More interesting comments below. Yes, the choice of prop certainly ha an effect on the durability of the engine/prop combination. I'm never sure what is meant by "harmonics", but the important factor is the torsional resonant frequency and how it relates to the frequency of the torsional impulses produced by the engine (not the prop). The torsional impulse is radically reduced by increasing the number of cylinders, as the impulses then overlap to a degree. I've seen numbers (which I've
forgotten), but I'll bet that a 6-cylinder engine has an impulse amplitude that is less than half that of a 4-cylinder. And an 8 cylinder engine is probably much better than a 6. So I would not be too worried about a prop directly connected to V8 crankshaft (the bending moment from gyroscopic precession loading is another matter). The big problem comes from adding a speed reduction unit between the engine an prop, as that first reduces the resonant frequency of the system and is often too fragile to tolerate the torsional loads. Yes, the blades themselves have a resonant frequency and that can cause problems. Props with a larger number of blades tend to be designed with smaller, lighter blades and therefore exhibit higher resonant frequencies.
Another factor is noise. Props tend to produce cabin noise via the pressure pulses that strike some portion of the cabin - like the windshield. These pulses, one per blade passing, can coincide with exhaust pulses, amplifying the perceived noise. So, the "bad" blade numbers are 2 for a 4-cylinder engine, 3 for a 6 and 4 for an 8. Guess which are the most common blade numbers :-)
A complex subject, to be sure.
Gary Casey
ES #157, 6-cylinder engine, 3-blade prop
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
From: Jack Morgan [mailto:jmorgan1023@comcast.net]
Sent: Thursday, June 02, 2011 8:29 AM
Subject: Re: [LML] Re: L-IV Choice of Engine
The most important issue being missed is the torsional resonance the auto crankshaft receives from the propellor. The
application of viscous couplings is often tried to isolate the auto crank from the propellor with some success but the weakness still remains. . . Jack Morgan