Propeller Selection Assistance

[David Z]

I'm happy to be wrong, and please tell me if I am.

It's looking like the 370/375 engines are similar in propeller needs to the angle valve 360/390 engines. Basically meaning that an un-dampened (counterweights) 370/375 is equally hard on equipment as an un-dampened angle valve engine. It makes sense to me because a ~200hp 4 cyl engine probably makes very similar power pulses regardless of crankshaft stroke or cylinder head design. It's still the same size power pulses.

Un-dampened angle valve engines aren't common but they are around. Whereas un-dampened 370/375 engines are a lot more common.

Basically, if you want 200hp, get a dampened crank or one of the robust (expensive) props. MTV-9 or WW HRT.

The one thing I don't get, and may help out Champ with his engine/prop combo is this quote from the Hartzell Application Guide (link) for the the RV 4,6,7,8,14 with the prop C2YR-1BFP/F7497 (page 1557) "For undamped 200 HP IO-360: Continuous operation is prohibited above 24 inches manifold pressure between 2350 and 2550 RPM". It's an approved combination with the restriction. Why would it matter if it's an un-dampened 200hp IO-360 or an un-dampened 200hp IO-370/375? They're both 200hp 4 cylinder engines. What am I missing?

[Cumulo]

Originally Posted by David Z

I'm happy to be wrong, and please tell me if I am.

It's looking like the 370/375 engines are similar in propeller needs to the angle valve 360/390 engines. Basically meaning that an un-dampened (counterweights) 370/375 is equally hard on equipment as an un-dampened angle valve engine. It makes sense to me because a ~200hp 4 cyl engine probably makes very similar power pulses regardless of crankshaft stroke or cylinder head design. It's still the same size power pulses.

Un-dampened angle valve engines aren't common but they are around. Whereas un-dampened 370/375 engines are a lot more common.

Basically, if you want 200hp, get a dampened crank or one of the robust (expensive) props. MTV-9 or WW HRT.

The one thing I don't get, and may help out Champ with his engine/prop combo is this quote from the Hartzell Application Guide (link) for the the RV 4,6,7,8,14 with the prop C2YR-1BFP/F7497 (page 1557) "For undamped 200 HP IO-360: Continuous operation is prohibited above 24 inches manifold pressure between 2350 and 2550 RPM". It's an approved combination with the restriction. Why would it matter if it's an un-dampened 200hp IO-360 or an un-dampened 200hp IO-370/375? They're both 200hp 4 cylinder engines. What am I missing?

With hat in hand, I'll offer this to the bright group of people on this thread regarding torsional vibration. Higher compression ratios or advanced timing will make "peakier" power pulses. Though the average of the power pulses may be the same to get, say, 200 HP, there will be more energy at the higher order frequencies to resonate the crank/propeller system as compression ratio is increased. Another factor is rod/stroke ratio. A stroker usually has a smaller rod/stroke ratio, and that will increase secondaries 2, 4, 6, etc. as well do to piston/crank geometry. So, these are two factors that could intensify a TV concern in comparing two equal horsepower engines and may be relevant to David Z's question.

By the way, and according to Den Hartog as well , much of the TV energy is dissipated by the prop. Props with heavy blades tend to dissipate less, so resonances will be more intense. Relatively light wood blades DO have advantages and will be easier on crankshafts as well.

Here is a very easy reading source on the subject of torsional vibration which reads a bit like a good novel. It is framed in the development of the P&W R-2800, and the subsequent R-2800-CB16 - maybe the best round engine of its era: http://www.enginehistory.org/NoShortDays/TV.pdf

Ron

[Toobuilder]

Theres a few things going on with this thread, and I actually have recent, personal experience with some of it.

First off - propeller "cruise performance"... Better define what that is because some props are better than others in certain RPM ranges. For instance, I bought the Hartzel BA 2 blade for my Rocket because data suggests its the "fastest" out there. That has indeed been demonstated to be the case compared to the MT 3 blade. But thats at 2700 RPM. At 2300, where I spend 99% of my time, the MT handily crushes me. Been there, done that. Should have bought the MT.

Also, dynamic counterwights do break in service. I had one overspeed event crack the ears. 4.7 hours earlier, the crank was installed and good. 3 seconds of overspeed and it was junk.

[Karetaker]

Howdy everyone. I have made my decision, but am glad to see this thread continue down this road of discovery!

Toobuilder makes a good point about cruise performance. My personal cruise settings I plan to use in my (hopefully light weight) RV-7 and IO375-M1S engine should play very nicely with this MT prop. Others may cruise with different setting, different engine, different aircraft and find the 3 blade MT unsuitable.

[Champ]

I spoke with AeroSport on this. They’ve had no reports of problems with 375/prop combos. They did suggest to dynamically balance the prop at CIs and do frequent tap tests. Also putting in a cw crank would add 12lb to my engine. Haven’t heard from Hartzell yet. I will start looking into an MT or WW HRT replacement though. One concern is resale value with my Hartzell prop. Anyone need a low time Hartzell BA?

[David Z]

I compiled the data from this thread and some research. The prop weight and prices are accurate as of today. The engine weights are approximate and 12lbs apart for a counterweight/dampened crank based on the post above. The engine prices (except for Van's) is a wild inaccurate guess. I'm happy to update if numbers are provided.

I take no liability for incorrect information. Verify everything for yourself, it's merely meant as a discussion point.

Looks like the the extra 15 cubic inches and ~20hp is worth about 7-9 grand with insignificant weight difference. The biggest variance was 8lbs. They all seem like good options to me, and it's really up to personal preference.

[Taltruda]

Originally Posted by Toobuilder

Theres a few things going on with this thread, and I actually have recent, personal experience with some of it.

First off - propeller "cruise performance"... Better define what that is because some props are better than others in certain RPM ranges. For instance, I bought the Hartzel BA 2 blade for my Rocket because data suggests its the "fastest" out there. That has indeed been demonstated to be the case compared to the MT 3 blade. But thats at 2700 RPM. At 2300, where I spend 99% of my time, the MT handily crushes me.

Interesting that you mention this. I delivered an angle valve IO-360 RV-7 that had the 3 blade MT prop. It was slower than I expected, but I noticed something interesting. While cruising, I experimented with rpm from 2500 down to 2200, leaned each to 75 ROP for each setting. (MP was wide open at 11,500) and I found NO speed loss at the lower rpm settings! The more I pulled the rpm down, the egt got cooler, allowing me to lean more, helping fuel economy..

[DanH]

Originally Posted by Champ

I spoke with AeroSport on this. They’ve had no reports of problems with 375/prop combos.

If you're new to this game, note EAB's classic Catch 22. Owners tend to keep issues between themselves and the vendor (in this instance, the prop guy), usually fearing loss of support. The community at large doesn't hear about the issues until the support stops, or patience and checkbook run dry.

They did suggest to dynamically balance the prop at CIs and do frequent tap tests.

Dynamic balance has nothing to do with blade vibration. And a tap test tells nothing about fatigue and fretting of the aluminum ferrule at the root of the composite blade, which requires a teardown to detect.

[David Z]

For all those interested. With the -375, the counterweighted crank is 6 lbs heavier and costs a bit over $2300. This is of course on a new build engine, and not a retrofit to an existing engine.