Compression vs harmonics

[gcrane01]

Anyone ever heard anything about regular Hartzell props not being stressed/may fail if you use them on high compression engines in the 10 to 1 range. The engine supposedly sets up harmonics which damage the props.

[DanH]

......Yes.

[Bob Axsom]

I personally know pilots the are running 10:1 pistons, winning races and their props are not failing. When you talk about frequencies and harmonics you have to consider the wide range of physical materials, tolerance variations and shapes of props even those made in Hartzell's Piqua, Ohio plant. I have toured the plant personally and I am somewhat experienced in production and testing and I believe any test of a propeller made there cannot be applied to all propellers made there - the potential for variations does exist in my opinion. Also, the variations in propellers applied to RVs even from Hartzell does exist. For example I specified F7496 blades on my recent prop instead of the F7497 currently being sold for RVs. It is a thinner airfoil for one thing and is projected by Hartzell to be more efficient at higher airspeeds.

I assume there is some story out there about some analytical study or specific test case or field failure that the original poster is trying to track down. I would like to hear the story too. Maybe something like Kevin Eldridge's prop failure at Reno? I don't have the answer to the original question but I believe Hartzell props and 10:1 pistons do not automatically equal an unsafe application based on compression and harmonics.

Bob Axsom

[William Slaughter]

The issue is not inherently that it's high compression, just different from that of the stock engine used to complete the torsional vibration survey for the "approved" engine/prop combinations. The metal blades are like big tuning forks, and if the excitation approaches a resonance frequency, well, bad things can happen. While not immune to the principle, composite blades tend to be well damped, and to have much higher resonant frequencies. Torsional vibration issues have plagued many a drivetrain over the years and are often quite labor intensive to engineer out once they've raised their ugly head.

[rv6ejguy]

Originally Posted by Bob Axsom

I assume there is some story out there about some analytical study or specific test case or field failure that the original poster is trying to track down. I would like to hear the story too. Maybe something like Kevin Eldridge's prop failure at Reno? I don't have the answer to the original question but I believe Hartzell props and 10:1 pistons do not automatically equal an unsafe application based on compression and harmonics.

Bob Axsom

Kevin's failure looks like it was caused by no oil pressure because the engine exploded and oversped the prop- about 4500 rpm.

[elippse]

I have nothing to back this up except the story about when troops walk across a bridge they should break stride to prevent the bridge from getting into resonance and failing. My opinion is that if you have an EI with precise timing that there might be more of a possibility of a prop resonance failure than when using the somewhat scattered timing of a magneto. I wrote to Les Dowd of Hartzell about this several years ago in response to an e-mail of his about EIs. Again, this is just an opinion without any testing to back it up. With a higher compression ratio, the back and forth loading of a blade increases. I also see some merit in Bob's discussion of material and process variations. A tuning fork will change frequency with temperature, and a metal propeller is a good representation of a tuning fork.

[Andy Hill]

See Vans Link as an example.

I am not sure there is "evidence" the Hi Comp pistons cause a problem... just as above, they have not been tested.

NB the F7497 is the current blade, and will have some test reports out there for certain engines / specs, but not sure about 10:1?

Andy

[Alan Carroll]

Originally Posted by elippse

My opinion is that if you have an EI with precise timing that there might be more of a possibility of a prop resonance failure than when using the somewhat scattered timing of a magneto.

Not doubting this, but not sure I understand - why is the timing of a magneto less precise? Don't the points open at the same point of every revolution?

[mcencula]

Unless your specific combination of all the variables has been vibration tested, it cannot be assured that your prop will be free from harmonics. Some of the variables are:

Engine make (Continental / Lycoming / other)
Engine displacement
Compression ratio
HP
Carbureted or fuel injected
Turbo / super / normally aspirated
Mags or EI and what manufacturer and model
Prop make and model
Blade material
Exhaust system
Intake arrangement
Engine mount
Airframe to which it's attached
Tractor vs. pusher

I'm sure there are more that I'm not thinking of.

Using an untested combination doesn't mean you're unsafe...it just means you don't know if there is or is not a problem.

[elippse]

Originally Posted by Alan Carroll

Not doubting this, but not sure I understand - why is the timing of a magneto less precise? Don't the points open at the same point of every revolution?

Unfortunately, the points don't always open at the same precise interval due to the changes that take place on the points metal buid-up, the cam is not precise, the cam-follower can have some bounce, and you have a gear train with its slop and variations. I have heard that typically you can expect 1? to 3? variation from cylinder to cylinder. Haven't you ever noticed that with a timing light on a car that the timing mark will bounce around a little bit? It only takes a small variation to reduce the power pulses into the prop resonance cycle to keep the resonance from building up destructively. A crank-triggered EI, especially with capacitive discharge, will always make the power pulses fall at very precise intervals. I say especially CD since the typical delay from trigger to the high voltage pulse is usually just 3-4 microseconds, whereas in an inductive EI or magneto the voltage rate-of-rise can be subject to losses on the plug insulator due to carbon or lead buildup.