# [Mechanics] Friction decelerating a helicopter rotor assembly

• DanRow93
In summary: I would want to know how much the blades and hub (I) are resisting the deceleration. In summary, the Chinook helicopter's rotor blades and hub have a top speed of 300 revs/min and a combined mass of 300 kg. During a maintenance test, the blades take 48 seconds to stop without applying the brake, with an effective radius of 6.8 m. Using the equations for acceleration and angular displacement, the deceleration of the rotor blades can be calculated to be -0.6545 rad/s^2. This results in a total of 120 revolutions for the rotor assembly. To calculate the frictional resistance at this deceleration, we can use the equation Frictional torque
DanRow93

## Homework Statement

A Chinook helicopter’s rotor blades and hub have a top speed of 300 revs/min and a combined mass of 300 kg.
On a maintenance test the blade assembly is allowed to stop without applying the brake, in this condition the blades take 48 seconds to come to a standstill. The effective radius off the rotor is 6.8 m.

I need to calculate the following:

a. The deceleration of the rotor blades, assuming a constant rate of deceleration.

b. The number of turns the rotor assembly will make.

c. The frictional resistance at this deceleration.

## The Attempt at a Solution

I have completed the first two parts:

300 rev/min * (2pi)/60 = 31.416 rad/s

Acceleration = (ω2 - ω1)/(t2 - t1) = (0-31.416)/(48) = -0.6545 rad/s^(2)

ϴ = ω1 * t + 1/2 * a * t^(2) = (31.416*48)+(0.5x(-0.6545)*48^(2)) = 753.984 rads

753.984/(2pi) = 120 revolutions

For the final part, all I have came up with so far is that the friction will be acting downwards as a result of gravity, F = mg = 300*9.81 = 2943N

I'm not sure if this is the right thing to do though, as the question asks the friction 'at this deceleration', so I guess I need an equation that contains acceleration in it.

Thank you for any help!

EDIT: Do I need to use the equation:

Frictional torque = angular acceleration * Inertia?

Last edited:
DanRow93 said:
EDIT: Do I need to use the equation:

Frictional torque = angular acceleration * Inertia?
Yes, or something similar. The force downward due to gravity would not enter into the calculation, IMO.

## 1. What is friction deceleration?

Friction deceleration refers to the slowing down of an object due to the resistance or force of friction acting against its motion. In the case of a helicopter rotor assembly, this refers to the decrease in rotational speed of the rotor caused by the friction between the rotor blades and the air.

## 2. How does friction deceleration affect a helicopter's performance?

Friction deceleration can significantly impact a helicopter's performance. As the rotor blades slow down, the lift they generate decreases, causing the helicopter to descend. This can also affect the maneuverability and stability of the helicopter, making it more difficult to control.

## 3. What factors can affect the amount of friction deceleration on a helicopter rotor assembly?

The amount of friction deceleration on a helicopter rotor assembly can be affected by several factors, including the air density, the shape and design of the rotor blades, the speed and direction of the wind, and the weight and aerodynamics of the helicopter itself.

## 4. How can friction deceleration be minimized on a helicopter rotor assembly?

One way to minimize friction deceleration on a helicopter rotor assembly is by using materials with low friction coefficients, such as carbon fiber or graphite, for the rotor blades. Proper maintenance and lubrication of the rotor assembly can also help reduce friction and maintain optimal performance.

## 5. Are there any safety concerns related to friction deceleration on a helicopter rotor assembly?

Yes, friction deceleration can pose safety concerns for helicopter operations. If the rotor blades slow down too quickly, it can cause the helicopter to lose lift and enter an uncontrolled descent. It can also put additional stress on the rotor assembly, potentially leading to mechanical failures. Therefore, proper monitoring and management of friction deceleration are crucial for safe helicopter operations.

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