Trim Changes Due to Compressibility

One compressibility effect on airplane stability and control that was not done away with by wing sweepback or thin wings was the longitudinal trim change when accelerating or decelerating through the speed of sound, or Mach 1. This was a much more severe problem for the Lockheed P-38 and its contemporaries. But along with swept wings designers began to have the hardware available for electronic trim change compensation. Ideally, this is done either on a different longitudinal control surface than the one hooked up to the pilot’s cockpit controller or by a series connection for the compensation system between the artificial feel system in the cockpit and the actuator at the control surface.

The nose-down longitudinal trim change or “tuck under” near Mach 1 was a particular problem for the Douglas F4D-1 Skyray, used by the U. S. Navy in 1953 for an assault on the world’s speed record. The F4D-1, later called the F-6, had fully powered elevon controls but no Mach trim change compensation.

The speed record flights were made by test pilot Robert O. Rahn at very low altitudes over a measured course at Edwards Air Force Base in California. The low altitudes at which the compressibility trim change occurred exaggerated its effect. At Mach 1, sea level, the F4D-1 changed load factor or g by about 1.5 for each degree change in angle of attack. At the highest speed attained, Rahn used a pull force to overcome the nose-down trim change. At the end of the runs, turning to return to the course, speed dropped off and a push force was required. This of course was contrary to the usual pull control forces required in turns.

When the F4D-1 was fitted with a higher powered engine, the J57-P-2, Rahn flew the new version to maximum speed at low altitude. The airplane reached a Mach number of 0.98, 500 feet over the ocean. This time Rahn used the F4D’s trim surfaces in the nose-up direction to overcome the diving tendency near Mach 1. This provided more precise path control at that tremendous speed close to the water. However, when Rahn cut off the afterburner to decelerate the airplane, the nose-up trim setting produced an uncontrollable pullup to 9.1 g. The airplane was overstressed and badly buckled but landable.

Flight tests of a well-instrumented North American F-86 Sabre provide an unusually good look at the transonic trim change problem (Anderson and Bray, 1955). The measurements show marked increases in longitudinal static stability and decreases in elevator control power as the Mach number increases from 0.94 to 0.97. The record of a dive pullout (Figure 11.12) shows a trim change when the F-86 traversed the same Mach number range, slowing down in a dive pullout.

The transonic trim change problem also was experienced with the North American F-100 Super Sabre, although less dramatically In unpublished correspondence Paul H. Anderson recalls these events:

The first complaint was that the airplane could not be trimmed at cruise speed. Much time and effort was spent redesigning and flight testing modifications to the trim system (with no improvement) until we finally recognized what was happening. The answer, of course, was to feed back Mach number to the flight control system. . . .

Prior to that, the slope of stabilizer position versus speed was called static longitudinal stability. When the aerodynamic center shift [with Mach number] was encountered some people said that the airplane was statically unstable, when it was actually more stable than before. We finally changed the name of [the] stabilizer position versus speed to Speed Stability and the problem went away.

Mach trim compensators as separate systems continued to be features of transonic airplanes for many years, up to the advent of integrated fly-by-wire control systems. As an

Trim Changes Due to Compressibility

Figure 11.12 Transonic trim change of the North American F-86 Sabre. At time equal to 5 seconds, normal acceleration continues to increase although the elevator is moving down. This is because the Mach number is reducing from 0.97 to 0.94 and the airplane is becoming less stable. (From Anderson and Bray, NACA Rept. 1237, 1955)

example of one of the older separate Mach trim compensators, the Boeing 707 transport has an automatic Mach trimmer that puts in 2 degrees of nose-up stabilizer trim starting at Mach number 0.82. In modern integrated fly-by-wire control systems Mach trim change compensation is just one of the many stability augmentation programs in a flight control computer.

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