Question

Which of these is an important parameter for selecting the rotor blade based on aerodynamic efficiency?

(a) Solidity ratio

(b) Structural ratio

(c) Figure of merit

(d) Hovering ratio

I got this question during an interview for a job.

My question is based upon Blades and Propellers in section Helicopter Dynamics of Aerodynamics

Answer 1

Correct option is (c) Figure of merit

The explanation: Figure of merit is one of an essential measure for choosing the rotor based on the aerodynamic efficiency. It is given by the formula:

M=\(\frac {1}{Κ+\frac {3}{4}\frac {c_{d_0}/\overline {c}_l}{\lambda _h}}\)

Where, Κ is an empirical factor which accounts for losses in the rotor

cl is mean lift coefficient

cd0 is profile drag

High value of the figure of merit is achieved when the airfoil has low drag for high lift coefficient.