Hello...
>
> If our engines are smooth and the entire engine is
> well balanced, why is it necessary to isolate the
> engine from the engine mount/airframe?  Why isn't it
> appropriate to simply place a square of an old auto
> tire( or some similar material ) between the two
> points where I bolt the engine to the engine mount?
>
> What are we trying to protect the engine
> mount/airframe from the engine?

Very smooth relative to a piston engine. Not very smooth relative to an electric motor. If you leave out the two explosions and torque pulses per revolution, then it's closer to smooth, but with the two 9 pound rotors offset on the crank still has a huge rocking couple.

 

If the counter weights were the same weight as the rotors, then it would be very smooth, but the shaft would have to be much stiffer than it is now. So, the counter weights are outboard of the very large main bearings, and take out enough rocking couple to make the whole mess seem rather smooth.

 

The mass of the oil inside the rotors is accounted for by the builders that have made that determination, for spin balancing the rotating assembly. That mass changes with temperature and oil foaming as the engine operates. As you can imagine, during acceleration, the rear rotor might retain slightly more oil than the front rotor. How then can they balance each other?

 

So, there is some imbalance. And some rocking couple. Since the engine design (short throws) has little torque, it performs much better with a reduction system that allows for operation in the 5,500 to 6,500 RPM range. So, while the amount of energy expended on imbalance is nothing close to the piston engine, it is still impressive.

 

The lord style dampers can absorb that energy over most of the frequencies involved.

 

Without that damping, the energy is free to excite sympathetic responses in other parts of the system (airplane). From a loose pop rivet spinning in its hole for years on end until one day it falls into that one gap that locks up that turn buckle, and ...............................!

 

Or the unexplained failure of the third fuel pressure gage. Because it has one small part that is tuned (sympathetic to) the 16th harmonic of your cruise RPM. I have my engine mounted rigid to the frame, and one of two tachs mounted rigid to the dash. The rigid one dies every year. Usually it just quits working. Once the needle fell off.

 

Adding or removing mass to the dash changes the range of frequencies it will respond to.

 

Isolating electronics and instrumentation from vibration is a common practice.

 

Years ago the instruments were so bad that there was a shaker attached to the dash on big airplanes that shook the dash board to keep the needles free. So, the dash was mounted in soft isolators and then shaken at a frequency that did just a small amount of long range damage.

 

Small displacement high frequency vibrations take a long time to fatigue a part. So the failure may be a bigger surprise, after years of trouble free operation.

 

All vibration adds to human fatigue. The smoother the ride the less stressed, more alert the pilot. And the wife might go with you more than one time.

 

 

Lynn E. Hanover