Tach Attack

A month or so back, I asked about re-setting the reading of a standard
aircraft tach to reflect the time the airplane spent with a temporary
replacement.

Unfortunately, as it turns out, there was nothing wrong with my
tach...or with the tach cable. The tach-drive section on the engine
itself (Continental C85) was bad.

Once I was convinced the engine-side tach drive was munged, I started
looking into the options for electronic tachs. Most installed a
transducer on the engine tach drive...which I figured wouldn't work in
my case, since they probably would have the same problems with my
damaged drive head. Others connected to a Bendix or Slick
magneto...neat option, but I have Eisemans.

Finally, I noticed the "Tiny Tach" on the Aircraft Spruce web page.
Seemed pretty good...got its signal by wrapping a wire around a spark
plug cable. From the description, I wasn't completely sure which model
to buy. But when I went to the company's web page, I found they had a
universal "Commercial" model that also updated faster than the standard
units.

http://www.tinytach.com/tinytach/commercial.php

Only $65, so I went ahead and ordered one. It had both a tach function
as well as an hourmeter and two service timers. It didn't need ANY
other connection other than to wrap the sensor wire around a spark plug
cable. It has a built-in battery.

When it came in, I went to the airport and tried a temporary
installation. No go...tach read zero. I suspected the shielding on the
tach cables, and a call to the tach vendor confirmed it. The tech
recommended peeling the shielding back near a plug, but I didn't want to
run my tach wire out into the slipstream.

One of my spark plug wires is about 9" too long, and a previous owner
had wrapped up the excess and tie-wrapped it to the engine mount. I
figured I'd modify the wire there, so if I botched it, there'd still be
enough cable to connect to the mag. So I picked at the shielding with a
toothpick to make some openings, then carefully plucked away at it with
an exacto knife. A few minutes work, and I had a 1" long section of
cable free of the braid.

The tach instructions said to wrap the red wire around the cable 3-4
times. I gave that a shot, and the tachometer worked.

I made a mount for the Tiny Tach from some leftover 1/8" plywood. I
could have just used a flat sheet to attach behind the panel, but I
ended up with a "sandwich" construction where the outer face was
circular with the diameter just less than a standard instrument hole. I
felt this gave a lot nicer look to the installation....it looked more
like an instrument sitting "in the hole" than just a flat sheet behind
the panel.

http://www.bowersflybaby.com/tech/tach_done.jpg

One thing I had to do was plug off the old tach cable port on the back
of the engine. Aircraft Spruce had a cap for Lycomings that had the
same thread sizes, so I took a chance that it'd fit my Continental.

It did, but the hardest part was safety-wiring it in place. The cap
sits between the generator and the right mag, and there aren't any good
places nearby to connect the safety-wire to. I finally settled on a
fitting eight or so inches away, and fed in a long piece of safety wire
through pre-drilled holes in the end of the cap.

I was wiggling the wire around, trying to curve the other end back
around, when I heard a crackling sound. I stepped back and saw smoke
rising. The loose end of the safety wire had flipped up and had shorted
a +12V terminal to ground!

My arm shot out and I grabbed the wire. Bad move. It was a 0.032
stainless steel wire with about 20 amps flowing through it. I turned it
loose and grabbed a pair of pliers.

Problem solved...but now I had a nice little burn across the last joint
of one of my fingers. Got a beau-ti-ful blister, now.

http://www.bowersflybaby.com/tech/tach_owie.jpg

The cap was finally safetied, and I ran the tach wire ran over the same
route as the old tach drive cable. Wrap the red sensor wire around the
open area between the braid four times, then lay the sensor wire
parallel to the spark plug wire and wrap the whole area in copper tape.
I was concerned about the open braid causing radio noise.

No radio noise...but no operation of the tach, either.

I figured it was the copper tape, so I stripped it away and straightened
the sensor wire.

