Porterfield Airplane Club

Keep the Skinny Birds Flying Safely

Hello, I thought I’d ask for opinions on what seems to be the best all-around propeller for the Continental A65 on a collegiate. We’ve been running a wood Sensenich, 72/42 for quite some time, and I am considering moving up to a 44 pitch. I know I would be giving up some climb performance but my cruise speeds are about 83 miles an hour. Not too shabby by any means but some documentation is touting 92 or 93. Any and all thoughts would be appreciated!

K T S B F!

Mark

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A pitch of 44 or 46 will allow you to see the speeds that the M-6 airfoil affords, while marginally limiting your climb performance. TCDS A-720 specifies, for the CP-65, static rpm at full power for a Landplane: Not over 2165 rpm, not under 1860 rpm. Diameter: not over 85 in., not under 68 in.

Your rpm at full power with a 46 pitch should be over 1860, unless your engine's not developing full power for reasons other than prop pitch.

The props specified in the TCDS are:

Propellers and Propeller Accessories 101. Propeller – wood, fixed or adjusted pitch, with hub

109. Propeller – Sensenich M74CK, fixed pitch metal (a) Model M74CK (CP-65 landplane only) 21 lbs (-49)
Static rpm at maximum permissible throttle setting:

Not over 2165, not under 1860

No additional tolerance permitted

Diameter: not over 74 in., not under 72 in.

This diameter range for a metal prop is more limited than for the wood prop. Pitch is still determined by static RPM only.

Something I failed to mention is the airplane was built as an LP65 and the engine was changed to the Continental A65-8 with a field approval. How does one apply the TCDS when the type of engine was changed with approval? Other than the obvious specifics of the engine and propeller, there doesn’t appear to be any other differences between the LP and the CP. The Continental was just reinstalled after major overhaul to factory new specs with new Superior cylinder assemblies so we should have a good strong power plant. Still awaiting paperwork from my A&P/IA so I can do the break in flights.In regards to the TCDS stated static min/max rpm range with respect to propeller selection, It makes it difficult to commit to buying a propeller and than finding out it’s out of range rpm wise. Sounds like what I need to do is reinstall my W72CK42 and fly it and seat the rings, and then do a static full throttle rpm check to see where i am with the new engine. At that point I should be able to “guess” if adding 2” of pitch will keep me in range.

Thanks Andy for your insights…

Mark

Your mechanic is supposed to be submitting the 337 to OK City within two days of completing the work, which means he should also have the logbook entries completed and have the logs back to you as well.

Just because the data plate says LP, you have a CP so the above specs apply. I concur with your decision to see how she flies with your current prop. You may find her performance satisfactory with the new cylinders and not need another prop.

Welcs!

I have a 72” 50 pitch on mine. Just finished flying the first 10 hours since its last flight in 1965. I can tell you it meets the minimum RPM but really lacks climb performance. Highly suggest something with less pitch. One that note does anyone have a prop for sale?

Could you have meant to say 72” 40 pitch? Our CP 65 is only approved for a 72 inch 42 or 44 inch pitch. If you have a 50 inch pitch on there, that would explain the terrible client performance.

Mark

Meant to say climb performance. 

Mark

Yes for metal props the TCDS gives only a couple options but with wood or adjustable hubs by my interpretation you have a very wide scope of options as long as it meets the length and static RPM requirements. We put the current prop on as a test prop during restoration and it met the minimum static RPM so we decided to give it a try. Long story short… it’s too much pitch. That said, I’m still going to go fly it today.

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