# Centrifugal twisting moment of a propeller

• asynchronous13
In summary, to calculate the centrifugal twisting moment of a variable-pitch propeller blade, you first need to know the material density, RPM, and angle between the minor and major axes. You then use the moment of inertia to calculate the centrifugal twisting moment.

#### asynchronous13

I am interested in calculating the centrifugal twisting moment (CTM) of a variable-pitch propeller blade. It's been a long time since I did anything with moments of inertia, so I'm looking for pointers to good info, or direct help if someone here can provide it.

Assume there is a propeller with two blades. For simplicity, each blade is a rectangular solid with length greater than width and negligible thickness. The blades spin around the main axis of rotation at the center of the propeller. In addition, the blades twist (variable pitch) about an axis that is perpendicular to the main axis of rotation, extends radially away from the main axis, and passes through the center of gravity of the propeller blade.

[ That was my attempt to describe the system. In plain english, it's a propeller with variable pitch blades. Hopefully my intention is clear even if my description is not. ]

Ignore aerodynamic forces (the centrifugal twisting moment is much greater than the aerodynamic twisting moment).

When the propeller is spinning, the CTM acts to reduce the blade pitch. In other words, when the propeller is spinning with no other applied forces, the blade pitch will go to zero degrees. For an actuator to hold a pitch angle in the blades, the actuator must be able to provide more force than CTM.

If the moment of inertia, mass, center of gravity, and RPM are known -- how does one calculate the centrifugal twisting moment?

http://www.scribd.com/doc/45224522/292/Propeller-Twisting-Moments [Broken]

Last edited by a moderator:
Fig 10 in your link makes it pretty clear how to compute it, using vector math. For a simple approach put the points A, B into the centers of mass of the blades halves, separated by the pitch axis. In general you would have to integrate of the blade volume.

It may be clear how to compute it for someone who is more familiar with these methods than I am. However, this is outside my normal field of work, so I could use some pointers.

While I could do an integration over the entire blade, I thought it would be simpler to use a moment of inertia that describes the blade.

Right now, I'm thinking along the lines of the following:
(material density) * (rpm^2) * (Imajor - Iminor) * sin(alpha) * cos(alpha)

alpha - angle between minor axis and plane of revolution

I'll keep crunching to see if I can get the numbers to match the data I have. Any tips in the meantime would be appreciated.

## 1. What is centrifugal twisting moment?

Centrifugal twisting moment, also known as centrifugal torque, is the force that causes a rotating object to twist or rotate around its axis.

## 2. How does a propeller create a centrifugal twisting moment?

A propeller creates a centrifugal twisting moment due to the distribution of mass and shape of its blades. As the propeller spins, the blades create a lift force that is perpendicular to the rotational axis, resulting in a twisting force.

## 3. What factors affect the centrifugal twisting moment of a propeller?

The centrifugal twisting moment of a propeller is affected by the number and shape of the blades, the rotational speed, the distribution of mass along the blades, and the angle of attack.

## 4. Why is the centrifugal twisting moment important for propeller design?

The centrifugal twisting moment is important for propeller design because it affects the performance and efficiency of the propeller. A properly designed propeller will have a balanced centrifugal twisting moment to avoid excessive vibration and ensure smooth operation.

## 5. How is the centrifugal twisting moment of a propeller calculated?

The centrifugal twisting moment of a propeller can be calculated using the formula T = r x F, where T is the twisting moment, r is the distance from the center of rotation to the point of force application, and F is the force causing the twisting moment. This can also be represented as T = I x α, where I is the moment of inertia and α is the angular acceleration.