RPM at Run-Up and take-off Roll

[Jim Collins]

I am wondering if someone in the group who has a similar engine and prop can answer this question.

The annual on my 1952 170B with the original engine (145hp) and a McCauley 7655 (cruse prop) was finished and when I went to do the take-off roll, it seamed sluggish and the RPM only got to about 2150-2200. So the take-off was aborted and repeated several times with my A&P/IA with the same results.

Everything on the engine was checked several times (Fuel system & flow, carburetor mixture both lean and rich, valves, compression, spark system, bore scoped) and I can describe all the checks if needed, but nothing was found.

My basic question is I remember getting more power before and the take-off RPM was around 2300, what is the full power static RPM you are supposed to get standing still and on the take-off roll with this engine and prop?

Because we can't find anything, I am starting to think I am imagining everything but I don't think I am.

Thanks

Jim Collins
C170B 1952 N2488D

[Bruce Fenstermacher]

Jim,

Taxi your plane to a point there is nothing blocking air-movement behind the airplane. LIke in the middle of a taxi way. I always face into the wind.

Hold the brakes and go full throttle. Do not allow the plane to move. Look at the rpm developed. This is static rpm.

For the McCauley 1A170 prop with a diameter not over 76" or under 74.5", your static rpm must be between not over 2330 and not under 2230 rpm. As you have a 55' pitch prop your static rpm would be very close to 2230. If you had a '53 pitch prop static would be right about 2280 and if you had a 51" pitch it would be close to 2330. These numbers are right out of the TCDS.

You can /should lean for best performance when doing this. Be careful what might get sucked into the prop like dirt or gravel.

Everyone should do this periodically and record the result. It is really the only performance test easily done anytime power is in question.

[voorheesh]

Did your mechanic check the rpm gauge for accuracy?

[n2582d]

... I always face into the wind. ...

Bruce,
That's cheating! You're going to get a higher static RPM than "normal" by facing into the wind. Here's how Cessna says to do it in the C-172R 1996 and On Service Manual:

B. Static Run-Up Procedures.
(1) Align airplane 90 degrees to the right of wind direction.
(2) Run up engine at full throttle in accordance with procedures outlined in the Pilot's Operating
Handbook and FAA Approved Airplane Flight Manual.
(3) Record RPM.
(4) Realign airplane 90 degrees to the left of wind direction and perform second run-up.
(5) Record RPM from second run-up.
(6) Average the results of the RPM from the two run-ups.
(a) For the 172R, RPM must be from 2065 to 2165 RPM.
(b) For the 172S, RPM must be from 2300 to 2400 RPM.

NOTE: Variances in atmospheric pressure, temperature and humidity can have a significant impact on run-up RPM. Low static run-up RPM information should be used only in conjunction with other troubleshooting procedures to determine if a problem actually exists.

(7) If run-up indicates low power, check the following items:
(a) Do a check of the operation of the alternate air door and make sure the door remains closed in normal operation.
(b) Do a check of the magneto timing, spark plugs and ignition harness for settings and condition.
(c) Do a check of the fuel injection nozzles for restriction and check for correct unmetered fuel flow.
(d) Do a check of the condition of the induction air filter. Clean or replace as required.
(e) Do an engine compression check.

I'd add: (f) Calibrate your tach with a digital tach checker. Edit: Looks like Harlow beat me to this suggestion.

[Bruce Fenstermacher]

Bruce,
That's cheating!

Caught me Gary. Once again you have found a written procedure I've looked for but never found. Searching for the definition of static rpm finds the simple explanation it is the attainable rpm with the aircraft not in motion.

I'm rarely flying in winds that matter. Nor would I take a single check as absolute. Why not make a check into the wind and then repeat tail into the wind and average the result.

[GAHorn]

I think “static” RPM should be checked in “static” conditions at sea-level, standard atmosphere. This would be in a No Wind condition…which is what Cessna is attempting in that procedure.

Tachometers in these airplanes are typically mechanical and prone to IN-accuracies and their mounted positions can have Parallax errors. Use a DIGITAL TACH-CHECK to confirm your tachometer accuracy (as has already been suggested). I have one I can loan-out if anyone will pay postage…but by the time you do that..you can OWN ONE …and SHOULD own one for your toolbox anyway.

https://www.amazon.com/Hangar-Micro-Dig ... 1iEALw_wcB

[n2582d]

Why not make a check into the wind and then repeat tail into the wind and average the result.

According to this entry that is how McCauley recommends doing it. I don’t find that information directly from McCauley now but it looks like MT-Propeller has identical instructions:

Click to Enlarge

[Jim Collins]

Thanks everyone for the valuable information. This organization is always most helpful.

The almost new tach does have a problem and was factored into the RPM numbers I gave as confirmed by the tach on a prop balancer tool we used to compare against. The tach reads 130RPM low.

I suspect the problem might be in my head as the RPM numbers supplied by Bruce match almost exactly to what was measured.

My plan is to fly the airplane and if the cruse speed is still the same, then there is no problem (other than the tach). if the plane is slow, we'll keep looking.

I suspect but can't prove, that the tach was reading correctly and is now not, which through me off. Again, a flight will show it one way or another.

Thanks again.

Jim Collins
170B 1952 N2488D