Good question
Bill
I also have a high compression parellel valve 360 Lycoming that I had built a few months prior, and if I knew then what I know now I would have not went any higher than 8.7 to 1.
As far as i am aware Hartzell propellors has not tested blended airfoil propellors running high compression. Even though you will be within tolerable horsepower range your engine still could overstress the blades running those pistons. I am told it is harmonics and resonance (torsional vibration)that damage blades and cause blades to disappear from the nose of your aircraft not horsepower.
So what are we to do? Run MT because the wood/composite is less likely to be damaged by the effects of high compression because of its ability to dampen resonence. Or rip the cylinders back off of our 360's and through in those 8.7's and fly safe behind a Hartzel.
Like Bill; I also am looking for some advice or experience on this subject
That is true Hartzell has not tested higher compressions. I talked to the engineers and high compression may change the harmonic characteristics of the prop/engine combo, but the affect is not drastic. It makes a little more power but does not change the frequancy or pulse drastically like going from Mag to EI.
The affect of the electronic ignition (they only tested one brand BTW lightspeed) has a greater affect. However you're absolutly right, as you say its an unknown. Well actually its a partial unknown. The engineer I talked to said a bump in compression might expand or move critical RPMs up slightly higher.
Just stay out of the corner of the envelope. Look at the typical limits for other props. Its 2,700 rpm and at lower RPM's in the 2,250 rpm under range, especially with over MAP pressure. Keep it in the middle of the normal operating range 2,300-2,600 rpm and MAP approx square or under. That is wild freaking guessing on my part, but it comes from the conversation I had with Hartzell about this topic. Again I encourage you to call them. They did not express concern BUT the only way to know for sure is do flight test. None have been done and none planned as far as I know.
I think you are over stressing (pun intended).
8.7:1 compression is very very mild and the affect will be limited. Stock is 8.5:1. If you get the better 7497 blade (better in that it's thicker, heavier and less susceptible to harmonic limits) and try not to fly at 2,700 rpm all day, all the time, you will be fine. Even the 7496 is fine in my opinion, but guaranteed unlimited fatigue life with out testing is not possible.
The older HC2YK/F7666 with the most restrictions with EI (or even with Mags) is good for I recall 8,000 hours. The new BA airfoil, specifically the newer of the new 7497 is designed for EI and well tested and understood. Criteria is unlimited fatigue life or hours. In fact the 7497 works on the IO360 angle valve with 8.7:1 compression. That is what it was designed for.
Are you also using electronic ignition. The affect of the compression alone from my understanding is not as drastic the change for adding EI. Call Hartzell engineering. They will be glad to help you. I have a feeling they will give you warm fuzzies, but not scientific absolute.
Now the guys going super radical high compression, different cam grinds, dual lightspeed's, cold sump, 4-into-1 exhaust are pushing the unknown scale much more. The last item, 4-into-1 is an unknown I thought about, which affects me. I realized Power Flow makes 4-into-1 exhaust for factory planes as an aftermarket STC. They claim higher hp. The FAA did not require a prop change. I don't think they check the prop harmonics. For some reason the EI makes the biggest change. Its not as much the greater power as much as the PULSE. Compression tends to possibly aggravate but not make big changes, with in limits.
Hope I calmed you down a little.
I'm pretty conservative. I wouldn't hesitate to fly in you plane this year or 5 years from now. As long as you maintain the prop and engine and operate it properly, the prop will wear out from erosion well before fatigue is factor. Be sure to keep the prop free of nicks and gouges, grease the hub per hartzell's recommendation and fly regualarly. There are a lot of factors of safety and margins built into the prop and engine that make existing limits for the know configurations conservative. Also there are lots of guys with way more radical compressions and set ups flying with BA props. One of their props will fly off well before yours does.