The tach worked now, but was very erratic at low RPM. I wrapped the end
of the sensor wire around the plug wire one more time, and the problem
got better. So I wrapped the rest of the red wire around the plug
(about six turns). The idle was now stable, except it would glitch
momentarily every five or ten seconds.

http://www.bowersflybaby.com/tech/tach_coil.jpg

I'd brought a headset with, and noticed that the open section in the
braid produced only a very minor bit of popping into the radio. But I'd
started to wonder if maybe my copper tape wrapping HADN'T been the
problem, earlier. I had laid the rest of the sensor wire horizontally
along the plug wire under the copper tape. The diagram from the tach
manufacturer shows the sensor wire coming straight away from the plug
wire, instead. So I re-wrapped the section with copper tape, taking
care to let the sensor go directly away from the plug wire instead of
wrapping it in the copper tape.

And lo and behold, it worked.

http://www.bowersflybaby.com/tech/tach_copper.jpg

Wrap the area in electrical tape for a bit of durability, then re-secure
the spark plug wire to the motor mount with tie-wraps, as before.

http://www.bowersflybaby.com/tech/tach_wrap.jpg

A test flight produced very nice operation. There's still a bit of
instability at low power settings, but once the RPM crosses 1000, the
tach is rock-steady. The copper tape shielding is working great; no
ignition noise is audible.

Since it's connected to the left magneto, conventional "Mag checks" are
a thing of the past. When the mag switch goes to "R", the tach goes to
zero...it can't register if the magneto isn't firing.

I don't see this as much of a drawback. I've been less concerned with
the actual RPM drop during a mag check than with rough running, instead.
I figure that any real problem will manifest itself as missing and
stumbling, rather than hitting 155 RPM on a mag drop.

In any case, I own a small handheld tach, and will use it at annual time
to make sure things are hunky-dory.

The hourmeter function is pretty slick; it registers hours until the
engine starts, then automatically switches to the tach mode. However,
the hour meter shows hours and minutes only until the tach reaches
199:59; after that, it reads whole hours only.

Not a real drawback, as the tach also includes a secondary counter that
can still be used to keep track of minutes. However, 200 hours is about
five years of flying for me, and the built-in battery of the Tiny Tach
is only good for 5-8 years. At that point, I'd better hope the company
is still operating...or come up with some other solution.

Ron Wanttaja

Ron Wanttaja wrote:


I was wiggling the wire around, trying to curve the other end back
around, when I heard a crackling sound. I stepped back and saw smoke
rising. The loose end of the safety wire had flipped up and had shorted
a +12V terminal to ground!

My arm shot out and I grabbed the wire. Bad move. It was a 0.032
stainless steel wire with about 20 amps flowing through it. I turned it
loose and grabbed a pair of pliers.

Problem solved...but now I had a nice little burn across the last joint
of one of my fingers. Got a beau-ti-ful blister, now.

http://www.bowersflybaby.com/tech/tach_owie.jpg

Ouch. Now I have another argument to put in my bag of retorts when
people bash me for not having an electrical system in my plane! I've
never shorted the battery supply to ground in any of MY planes!

Hope your finger gets better soon...you may need it for driving!

On Aug 7, 9:52*pm, Ron Wanttaja wrote:

. . . Unfortunately, as it turns out, there was nothing wrong with my
tach...or with the tach cable. *The tach-drive section on the engine
itself (Continental C85) was bad. . . .

And, what do you plan to do about the stray pieces of the old tach
drive which are floating around in your oil? )

Rich S.

Rich S. wrote:
On Aug 7, 9:52 pm, Ron Wanttaja wrote:

. . . Unfortunately, as it turns out, there was nothing wrong with my
tach...or with the tach cable. The tach-drive section on the engine
itself (Continental C85) was bad. . . .

And, what do you plan to do about the stray pieces of the old tach
drive which are floating around in your oil? )

That's why the engine has the oil screen, right? :-)

Actually, the tach drive is external to the engine. It's at the end of
a longish shaft on an idler gear inside the accessory case.

