Validation of a Method for Air Resonance Testing of Helicopters at Model Scale Using Active Control of Pylon Dynamic Characteristics

A basic problem inherent in the testing for helicopter air resonance at model scale is the design and fabrication of the pylon support structure to effect both a proper (Froude) scaling and adequate variability of the pylon parameters. Generally, provision for suitable pylon parameter variability (especially the inertias) through passive properties within a properly sealed configuration is often difficult, at best. One method of overcoming this difficulty is to provide for active control of the pylon properties. This is potentially achievable using suitably controlled hydraulic feedback servo‐actuators acting in response to the measured motion of the pylon. The objective of the present study is to investieate analytically the validity of such an apporoach. Formulations are presented describing the modifications required of a basic air resonance theory to account for the dynamics of the selected feedback control network. The results presented compare the air resonance eigensolutions obtained far a full‐scale, free‐flying helicopter to those obtained for various approximations inherent in such a method of model testing.