Helicopter Downwash
As one who has had the experience of standing beneath a Sea Stallion Helicopter as it turned through 90 degrees at 50ft altitude just before landing,
I can vouchsafe that it felt to me as if the air mass equivalent to the whole weight of the machine plus a percentage more to allow for the turn was washed down by the rotors as cyclic pitch plus power was applied by the pilot. Not very scientific, I agree, but a practical demonstration I shall never forget.