Tachometer problem

DavidWhite

DavidWhite

Final Approach
Joined
Apr 19, 2011
Messages
7,299
Location
49
Display Name
DW
My plane has always been a touch off on the tachometer, about 100RPMs to fast, but recently it has gotten more aggravted - About 300RPMs high on the tach. It's not a pressing issue, but it is annoying. Is there any way to fix this?
 
DavidWhite said:
My plane has always been a touch off on the tachometer, about 100RPMs to fast, but recently it has gotten more aggravted - About 300RPMs high on the tach. It's not a pressing issue, but it is annoying. Is there any way to fix this?
Click to expand...


Yep, you take the instrument to an instrument shop, or a Speedometer shop, and they'll calibrate it for you. Is it cable or electric? The electrics often have an accessible screw on them. Cable drive they have to tweek the magnet separation.
 
I believe it is cable. I'll take it to my mechanic tomorrow and see what he thinks
 
That explains why the cruise numbers you've told me seem too low.
 
Tom-D said:
binding cable will make it jerky, not slow or fast.
Click to expand...


The reference to cable was in answer to me whether cable or electric for when it comes time to adjust what was required. Electrics are typically eazy as you just trim a pot to get it on. With a cable you have to adjust the spacing between the drive couple magnets to make up the difference.
 
DavidWhite said:
TT? Total Time?
Click to expand...

the tach is also an hourmeter which bases its time on total revolutions. The hourmeter isn't affected by the magnet spacing as it is directly geared to the cable drive on cable type tachs. So based on that his TT shouldn't be affected by the rpm error unless he's changing rpm based on the tach.
The tach hourmeter reads real time only when its turning at the rpm for which the tach is set. That varies by engine cruise rpm so replacing the tach requires more than just making sure it fits the panel.....


Frank
 
BiffJ said:
the tach is also an hourmeter which bases its time on total revolutions. The hourmeter isn't affected by the magnet spacing as it is directly geared to the cable drive on cable type tachs. So based on that his TT shouldn't be affected by the rpm error unless he's changing rpm based on the tach.
Click to expand...
?????
At a given throttle setting, he's the tach is recording more rpm than actual, hence he's getting more hours recorded than he should.

Either you set the throttle based on target rpm and you're lower than indicated while also going slower than you should be and also spending more time in the air, our you set throttle to target speed and the rpm are still higher than actual and the hours click over faster than they should.
Either way, you're building TT faster than if your tach was reading correctly.


Sent from my DROIDX using Tapatalk
 
PilotAlan said:
?????
At a given throttle setting, he's the tach is recording more rpm than actual, hence he's getting more hours recorded than he should.
Click to expand...

No, the error in the system comes PAST the counter wheels. the flow is:
cable->shaft->wormgear driving counter wheel-> drive magnet->calibration gap-> couple magnet->shaft with perpendicular "clock spring"->shaft through the face-> needle. The error is adjusted out in the calibration gap and comes from the needle side of it.

That is the basic format for every cable driven instrument of that type, speedometer or mechanical tach.
 
Gonna swap tachs sometime this week, and see if its the tach or something else.

If that doesnt work I'm going to have an electric one installed
 
Henning said:
No, the error in the system comes PAST the counter wheels. the flow is:
cable->shaft->wormgear driving counter wheel-> drive magnet->calibration gap-> couple magnet->shaft with perpendicular "clock spring"->shaft through the face-> needle. The error is adjusted out in the calibration gap and comes from the needle side of it.

That is the basic format for every cable driven instrument of that type, speedometer or mechanical tach.
Click to expand...

It's called a "Hysteresis" disk.. http://en.wikipedia.org/wiki/Hysteresis
 
DavidWhite said:
Gonna swap tachs sometime this week, and see if its the tach or something else.

If that doesnt work I'm going to have an electric one installed
Click to expand...

BEEP BEEP, Bad mistake, electric Tach is yucky....

get a new mechanical tach and have it set at the TT of your aircraft/engine.
 
Tom-D said:
BEEP BEEP, Bad mistake, electric Tach is yucky....

get a new mechanical tach and have it set at the TT of your aircraft/engine.
Click to expand...
Could you explain, perhaps in better words, as to why an electric tach is "yucky"?
 
jesse said:
Could you explain, perhaps in better words, as to why an electric tach is "yucky"?
Click to expand...

most engines will not hold a steady RPM so the electronic tachs will cycle the numbers too fast to get a good mag reading, and they are short lifed.
 
Tom-D said:
most engines will not hold a steady RPM so the electronic tachs will cycle the numbers too fast to get a good mag reading, and they are short lifed.
Click to expand...

I have an EI tach with LEDs around the outer edge, for an analog presentation, and digital in the center.
The resolution of the digital tach is filtered to 10 rpm, to prevent the numbers from constantly changing.

For mag checks, you press the two buttons simultaneously, and it resets the rpm to zero. Flip to a mag or pull carb heat and it shows you the drop from the zeroed rpm.

EI_R4.jpg
 
PilotAlan said:
I have an EI tach with LEDs around the outer edge, for an analog presentation, and digital in the center.
The resolution of the digital tach is filtered to 10 rpm, to prevent the numbers from constantly changing.

For mag checks, you press the two buttons simultaneously, and it resets the rpm to zero. Flip to a mag or pull carb heat and it shows you the drop from the zeroed rpm.

EI_R4.jpg
Click to expand...

And when you are required to turn off the electric power in an emergency?
 
Tom-D said:
And when you are required to turn off the electric power in an emergency?
Click to expand...

I don't need the tach to tell what the engine is doing.
Pitch for performance, power for airspeed at that pitch.

Other than runup, initial throttle up for takeoff, and setting power for cruise, how often do you actually need the tach?
If you had an emergency and I put a post-it over the tach, would you really have a problem?
 
I've flown behind both and there's hysteresis designed into most of the electric ones so they're not jumping all over the place. It's a non-issue.

The benefit to mechanical ones is that you don't have to turn the Master back on to read it every time you forget to write the tach time down at shutdown. ;)

The benefit to the electric, you don't typically need to have it recalibrated after a while. It also has a bright obnoxious red overspeed LED which is very hard to ignore or miss.

Some can be programmed to have yellow LED at takeoff RPM for those flying behind constant speed props that switches to green when you're in the aircraft's cruise RPM range. That's nice if you're shooting for the top-of-the-green with a vernier prop control. Just twist until green. Easier than dealing with parallax if your RPM gauge is a long way across the panel.

Either type is fine by me, though. Fly whatever you got! All the fancy glass cockpit folks have all sorts of fancy ways to display the tach and warn of out-of-spec numbers...
 
You must log in or register to reply here.
Back
Top