<< Picture after I get it
 powdered coated. >>

Paul,
discussion of cracked engine mounts on the canard-aviators forum brought

out

a recommendation against Powder coating an engine mount because it is

likely

to make it impossible to detect cracks visually. The coating tends to

stay

intact even though the underlying metal has cracked.
Ed.           pebvjs@aol.com

Yes I have heard that before. I personally don't buy it. But to each his
own.

Motor mounts are designed to extreme limits. Namely six G in a
severe pull up or hard landing. Or if the engines suddenly seizes
a high torque load is applied. As such the failure mode is not likely
to be cracking. The failure mode is likely to be bending when the 6 G

occurs.

Most of the time the stress is sleeping by comparison.

The rotary is low vibration so a large diameter tube (as opposed to a

long,

small diameter bolt) resonating in the guitar string mode is unlikely but
possible I guess. The Mazda torque forcing frequency is zero to 200 cps
to 6000 RPM. Anybody care to calculate the resonant guitar string mode
for a 3/4  inch diameter, 0.049 wall steel tube 20 inches long?

Paul Lamar

Cliff Manzke wrote:

Paul, answer to your question:

Anybody care to calculate the resonant guitar string mode
 for a 3/4  inch diameter, 0.049 wall steel tube 20 inches long?

395 Hz for the first mode, 1583 Hz for the second mode, 3562 Hz for the
third mode, all assuming the ends are simply supported.

Your forcing frequency of 200 Hz at 6000 rpm is correct for the first
harmonic, but there may be higher harmonics (2nd, 3rd, etc) present.  I've
never done a Fourier analysis of the rotor motion on a rotary so I can't say
for sure which (if any) higher harmonics are present.  Maybe Jeff could add
to this?

Cliff Manzke

Hey thanks Cliff. Glad to have you back. 
Ends are fixed more or less so the frequencies may be higher.

Can you figure out what it is for a tension bolt with 30 foot 
pounds of torque? Don't have one in front of me but I would
guess it is 12 inches long about 3/8 inch in diameter.

It always amazed me that ultra lights like Kitfoxes did not have
more trouble with cracking tube joints since their fuselages are made
mostly from 1/2 inch 0.035 wall tubing up to 30 inches long.

Paul Lamar
 
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