High compression and prop limit

[Beancounter]

Can anyone comment on the effect of higher compression pistons 9:1 in a 0-320 and the 2600 rpm limit with the Sens fp prop. I assume the limit is due to a torsional vibration issue. Does compression ratio alter TV in any way.

[jrs14855]

I have flown Pitts with both fixed metal and hartzell and know of no change in r/m limits due to compression ratio. 10 to 1 on both parallel and angle valve 360 with no issues. The Sens RV metal is a wierd prop. having flown one 50 plus hours on a Glasair I would not take one as a gift. Catto is so much better as well as several other prop manufacturers.

[DanH]

Originally Posted by Beancounter

Can anyone comment on the effect of higher compression pistons 9:1 in a 0-320 and the 2600 rpm limit with the Sens fp prop. I assume the limit is due to a torsional vibration issue. Does compression ratio alter TV in any way.

Yes. You may safely assume the Sensenich metal has a resonant vibratory mode at the prohibited RPM. Increasing the amplitude of the forcing frequency will increase vibratory stress. Put another way, if it is bad with standard compression it will be worse with higher compression, assuming the forcing frequency is a crankshaft order.

Some other metal prop or a wood prop or wood/glass prop may or may not exhibit a resonant vibratory mode at the same RPM. It has nothing to do with compression, but rather different blade shape, mass, elastic modulus, and damping, the parameters which set its natural frequencies. Put another way, if a resonant mode doesn't exist with standard compression it probably won't exist with higher compression.

Which brings us to the wild card, the prop which does exhibit some level of significant stress when subjected to a particular forcing frequency, but not so much stress as to make the propeller test engineer nervous. Bumping compression, or ignition advance curve, or other performance mods may increase the moderate stress to an unhappy level. That's why the responsible metal prop folks run strain gauge tests, and why some have issued various bulletins about engine mods.