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Went for a prop balance yesterday and the phototach on the balance instrument was showing significantly off from my installed tach.  Looking through the SM I saw no mention of an adjustment.  But being electronic counters, how could it be off 8%

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Often electronic doesn’t mean digital, and often analog electronic devices can be quite inaccurate. It dives me nuts that people will buy something that displays to two decimals so they are sure it’s accurate to that, and that’s not always true.

I have no familiarity with your tach though, but if it’s a hall effect digital system, it really can’t be off, unless of course the sensor’s gap or whatever is off, and that usually shows erratic readings. not a steady inaccuracy

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Mooney has used several tachometers. The B&D electric tach on my 1994 M20J uses a tach generator that puts out a voltage proportional to rpm. So, it is electric, but not digital.

Skip

 

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Expect any instrument to be sensitive to some environmental factor...

The lighter, and lower cost the instruments is... the more errors they can be susceptible to...

Find the manufacturer of the instrument, to find its calibration method...

Trust no instrument be accurate... and you won’t be disappointed. :)

 

Using an independent light based rpm sensor can be very accurate...

Each smart phone has the ability to do something similar... using sound...  find your App Store.

A Hall effect sensor is essentially counting electrical events...

Look for what is causing the inaccuracy... might be as simple as a dirty sensor... not being counted properly...

 

When it comes to not seeing the proper rpm...

When my engine delivers...

  • 2500rpm instead of 2700...
  • It makes 280hp instead of 310hp
  • a 10% difference in output...
  • T/O roll goes from 800’ to 1200’ under the same conditions
  • a 50% difference in runway used...
  • FF 25gph instead of 28gph

 

To put this into perspective with the OP’s 8% rpm shift...

That’s nearly the same as my example above... 200rpm off from 2700rpm.

 

With a smartphone in your pocket... don’t forget to check your rpm reading every now and then...

 

On each T/O a quick scan of MP, RPM, and FF... the first two tell you have full power... the third piece of data confirms the first two are reading correctly... :)

Expect that your T/O performance was off from book values... climb rate was a bit off as well....

Proof that all of our instruments are supplying important information...

 

OK... final important thought...  was that 8% error... an overspeed or an under speed?

Expect it was an under speed...  An overspeed may be showing an impressive bump in unexpected hp...

There may be some amount of corrective action to go with this anomaly...

Has the engine been set up to run with this error in rpm?  Make sure with proper rpm, the proper FF is there...

 

PP thoughts only, not a mechanic...

Best regards,

-a-

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Service manual mentions two options.  231s apparently used one that was counter based connected to the leads from the mag switch, not unlike a Horizon I assume.  252s apparently had one driven by a Hall sensor in one mag.  Not sure which I've got with a 12V plane and a -MB engine with pressurized Bendix mags.  Need to look more.  And get another photo tach to check again.

180 rpm difference between my tach and balance rig has me concerned.

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Get tach app.

Use phone to independently read rpm.

Compare to ships tach.

There is a high probability of two things...

One worse than the other...

1) Something funky happened when they set up the balancing machine.... some old setting was left in the machine.

2) Your seat of the pants sensor needs to be checked... :)

180rpm is a big difference...  Chevy Malibu acceleration instead of Chevy Corvette...

This should be noticeable in the acceleration from 0 to Vr... as the thousand foot markers go by...

When discussing prop related things... I like to invite @Cody Stallings to the conversation... (8% rpm error, 180rpm)

PP thoughts only...

Best regards,

-a-

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2 hours ago, geoffb said:

Service manual mentions two options.  231s apparently used one that was counter based connected to the leads from the mag switch, not unlike a Horizon I assume.  252s apparently had one driven by a Hall sensor in one mag.  Not sure which I've got with a 12V plane and a -MB engine with pressurized Bendix mags.  Need to look more.  And get another photo tach to check again.

180 rpm difference between my tach and balance rig has me concerned.

Maybe. My M20J service manual doesn’t list the B&D tach I’ve got. But it’s in the IPC. 

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I woudnt use a phone app for anything except curiosity, I wouldn’t adjust aircraft instrumentation to a phone.

‘There are two types of optical tachometers passive and active, active is like the prop balancer in that it transmits a light and reads the reflection. Passive just works off of reflected light of course, passive tachs are relatively inexpensive.

Funny thing is any passive tachometer I’ve ever seen can be validated by pointing it at a fluorescent light, it will indicate 3600 RPM as it sees 60 flashes per second. It won’t work on an incandescent bulb though. If you feel the need validate the Hz rate with your multimeter, places with 50 Hz rate electricity will show 3000 RPM of course.

‘This is my tachometer, Circa mid 1960’s from North American Rockwell, last calibrated in 1996.

‘It’s surprising how accurate it is, even though it’s an analog device of course.

Pretty much all old aircraft instrumentation should be treated as suspect, best use is for trend monitoring, meaning it is indicating the same as always or has it changed? If it’s different I need to verify it’s correct before anything else.

But please don’t do things like adjust your engines oil pressure so your 40 year old  never been calibrated instrument indicates what you want to see.

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Edited by A64Pilot
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I haven't used a strobotach in something like 30 years.  Fun artifact you have there.

I'm going to put a optical tach on it as soon as I have time.  My question is more related to adjustment of the tachometer if I find out it's really reading 8% low.

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