Checking Tach

[ahramin]

You can always remove the tach cable from the engine, drive it with a drill and use the laser tach on the drill .. Cover the . with tape and put a slip of relective tape on it that the laser will pick-up.
It's cheap and dirty but I've done that to troubleshoot before and it worked for me...you will need someone in the cockpit though to help.
You could also remove the tach itself and drive it with the drill but you really need to include the tach cable in your test...4% is not alot.

I would suggest to any aircraft maintainer that this is a bad idea. Why disconnect and reconnect parts to your engine when there is no need? Seems like a lot of opportunity to damage stuff. Especially driving the tach cable with a drill. If you maintain aircraft I would look up the Waddington effect, which may lead you to the concept of reliability-centered maintenance. Same for removing the tach from the aircraft and sending it to a shop. If the tach is reading within 4% of the engine rpm, why introduce the possibility of more maintenance errors for something that is within spec?

The trutach goes off of the prop...somehow

...on 9 volts, or 8.5 volts, or 8.0 volts...somehow

The somehow you are referring to is a quartz crystal. https://en.wikipedia.org/wiki/Quartz_clock. Accuracy of 0.025% (almost 1 rpm) in this application (regardless of voltage) to check an instrument with markings every 100 rpm. Not really an issue.

Thanks everyone for the help and suggestions. I'm considering throwing one of these in as a backup because at that price, why not? Plus an hour meter would be nice. Anyone familiar?

http://www.aircraftspruce.ca/catalog/in ... ckkey=3574

[NeverBlue]

I would suggest to any aircraft maintainer that this is a bad idea. Why disconnect and reconnect parts to your engine when there is no need? Seems like a lot of opportunity to damage stuff. Especially driving the tach cable with a drill. If you maintain aircraft I would look up the Waddington effect, which may lead you to the concept of reliability-centered maintenance. Same for removing the tach from the aircraft and sending it to a shop. If the tach is reading within 4% of the engine rpm, why introduce the possibility of more maintenance errors for something that is within spec?

What a load of crap...sorry but it is.
..
The one thing I know like the back of my hand is electronics and the Quartz xtal you're referring to Ahramin is used for nothing but a reference clock for the RF signal...it has nothing to do with the accuracy of the tach.

I'd bet my house if you had 5 Trutachs all checking the same prop rpm they'd all give a different reading...

 It's so easy just to run the engine, and use the appropriate tach checker as it was designed to be used.... 

of course it is...but he only had one that worked from six inches away when i made the suggestion

Power supply has a huge effect on electronics...and you all saying it doesn't is laughable....

[hoptwoit]

What a load of crap...sorry but it is...The one thing I know like the back of my hand is electronics

Power supply has a huge effect on electronics...and you all saying it doesn't is laughable....

True tachs work great in the day time. I have used them for years. They are reliable and read from a long distance. Most importantly for what you are trying to do they are the simplest, least time consuming and therefore most cost effective way to get the job done. Which is what the OP was asking. I have loaned mine out to a few pilots that ended up becoming customers when they needed the indication error trouble shot and repaired. Hats off to the OP for taking an interest in your airplane and maintaining it.

[photofly]

The one thing I know like the back of my hand is electronics and the Quartz xtal you're referring to Ahramin is used for nothing but a reference clock for the RF signal...it has nothing to do with the accuracy of the tach.
...

What RF signal?

Power supply has a huge effect on electronics...and you all saying it doesn't is laughable....

If you do know electronics at all you'll know that it's almost impossible to design a crystal clock that's not stable and accurate to better than .1% across a range of supply voltages. Frequency is about the easiest thing to measure accurately.

TruTach advertises accurate to 1rpm. That's such a trivial target to meet that I believe it.

[PilotDAR]

I'd bet my house if you had 5 Trutachs all checking the same prop rpm they'd all give a different reading...

My experience has been just the opposite over about 3 decades of doing such checks. I've used several different Trutachs, and several other variants, calibrated each one before use, and never seen an error. I've had batteries discharge, until the unit stops working, without error.

For me, a "looking through the prop blades" optical tach is the very best way to check your installed tach, and entirely removes the need to disassemble entirely airworthy system components, and risk that damage.

[PilotDAR]

You calibrated the trutach before you used it each time?...sure you did.

Maybe I did not wrote "EVERY" with enough capitals to make it convincing... Yes, every time I have used a Trutach or similar product for optically checking engine propeller RPM, I have directed it at a florescent light source just before and confirmed "3600" [RPM]. Doing this conforms to the instructions for use, and is remarkably easy. I have no recollection over all the time I have used one, of ever seeing deviation from this value of any consequence, and certainly not the possible 1% or 1RPM which might be allowed.

