Prop Question

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Pat Weeden
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Prop Question

Post by Pat Weeden »

Received via email today at BPA headquarters.
Sir / Madam,

I have about 40 hours on my recently finished Piet. It’s flying with an aluminum propeller I had lying around, and it’s proven way too coarsly pitched. I am going to buy a new (wood) prop, and would like as to proper length / pitch.

Engine:
Continental C-85
85 HP
Max. RPM: 2575

Current Prop:
McCauley CM6850

Please feel free to share this with anyone whom might be able to help.

Thanks,
Pete W.
Pat Weeden, Site Admin
Brodhead Pietenpol Association
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taildrags
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Re: Prop Question

Post by taildrags »

Pat; when we post replies to this type of inquiry, how do we know that the person who raised the question gets the replies? If they are not subscribed to the list, surely you don't have to forward them back and forth?

Oscar Zuniga
Medford, OR
Air Camper NX41CC, A75 power
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taildrags
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Re: Prop Question

Post by taildrags »

Since no one else has replied, I'll throw in my opinion: connect with one of the prop manufacturers and get their recommendation. My prop is a Tennessee Props (now Culver) 72x36 and my engine is a Cont A75. This engine develops its 75 HP at 2575 RPM, which is quite close to where the 2600 that the C85 develops its max power, so my guess is that you can use the same 72" diameter but can handle more pitch than mine has. Looks like you are currently swinging a 68" aluminum prop, so depending on the landing gear on your airplane and how much prop clearance you have with your current prop, you may be able to swing a longer prop when you get a new wood prop.

When I got ready to order my Culver, I just emailed them for a recommendation and what they sent me has been just about perfect... no trimming or re-pitching was needed. As another suggestion, perhaps another builder on this list has a spare wood prop that you can try to see how your engine turns up with it.

There are also some data points for static thrust with different engines and props, although those are going to be less helpful due to differences in prop maker's nomenclature, tach calibration, and other variables.

Oscar Zuniga
Medford, OR
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Pat Weeden
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Re: Prop Question

Post by Pat Weeden »

Good question, Oscar. In most cases, I post the question, then reply to the person with a direct link to the thread. They can follow along or choose to join the forum and chime in. Perhaps I should have them join and post their own questions?
taildrags wrote: Mon Feb 04, 2019 9:53 pm Pat; when we post replies to this type of inquiry, how do we know that the person who raised the question gets the replies? If they are not subscribed to the list, surely you don't have to forward them back and forth?

Oscar Zuniga
Medford, OR
Air Camper NX41CC, A75 power
Pat Weeden, Site Admin
Brodhead Pietenpol Association
PeterAlexander
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Joined: Tue Feb 12, 2019 1:59 pm

Re: Prop Question

Post by PeterAlexander »

Thanks for the help. I suppose I'm a leviathan... I've built two airplanes (Piet and a Sonex), lots of cars and motorcycles, yet I have never before used a message forum such as this. Good to be part of the 21st century!
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