Wayward Tach

[O. Sami Saydjari]

Based on advice from a column I read on AVWEB by John Deakin, I had my
Tach checked since it was in the shop anyay. I was surprised to find
out that it was reading 150 RPM lower than the true RPM. That seems
pretty bad. I said, OK, no problem, just adjust it. No dice. I was
informed that it was a "mechanical type" tachometer and that there was
no adjustment possible. I really do not care for the idea of having to
remember to add 150 to the tach reading everytime I do my settings. Is
there really no way to adjust these things? If not, is there an
alternative?

I read an add for a digital tach called a P-1000 by Horizon Instruments.
Seems really good, but at $510, it seemed a bit steep to avoid doing a
little math everytime I set my RPMs. Are there alternative replacement
tachs that are less expensive that folks here have been happy with?

-Sami
N2057M, Piper Turbo Arrow III

[John Kunkel]

"O. Sami Saydjari" > wrote in message
...
> Based on advice from a column I read on AVWEB by John Deakin, I had my
> Tach checked since it was in the shop anyay. I was surprised to find
> out that it was reading 150 RPM lower than the true RPM. That seems
> pretty bad. I said, OK, no problem, just adjust it. No dice. I was
> informed that it was a "mechanical type" tachometer and that there was
> no adjustment possible. I really do not care for the idea of having to
> remember to add 150 to the tach reading everytime I do my settings. Is
> there really no way to adjust these things? If not, is there an
> alternative?

Ah, the mechanical tach, my favorite "soapbox" subject. If there ever was a
piece of crap installed in an aircraft, this is it.
That 150 rpm error might be at one setting only, it could be off a different
number in a different range. It could also show a different error when
advancing the throttle than when retarding the throttle.


> I read an add for a digital tach called a P-1000 by Horizon Instruments.
> Seems really good, but at $510, it seemed a bit steep to avoid doing a
> little math everytime I set my RPMs. Are there alternative replacement
> tachs that are less expensive that folks here have been happy with?

I think the Horizon is a good investment since it shows more than merely
engine rpm. An alternative would be to purchase a hand held optical unit
like TruTach which the shop probably used to check your mechanical tach and
use it to set your cruise rpm.
John

[Michelle P]

Sami,
it is allowed to be off by 5%. this is 40 rpm at 2000 or 50 at 2500.
While it is outside limits most places will placard the difference and
call it good. getting a mew one it may not be much better.

Michelle

O. Sami Saydjari wrote:

> Based on advice from a column I read on AVWEB by John Deakin, I had my
> Tach checked since it was in the shop anyay. I was surprised to find
> out that it was reading 150 RPM lower than the true RPM. That seems
> pretty bad. I said, OK, no problem, just adjust it. No dice. I was
> informed that it was a "mechanical type" tachometer and that there was
> no adjustment possible. I really do not care for the idea of having
> to remember to add 150 to the tach reading everytime I do my
> settings. Is there really no way to adjust these things? If not, is
> there an alternative?
>
> I read an add for a digital tach called a P-1000 by Horizon
> Instruments. Seems really good, but at $510, it seemed a bit steep to
> avoid doing a little math everytime I set my RPMs. Are there
> alternative replacement tachs that are less expensive that folks here
> have been happy with?
>
> -Sami
> N2057M, Piper Turbo Arrow III
>

--

Michelle P ATP-ASEL, CP-AMEL, and AMT-A&P

"Elisabeth" a Maule M-7-235B (no two are alike)

Volunteer Pilot, Angel Flight Mid-Atlantic

Volunteer Builder, Habitat for Humanity

[Dan Thomas]

Michelle P > wrote in message et>...
> Sami,
> it is allowed to be off by 5%. this is 40 rpm at 2000 or 50 at 2500.
> While it is outside limits most places will placard the difference and
> call it good. getting a mew one it may not be much better.

5% of 2500 is 125 RPM, not 50. His tach is still out of limits,
though. In Canada the limit is 4%, measured at the centre of the
cruise RPM range.
Most tachs in singles are the magnetic-drag type, and the magnet
weakens with age, so the tach underreads. Replace it, but I think you
need the green and red markings to identify safe ranges; does the
electronic tach have such warning devices?

