Robert,

That's a most interesting comment. Essentially what you are saying is that a prop of
1. non-metallic construction
2. relatively prime number of blades compared to cylinders
would lessen the effects of harmonic vibration since (1) would damp the vibration modes and (2) would limit the possibility of them feeding back to damage the engine.

I guess that means I'm in good shape, since I have an AeroComposites 3 blade prop rated for 800HP which satisfies both criteria. It is made of a foam core with carbon fiber over the foam for strength and fiberglass for final shape. The only metal is the nickel leading edge and mating structure in the hub.

On top of that, I have a question for the physicists in the discussion. How important is blade weight? I can make an argument that a heavy blade (typical aluminum, about 11#) would damp vibration better due to mass effect. I can also argue that a light blade (AC, about 7#) would incite less vibration. We might even suggest that the vibration frequency of the light blade would be higher, and might be less damaging? But the foam core would more easily damp higher frequencies? Which is the case? Help me out here.

Ted Noel

On 6/4/2011 9:52 AM, Lancair wrote:

I have read some discussion on the effects of different props, reducing harmonic vibration by changing the number of blades (4 blades on a 6-cylinder engine for example, vs. 2 or 3 blades which have lower harmonics) and this might be beneficial.  That thought process might lead one to a 3 blade or 5 blade on an 8-cylinder engine.

 

Another  concept which might be considered is the prop material itself, with composite props like the MT damping out the combustion shock more than an aluminium prop.

 

Guess that’s what makes these planes “experimental”.

 

Robert M. Simon

ES-P N301ES