Why isn’t my rpm at 2400 during full prop/full power

Nikhiln25

Nikhiln25

Pre-takeoff checklist
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nikhiln25
I’ve noticed this the last few flights. The engine prop are only a 100 hours old. Could this be a gauge issue? At full power full prop my prop only reads 2300 while it’s a 2400 max power prop.

Cessna 182 O470-U with McCauley CS prop
I’m taking it into annual in 2 weeks and will highlight to the A&P but trying to figure out what’s going on
 

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Did it used to? Did this develop in a short time frame? What is the static RPM listed in the POH?
 
Could be the governor setting, but most likely the gauge. Common for old mechanical gauges like that to be off. For example the tach in my plane reads exactly 100 RPM lower than the engine monitor at rated RPM. The engine monitor is measuring ignition pulses, so it's definitely right. It's also the "primary" gauge, so a little inaccuracy on my backup is NBD.

I THINK there's an optical tachometer phone app that can read propeller rpm, but I've not personally used it.
 
RyanB said:
Did it used to? Did this develop in a short time frame? What is the static RPM listed in the POH?
Click to expand...
Not 100% sure. I bought this plane 2 months ago with about 80 hours on the engine and prop. The A&P that did the pre buy did a static rpm check but I’m not sure where in the POH it says what it should be.

I did look at a video of a takeoff I did about a month ago and it was reading the same. I don’t think I’ve noticed it until now
 
Jim K said:
Could be the governor setting, but most likely the gauge. Common for old mechanical gauges like that to be off. For example the tach in my plane reads exactly 100 RPM lower than the engine monitor at rated RPM. The engine monitor is measuring ignition pulses, so it's definitely right. It's also the "primary" gauge, so a little inaccuracy on my backup is NBD.

I THINK there's an optical tachometer phone app that can read propeller rpm, but I've not personally used it.
Click to expand...
What would be the reason to set governor to a lower rpm than engine is rated for?
 
Nikhiln25 said:
What would be the reason to set governor to a lower rpm than engine is rated for?
Click to expand...
The way these gauges work is a cable spins a little magnet that induces a field that makes a cup that's attached to the needle move.


Over time, the magnet gets weak, the spring gets weak, the cup gets dirty, the bearings get dirty...a bunch of things stack up to reduce the accuracy. Your choices are to overhaul/replace the gauge or live with it.

Verify that's the actual problem though, as it could be correct and the governor could be out of adjustment.
 
The IA I use for annuals has a tachometer he just points at the prop. It has a selection for 2/3/4/5 blades. Not sure where he got it, but I've seen some on Amazon.
 
If I were to diagnose this....I'd start with an optical tach (Spruce sells them) and verify the RPMs. After that I'd make sure you have warm oil. Thick cold oil will not flow well and cause the governor to perform subpar. If both of those are fine....an adjustment at the gov can change the max RPM. Keep in mind you may not see max at static....you may need to be on the move to see red line.
 
When I first got my plane I bought this optical tach that I can put on the glare shield to double check mechanical tach accuracy.

www.aircraftspruce.com

Trutach II Optical Digital Tachometer | Aircraft Spruce

Trutach II Optical Digital Tachometer An inaccurate tachometer can waste performance, fuel and engine life. Mechanical tachometers are notoriously inaccurate and should be checked at each annual and 100 hour inspection or any time ...
www.aircraftspruce.com www.aircraftspruce.com
 
Checkout_my_Six said:
If I were to diagnose this....I'd start with an optical tach (Spruce sells them) and verify the RPMs. After that I'd make sure you have warm oil. Thick cold oil will not flow well and cause the governor to perform subpar. If both of those are fine....an adjustment at the gov can change the max RPM. Keep in mind you may not see max at static....you may need to be on the move to see red line.
Click to expand...
Thanks for this and to everyone else for all the suggestion

I’ll talk to the A&P see if he has a optical tach to verify the RPMs and if not I will order tthat

I don’t think it will be the oil because this pic is after a 2 hour flight where I did another takeoff so the oil is sufficiently warm
 
If you take a 60fps or 120fps video of your prop at 2400RPM, do the blades appear to stand still or move very slowly in the video? How about 1800RPM?
 
Nikhiln25 said:
I’ve noticed this the last few flights.
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Have you tried to obtain 2400 on the ground yet? And to note most rpm limits will have allowable variances like 2300-2400 and still be acceptable.
 
asicer said:
If you take a 60fps or 120fps video of your prop at 2400RPM, do the blades appear to stand still or move very slowly in the video? How about 1800RPM?
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I have a 30 fps video for takeoff where the blades are moving. I’ll take a test ar 60 and 120
 

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Bell206 said:
Have you tried to obtain 2400 on the ground yet? And to note most rpm limits will have allowable variances like 2300-2400 and still be acceptable.
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I have not tried full rpm on the ground yet will check that next flight
 
This is from the '77-'86 C182 service manual:

1735406381723.png

The TCDS for constant-speed prop-equipped airplanes will often not have a max/min static RPM specification due to that. The max RPM achievable in flight is what matters. Normally, it will come up soon after the start of the takeoff run.

So the FIRST thing is to get that tachometer checked. They do get weak and suffer various inaccuracies. In Canada we have a regulation regarding that:

1735406723557.png

(Ref. Car 625 Appendix C)
 
IR optical tach, or (acoustic, I believe) tach app on your phone. I don't know if the trick of backlighting the prop against 60Hz airport lighting will work with LEDs...
 
What’s your altitude on ground? I never see full power on the ground at Denver.
 
Dan Thomas said:
This is from the '77-'86 C182 service manual:

View attachment 136610

The TCDS for constant-speed prop-equipped airplanes will often not have a max/min static RPM specification due to that. The max RPM achievable in flight is what matters. Normally, it will come up soon after the start of the takeoff run.

