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From: Brent Regan <brent@regandesigns.com>
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Subject: Re: L-IV Choice of Engine
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All matter is composed of tiny bits of stuff in mostly empty space held 
together by elastic forces, therefore all matter is elastic and all 
chunks of matter will resonate at a specific frequency or multiple 
(harmonic) of that frequency. From church bells to people to the Tacoma 
Narrows Bridge, everything has a resonate frequency. Shake that chunk of 
matter at its resonate frequency and the displacement of the chunk of 
matter will exceed the displacement of the shaking. The magnitude of the 
displacement will increase until the energy carried away by damping 
equals the energy input by the shaking.

If you couple two vibrating "systems" of different frequencies together 
some interesting things happen. One is that more frequencies are spawned 
that are the sum or the difference of the the two principle frequencies. 
This is also true for any harmonics present. FM radio uses "beat 
frequencies" to deliver information.

Another thing that happens when two vibrating systems are coupled 
together is that the vibrational energy transfers from one system to the 
other and then back. If you want to see this effect hang a weight from a 
string to make a pendulum then hang a second pendulum with a shorter 
string or lighter weight from the first. Swing one of them and watch the 
energy (displacement) go from one to the other.

Now take an 8 cylinder engine that has a primary frequency equal to a 
rotation, a camshaft frequency 2 times that, a combustion frequency 4 
times that, a piston frequency 8 times that, a combustion event rise 
time frequency many times that and all the associated harmonics PLUS all 
these frequencies vary with RPM and load and you quickly realize that 
there is no way to fully characterize all the vibrations coming out of 
an engine.

Now couple all this shaking to a big flywheel (propeller), but not a big 
stiff flywheel, one made of lengths of chain (blades) that increase in 
torsional stiffness with RPM.

NOW you want to transfer 800 horsepower from one vibrating system to the 
other but that power is not in the form of a smooth torque. It is a 
product of hammer blows on the end of a lever arm. So you have big 
pulses, lots of little pulses and lots of energy that is going back and 
forth between the two big vibrating systems (engine and propeller). And 
what is all this energy passing through? Two little chunks of steel that 
together are about the size of a pencil (gear teeth).

Push on a chunk of anything and it will bend. Push harder and it will 
yield. Push harder yet and it will fail. But push some materials (most 
metals) repeatedly at less than their yield strength and they will 
eventually fatigue and fail. Fatigue failure is one of the least 
understood phenomenon in mechanical design and one of the more common 
failures in aircraft.

Any successful mating of an engine to a load involves either some 
massive connection or some damping mechanism that is carefully designed 
to remove or distribute energy. In cars it is the flywheel and torque 
converter or clutch disc springs and usually a harmonic damper at the 
other end of the crankshaft. In aircraft engines there is frequently a 
pendulum damper attached to the crankshaft. These mechanisms "filter" 
the vibration "noise" and quiet the party down to acceptable levels.

How do you accomplish the filtering of the forces that will invariably 
tear your little gears to shreds? Unfortunately the only acceptable 
solution involves mass and airplanes hate mass. Years ago at OSH I had 
dinner with the guy that designed the original Enginair gearbox. His 
solution involved a flywheel and a  torsional spring that would isolate 
the engine vibrations from the gears but because of the limited space he 
had to fold the spring back on itself such that it formed a torsion tube 
within a torsion tube. Cleaver in concept but a nightmare to produce as 
the the performance of the part is a function of the cube of some 
critical dimensions plus some tricky metallurgy and multiple process 
steps. IMHO, the guy delivered the carefully built prototype and then 
had an "Oh S&*T!" moment when he realized that the design was not 
manufacturable. If he admitted the problem he would get sued. If he 
delivered the design and people got hurt, he would get sued.  The best 
option was to not deliver any more hardware in the hopes of finding a 
solution.

Can you connect a propeller to an auto engine? Yes. Will it work? Yes. 
Will it fail? Yes.  We have many examples of all these cases. The only 
question seems to be "When will it fail?". The problem defies analysis. 
You can have armchair debates as to whether composite is better than 
metal or if three blades are better than four but being a "better" 
choice does not address the critical question. Are ANY of the choices 
presented good enough?  Only time and testing will tell.

Regards
Brent Regan