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Low Frequency Instabilities of Free Systems

Report Number: ASD TDR 63-655
Author: Robert G. Schwendler and Jack H. Hill
Corporate Author: Computer Engineering Associates
Laboratory: AF Flight Dynamics Laboratory
Date of Publication: 1963-11
Pages: 127
Contract: AF 33(657)-8293
Project: 8219
Task: 821901
AD Number: AD0426543

This report contains an investigation of "mode interaction" instabilities of aircraft. "Mode interaction" refers to coupling between an elastic mode and a rigid body mode of the free system. Three rather general airframe configurations are analyzed in detail. It is shown that systems which tend toward steady state divergence are particularly susceptible to mode interaction. Also, that aerodynamic damping terms can have a destabilizing effect upon a free system. The analyses show that aeroelastic systems which possess no finite frequency elastic mode can be susceptible to a finite frequency instability. Computer studies of several different airframe configurations are discussed. An appendix to the report contains stability charts for an aircraft having two rigid body modes and one elastic mode.

Lockheed Martin Missiles & Fire Control
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