rotorfossil, this old saw will not die, and it is fun every time! The old, pop-corn texts used to train today's instructors were written by pilots who did not understand the issues. An expert can find a dozen substantively wrong statements in the typical "aerodynamic" discussions in training manuals.
Your observation is a continuation of the urban myth. Actually the altimeter jumps because the elephant is screaming and the grass waves because the invisible elephant is struggling below the aircraft.
Seriously, the altimeter is affected in both directions on many helicopters, it goes up first during the lift-off! The effect is mostly due to the screwy hover flow past the static ports, which are not designed for hover readings. Try two things:
1) calculate the amount of equivilent lift due to 30 feet of altitude's worth of pressure, to see if it is the 15% that IGE gains you
2) Look at the altimeter in an OGE hover compared to a fly-by at that altitude to see if the "pressure" is the reason for the movement IGE
3) If there were truly 30 feet worth of pressure under an R-22, and the aircraft were "packing the air down" with that much force, the force exerted on the helo would be nearly enough to shut off the engine and just sit there on the ball of air!
Last edited by NickLappos; 12th Jun 2005 at 19:18.