Bouncing Tach?

[Paul Taylor]

Is it normal for the tach to jump a hundred rpm? When breaking in this new engine the tach does not hold a steady number. Neither does my other LAs. Is this normal?
Tach is a 20$ from tower.

[Tim Wescott]

Mine certainly does.  It can't even decide how fast the grass is turning, much less the engine.

Although, the lights do seem to turn a constant 1800 RPM.

[Andrew Borgogna]

Mine does the same thing.  I sure wish we could get better tachs, but until the R/C guys need them it won't happen.

[Paul Taylor]

OK, Thanks guys. Glad it is not just me.

[Dan McEntee]

   The tach is jumping because the engine RPM is jumping. 100 RPM isn't very much but the tach is sensitive enough to pick it up. Turn on your tach, set it for two blade prop, and point it at a florescent light. It should read 3600 RPM and be steady unless you move your hand and it will change. Your unsteady hand while tacking the engine will cause a change also. For twenty bucks, these things are pretty darned accurate and sensitive. I really don't care the RPM indicated is exactly what the engine is turn though. I just want it to be reliably consistent and repeatable to establish the baseline RPM I want at take off. I let my stop watch and lap times verify the take off RPM.
  Good Luck and have fun,
   Dan McEntee

[Bob Reeves]

I have a $100.00 TNC, it doesn't jump around quite as much as others I have observed at the field but yes they all jump around and for all the reasons stated above.

[Steve Thomas]

This might be of interest.  It's an acoustic tach app for iPhone, sold as 'Engine RPM'.  It cost about $10, which is a bit pricy for an app, but cheap for a tacho.  It's not as intuitive to set up as a regular tach, but once you've figured out how it works, it really is very good (to the extent that I just don't use my 'regular' tacho any more).  There are a number of things you can select, such as number of cylinders, 2 or 4 stroke, and which harmonics you want to look at. One drawback is that you generally need to know very roughly what rpm to start with; you then use a slider control to get the thing in the ball park, after which it will lock on to the reading.  On the plus side, you can use it in any light conditions and at a considerable distance from the engine - in fact, I've found that I can get a really good reading while I'm flying, with the model nearly 70' away.  I've tested the accuracy against a good ordinary tach, and it's spot-on.

In the picture, the phone is sitting on the seat of a car, about 30' away from a 5cc diesel which is running on the bench. The green graph down the bottom has frequency along the x-axis; there are regular spikes corresponding to the harmonics of the engine.  I've selected harmonics 1 to 5 for the tacho to use - this is indicated by the red lines overlaying the graph.  These red lines 'lock on' to the green spikes.  I could have used more harmonics if I'd wanted, as there are good solid spikes right across the graph.

This all sounds rather complicated, and it mightn't be for everyone, but I can assure you that once you have it set up and understand it, it really is very simple and easy. It's also handy to always have a tacho in your pocket!

[Brett Buck]

Is it normal for the tach to jump a hundred rpm? When breaking in this new engine the tach does not hold a steady number. Neither does my other LAs. Is this normal?
Tach is a 20$ from tower.

    I have original-version TNCs and they are dead-nuts, but the engine isn't all that steady. A hundred RPM +- would be a pretty steady run.

     If the question is about tach function, most of the current ones work reasonably well most of the time. The old Tower version would randomly vary for whatever reason over sometimes thousands of RPM and just give nothing sometimes. That's when the TNC was invented and we all got them. The battery cost saving alone was worth it, never mind that my first TNC is still working dandy after maybe 15-16 years with *no* drift in the calibration.

    Brett

[qaz049]

This might be of interest.  It's an acoustic tach app for iPhone, sold as 'Engine RPM'.  It cost about $10, which is a bit pricy for an app, but cheap for a tacho.  It's not as intuitive to set up as a regular tach, but once you've figured out how it works, it really is very good (to the extent that I just don't use my 'regular' tacho any more).  There are a number of things you can select, such as number of cylinders, 2 or 4 stroke, and which harmonics you want to look at. One drawback is that you generally need to know very roughly what rpm to start with; you then use a slider control to get the thing in the ball park, after which it will lock on to the reading.  On the plus side, you can use it in any light conditions and at a considerable distance from the engine - in fact, I've found that I can get a really good reading while I'm flying, with the model nearly 70' away.  I've tested the accuracy against a good ordinary tach, and it's spot-on.

In the picture, the phone is sitting on the seat of a car, about 30' away from a 5cc diesel which is running on the bench. The green graph down the bottom has frequency along the x-axis; there are regular spikes corresponding to the harmonics of the engine.  I've selected harmonics 1 to 5 for the tacho to use - this is indicated by the red lines overlaying the graph.  These red lines 'lock on' to the green spikes.  I could have used more harmonics if I'd wanted, as there are good solid spikes right across the graph.

This all sounds rather complicated, and it mightn't be for everyone, but I can assure you that once you have it set up and understand it, it really is very simple and easy. It's also handy to always have a tacho in your pocket!

Steve,

that's really interesting. I have a fancy Russian Audio tacho which is effectively useless for diesel powered c/l models. I'd previously used Stuart Sherlock's audio recording method with the Dopler equations with excellent results, except for diesel powered models. It looks like the Fundamental Harmonic isn't picked up.

How's your Ap with diesels?

Also the documents on it on the web suggest an accuracy of within 25%. You've found it to be much better than that?

Pity I bought an Android phone :-)

Ray

[Steve Thomas]

Hi Ray, it works just fine on diesels, as long as they're 'on song'.  If they're running undercompressed and crackly, then the waveform can be a bit useless.  I've used it on everything from a PAW 35 down to a Schlosser .25 that I was messing about with this morning, with consistently good results.  I think you're right about the fundamental harmonic, though - you can even see on the photo I took that the fundamental is pretty tiny, with the next one up being much stronger.  I found this a bit surprising. (This thing allows you to use up to the 18th harmonic, if you want.)

Accuracy-wise, I've compared it a few times with a good optical tacho, and can't pick any appreciable difference in the readings.

Steve

(Shame about the Android issue.  You could try this - http://digitizeit.de/heli/ which is for helicopters but might work on proper aircraft as well.  )

[t michael jennings]

Gentlemen,

Which is the best tach for your moneys worth?


T Michael Jennings
Knoxville, TN

[FLOYD CARTER]

Well, I have a Glo Bee tach that works well.  I like because it uses a cheap 9 volt transistor radio battery.  Bad part is:  you have to turn it off or the battery will run down after a few days!  Available from Tower Hobby.

Floyd