Prop pitch question

[Richgj3]

I’m currently running a 76/54 on my 1952 170B with the C145. At least that’s what the stamp on the hub says. I’d like better climb performance without sacrificing too much cruise. Right now on a standard day at sea level I indicate 120 mph at 2450 rpm.

If I went to a 76/51 what’s the guess on cruise? And what’s the guess on static rpm? Now I barely make minimum static on a good day.

Another way to ask is what’s the expected change in static rpm for each one inch change in pitch, all else being equal?

Thanks

Rich

[Bruce Fenstermacher]

What exact prop blade design do you have? A 54" pitch prop is not a common pitch for a McCauley 1A170 DM prop. Or do you have a 1C172MDM? there is a difference.

A rule of thumb is between 50 rpm change for every 2" of pitch change. It is not an exact science though. No two props are the same even though they may be marked the same because there are variances in prop manufacturing and then if used, wear is different.

On a 170, with a McCauley 1A170 DM prop at 76" a 51" is climb, 53" pitch is standard and a 55" is cruise. On a 1C172 MDM its 52", 54 and 56". The 1C172MDM has a thinner blade profile and it is often referred to as a "California Twist" prop.

As for speed differences, there are also lots of variables very hard to determine. However you can create a formula that should give you a pretty close idea. A 53" prop means in theory, the prop would move forward 53" every turn, multiply that by RPM and you have the distance traveled in 1 minute, multiply that by 60 to get distance traveled in one hour. Divide that by 62760 to convert from inches to miles per hour. Inch x RPM x 60 / 62760. In your case plugging in your reported numbers you get 126 mph. You say you see 120 mph or 94.8% less so multiply your number by .948. or (Inch x RPM x 60 / 62760) x .948. Now you can change any variable a and get a good idea what will happen.

Drag, atmospheric conditions and inaccuracies in your tach and airspeed are what is being compensated for by the last factor.

And last thing. If you decide to get your current prop repitched, ask the prop shop to measure your prop pitch and may a correction say of 1" for example, to it if that is what you desire. Don't say change this to a 53" prop cause you don't really know what you have to begin with.

Also discuss with the prop shop, the entire process you used to come up with the number you came up with for your desired change and your expected result. Prop shops do this all the time. Make sure they concur. They just may know what they are doing.

[GAHorn]

And then…there are the six-bolt EM series of McCauley props which add 1” more to the ratings, 53’, 55”, 57” for climb/std/cruise.

[Richgj3]

Sorry for not being clear. I have a McCauley 1A70 76/53 that sometime in the dim, undocumented past was repitched and restamped 54.

It is an 8 bolt prop. Thanks for the formula. Speaking of the dim past, I knew that once. If I remembered that I could have saved that part of the question. And thanks for rpm approximation. That helps.

Rich

[Richgj3]

Bruce

Your explanation about the difference between the 170 props and the 172 props may explain another question. I owned a “170C” from 1986 until 1992. It was a 1958 C172 with the Bolen TW STC. All in all, if my memory serves, it was slower in cruise than my 170B. I can’t remember or maybe never knew what prop was on there. But I can certainly see a difference when going out of the same grass field today with the B that I used to base the 172 at. And I weigh 50 pounds less now than I did then!

The 172 was a better short field performer to be clear.

[GAHorn]

As a comp, I changed my EM7655 last year for the Convention/AirVenture trip to an EM7652 hoping for better short-field performance (and expecting slight loss of cruise)…. and frankly ….only IMAGINE there’s a difference…. it is NOT obvious.

I raised my cruise RPM from 2450 to 2500…as an experiment….and as far as i can tell…the only chg was aa fuel-burn chg from 7.8 gph to 8.