Aircraft elastic mode control

I 1914, a patent was granted for a "Stabilizing Device for Flying Machines", which would "counteract the disturbance (gust) and prevent it from having an injurious effect on the stability of the machine." Since then over thirty other U.S. patents related to gust load alleviation have been issued. During the same time period, advances in aerodynamic, structural, propulsion, and control systems technology have resulted in two orders of magnitude increase in speed and altitude regimes of aircraft flight. This performance increase has not been matched by improved aircraft gust response characteristics. Since atmospheric turbulence cannot be avoided entirely, airplanes have been constructed to withstand gust loads. The aircraft dynamic response resulting from gust inputs is usually considered only for determining the estimated additional structural strength needed for survival. At the present time about one structural failure in 10 flying hours is expected and accepted. As yet, the benefits to be gained from gust alleviation control systems have not been realized since such systems have not been applied to a production airplane. A spectacular illustration of the effects of gusts on aircraft occurred on January 10, 1964. A B-52H flying at low altitude encountered a patch of severe turbulence. During the first 5.7 sec, its rigid-body and elastic mode dynamic responses built up until, under a combination of rigid-body and elastic mode excursions, the tail loads exceeded ultimate design values. This B-52 lost 85% of its vertical tail because of the gusts with an estimated peak velocity of 120 fps. Under these circumstances its yaw damper was saturated, resulting in virtually an unaugmented rigid-body dynamic response. The gusts did not fail the tail, but they excited responses that did. Amazinglyj the pilot was able to land this airplane. The effects of turbulence on large flexible aircraft have led to concern in four major areas: ride qualities (crew and passenger comfort as a function of vibration level), structural fatigue life, peak structural loads, and handling qualities. There is also the additional concern of aircraft upsets which on occasion has resulted in disastrous vehicle-pilot-control system interaction.' With the advent of highly flexible airplanes, such as the B-52, XB-70, C-5A, and SST, there is a need for more than rigid-body gust alleviation. Turbulence feeds as much or more energy into the lower-frequency normal elastic modes of these type airplanes as it does into the rigidbody dynamics. Thus, there is a demonstrated need for elastic mode control systems which can suppress the level of normal accelerations at selected locations on the structure. This paper will attempt to survey the research and development which has been done on such systems. It is limited to results in the open literature; although, significant work has been done and documented in several classified reports. Rigid-body gust alleviation research over the years has been extensive and varied, notwithstanding the fact that no system has been incorporated into a production airplane. In 1938, Reiie Hirsch worked out a system which was test flown much later in 1954. In 1950, Douglas Aircraft Company was flying a C-47 equipped with gust alleviation flaps. In 1954, Cornell Aeronautical Laboratory flew a lateral-directional system in a PT-26. NACA performed a significant series of demonstration flights in a C-45 starting in 1952.In England, a Lancaster bomber was flown with a flap gust