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