Interestingly in the book "Dynamics of Helicopter flight" By G Saunders, there is a graph which shows, presumably as a result of tunnel investigation, that the most severe symptoms of VR occur when the helicopter is descending at angles between approximately 80 and 40 degrees to the vertical, not in true vertical descent relative to the air but without any explanation of why. He quotes a reference to J Wolkovitch and R P Walton, "Stability and Control of Helicopters in Steep Approaches. USAAVLABS Technical Report 70-74A.
Last edited by rotorfossil; 3rd Jul 2012 at 15:06.