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 Post subject: Checking Tach
PostPosted: Mon Jul 27, 2015 11:04 pm 
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Tried to use the club's laser photo tachometer to check my RPM readout and couldn't get it to work even with reflective tape on the prop. A little research later I find it's good for distances up to 6" away! I'd like at least 6 feet.

Can anyone recommend a model suitable for measuring aircraft propellor rpm?



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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 7:23 am 
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Is it the prop RPM you're wanting to check or the Tach?


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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 9:23 am 
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Thought it was the same thing :oops:. I'm trying to
Quote:
The accuracy of mechanical drag cup type tachometers, for fixed wing propeller driven aircraft, shall be checked on site annually, and be accurate to within the tolerances established by the aircraft manufacturer or, where no tolerance has been specified by the aircraft manufacturer, to within +\- 4% of engine RPM at mid-point of the cruise range.



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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 12:10 pm 
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I've used a "One Touch Tach" by Cermark before. You can do it in the cockpit without fear of getting prop whacked! You can check it's accuracy by pointing it at an incandescent light (fluorescent won't work), pressing the button and it picks up on the 60 Hz AC frequency.


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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 12:22 pm 
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It is the same if you have a direct drive prop...which I assume you have.

You can always remove the tach cable from the engine, drive it with a drill and use the laser tach on the drill chuck. Cover the chuck 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.



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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 1:18 pm 
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Has anyone had any luck with the iPhone apps that measure RPM through the microphone? You configure the number of cylinders, blades etc. I will experiment and report back.

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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 2:19 pm 
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ahramin wrote:
Can anyone recommend a model suitable for measuring aircraft propellor rpm?
NeverBlue wrote:
Is it the prop RPM you're wanting to check or the Tach?
ahramin wrote:
Thought it was the same thing :oops:
NeverBlue wrote:
It is the same if you have a direct drive prop...which I assume you have.

You can always remove the tach cable from the engine, drive it with a drill and use the laser tach on the drill chuck. Cover the chuck 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.
Attachment:
Ratio.png
Ratio.png [ 50.18 KiB | Viewed 6794 times ]



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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 2:55 pm 
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I guess " direct drive" was the wrong term...

With that engine you would actually need 2 pieces of reflective tape on the drill chuck 180 degrees apart.



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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 4:16 pm 
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This method works well, and "freezes" the prop quite accurately at certain combinations of RPM and number of blades. Once you've "stopped" the apparent motion of the blade, you can check the tach indicated value.

Works like a hot-damn once you get the hang of smooth operation of the throttle to sync it with multiples of the frequency of the light .... which switches on and off at 60 hertz and functions like a strobe light.

This entry in a forum explains the concept.

http://www.pilotsofamerica.com/forum/ar ... 42166.html

Then about half way down the page at this link, gives a chart with the RPM and blade count combos ....

http://www.grtavionics.com/documents/EI ... manual.pdf



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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 5:23 pm 
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You guys are making this all way too difficult. Use this.

https://www.aircraftspruce.ca/catalog/t ... rutach.php



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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 6:39 pm 
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But he's only trying to check his tach for his annual...cheaply.


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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 6:53 pm 
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Pretty tough to get cheaper than pointing the airplane at a light bulb.


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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 7:01 pm 
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True dat!


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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 7:37 pm 
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I'm guessing you're not going to be up for the $260 version.
If you look on Amazon.ca, there are a bunch claiming range of up to 6 feet for between $15 and $50.
Can't vouch for them though.
As previously mentioned though, you can check the function/accuracy of the optical tach by pointing it at a lit fluorescent tube and verifying a reading of 3600 (or 7200, depending on how many blades/reflectors the tach is set for). Works the same for both the high and low end models.
Make sure you put tape on both blades and set the tach for two bladed (or do the math afterwards if it doesn't have the option) so that it doesn't inadvertantly read the non-taped blade part of the time.



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 Post subject: Re: Checking Tach
PostPosted: Tue Jul 28, 2015 7:58 pm 
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hangarline wrote:
You guys are making this all way too difficult. Use this.

https://www.aircraftspruce.ca/catalog/t ... rutach.php

Looks like you either spend the big bucks or put up with stuff that doesn't work. Ended up "borrowing" one of these from a local AMO in the same building as my avionics guy. Slipped him $10 (the AME who lent it to me, not my avionics guy). Wow did that work well. Unfortunately I'm pretty close to the 4% limit with 2300 on the tach reading 2380 on the trutach. Think I'll consider 2600 as the redline instead of 2700 from now on :shock:.

Someone did mention "there should be an app for that". Found a couple that looked neat but I think I'll stick to proven technologies for now.



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 Post subject: Re: Checking Tach
PostPosted: Wed Jul 29, 2015 7:54 pm 
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Don't forget, prop rpm may not be engine rpm in your case...
The trutach goes off of the prop...somehow

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



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 Post subject: Re: Checking Tach
PostPosted: Thu Jul 30, 2015 7:42 am 
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Quote:
Don't forget, prop rpm may not be engine rpm in your case...
The trutach goes off of the prop


Yes. This is the type you would like. There are two types of optical tachs, One "looks" at the flashes of light through the propeller blades. You'll know that you have this type, if there is a switch on it to select 2, 3, or 4 blade prop. Set to two blade, and aimed at a fluorescent bulb, it'll indicate 3600RPM (60 cycles per second X 60 to minutes). Just shine it through the turning prop, at a good light source, like the sun. I've had great luck doing this in flight. Do correctly switch to the number of blades, or you'll have math to do.

