propeller efficiency curves

[Sixracer]

Bringing this thread back to life.
I found a listing for my 172:
Prop listing for 172 Climb Normal Cruise Max Min Static RPM
172B O-300-C (TCM) 1C172/EM7652 1C172/EM7653 1C172/EM7655 76.0 74.5 2350 2230
172B F O-300-C (TCM) 1A175/SFC8040 80.0 78.4 2480 2380
172B O-300-D (TCM) 1C172/EM7652 1C172/EM7653 1C172/EM7655 76.0 74.5 2350 2230 XXXXXXXX
172B F O-300-D (TCM) 1A175/SFC8040 80.0 78.4 2480 2380
F= foreign STC approval
Mine is marked with the XXX
What is or how is the static RPM tested?
Is the RPM range due to DA differences or engine efficency? Or is that the difference in Climb, Normal & Cruise props?
I have checked the prop flange on my engine is a 6 bolt. I guess the next step is to pull the spinner and see if I can determine what prop I have. Then I can do a static test, after someone tells me what it is>> :~)
I'm interested in setting up for some 400 mile trips. I have no need for STOL performance. Anyone have any ideas what mods & prop I need?

[Bruce Fenstermacher]

Four things determine the speed an engine will spin a prop. The atmospheric conditions at the time, engine performance or health, prop diameter and prop pitch.

172B O-300-C (TCM) 1C172/EM7652 1C172/EM7653 1C172/EM7655 76.0 74.5 2350 2230

You will note that in the above example static RPM is given for a range of prop pitch in this case from 52-55 and prop diameter 74.5"-76".

If you read the TCDS for your 172 you would probably find the pitch of the prop is not a limiting factor but the static RPM and diameter are. So as long as you have a 1C172/EM prop that is at least 74.5" but not more than 76" in diameter and the static RPM is within the range of 2230-2350, you have a legal prop. A 1C172/EM prop pitched between 52 and 55 should fall into the static rpm range available. A 52 pitch would be near or at 2350 and the 55 pitch would be near or at 2250 all depending on the diameter of the prop.

So what and how do you find your static RPM? Well remember the atmospheric conditions at the time not only determine drag on the spinning prop but engine performance. I don't know for sure but would bet the static RPM given is for a standard day and no wind. A standard day being sea level altitude, 29.92 on the barometer and 59 degrees. The important thing to keep in mind is any variance from that of the conditions for the limits set may render a different result. In reality unless you are extremely far away from the set conditions you aren't likely to see much variance, specially with our bouncing mechanical tachometers. Which brings up another point. No sense in doing this test unless you know your tach is accurate and since most mechanical tachs are not at some point in their range a digital tach is almost a must.

So how do you do a static test. Well static means not moving. So find the atmospheric test conditions or as close as you can and with the airplane not moving, run the throttle to full power and note the maximum rpm. That is basically it. Of course you may want to lean your engine to max rpm depending on the result and what your trying to accomplish.

Cautions are warranted when doing this test. One your creating a lot of thrust. Make sure your not blowing it on anything or that anything will be sucked into the prop. Second you will likely have to secure the aircraft to something because the brakes are not likely to hold it at full throttle or at least this is the case with lots of airplanes. If you secure it it should be in a open area, don't tie the tail to your hanger building directly behind the airplane.

Sixracer you say you are setting up for some 400 mile trips and don't need STOL performance. Well to be honest there probably isn't any economy changing what ever prop you have. If you happen to have a 52 pitch prop you can probably cruise at 100 mph at least making the trip 4 hours. If you repitch to 55 and you cruise at 115 mph you'll get there in 3.5 hours. Your potty breaks will make more difference in time of travel. Now if it was 4000 miles then I might tweek the prop with more pitch but only if it were a 52 pitch.

Of course no matter what it is always nice to know more about how your plane is equipped.

[Sixracer]

Yesterday while finishing up the baffles I did do an inspection of the prop. It measures very, very close to 76" It doesn't have any visible numbers. I think the spinner mounting plates are covering them up. I did recount the bolts and as Aryana said it IS a 6 bolt prop. I didn't do a static RPM test. I was too interested in doing some pattern work and in trying out the cooling mods before dark. I might go down today and get things warmed up and do a static RPM test. In all my Prop searching I did find a 1 C 172 EM 7654 number. I'm thinking that would be between a Std prop and a cruise prop.
I also found a listing for a SENSENICH prop that was smaller diameter 74" (that is 2 inches shorter) but it was a 56 pitch. The pitch is more than the cruise prop that McCauley offers for the 172 (55 vs 56) wouldn't a smaller diameter prop be a lot lighter and take less power to swing? Any advantages to the smaller diameter??
I did get some good test results in the cooling mods. (results posted in other thread)
I'm thinking about some instrumentation, like fuel flow and O2's, to be able to monitor in flight economy/effiency. Forum, any thoughts??

[Bruce Fenstermacher]

Sixracer, if the Sensenich prop is not listed on the TCDS or you have another bases of approval like an STC, you can't use it no matter how it would perform. If it is listed and the 74/56 is a combination in the TCDS I'd bet it performs no better than the McCauley. If it is anything like the McCauley vs Sensenich props allowed for the 170, the McCauley outperformed the Sensenich.

A Sensenich prop will not be the same blade design as the McCauley and so an apples to apples comparison of just the length and pitch is not possible.

Yes if you cut your McCauley down to it's minimum length of 74.5" your engine might be able to swing it faster however it will likely not be as effective as it was at full length so you've gained nothing. We could fill books (it's been done) on prop aerodynamics it is not a simple subject.

[Sixracer]

Thanks for the good info. No sense spending time and money chasing the wrong rabbit.

[Bruce Fenstermacher]

No sense spending time and money chasing the wrong rabbit.

I'm not really sure what rabbit your chasing.

Understanding what you have and how or why it performs as it does compared to others is fine. It seems you are bent on changing your prop. Why?

[Sixracer]

I was, but since I have had some good answers to my questions I'm not hunting that Rabbit any more. Thanks...I'm going to stick to restoration issues like dash, paint and interior. Cessna & Mc Cauley have engineers that are trained in the aircraft field, I'm not. I will determine if I have a cruise or climb prop. Just like you said: For the purpose of knowing what I have. I appreciate all the info.