Ron Wanttaja

On Fri, 07 Aug 2009 21:52:49 -0700, Ron Wanttaja
wrote:


Finally, I noticed the "Tiny Tach" on the Aircraft Spruce web page.
Seemed pretty good...got its signal by wrapping a wire around a spark
plug cable. From the description, I wasn't completely sure which model
to buy. But when I went to the company's web page, I found they had a
universal "Commercial" model that also updated faster than the standard
units.

http://www.tinytach.com/tinytach/commercial.php

Only $65, so I went ahead and ordered one. It had both a tach function
as well as an hourmeter and two service timers. It didn't need ANY
other connection other than to wrap the sensor wire around a spark plug
cable. It has a built-in battery.


it is indeed a pity that tinytach cant be given the hint that a
replaceable lithium battery would see an aviation market open up for
them. two lithium batteries so that they can be replaced individually
without loss of power would seem a simple mod to make the tach really
useful.
my hp calculators have used the two battery idea for ages.(ever since
I bought them :-) )

Stealth Pilot

Stealth Pilot wrote:
On Fri, 07 Aug 2009 21:52:49 -0700, Ron Wanttaja
wrote:

it is indeed a pity that tinytach cant be given the hint that a
replaceable lithium battery would see an aviation market open up for
them. two lithium batteries so that they can be replaced individually
without loss of power would seem a simple mod to make the tach really
useful.

They're losing the vast aviation market? :-)

From what I can see, the product is mostly aimed at the industrial
engine market, especially rental equipment. They know it's being used
on aircraft engines, but I suspect the number of actual installations in
airplanes is pretty small. Probably mostly in the ultralight world,
since anything with a Lycoming or Continental is already going to have a
tach drive.

As far as the non-replaceability of the battery, we'll see when the time
comes. The case is sealed plastic, so it should be possible to cut it
open. If nothing else, adding an external pack for a battery shouldn't
be tough.

It looks like Tiny Tach did, in fact, recently upgrade its commercial
tach from using external power to using solely the internal battery.
The version of their installation instruction that's online shows a
battery connection....

http://www.tinytach.com/tinytach/ima...structions.pdf

When I ordered the tach, I thought I'd have to supply power to it.
Since I thought I'd have to deliver power to the meter, I figured I'd
add a small voltmeter as well, since my voltmeter is a short-arc analog
gauge mounted just above the floor (tough to read).

I ordered the Martel Tools QM-100V....

http://www.martelmeters.com/pdf/QM_100V.pdf

....and this is one slick device. Self-contained LCD voltmeter in a
package the size of two postage stamps, less than a quarter inch thick,
surface mounts using a 7/32" stud on the back of the package. Didn't
use it, since the tach didn't need power after all, but I'll probably
add it the next time I rework the power.

Ron Wanttaja

Gee Ron,

Your on a slippery slope. Digital displays and already planning your
next electrical upgrade! Could be evidence of that dreaded
psychological disorder known as Airventure Syndrome. We'll keep an
eye out for the major symptoms, obsessive urges to install IFR glass
panels, researching the installation of autopilot systems and weather
radar, and the most serious sympton requiring medication a small
little black box that costs more than your last house.

There is only one cure. Quick, rush to the airport and throw that
generator and starter into the bush, recycle the battery, and strip
all that tangled wire. Think back to the days when your electrical
system was 2 wires to a switch. There is hope still for a complete
recovery! ;-)

Bob

Ron Wanttaja wrote:

They know it's being used
on aircraft engines, but I suspect the number of actual installations in
airplanes is pretty small. Probably mostly in the ultralight world,
since anything with a Lycoming or Continental is already going to have a
tach drive.

Except yours!!!

Scott
Corben Junior Ace WITH built in tach drive (so far)!!!

canuck_bob wrote:
Think back to the days when your electrical
system was 2 wires to a switch. ;-)

Bob


And some of us STILL do!

Scott
Corben Junior Ace
We don't need no steeenkin' electrical system

Ron Wanttaja wrote:

Not a real drawback, as the tach also includes a secondary counter that
can still be used to keep track of minutes. However, 200 hours is about
five years of flying for me, and the built-in battery of the Tiny Tach
is only good for 5-8 years. At that point, I'd better hope the company
is still operating...or come up with some other solution.

Ron Wanttaja

Here is a picture of the insides of a tiny tach. The gray stuff is soft potting compound similar to BMS5-95. The battery was a common
3v Lithim Ion button cell welded to the tabs before potting. If I ever find another tiny tach that needs a new battery, I will cut a
hole right over the battery with a rotabroach or a mill instead of splitting the case. You can't open the case without breaking it

http://tinyurl.com/tinytach

--
John Kimmel


So in this world
Of the simple and odd,
The bent and plain,
The unbalanced bod,
The imperfect people
And differently pawed,
Some live without love...
That's how they're flawed.