The other type of optical tach will emit a laser beam. You'll know you have that type, if there is no 2, 3, 4 switch, but a "warning laser" sticker on it. That type of tach requires that the laser beam be reflected back to the sender unit. Hence the 6 feet. They are more designed to check the shaft speed of machines in dark places. They will work on a prop, but are awkward. I do have reflective tape around exactly one half of the prop hub of one of my planes for this purpose. On that plane, you're not looking through the prop in flight, but rather at the hub. My other plane with a nice painted spinner does not welcome stuck on reflective tape, so I use the Trutach on it.

As suggested, the tach instrument itself will probably operate at a ratio to the engine/propeller speed. Based on experience with delicate and expensive tach cables, I highly recommend that you not drive it with a drill. You won't find a manufacturer's maintenance instruction which tells you to do that, so why would you? If you're at that stage, take the tach out of the panel, and send it to the instrument shop.



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 Post subject: Re: Checking Tach
PostPosted: Thu Jul 30, 2015 9:42 am 
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Thanks for the clarification, PilotDAR.

I can't recall the make, but borrowed a laser emiting tach once that had a blade number selector function. I recall it requiring reflective tape on the back of the blades to get a reliable reading. (possibility exists that time has muddied the recollection though)



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 Post subject: Re: Checking Tach
PostPosted: Fri Jul 31, 2015 5:01 am 
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Quote:
You won't find a manufacturer's maintenance instruction which tells you to do that, so why would you?


Don't tell me the manual tells you to use a Trutach?...so why would you?

I would definitely agree the best way is to remove the Tach and have it checked at an instrument shop. But the last forum I suggested that I had my head bit off here.
CARs wants the Tach checked...not the Prop RPM...not the Engine RPM...the TACH.

The only reliable way to do that is to have a calibrated device to measure something that is driving the tach. A 9 volt battery operated device is NOT reliable in my experience. How do you know the battery is operating at 9 volts?
Holding something in your hand and pointing it at the prop looking for reflections is not reliable either...there are too many variables.

Driving the Tach cable with a drill is totally acceptable...it's just an electric motor...which every instrument shop has rigged up somehow to check tachs and tach generators.
Tach cables are not delicate...they're extremely robust...they work sometimes for 1000s of hours routed and bending from the engine compartment through the firewall behind the instrument panel.
They are really no different than a flex-drive for the drill.

The job however should be left to those that know what they are doing.

No disrespect Ahramin, but if you didn't know that your prop speed isn't your engine speed, you shouldn't be checking your tach at annual at all.

The best way to check engine RPM on the aircraft is using the ignition as your timing source with a calibrated reliable device.

But without a doubt in my opinion the tach should be checked by an instrument shop with a calibrated test set.



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 Post subject: Re: Checking Tach
PostPosted: Fri Jul 31, 2015 7:22 am 
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Happily, the optical tachs are extremely precise (and voltage does not affect their accuracy, as long as they are indicating at all). The "see through the blades" type are calibrated with a 60 cycle light source. If there is an error, you'll see it. If the continent's 60 cycle power grid becomes other than exactly 60 cycles, we'll have way more serious problems than an error in a tach!

Before the advent of the optical tachs, we checked tachs with a mechanically driven strobe disc running with the tach, I still have one here, which I no longer use. They also were very precise, but more fussy to use. We also used a calibrated tach which could be driven off the spinner, though holding this was a little un-nerving, and I'm sure would violate every workplace safety rule these days! Thus, the manufacturer's instructions when written, might not account for newer technology.

Using an aircraft engine to drive the tachometer for that engine, to enable a test, is a perfectly appropriate way to do it, as long as your means to measure is also appropriate. A correctly employed optical tach is. Though driving a tach cable with a drill is certainly possible, it is a less desirable means. The tach speed may not be one to one with the engine, so you have to account for that, as your drill speed will not be the same as the indicated tach speed in that case. If the tach drive cable is accidentally pulled out while it is running, it will disengage from the tach, and pushing it back in to re-engage it could end in disappointment. If I saw a person doing this under my supervision, I would stop them.

Honestly, for many engine installations, by the time you got a drill squeezed in between the back of the engine, and the firewall, and tried to hold an optical tach pointed perpendicular to the turning chuck, keeping the tach drive cable steadily in a good position could be very difficult. It's so easy just to run the engine, and use the appropriate tach checker as it was designed to be used....



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 Post subject: Re: Checking Tach
PostPosted: Fri Jul 31, 2015 10:00 am 
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NeverBlue wrote:
You can always remove the tach cable from the engine, drive it with a drill and use the laser tach on the drill chuck. Cover the chuck 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?

NeverBlue wrote:
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



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 Post subject: Re: Checking Tach
PostPosted: Sun Aug 02, 2015 10:01 am 
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Quote:
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...

Quote:
 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....



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 Post subject: Re: Checking Tach
PostPosted: Mon Aug 03, 2015 2:12 pm 
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NeverBlue wrote:
What a load of crap...sorry but it is...The one thing I know like the back of my hand is electronics
NeverBlue wrote:
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. :smt023


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 Post subject: Re: Checking Tach
PostPosted: Mon Aug 03, 2015 6:35 pm 
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NeverBlue wrote:
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?
Quote:
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.


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 Post subject: Re: Checking Tach
PostPosted: Tue Aug 04, 2015 3:27 am 
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Quote:
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.



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