Hand held tachometer - need recommendation

[Yourpilotincommand]

I wanted to check the accuracy of my tach. Is there an inexpensive RC model plane tach I could stick out the window and aim at the prop while I throttle up the engine?  I’m not getting the full 2700 rpm during takeoff roll. I’m seeing only 2550,  but I always do feel very good power. I may not be getting the most power that I should, and I want it all.  I first wanted to eliminate the tachometer before I have my mechanic make governor adjustments, etc.. I’ll also check the governor cabling for slippage, etc. just trying to eliminate the simple things first.  thanks, Chris

[EricJ]

6 minutes ago, Yourpilotincommand said:

I wanted to check the accuracy of my tach. Is there an inexpensive RC model plane tach I could stick out the window and aim at the prop while I throttle up the engine?  I’m not getting the full 2700 rpm during takeoff roll. I’m seeing only 2550,  but I always do feel very good power. I may not be getting the most power that I should, and I want it all.  I first wanted to eliminate the tachometer before I have my mechanic make governor adjustments, etc.. I’ll also check the governor cabling for slippage, etc. just trying to eliminate the simple things first.  thanks, Chris

Just search Amazon or your favorite supplier for "optical tachometer" or "non-contact tachometer" and you should get a long list of options going up from around $20.   I've used the cheap ones on various instrumentation installations for different rotating things and in my experience they all work about the same.    Some will pick up both blades, even if only one has a reflector, but that's easy to just divide by two to get a reliable estimate. 

It might be tricky to get one with that you can use from the cockpit, since that's a bit more distance (plus through a windscreen) than most usually handle easily.   In the applications where I used them the gap from the sensor to the test item was never more than a few inches.

[ZuluZulu]

https://www.aircraftspruce.com/catalog/topages/trutach.php

[Andy95W]

Here's one for 20 bucks.  Even mentions airplanes in the write up.

https://www.amazon.com/Non-Contact-Handheld-Tachometer-Rotational-Measuring/dp/B07FM2Y6Z9/ref=asc_df_B07FM2Y6Z9/?tag=hyprod-20&linkCode=df0&hvadid=343187910376&hvpos=&hvnetw=g&hvrand=1629442965056569980&hvpone=&hvptwo=&hvqmt=&hvdev=t&hvdvcmdl=&hvlocint=&hvlocphy=9016902&hvtargid=pla-724128135766&psc=1&tag=&ref=&adgrpid=69721953515&hvpone=&hvptwo=&hvadid=343187910376&hvpos=&hvnetw=g&hvrand=1629442965056569980&hvqmt=&hvdev=t&hvdvcmdl=&hvlocint=&hvlocphy=9016902&hvtargid=pla-724128135766

[Jeph357]

2 hours ago, ZuluZulu said:

https://www.aircraftspruce.com/catalog/topages/trutach.php

Plus 1. I have this one. Calibration  is done with any fluorescent light

[SSimpson77]

https://hackaday.com/2018/02/08/the-tachometer-inside-your-smartphone/
 

this app is actually really neat. I’ve used it to diagnose different vibrations and hertz fq to find out which bearing are getting worn out on automotive and race motors. Every rotating part has some sort of frequency and harmonics to it. I wouldn’t calibrate my tachometer to it, but will give you an idea how far off you “could” be. 
Steve

[Bolter]

The $20 amazon special specifies a distance up to 500 mm/20".  Not sure how well it will work through windshield and then a longer distance.  Maybe worth $20 gamble?

I have installed a free audio analyzer app on my phone.  Does an FFT of the sound into the microphone.   This means it breaks up the total sound into its frequency components.  If you know a bit how this works, you can quickly zoom in on the engine or prop sounds and get your real RPM.  It is surprisingly accurate.

One example: Spectrum Analyzer from Keuwl on the Google Play store.  

[N201MKTurbo]

I used to have one for the same reason as you.

It depends a lot on the lighting and the background. You might try doing it with plane facing a hangar wall or something.

I always got the best results holding it out the storm window and pointing it at the prop tips.

[EricJ]

 

2 hours ago, Bolter said:

I have installed a free audio analyzer app on my phone.  Does an FFT of the sound into the microphone.   This means it breaks up the total sound into its frequency components.  If you know a bit how this works, you can quickly zoom in on the engine or prop sounds and get your real RPM.  It is surprisingly accurate.

One example: Spectrum Analyzer from Keuwl on the Google Play store.  

These work best when the fundamental (the blade noise at 2x the rpm) is the dominant sound.   My thesis was on estimating the speed and distance of an aircraft passing a ground station from the spectrum of the sound and the doppler characteristics.   Worked best for propeller airplanes where there was a dominant tone, not so well on turbojets or when an afterburner was making broad-spectrum noise.   I also developed an algorithm that is widely used to more finely estimate the frequency from the shape of the peak in the Fourier transform.   It, or one of the subsequent improvements, may be used in that app.   Yes, they can be very accurate, and also the ones that use the vibration spectrum from an accelerometer can be very accurate as well, as they use the same principle.

http://www.ericjacobsen.org/Files/FastFreqEstimators_SPTnT.pdf

 

19 minutes ago, N201MKTurbo said:

I used to have one for the same reason as you.

It depends a lot on the lighting and the background. You might try doing it with plane facing a hangar wall or something.

I always got the best results holding it out the storm window and pointing it at the prop tips.

Yes, or in the shade or at night when the sun isn't interfering.

 

Edited August 6, 2020 by EricJ

[toto]

These are both quite usable:

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

https://apps.apple.com/us/app/engine-rpm/id340401811

(The android one is free, the iOS one is ten bucks or something.)

[MooneyMitch]

Seemed to do the deed. Inexpensive if I recall correctly.

[carusoam]

+1 Smart phone apps...

MS math word of the day....  Fourier Transforms...

Where else would this be used in a casual conversation...?
 

I thought I would put a wiki here to simplify things.... But that didn’t work very well....
 

https://en.wikipedia.org/wiki/Fourier_transform
 

who would talk math without inviting a mathematician..? @aviatoreb 

 

Go MS!

Best regards,

-a-

[Niko182]

10 hours ago, ZuluZulu said:

https://www.aircraftspruce.com/catalog/topages/trutach.php

I also have this one and highly recommend it.

[aviatoreb]

Did someone call?

[MooneyMitch]

I believe all the tach units others have shared are quite good.

I’ll just add for further information, my little Hangar 9 unit does perform inside the cockpit through my windscreen in all phases of propeller movements..... idling, run up, climb, cruise and decent.

[Yourpilotincommand]

Thank you all! I bought the Hangar 9, and it worked perfect from the cockpit thru the windscreen.  So, After taking readings my Mooney tach is indicating lower than the Hangar 9. So I believe I may be getting the full power rpm and not needing a governor adjustment. So now my tach is suspect. Can tachs be adjusted/calibrated?

[carusoam]

They can also be lubricated...

Are you familiar with the oil cup at the back of the tach? An annual event...

EI makes a great electronic replacement if you prefer modern...

Instrument shops can OH and remark properly...
 

Good instrumentation is far better than aged worn or unknown quality of info...

PP thoughts only, not a mechanic...

Best regards,

-a-

[bradp]

This works 

[carusoam]

Brad did you see or get some noise in your data..?

The max rpm is an interesting number...

Best regards,

-a-

[FloridaMan]

Vibration app on the App Store.