Which of these conditions is essential for a rotor airfoil when the helicopter has high forward speed?

High angle of attack

Low drag divergence

High critical Mach number

Low critical Mach number

Correct Answer : High critical Mach number

Explanation : When the helicopter moves at high forward speeds, the front tip of the rotor blade usually has a very high Mach number. For this reason, it is essential for the rotor blade airfoil to have a high critical Mach number so that it doesn’t reach stall conditions very fast.

Recently Updated in Aerodynamics Questions

What is rotor polar?

Plot of power coefficient as a function of thrust coefficient

Plot of power coefficient as a function of lift coefficient

Plot of power coefficient as a function of drag coefficient

Plot of thrust coefficient as a function of power coefficient

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.

What is the inflow ratio for hovering?

1-1.5

0.6-1

0.01-0.05

0.05-0.07

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.

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

Model thrust lag

Neglects span wise flow

Account for varying blade geometry

Assumes flow inside stream tube as constant

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.