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Torsional Dampening

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rv7charlie

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Most of this thread, we have been talking about torsional vibration of the rotating parts. Lycoming's (the direct drive ones anyway) rotating parts along with the prop are a stiff system. In stiff systems, the lowest natural frequency in torsion is significantly higher than 2x the firing frequency from the engine. In a stiff system, everything vibrates together.


Billski
You know, many c/s props require a yellow arc (no continuous operation) around 2k rpm. And the Sensenich metal fixed pitch for 160 hp lycs (designed for RV's & other fast homebuilts) is placarded against operation above 2600 rpm, even though the engine requires 2700 rpm for 75% power at 8k'. Dangerous, I know, but I'd assume that both are due to vibration issues.

Charlie
 

BJC

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You know, many c/s props require a yellow arc (no continuous operation) around 2k rpm. And the Sensenich metal fixed pitch for 160 hp lycs (designed for RV's & other fast homebuilts) is placarded against operation above 2600 rpm, even though the engine requires 2700 rpm for 75% power at 8k'. Dangerous, I know, but I'd assume that both are due to vibration issues.

Charlie
Example from a type certificate data sheet for a FP: "Avoid continuous operation between 2150 and 2350 RPM." Similar warnings are for CS propellers. See http://richgoodwin-airshows.com/docs/Pitts_type_certificates.pdf

Those limitations do not present any operational difficulties.


BJC
 

wsimpso1

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You know, many c/s props require a yellow arc (no continuous operation) around 2k rpm. And the Sensenich metal fixed pitch for 160 hp lycs (designed for RV's & other fast homebuilts) is placarded against operation above 2600 rpm, even though the engine requires 2700 rpm for 75% power at 8k'. Dangerous, I know, but I'd assume that both are due to vibration issues.

Charlie
Thread drift, but also interesting...

I also know of fixed pitch props on early PA28-180's with a little yellow band at 2200 rpm. What those yellow arcs in the middle of green mean is that some forcing vibration goes into resonance with some very specific vibration mode at that speed, and will break off pieces of prop blade if run enough hours there.

My best guess is one or more of the block yawing modes (left and right banks are offset from each other, and gives block yaw vibration at 1, 2, and 4 per rev) are the forcing functions, and the prop blade flex mode (the thin way) coincide there, but it could be torsional vibe of the crank interacting with blade modes too.

I am guessing, but If I had to bet on this, I would bet that they found it during the cert process for the airplane, so elected to placard the tachometer rather start over on stiffer blade design and restart certification.

Billski
 

rv6ejguy

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Metal props introduce another dimension to resonance concerns. A number of GA aircraft have yellow arcs on the tachs as ranges to avoid. All the major prop makers do instrumented flight tests on various engine/ prop combinations. You're asking for trouble if you don't. They've done a few on some popular Experimental combinations as well.

The point being here is that some certified engine/ prop combinations never had a fully developed fix applied to TV or propeller resonance and just have an rpm range restriction in place. Not unusual. This would not be acceptable in modern automotive releases but it is in aviation sometimes.
 
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TFF

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It became very popular in the 60's to clip metal props at home for planes like T-18s. The resonance change started them pitching blades.
 
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