery from the oscillation. At about 4° angle of attack indicated, it was observed that both the lateral and directional modes were becoming damped and the angle of attack was dropped on down in order to accelerate this damping.
As soon as the oscillation was down to a bare minimum, angle of attack was again increased to the vicinity of 7° angle of attack and it was noted that the oscillation built up and continued to diverge in spite of strenuous effort to make certain that the stick was not disturbed laterally. I believe the first oscillation started to damp in the neighborhood of 2.7 Mach number and the second pullup to 7° made at about 2.6 Mach number. As soon as the oscillation amplitude became excessively large the roll and yaw dampers were turned on for recovery and the aircraft was returned to level flight. The dampers were turned off at a low angle of attack and then the pullup to 7° initiated in the neighborhood of 2.4 Mach number. This time there was no oscillation at the elevated angle of attack. This angle of attack was held to approximately Mach 2 at which time essentially 1 g level flight was resumed and the full left rudder sideslip performed at indicated Mach 1.9. There was at least an observable requirement for a right lateral stick deflection from neutral, a very small deflection but at least a movement to the right from neutral necessary to counterbalance the left rudder input initially and at no time did the sense of lateral control to balance the left rudder input change to the left.
Following the sideslip the airplane was returned to trim flight. Pushdown to zero angle of attack, followed by gradual pullup to 12° angle of attack, then a pushdown again to near zero angle of attack followed by a return to near trim was made. At about 1.5 Mach number and near 50,000 ft. the speed brakes were extended to full deflection. The drag effect at this speed from extension of speed brakes was not noted to be excessive, however, considerable lowering of the nose of the aircraft was required to avoid a rapid speed bleedoff and hence altitude was lost during this maneuver, At about 1.2 Mach number the speed brakes were retracted and the remainder of the flight devoted to positioning for landing on the north lakebed runway 18, which landing was of a standard uneventful nature. Estimated touchdown speed was between 190 and 200 kts.
I should like to repeat that during the course of the turns at about
7° angle of attack with the roll and yaw dampers off particular attention
was paid to trying to make sure that inadvertent disturbance of the stick
laterally was not taking place and hence feeding the oscillation. If they
were so taking place in spite of this effort, I am convinced that the aircraft
control system-stability combination is such that it could be considered
to be dynamically unstable in this range regardless.
Aeronautical Research Pilot