vfr: It's down to energy consumption.. .. .To get the same reaction, you can change the momentum of a large mass of air by a small amount, or you can take a small mass of air and change its momentum by a large amount.. .. .Since momentum = mv but kinetic energy = 1/2 mv^2, the second method uses much more energy to achieve the same amount of lift.. .. .The most extreme example of this is the Harrier, which shoves a tiny mass of air out of the nozzles at very high speed - but you'll run out of fuel after just a few minutes' hover.. .. .In forward flight, the Osprey and Harrier use the wings to deflect a larger amount of air, improving fuel efficiency. . .. .In a helicopter you need a large diameter rotor disc to achieve the same efficiency.