I also have other aircraft instrument shop certified equipment for checking tachs, which dates back more than a half century, but really, the optical tach is simply easier and more reliable. My other plane mush be calibrated with a laser tach, as you cannot look through the prop blades while you are running the engine. That optical tach works too, though is a bit more fussy to get a reading.

If there were no other means than disassembling to calibrate, I would do that. But I cannot imagine fighting your way into bending a disconnected tach cable around the corner, so a drill . could be connected. Then, possibly pull out the turning cable, to have it disengage, and then either jam then, or jam when being reinserted. Plain silliness, when optical is so reliable and simple to use....

[GyvAir]

Maybe I'll design one and post the details here so we can all build one. Shall we have some fun and try to keep the part cost to say, $10? Including the batteries?

If you actually did that, I'd probably actually build it and get back to you with the results.

Yes, every time I have used a Trutach or similar product for optically checking engine propeller RPM, I have directed it at a florescent light source just before and confirmed "3600" [RPM].

I'm not sure I've ever caught anybody not doing this. Why wouldn't you? Most hangars, tool rooms, terminals, etc are filled with fluorescent lighting fixtures. What could be easier?

I've never seen anyone try to check calibration by pointing at an incandescent bulb, yet I see people online saying that's how to do it, even in this thread. Is it possible, or is it just people accidentally typing the wrong word?

[robertw]

The ones I've used do not work on a fluorescent bulb. Had to be an incandescent. I tried both and got no reading on the fluorescent. It was a bit of a task actually finding an incandescent bulb in a mostly fluorescent hangar, but eventually I found one!

[KISS_MY_TCAS]

http://www.proptach.com

Have had one of these in my toolbox for years, and check its accuracy every time I do a dynamic balance with a calibrated Chadwick Helmuth balancer. Have yet to see more than a 5 rpm difference from the calibrated optical pickup on the CH.

[ahramin]

Funny you should mention that, I just did the dynamic balance and the rpm was matching the Trutach calibration as far as I could tell with the analog gauge.

[photofly]

Maybe I'll design one and post the details here so we can all build one. Shall we have some fun and try to keep the part cost to say, $10? Including the batteries?

If you actually did that, I'd probably actually build it and get back to you with the results.

I've written the code; I'm just waiting for various parts to arrive to build a prototype. It looks like the cost target will be met.

[torquey401]

http://www.princessauto.com/en/detail/d ... -p8500647e

On sale this coming week. $24.44.

[ahramin]

http://www.princessauto.com/en/detail/d ... -p8500647e

On sale this coming week. $24.44.

Measuring Distance 50 to 500 mm

[PilotDAR]

Not the important difference between "laser" tachs, and "optical" tachs. You'll know that you have an optical tach, designed for checking aircraft engines, if it has a "2, 3, or 4" blades switch. These work by looking through the propeller, which quite easy on most planes.

Laser tachs will require a bit of reflective tape on the spinner or prop hub (my experience is that reflective tape on a blade is not enough). The laser has to reflect off that bit of tape. Possible to do, but more awkward, and near impossible for most aircraft while seated in the cockpit, so it becomes a two person job.

I tach check one of my planes with a laser tach, as I cannot look through the prop in flight, but I would prefer to use an optical tach were I able to, the laser tach is really fussy to get a reading.

[photofly]

Here's a breadboard of the device, showing the, er, RPM of an LED table light overhead:

FullSizeRender.jpg (41.88 KiB) Viewed 1910 times

Anyone interested in a PCB?

[ahramin]

Neat. What is the time source and how did you tune it?

[photofly]

It's a $1 4.000 MHz crystal. Typically good for 50ppm and they don't need adjustment. Not even sure how you would.

[GyvAir]

Looks pretty cool.. maybe a little awkward to hold in the cockpit while running the engine though.
Is the pickup just a common photocell?

[photofly]

Looks pretty cool.. maybe a little awkward to hold in the cockpit while running the engine though.
Is the pickup just a common photocell?

The finished article should be just a little bigger than a pair of AA cells, so it might be tricky to hold because it's small. But you can always glue it to a piece of wood if you want it bigger.

At the moment the pickup is a phototransistor ($1). The pickup circuit is tricky - adjusting for different light levels automatically, so I might have to redesign that part of it, in which case I will try a photodiode as they're likely cheaper.

[torquey401]

Looks cool. I would be interested in one for my toolbox. I love little gadgets that makes my job easier. Keep us informed.