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?
Checking Tach
Moderators: sky's the limit, sepia, Sulako, North Shore
Re: Checking Tach
Is it the prop RPM you're wanting to check or the Tach?
Re: Checking Tach
Thought it was the same thing 
. I'm trying to
. I'm trying to
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.
Re: Checking Tach
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.
Re: Checking Tach
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 .. 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.
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.
Re: Checking Tach
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.
DId you hear the one about the jurisprudence fetishist? He got off on a technicality.
Re: Checking Tach
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
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 .. 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.
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Re: Checking Tach
I guess " direct drive" was the wrong term...
With that engine you would actually need 2 pieces of reflective tape on the drill . 180 degrees apart.
With that engine you would actually need 2 pieces of reflective tape on the drill . 180 degrees apart.
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single_swine_herder
- Rank 7
- Posts: 627
- Joined: Tue Feb 07, 2012 9:35 pm
Re: Checking Tach
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
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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hangarline
- Rank 3
- Posts: 114
- Joined: Mon Jul 23, 2007 11:22 am
Re: Checking Tach
You guys are making this all way too difficult. Use this.
https://www.aircraftspruce.ca/catalog/t ... rutach.php
https://www.aircraftspruce.ca/catalog/t ... rutach.php
Re: Checking Tach
But he's only trying to check his tach for his annual...cheaply.
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single_swine_herder
- Rank 7
- Posts: 627
- Joined: Tue Feb 07, 2012 9:35 pm
Re: Checking Tach
Pretty tough to get cheaper than pointing the airplane at a light bulb.
Re: Checking Tach
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.
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.
Re: Checking Tach
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 onhangarline wrote:You guys are making this all way too difficult. Use this.
https://www.aircraftspruce.ca/catalog/t ... rutach.php
.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.
Re: Checking Tach
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
The trutach goes off of the prop...somehow
...on 9 volts, or 8.5 volts, or 8.0 volts...somehow
Re: Checking Tach
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.Don't forget, prop rpm may not be engine rpm in your case...
The trutach goes off of the prop
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.
Re: Checking Tach
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)
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)
Re: Checking Tach
Don't tell me the manual tells you to use a Trutach?...so why would you?You won't find a manufacturer's maintenance instruction which tells you to do that, 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.
Re: Checking Tach
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 ., 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....
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 ., 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....
