Thank you all for these fascinating and informative replies. It's good to know I'm not alone in this.. .I'm not sure that theories about the lack of apparent groundspeed hold good. After all, this phenomenon does not affect me when I'm flying a fixed-wing. I suspect it's something to do with the instability of the helicopter, the feeling that you're balancing on the head of a pin.... .A guy I spoke to last weekend about this says he had an instructor who looked over the side, on one of his early student flights, and said: "We're about four screams up here... go lower."