Dan

[Michelle P]

Dan,
ok so I had a weak math moment. 5% at 2000 is 100 RPM, at 2500 is 125.
Weather or not is Tach is out of limits depends on how accurate the
reference device.
Michelle

Dan Thomas wrote:

>Michelle P > wrote in message et>...
>
>
>>Sami,
>>it is allowed to be off by 5%. this is 40 rpm at 2000 or 50 at 2500.
>>While it is outside limits most places will placard the difference and
>>call it good. getting a mew one it may not be much better.
>>
>>
>
> 5% of 2500 is 125 RPM, not 50. His tach is still out of limits,
>though. In Canada the limit is 4%, measured at the centre of the
>cruise RPM range.
> Most tachs in singles are the magnetic-drag type, and the magnet
>weakens with age, so the tach underreads. Replace it, but I think you
>need the green and red markings to identify safe ranges; does the
>electronic tach have such warning devices?
>
> Dan
>
>

--

Michelle P ATP-ASEL, CP-AMEL, and AMT-A&P

"Elisabeth" a Maule M-7-235B (no two are alike)

Volunteer Pilot, Angel Flight Mid-Atlantic

Volunteer Builder, Habitat for Humanity

[Dave Butler]

Dan Thomas wrote:
> Michelle P > wrote in message et>...
>
>>Sami,
>>it is allowed to be off by 5%. this is 40 rpm at 2000 or 50 at 2500.
>>While it is outside limits most places will placard the difference and
>>call it good. getting a mew one it may not be much better.
>
>
> 5% of 2500 is 125 RPM, not 50. His tach is still out of limits,
> though. In Canada the limit is 4%, measured at the centre of the
> cruise RPM range.
> Most tachs in singles are the magnetic-drag type, and the magnet
> weakens with age, so the tach underreads. Replace it, but I think you
> need the green and red markings to identify safe ranges; does the
> electronic tach have such warning devices?

The Horizon P1000 electronic tach has a red LED that comes on when you exceed
the maximum RPM. It also has LEDs for grounding of the P-leads, probably some
other warning indications I've forgotten.

http://www.horizoninstruments.com/prod01.htm

Google for my postings about installing on my Cherokee long ago.

- 1 RPM resolution and accuracy.
- no more worries about deteriorating accuracy.
- totalizer runs 1:1 in real time any time RPMS over 800.
- you get used to the jitter in the display (caused by 1 RPM resolution).
- directly displays RPM loss during runup as you ground each P-lead.

Dave
Remove SHIRT to reply directly.

[Dan Thomas]

We use an electronic optical tach designed for the purpose. Costs $300
from an aviation supplier. If I hold it near a fluorescent light, it
will read 3600 if set on two-blade mode. 60 hertz for one minute is
3600 two-pulse cycles, and since the 60 hertz supplied by power
companies is very accurate (some clocks rely on it), this instrument
is pretty good. It measures in tens (have to add a zero).
Now, my 17-year-old son bought a model airplane engine tach for
$50, and it measures in ones. And equally accurate. Go figure.

Dan


Michelle P > wrote in message et>...
> Dan,
> ok so I had a weak math moment. 5% at 2000 is 100 RPM, at 2500 is 125.
> Weather or not is Tach is out of limits depends on how accurate the
> reference device.
> Michelle
>
> Dan Thomas wrote:
>
> >Michelle P > wrote in message et>...
> >
> >
> >>Sami,
> >>it is allowed to be off by 5%. this is 40 rpm at 2000 or 50 at 2500.
> >>While it is outside limits most places will placard the difference and
> >>call it good. getting a mew one it may not be much better.
> >>
> >>
> >
> > 5% of 2500 is 125 RPM, not 50. His tach is still out of limits,
> >though. In Canada the limit is 4%, measured at the centre of the
> >cruise RPM range.
> > Most tachs in singles are the magnetic-drag type, and the magnet
> >weakens with age, so the tach underreads. Replace it, but I think you
> >need the green and red markings to identify safe ranges; does the
> >electronic tach have such warning devices?
> >
> > Dan
> >
> >
>
> --
>
> Michelle P ATP-ASEL, CP-AMEL, and AMT-A&P
>
> "Elisabeth" a Maule M-7-235B (no two are alike)
>
> Volunteer Pilot, Angel Flight Mid-Atlantic
>
> Volunteer Builder, Habitat for Humanity

[thnksno]

When I have a snowmobile tach calibrated, I add hash marks or tape to
the glass to indicate the correct rpm... Clutching dictates rpm and is
sometimes hard to read at WOT or 8000-8200 with vibrations and wind etc.
so even if the tach is spot on, a mark is handy. I don't see why cruise
setting couldn't be similarly "bugged."

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