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Checking tachometer accuracy

I have been trying to find a hand-held digital rpm meter to check the accuracy of the ageing mechanical engine rpm gauge in my SF260. There are various types used by model aircraft builders but all seem to have a limited range of about 500mm which obviously isn’t suitable for use in the cockpit. Does anyone have any suggestions for a supplier of a suitable meter that doesn’t cost more than a new tachometer from Adams or Aircraft Spruce?

El cheapo: mobile phone with free guitar tuning app. Let the engine run, search for peak, multiply Hz by 60 for rpm. If necessary, divide by number of prop blades, cylinders, if applicable.

Last Edited by a_kraut at 12 Mar 13:22
Bremen (EDWQ), Germany

Is it for use in flight?

It might be fiddly in flight, but basically yes. You get a pleothora of resonance frequences, but you see the main frequencies peaking out

Last Edited by a_kraut at 12 Mar 14:18
Bremen (EDWQ), Germany

Something like this for $30 USD might work for you.

My experience with analog tachs was checking and replacing and old AC unit because it was 50 rpm low, and finding that the new Mitchell unit was inconsistently inaccurate and often 100 rpm high – in other words worse than the original. The good news is that I have an electrically controlled propeller which controls to the setpoint within about 5 rpm, regardless of what’s shown on the tach.

I have for perhaps 15 years used the “Proptach” from Cardinal Electronics, Lansing, Michigan. It run on a standard 9V battery and is as simple to use as can be. Switch it on, check the 2 or 3 blades switch, put it on the glareshield and let it look at the prop. Unless it is dark or the sun it right it its “eye”, it will provide a RPM readout with a 1 RPM solution and a 1 RPM accuracy (quartz). Whether it is still on the market, I have no idea. Back then it cost around $ 300, AFAIR.

In my experience, standard mechanical tachometers read between 0 RPM and 250 RPM low, with an estimated average error of around 40 RPM low. Reading high is very rare, I recall only one, and that was only at specific RPMs.

Having a tach read 200 RPM low has a number of implications, one of which was demonstrated many years ago when it was deemed partly responsible for a pilot running out of fuel. Leaning at a power setting too high to allow leaning (setting 85% thinking the engine is doing 75% pwr), and thus risking too high CHTs, is another problem.

An overreading tach is a very common cause of planes apparently overperforming, when compared to indicated RPM. Many owner pilots do not react appropriately to this. Beating POH speeds is something that makes some aircraft owners very, very proud, and so any suggestion – or worse, any proof – that a miscalibration could be involved, should be presented carefully, especially if the owner is a valued friend, in which case it could be considered to let someone else break the sad news.

Edit: no need to look for the “Proptach”, if the unit Silvaire mentions above does the job, it is very much cheaper. Same working principle, I think.

Last Edited by huv at 12 Mar 19:16
huv
EKRK, Denmark

Will those hobby ones work on an aircraft? It says the distance in the one in Post 05 is:

Distance: 50 to 500mm/ 2 to 20 inch

Buying, Selling, Flying
EIBR, Ireland

At Inverness EGPE the maintenance firm lent their device to allow an O200 tach to be checked. I saw the small box, but not the device. I believe it was placed on the dash and the engine run.
Your maintenance organisation might lend it to you, as they’ll get trade if it’s inaccurate.

Maoraigh
EGPE, United Kingdom

Following from A-Kraut’s post:

There are some mobile phone apps for model helicopter fliers to check their rotor RPMs based on sound. They may work for aircraft. My guess is that they may either be very precise, or wildly inaccurate – i.e. if they are picking up on a harmonic you may get a multiple of the RPM multiplied by the number of blades, or perhaps they will only hear the engine noise (which if you don’t have a gearbox will still give you a multiple of the RPM).

Google also shows some apps like this:

https://play.google.com/store/apps/details?id=com.javiery.rpmgauge&hl=en

Last Edited by kwlf at 12 Mar 23:02

I would not recommend the cheap handhelds. They are unreliable and inconvenient to use.
Either a Proptach or a Truetach II placed on top of your panel in plain sight will give you the comfort, accuracy and constant readouts you need.
https://www.aircraftspruce.com/catalog/topages/trutach.php

Last Edited by cessnatraveller at 13 Mar 08:42
United Kingdom
15 Posts
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