vfrpilotpb asked:
Why is there such a huge difference in the required HP that you quote from the spinning rotor to the vectored thrust, is it to do with aerodynamics of moving air,or in the case of the Heli is it that the MR can be so much larger than the fan on a vectored thrust system and will be therefore more efficient?
Nick sez:
Both, really. It is locked into the aerodynamics, where the momentum change that generates the lift can be imparted on a big package of air moving slowly (a large rotor) or a smaller package moving quickly. The greater the velocity, the much greater the power needs. Theoretically, if the rotor is infinately large, it could generate lift with virtually no power.
The configuration of the air mover dictates the range of disk loadings that can be achieved. Helos have the advantage, since they have a vast, overarching rotor. Tilt rotors must be packaged so that the rotors cna only have about 40% of the area of an equivilent helo, since the rotors must not pass over the cabin. If a Harrier had a big rotor, it would have an even bigger fuselage, and would weigh tons more than it does.