Heave Stability and Heave Damping of Ground Effect Machines - Thick Annular Jet and Plenum Types

摘要

The report describes an experimental investigation of the heave stability and heave damping characteristics in hovering flight of an electrically powered model Ground Effect Machine, arranged firstly with a thick annular jet and, secondly, as a simple plenum chamber. The lift, rise height and power relationship were determined and the fan characteristics of the thick annular jet were measured. Air was bled from and fed into the air cushion to evaluate the GEM's change in lift. Finally, the machine was allowed to hover at a specific height and then forced to oscillate. From this the heave stiffness and damping ratio were determined. The results show that the physical basis of the theory is correct and that good estimates may be made of the heave characteristics. However, it is suspected that other derivatives may be present, especially in the case of the plenum chamber, where unstable oscillations in a narrow attitude band were observed, and in any case the measured value of 2 damping ratio/undamped frequency in heave was much less than theory would predict. It is considered that the latter result was not due to an unstable fan characteristic, but rather to an instability of flow in the plenum chamber inself. (Author)