Tach Vs. Hobbs Time

Hello

Im trying to figure out the Tach on our clubs Archer is screwing us. I
just flew down to the Centenial of Flight FFA from Planville CT. Round
trip with all diversions and ATC rerouts was 975nm. The tach said 12.4
hr. I never checked the Hobbs but I do know that for this plane the Tach
usually runs faster than the hobbs on cross countries. Is this normal. I
also did another flight previously at 65% power from Meriden MMK to
Williamsburg W94 in 4.6 Tach time. My watch said 3.9. Also based on the
4.6 my fuel burn was only 8 GPH.

Is there any text book way to check this. Our FBO seems to feel that
calibrating the Tach is a big deal. I'm currently thinking of just
replacing the Tach without tring to calibrate. Right now it looks to me
as if we just replaced and engine at 1700 hr thinking it had 2000 hr.

Thoughts ideas?

On Sun, 21 Dec 2003 02:04:05 GMT, John Roncallo
wrote:

Hello

Im trying to figure out the Tach on our clubs Archer is screwing us. I
just flew down to the Centenial of Flight FFA from Planville CT. Round
trip with all diversions and ATC rerouts was 975nm. The tach said 12.4
hr. I never checked the Hobbs but I do know that for this plane the Tach
usually runs faster than the hobbs on cross countries. Is this normal. I
also did another flight previously at 65% power from Meriden MMK to
Williamsburg W94 in 4.6 Tach time. My watch said 3.9. Also based on the
4.6 my fuel burn was only 8 GPH.

Is there any text book way to check this. Our FBO seems to feel that
calibrating the Tach is a big deal. I'm currently thinking of just
replacing the Tach without tring to calibrate. Right now it looks to me
as if we just replaced and engine at 1700 hr thinking it had 2000 hr.

Thoughts ideas?

Is the tach like the speedometer in a car? Flexible shaft turns an
aluminum cup to run the needle, and a worm gear run by the shaft
counts up the "hours"?

If so, let's say you set the tach for 2500 rpm and mark the time.
Ignore the gear reduction between the crankshaft and the tach. After
the crankshaft has gone around 150,000 times, it is supposed to be one
hour later.

But your tach is telling you that it's actually 1.18 (4.6/3.9) hours
later. Seems to me, that's the same as telling you that your
crankshaft has made 177,000 revolutions. Eg. that your actual rpm was
2950.

That's unlikely on an Archer, but an error in rpm is something you
could check easily with a strobe and it probably ought to be your next
step.

Don

The tach is GEARED to the engine crankshaft. At some nominal RPM the
tack will read one hour in one clock hour. If you run the engine at
an excessive RPM then the tach will run fast. If you run at 60% power
with a fixed pitch prop then the tach should read slower than a
clock.
The hobb's is a clock that is started and stopped by a engine oil
pressure switch on most aircraft.

Tach hours are really the number of crankshaft revolutions times a
fixed scale factor which is the gear ratios from the crankshaft to the
counter wheels.

It sounds like you are running full throttle all the way since my tach
always reads slower than the hobbs but then I have to pay for the
maintenance so I take care of the engine.



On Sun, 21 Dec 2003 02:04:05 GMT, John Roncallo
wrote:

Hello

Im trying to figure out the Tach on our clubs Archer is screwing us. I
just flew down to the Centenial of Flight FFA from Planville CT. Round
trip with all diversions and ATC rerouts was 975nm. The tach said 12.4
hr. I never checked the Hobbs but I do know that for this plane the Tach
usually runs faster than the hobbs on cross countries. Is this normal. I
also did another flight previously at 65% power from Meriden MMK to
Williamsburg W94 in 4.6 Tach time. My watch said 3.9. Also based on the
4.6 my fuel burn was only 8 GPH.

Is there any text book way to check this. Our FBO seems to feel that
calibrating the Tach is a big deal. I'm currently thinking of just
replacing the Tach without tring to calibrate. Right now it looks to me
as if we just replaced and engine at 1700 hr thinking it had 2000 hr.

Thoughts ideas?

The tach should almost always run slower than the Hobbs. The Hobbs is a
clock, but the tach is connected directly to the engine and should only
click off one tach hour when the engine has turned 60 x max rpm.

In article ,
C J Campbell wrote:
The tach should almost always run slower than the Hobbs. The Hobbs is a
clock, but the tach is connected directly to the engine and should only
click off one tach hour when the engine has turned 60 x max rpm.

But it's not "max rpm". In my Comanche (260hp IO-540, redline 2700RPM)
it's 1:1 at 2300RPM. I think the "definition" of tach time is 1:1 at
cruise.

--
Ben Jackson

http://www.ben.com/

In article ,
wrote:

The hobb's is a clock that is started and stopped by a engine oil
pressure switch on most aircraft.

Some are connected to the battery Master switch. Hobbs starts when the
Master is turned on, stops when it is turned off.

"Ben Jackson" wrote in message
news:aJ8Fb.103545$8y1.318631@attbi_s52...
| In article ,
| C J Campbell wrote:
| The tach should almost always run slower than the Hobbs. The Hobbs is a
| clock, but the tach is connected directly to the engine and should only
| click off one tach hour when the engine has turned 60 x max rpm.
|
| But it's not "max rpm". In my Comanche (260hp IO-540, redline 2700RPM)
| it's 1:1 at 2300RPM. I think the "definition" of tach time is 1:1 at
| cruise.
|

That's right, he is talking about a Piper, there.

"C J Campbell" wrote in message
...
The tach should almost always run slower than the Hobbs. The Hobbs is a
clock, but the tach is connected directly to the engine and should only
click off one tach hour when the engine has turned 60 x max rpm.

The ratio of turns to "tach time" isn't necessarily set on max rpm.

"EDR" wrote in message
...
In article ,
wrote:

The hobb's is a clock that is started and stopped by a engine oil
pressure switch on most aircraft.

Some are connected to the battery Master switch. Hobbs starts when the
Master is turned on, stops when it is turned off.

And on some retracts it's connected to the squat switch so that it only
measures
"flight time." This saves you on maintenance a bit.

But the heart of the matter is that the hobbs is just an electrically driven
hour meter.
It has two terminals (hot and ground) and whenever voltage is present (from
whatever
source) it runs.

On my plane it is a seperately fused line (not through the master bus)
through the
oil pressure switch. This for rentals cuts down on any clock shenanigans
by running
with the master off (so a lot of rentals have it this way). My favoritle
rental was the
170 I used to fly that had the hobbs installed in where you could actually
see the
slide on contacts to it and pull it off if you were so inclined.

In article , John Roncallo
writes:

Is there any text book way to check this. Our FBO seems to feel that
calibrating the Tach is a big deal. I'm currently thinking of just
replacing the Tach without tring to calibrate. Right now it looks to me
as if we just replaced and engine at 1700 hr thinking it had 2000 hr.


I got a free program that uses the engine sound to give you RPM. I don't know
the web site but the program says email is . Just
take a laptop to the plane and check it out.

Chuck