1. Flight No. 3-9-18 was flown on August 14, 1962, by Joseph A. Walker for the purpose of evaluating a constant zero-degree pitch attitude entry from 220,000 feet and to investigate the effect of lateral control inputs on the lateral-directional stability at high angles of attack during reentry. It was intended that only the yaw damper would be operated on fixed gain with both pitch and roll dampers operating on adaptive. Because of an inadvertent disengagement of the roll damper, the research objectives were not accomplished as planned. However, data were obtained which will permit an evaluation of the lateral control input effects on lateral-directional stability. There were no malfunctions or component failures in the MH-96 FCS during this flight.

2. Pre-launch operations were normal except for a small pitch rate mistrim (side stick dial trim) which resulted in an analyzer indication of a temporary malfunction on a pitch response to a fixed gain command. (Test #8)

3. The flight, from launch to maximum altitude, was accomplished according to plan and the FCS performance was satisfactory. Handling qualities during launch and rotation were rated: pitch - 2, roll - 1, and yaw - 2. After burnout, the handling qualities were rated: pitch - 1.5, roll - 1.5, and yaw - 1. The maximum altitude was somewhat lower than expected with the pilot's indicator reading about 195,000 feet. The velocity at maximum altitude was an indicated 5400 fps.

4. Immediately before reaching the maximum altitude, at a pitch attitude of 8° and an angle of attack of about 1°, the pitch attitude hold mode was engaged. Subsequent to reaching the peak, the pitch attitude hold mode was trimmed to hold 5.5 to 6° nose up. During this part of the pitch hold operation, angle of attack increased from 1° to 26.5° while pitch attitude was 5.5° ±3°. The speed brakes were extended during this period also. After speed brake extension, the stabilizers periodically limited at the -35° limit with an average stabilizer deflection of about -30°.

5. As the entry progressed to the point where normal acceleration was one "g," fixed gain yaw damper operation was selected and the automatic reaction controls were turned off. A mild rudder pulse was then performed at an angle of attack of about 24°, which resulted in a mild but slowly diverging sideslip oscillation with a period of about 2.5 seconds. The peak-to-peak sideslip amplitude did not exceed 2° of diverge rapidly until lateral control inputs were made. However, the divergence was sufficient to induce the pilot to reduce angle of attack and reengage adaptive yaw damper. Coincident with reengaging adaptive yaw damper, the roll damper was inadvertently disengaged, resulting in a severe lateral-directional oscillation. The oscillation subsided when angle of attack was reduced to less than 10° and reappeared three times as the angle of attack was increased above 10° and lateral control inputs became large. The periodic increase in angle of attack was caused by the pitch attitude hold mode attempting to maintain an attitude of 5° nose up. The maximum values of pertinent parameters are listed below.
 
 

6. As a result of this experience with the MH-96 FCS, two modifications will be made to the system. The first modification will be the installation, on the primary instrument panel, of a flashing annunciator light that will give warning to the pilot that one or more damper channels are disengaged. The second modification will be the installation of open-channel switch guards for each of the damper engage-disengage switches to prevent inadvertent switch operation.
 
 

Elmor J. Adkins

Aerospace Engineer