So the FIRST thing is to get that tachometer checked. They do get weak and suffer various inaccuracies. In Canada we have a regulation regarding that:

View attachment 136612

(Ref. Car 625 Appendix C)
Click to expand...
Interesting thank you for sharing.

The picture on the top is while taking off, in the air. I’m going to see what I get in cruise at low altitude and also check for the accuracy
 
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455 Bravo Uniform said:
When I first got my plane I bought this optical tach that I can put on the glare shield to double check mechanical tach accuracy.

www.aircraftspruce.com

Trutach II Optical Digital Tachometer | Aircraft Spruce

Trutach II Optical Digital Tachometer An inaccurate tachometer can waste performance, fuel and engine life. Mechanical tachometers are notoriously inaccurate and should be checked at each annual and 100 hour inspection or any time ...
www.aircraftspruce.com www.aircraftspruce.com
Click to expand...
One for model airplanes works the same way and costs about 1/10 the price.

Hangar 9 Micro Digital Tachometer | Horizon Hobby

The Micro Digital Tachometer by Hangar 9 from our collection of the best RC plane Field Equipment, will help your dreams take flight. Find the RC airplane parts and accessories you need at www.horizonhobby.com!
www.horizonhobby.com www.horizonhobby.com
 
just like everyone else has said, the mechanical tachs can be off. Calculate your takeoff distance numbers and see how close they are. If you ate not making rated power and rpm I’d guess you’d see a noticeable increase from book numbers.
 
I recall from another post that you have a JPI 730. Buy the rpm sensor for that and install it. Less than $200 and dirt simple to install. You'll get a much more accurate indication than your analog tach. Not legal for primary but so what.
 
Nikhiln25 said:
What would be the reason to set governor to a lower rpm than engine is rated for?
Click to expand...
None. Just like anything with an airplane they can get out of adjustment after awhile. Tachs that read 100rpm low or a governor 100rpm out of adjustment are not unusual.
 
Ed Haywood said:
I recall from another post that you have a JPI 730. Buy the rpm sensor for that and install it. Less than $200 and dirt simple to install. You'll get a much more accurate indication than your analog tach. Not legal for primary but so what.
Click to expand...
Yeah I think I will be doing that now
 
Nikhiln25 said:
Yeah I think I will be doing that now
Click to expand...
While I'm spending your money, go ahead and grab an MP sensor too. It's even easier to install than the RPM, and I THINK they share a D-sub (check me on that one). Having the most accurate displays together will be handy for setting cruise power.
 
This IOS App works pretty well is most airplanes I fly. You have to estimate the RPM you expect and tell it what kind of engine i.e 4cycle, 4 or 6 cylinder and it will try to determine the RPM based on sound.
Some planes are so quiet it doesn’t work as well.

1735525891341.png
 
As others have said, old analog tachs often read 100 lower than actual RPM.
A better test is in cruise flight configure max power. Either full throttle, max RPM, mixture just rich of peak power, or the max allowable power configuration from the POH. Check your airspeed, compare with the POH and adjust for being lighter than max gross weight.
Another good test is what max climb rates the airplane achieves at altitudes up to 10,000', compared with what the POH says it should achieve, again adjusted for being lighter than max gross weight.
 
There is definitely a gauge issue. I did two tests today. The first is I did RPM at 1800 with a 60 fps video. The prop was not steady in the video

The second I did was an optical gauge app on the iPhone. I tested at various rpms and the props were only steady at a 100-150 above the rpms showed on the gauge.

So now this begs the question. Do I get this fix or just add probes to by 730 or upgrade to a primary engine monitor. Anyone have any idea how much the fix would cost?
 
Nikhiln25 said:
There is definitely a gauge issue. I did two tests today. The first is I did RPM at 1800 with a 60 fps video. The prop was not steady in the video

The second I did was an optical gauge app on the iPhone. I tested at various rpms and the props were only steady at a 100-150 above the rpms showed on the gauge.

So now this begs the question. Do I get this fix or just add probes to by 730 or upgrade to a primary engine monitor. Anyone have any idea how much the fix would cost?
Click to expand...
The model number of your tach will be on a sticker on the back or side of it. Look up the new price on Spruce and/or the exchange price on https://www.centuryinstruments.net/
That plus an hour or two of A&P labor is your cost to replace it. If you want to save a few bucks, an instrument repair shop can overhaul it, but the down time will be longer than a core exchange.

That illustrates the argument for going with a primary monitor. By the time you individually replace your tach, MP, OP, OT, and FP when each of them dies, you will have spent just as much as buying a primary monitor.

But since you already have the 730, IMO adding the RPM probe and ignoring the analog tach is the most cost effective solution for now.
 
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I would buy a new electronic tach, not another mechanical tach that will just make hassles. That tach likely also needs a new cable as well. More money. The electronic tachs run off P-lead signals, no moving parts to act up.

There are digital tachs, as well as electronic tachs with an analog display for those wanting to maintain the vintage look.

https://www.aircraftspruce.com/catalog/inpages/tso_tachometer.php
 
Interesting thanks I will look into that as well. I’m going to price out a primary upgrade and see what it will run me. Hard to know if any of the other mechanical instruments are accurate either (esp MP)
 
Suggest you check out the entire MAP system.

Some require a tiny bleed hole in the line.

Note that a leaking line to the instrument will present as a Higher

reading than actual.
 
Is the tach original? Replacing old tach you will have to document well so in the future someone can fig aircraft total time. Kinda shy away from buying aircraft with multiple tach replacements.
 
Yeah it’s an original Tach. I would imagine replacing it would require decent documentation of everything, esp replacing it as primary
 
New or overhauled tachs can be preset by the shop to match the correct hours, usually for a small fee.
 
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