I really miss Lu during these discussions....I can just read his response about helicopters rolling off the back of ships in the '50s, etc, etc, etc, etc, etc.......
I am finding this thread fascinating, with some good educational points for me. Firstly, I guess it is fair to answer Dave's original question of
"During autorotation to touchdown, does ground-effect reduce the final descent rate by any significant amount?"
with a widely agreed "NO, not significantly".
There are some points raised that I would like to hear more about, in particular, the ground effect Nick has likened to a low flying aeroplane. Having conducted a few hours low level (<10ft AGL) I have not noticed any reduction in power required to fly level Vs flight at say 200ft AGL (above translational speeds of course). But I will test this theory at my earliest. On the other hand, I have seen the effect demonstrated in a fixed wing, and seen the telltale swirls over the water, even when going more than 350kts. Over about 40 kts do not recall seeing the same on the water behind a helo, but I will certainly pay more attention in the future.
Does the ground effect we are talking about here have to do with the cord, span and camber of the wing? I.E. the helo blade is so much smaller than the fixed wing "blade", thus it produces an entirely different pressure wave below and behind the wing. Also, the very next blade is slicing through the vorteicey's created by the preceeding blades and mixing the pressure areas up (plus fusleage and tail rotor disturbances), unlike a fixed wing that has no interference and can build a more constant effect. In otherwords, does the helo in it's forward flight have too much mixing of vorticey's to allow a fixed wing like ground effect to develop?
Perhaps the concept of time, as many of you have stated above applies here. The helicopter in the IGE hover has had enough time for ground effect to develop due to the consistants such as downwash dissipation over close proximity ground, height, wind, and pitch. For example, go to an IGE hover and note the TQ. Now do a quickstop to the same spot with a termination to the same heigh, but only pull the same TQ as previously noted. Ouch!
But, if your helo could take it like the UH60 can (due to it's undercarriage design) you will eventually rise back up to the previous height. This demonstrates that the ground effect is always there, but it takes a finite time to develop into a power reduction that helps you. Thus in the auto, I reckon you would never notice it, and I would be suprised if it was even measurable. Besides, if I am going to take my mind of sex it will be to avoid heavy ground contact, not to try and notice any ground effect!!
Waddya reckon?