"Balancing" a Propellor

[mccarvil]

I'd like to model the dynamic forces on a blade.
Statically the moment of inertia is simple.
But dynamically (rotating) there appears to be so many different forces on the blade how would a single point of action (where all forces could be equivalently resolved through) be calculated?

Blade pitch, rotational speed, blade design, blade material, gyroscopic effects; it appears all these factors and more act on a blade.

What are the major effects? Are there basic models to approximate these calculations? How can these forces/factors be (approximately) calculated?

Thank you!

 

[cawiddison]

A good starting point would be "Theory of Flight" by Richard von Mises, Pub. by Dover. This covers the aerodynamic analysis of the propeller using the "blade-element theory." It is old (coyright 1945) but the the theoretical treatment is thorough and there is lots of practical detail. You'll need some understanding of differential and integral calculus.

As for structural and dynamic considerations, there are several text books on mechanics and strength-of-materials that will lead you through bending, torsion, vibration, etc.

As you observed, it is not a simple topic.

Fundamentally a propeller is a collection of wings attached to a hub about which they rotate; the lift that is generated by the wings (blades) is transmitted via the hub to the aircraft. The airspeed experienced by the propeller is different at each radial station and is the (vector) sum of the forward speed and the rotational speed; there are two other airspeed components that are the induced inflow that is caused by the lift on the blades and the swirl that is due to the drag of the blades. A properly designed propeller has blade sections that are airfoil sections but their chord-lines do not all lie in the same plane - the pitch of the blade section normally reduces as you move from root to tip in order to keep the actual angle-of-attack of each section near the ideal value for minimizing drag (and threfor, power required).

Because of bending and centrifugal loads at the root, the sections become relatively thick to provide sufficient strength.

Good luck

 

[Felix107]

A very reasonable prop modeling system is available from Don Bates see his website at -

http://home.earthlink.net/~bateseng/

You are absolutely correct about the complexity of forces operating on a propeller. Once you model the design nothing can replace building it and real world testing it to be sure it operates as predicted. This is particularly important to test for harmonics and vibration that can cause stress fracture. If it is made of metal or other stressing material, the prop has to be tested and certified on each specific engine/airframe combination.

 

[amorrison]

The above HTML address (Felix107)
has already vaporized (1 yr.after)post.