Question

In order to increase the hovering efficiency, which parameter must be taken care of?

(a) High power by thrust ratio

(b) High thrust by area ratio

(c) Low induced drag

(d) High rotor tip speed

This question was addressed to me during an interview.

The origin of the question is Momentum Theory topic in section Helicopter Dynamics of Aerodynamics

Answer 1

The correct choice is (a) High power by thrust ratio

The best explanation: According to the momentum theory, the induced power by thrust ratio is given by:

\(\frac {P}{T}=\sqrt {\frac {T}{2\rho A}}\)

Thus, for low inflow velocity and low induced power loss, air must be accelerated through the rotor by virtue of pressure difference. For improving hovering efficiency, the value of P/T must be small thus improving fuel efficiency and decreasing engine’s weight.