Why is blade element theory preferred over momentum theory for designing a propeller?

Correct Answer : Option (A) :   Plot of power coefficient as a function of thrust coefficient

Explanation : Plot of power coefficient Cp/σ as a function of thrust coefficient CT/σ is known as rotor polar. There is no loss of profile power for an ideal rotor and there is minimum induced loss. Due to profile power loss, the rotor polar for a real rotor is at an offset compared to the ideal polar, and power increases faster with thrust coefficient due to the larger induced power.

Correct Answer : Option (D) :   0.05-0.07

Explanation : Inflow ratio is a nondimensional quantity that is used to compare the results from different rotor blades. The formula for inflow ratio is given by:

Where, V is the climb velocity which is zero for hovering
v is the induced velocity
Ω is the rate of rotation of the blade
R is the radius
The value of inflow ratio ranges between 0.05 and 0.07 for hovering.

Correct Answer : Option (C) :   Account for varying blade geometry

Explanation : There are several reasons why blade element theory is preferred over momentum theory. It can account for varying blade geometry, allows torque estimation, allows non-linearities for example lift curve to be modelled. The other points are some of the disadvantages.