Things aren't always what they seem...

[48RagwingPilot]

Background:

'48 C-170
C-145 (400 SMOH) & running strong
McCauley 1A170 DM7653 prop (per logs & stamp)
Static run-up 2235 RPM (Prop Tach)
Climb 800 FPM from 500' to 2000' AGL (pilot, half fuel, no back seat)
Cruise 126 MPH IAS @1500 AGL

I've only been able to fly the airplane three times since I bought it last December, but always thought the takeoff and climb numbers were a bit below par and that cruise performance was a bit above par.

So, being the curious sort, and having educated myself a bit through this association, I took the prop into NW Propeller Service in Puyallup, WA for a checkup. To my surprise, I discovered the prop was actually a McCauley 1A170 DM7557, despite what was recorded in the logbooks and stamped on the prop. So, I had it it repitched to 53" (at least it didn 't have be restamped!) and hope to test it out this weekend, weather permitting.

Thanks to all who share their knowledge with the rest us!

[Bruce Fenstermacher]

If it is a 75" and that was not a typo I'd expect the RPM to be at or closer if not the max allowed static. Your climb will improve but of course your cruise will be less than it was for the same RPM.

Your original numbers were not that far out of wack. The wild card is what effect the 1" shorter prop will have. Hate to say it, and I hope it works out for you, but there might have been a reason for the courser than normal pitch. If it would have been my prop I might have only taken it back to 55 or not more than 54.

[48RagwingPilot]

Thanks, Bruce. I'll report back when I get real-world numbers so we can include them in the repository of knowledge.

[Sixracer]

I have a spare prop that I'm thinking about having re pitched and checked out for my 1961 172B 0-300-D.
I'm in East Tx /SW Ark. Have a: 1C172/EM7653 Thinking about taking it to a EM7654 or a EM7555 if it has to have tip rework. I cruise a lot more lengthy flights that T/O & landings (around the patch) and I do start the flights close to gross weight with full or almost full fuel.
Any suggestions.

[GAHorn]

I have a spare prop that I'm thinking about having re pitched and checked out for my 1961 172B 0-300-D.
I'm in East Tx /SW Ark. Have a: 1C172/EM7653 Thinking about taking it to a EM7654 or a EM7555 if it has to have tip rework. I cruise a lot more lengthy flights that T/O & landings (around the patch) and I do start the flights close to gross weight with full or almost full fuel.
Any suggestions.

I'm laughing at this situation.
I have an 1800-hour EM7655 installed and own a 200-hour EM7652 I'm thinking of installing to improve field performance. I'm just concerned what that will do to my cruise performance and afraid that in order to match-up the numbers, I'll have to paint the plane green.

(I'm toying with a top overhaul and installing the EM7652 or twisting it to 53" before convention this summer. If your 53-pitch is not what you want maybe we should trade it for my 55 after you try it and like it.)

[Sixracer]

Well, I guess that goes to show we all travel to a different drum beat!! LOL!
This extra prop has no tags or paperwork with it and it appears to be identical to the one on the plane. I can't find anything about it's history. That is one reason I was thinking about sending it in for a re pitch.
I guess we could meet at convention and swap props!! ?? LOL!
I'll bring my Green paint!

[Bruce Fenstermacher]

Here is my suggestion with my vast experience of twisting two propellors.

No two props are the same even if they are marked the same. I would put the prop you intend to modify on the airplane and fly it. Using a calibrated tach measure the actual RPM you get with it static, climb and cruise. Then using rules of thumb I'd have the prop twist to the pitch for the performance I wanted. With in applicable limits of course which is usually static RPM.

I always forget the rules of thumb but a good prop shop should be able to guide you as well as the prop manufacturer. I seem to remember for every inch of pitch you will gain or lose 50 RPM. There is a rule for changing diameter and I remember that to also be 50 RPM for every inch of length gain or loss. PLEASE check my memory with another source before applying these stated rules.

Changes in diameter are a bit harder to estimate change effect because of the change to the blade area. It is not an exact science. So if your changing length and pitch you will have to swag for each change. Be prepared to change the pitch twice to dial in the desired effect.

BTW as a matter of interest there is a STC for the Piper Cherokee to modify the tip of the Sensenich prop, in order to reduce drag. This is basically an under cut to the tip like a modern wing tip. Less drag means the engine can swing the prop faster with the same effort. So more pitch can be added to the prop to maintain the same RPM but now you will cruise faster. Or you can leave the pitch the same and in effect reduced pitch and have better climb performance. It is said to be like adding or reducing pitch by 2 inches. The basic idea has been used on formula one racers. My point of mentioning this is to show that the design or change to design of the blade, even ever so slight, can have a dramatic effect on prop performance.

[blueldr]

When you get to thinking about repitching a fixed pitch prop, shop around for repitching prices. I have seen a SERIOUS (read: Big Dollars) difference between prop shops.