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

One good thing about analog tach and MP: they do not require power. If you have an issue with your electrical system, you will still have engine and prop indication. Likewise for the original OP, OT, and FP.
 
If you have iphone or ipad, search the App store for “video tachometer”. Its a free app that uses the strobe effect, and allows one to adjust the video frame rate to measure rpm. Very simple to use. Simply adjust the rpm on screen, until the propeller stops turning.
 
The good news is, your current tach records hours slower, saving you 4% on all your maintenance and depreciation :cheers:

I'd put the RPM on your 730 and fly on, personally...
 
Jim K said:
The good news is, your current tach records hours slower, saving you 4% on all your maintenance and depreciation :cheers:

I'd put the RPM on your 730 and fly on, personally...
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I think this is the way I’m leaning. Though this brings up a question. If I have a known bad primary rpm gauge am I required to fix it? A tach is required equipment, can it have known deficiency ?
 
wsquare said:
If you have iphone or ipad, search the App store for “video tachometer”. Its a free app that uses the strobe effect, and allows one to adjust the video frame rate to measure rpm. Very simple to use. Simply adjust the rpm on screen, until the propeller stops turning.
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That’s the one I used - great app
 
Nikhiln25 said:
I think this is the way I’m leaning. Though this brings up a question. If I have a known bad primary rpm gauge am I required to fix it? A tach is required equipment, can it have known deficiency ?
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I don't think there's a regulation saying how accurate a gauge has to be. Might be worth looking at AC43.13 to see if there's guidance in there. As the PIC, you determine airworthiness. At least until annual time. It'd be worth having the discussion with the AP who's going to put the JPI sensor on and the IA who's going to sign off the annual. Their opinion will matter much more than SGOTI. My tach is off 100 RPM, but I have a JPI 930 as primary, so the mechanical tach is not really required anymore.
 
8-95. TACHOMETER INSPECTION.
Due to the exceptionally high stresses that may
be generated by particular propeller/engine
combinations at certain engine revolutions per
minute (RPM), many propeller and aircraft
manufacturers have established areas of RPM
restrictions and other restrictions on maximum
RPM for some models. Some RPM limits do
not exceed 3 percent of the maximum RPM
permitted, and a slow-running tachometer can
cause an engine to run past the maximum
RPM limits. Since there are no postmanufacture accuracy requirements for engine
tachometers, tachometer inaccuracy could lead
to propeller failure, excessive vibration, or unscheduled maintenance. If the tachometer exceeds 2 percent (plus or minus) of the tested
RPM, replace it.
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From AC43.13B, page 8-40, propeller section. So there's the official answer.
 
When a Tach reads LOWER than Actual it’s common for the pilot to push the Throttle to obtain the “ Normal “ readings anyway. This results in HIGHER power than indicated. Also AD’s , Inspections, etc will come around sooner.

My belief is that the reason some airplanes are “faster” than similar is because of poor instrumentation on one or both.
 
So now I’m question the accuracy of the MP as well. This is because I’ve recently noticed when I set 22/22 my engine runs a little rough. That in reality is 22/23 because my tach is off, so it shouldn’t be. But if I increase my rpm the engine runs smooth. So I wonder if my tach is the opposite way. If I pull out some MP it runs smoother as well. But if I pull my prop back to run 22/22 it runs a little rough. It could be something else but now I’m just worried about accuracy of every gauge
 
Jim K said:
The good news is, your current tach records hours slower, saving you 4% on all your maintenance and depreciation
Click to expand...
Unfortunately, it doesn't work that way. The tach's input shaft is spun by the drive cable at engine camshaft speed, and that shaft spins a magnet inside an aluminum cup. The cup is attached to the indicator needle. The magnet creates electron flow in the cup that creates its own magnetic field that interacts with the spinning magnet, and the drag created pulls the needle around against a spring. Faster input shaft rotation is translated into a higher reading.

But the hourmeter part of the tach is geared off that input shaft, which still spins at camshaft speed as usual even if the magnet goes totally dead and the RPM is no longer indicated. The hourmeter still racks up the time at the usual rate.
 
Dan Thomas said:
Unfortunately, it doesn't work that way. The tach's input shaft is spun by the drive cable at engine camshaft speed, and that shaft spins a magnet inside an aluminum cup. The cup is attached to the indicator needle. The magnet creates electron flow in the cup that creates its own magnetic field that interacts with the spinning magnet, and the drag created pulls the needle around against a spring. Faster input shaft rotation is translated into a higher reading.

But the hourmeter part of the tach is geared off that input shaft, which still spins at camshaft speed as usual even if the magnet goes totally dead and the RPM is no longer indicated. The hourmeter still racks up the time at the usual rate.
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So you're saying the tach time on my plane is accurate?

Damn :biggrin:

At least I'm still saving by pulling the RPMs way back. Actually I'm saving even more right now because my engine is in pieces scattered around the midwwest :(
 
Figured. So if I upgrade my 730 to add the tach, I gotta replace the existing tach. Or just upgrade to the primary
 
Bell206 said:
Yes. 91.405(a)

No. As it would not conform to its type design and thus be unairworthy which takes you to 91.7(a).
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I recall seeing flight school planes with tachs placarded as "add 75 rpm to indication" or similar. Would that be legal?
 
AV8R_87 said:
I recall seeing flight school planes with tachs placarded as "add 75 rpm to indication" or similar. Would that be legal?
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As long as the difference was within the acceptable accuracy tolerance limits of the indicator. The placards were/are an old-school shop solution to satisfy customers. Most indicators allow tolerances up to 100 rpm.
 
My A&P advice was to placard it and move on. He said no mechanical gauges are precisely accurate, and getting another mechanical gauge won’t solve it.

Since this plane is new to me, he also recommended waiting 2 years before making any avionics/EM upgrade as I may want to my panel to look different in 2 years and not be happy about the add on I make today. Seems to make sense
 
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Nikhiln25 said:
My A&P advice was to placard it and move on. He said no mechanical gauges are precisely accurate, and getting another mechanical gauge won’t solve it.

Since this plane is new to me, he also recommended waiting 2 years before making any avionics/EM upgrade as I may want to my panel to look different in 2 years and not be happy about the add on I make today. Seems to make sense
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Good advice ... but since you already have the 730 display, the RPM and MP sensors are a lot of bang for minimum bucks and won't charge the look of your panel at all.
 
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