Which of these is not an assumption made while formulating momentum theory for hover and clib of a helicopter?

Smooth slipstream

Non-uniform loading over the disk

Blade is modeled as an actuator disk

Slipstream rotation velocity is negligible

Correct Answer : Non-uniform loading over the disk

Explanation : There are various assumptions made while formulating the momentum theory for the helicopter in hover and climb condition. These are-the blades are modeled to be a thin circular actuator disk, there is uniform loading over the disk, the slipstream is well defined, the rotational velocity in the slipstream is neglected and there is uniform induced velocity.

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.