Tach Vs. Hobbs Time

[John Roncallo]

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?

[Don Tuite]

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

[[email protected]]

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?

[C J Campbell]

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.

[Ben Jackson]

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/

[EDR]

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.

[C J Campbell]

"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.

[Ron Natalie]

"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.

[Ron Natalie]

"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.

[PaulaJay1]

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

[Ron Natalie]

"PaulaJay1" > wrote in message ...

> 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.
>
Engine sound or prop sound? How does it work on geared engines?

(just kidding).

[Drew Dalgleish]

On Sun, 21 Dec 2003 17:56:56 -0500, "Ron Natalie" >
wrote:

>
>"PaulaJay1" > wrote in message ...
>
>> 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.
>>
>Engine sound or prop sound? How does it work on geared engines?
>
>(just kidding).
>
Someone suggested using a street light as a strobe to check your tach.
Do the math to figure out what speed will stop the prop and see if
your tach agrees
Drew

[John Roncallo]

Ben Jackson wrote:
> 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.
>

I guess this is the question I'm asking. Where does 1:1 occur. On the
particular trip I was running 65% which at that altitude was 2425 RPM.

John Roncallo

[John Roncallo]

Ben Jackson wrote:

> 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.
>

This sounds like it would be appropriate for a CS prop because you can
always load the prop up at these low RPMs and still get 65% even at 8000
ft. For example the C-172RG also with an O-360 180HP Lycoming will put
out 68% power at 8000 ft 2300 RPM and 22MP.

I would think that the Archer fixed pitch prop would be calibrated
differently. To get 65% on a standard day at 8000 ft requires ~2480 RPM.

John Roncallo

[Ben Jackson]

In article >,
John Roncallo > wrote:
>
>I would think that the Archer fixed pitch prop would be calibrated
>differently. To get 65% on a standard day at 8000 ft requires ~2480 RPM.

Why? Do the Archer's engine components wear less at high RPM than an
engine on an Arrow?

--
Ben Jackson
>
http://www.ben.com/

[G.R. Patterson III]

Drew Dalgleish wrote:
>
> Do the math to figure out what speed will stop the prop and see if
> your tach agrees

Any multiple of 60.

George Patterson
Great discoveries are not announced with "Eureka!". What's usually said is
"Hummmmm... That's interesting...."

[Doug]

It is 1800 for a 2 blade prop, and 1200 and 2400 for a 3 blade prop,
if my memory serves me.
I worked the math once. Light goes off 60 times per second, which is
3600 times a minute. And you have to account for the number of blades.
Just pull up with a street light to your back and try it. The prop
will stop. Should be at one of the numbers above.

It is not every multiple of 60.


"G.R. Patterson III" > wrote in message >...
> Drew Dalgleish wrote:
> >
> > Do the math to figure out what speed will stop the prop and see if
> > your tach agrees
>
> Any multiple of 60.
>
> George Patterson
> Great discoveries are not announced with "Eureka!". What's usually said is
> "Hummmmm... That's interesting...."

[Ron Natalie]

"Doug" > wrote in message om...

> It is not every multiple of 60.
>
Yep, and if you have a geared engine there's still a bit of math. We checked my tach
once with one of those strobotach things (looking at the prop). The tach is turning faster
than the prop by a 120:77 (or something like that) ratio.

[Sven]

Of course, this only applies in North America and other 60Hz areas. You
Europeans, Asians, etc., are a bit slower at 50Hz; 1,500rpm for 2 and
1,000/2,000 for 3 (non geared) blades. Right???

"Doug" > wrote in message
om...
> It is 1800 for a 2 blade prop, and 1200 and 2400 for a 3 blade prop,
> if my memory serves me.
> I worked the math once. Light goes off 60 times per second, which is
> 3600 times a minute. And you have to account for the number of blades.
> Just pull up with a street light to your back and try it. The prop
> will stop. Should be at one of the numbers above.
>
> It is not every multiple of 60.

[John Roncallo]

Ben Jackson wrote:

> In article >,
> John Roncallo > wrote:
>
>>I would think that the Archer fixed pitch prop would be calibrated
>>differently. To get 65% on a standard day at 8000 ft requires ~2480 RPM.
>
>
> Why? Do the Archer's engine components wear less at high RPM than an
> engine on an Arrow?
>

Yes they probably do wear less. They are under less load for the given
circumstance. Cylinder pressures are much higher in an engine at 65%
power 2300 RPM than they are at 65% 2480 RPM.

John Roncallo

[John Roncallo]

Drew Dalgleish wrote:

> On Sun, 21 Dec 2003 17:56:56 -0500, "Ron Natalie" >
> wrote:
>
>
>>"PaulaJay1" > wrote in message ...
>>
>>
>>>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.
>>>
>>
>>Engine sound or prop sound? How does it work on geared engines?
>>
>>(just kidding).
>>
>
> Someone suggested using a street light as a strobe to check your tach.
> Do the math to figure out what speed will stop the prop and see if
> your tach agrees
> Drew

I guess you are saying the street light flickers at 60Hz. Than a 2
bladed prop should look stopped at 1800 RPM? I might give this a try but
I think this would be better for calibrating the tach neadle rather than
the tach hour indicator.

John Roncallo

[Dan Raneri]

All those posts, and no one answered the question...

> "John Roncallo" > wrote
>
> I guess this is the question I'm asking. Where does 1:1 occur. On the
> particular trip I was running 65% which at that altitude was 2425 RPM.
>
> John Roncallo

It depends on the tach. Typically, you can buy one that's designed for 1:1
at 2300 or 2500... If it's not marked on the faceplate, check the model
number or s/n of the tach, and go to the manufacturer. That's the best way
to tell how the tach is calibrated.

If the tach is calibrated to run at 2300 and you're cruising at 2425, that's
a 5.4% increase, so a trip that took 3.9 would read 4.1, assuming no
takeoff and climb...


"John Roncallo" > wrote in message
. com...
> 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?
>

[Ron Natalie]

"John Roncallo" > wrote in message . com...

> I guess you are saying the street light flickers at 60Hz.

It flickers at 120 Hz.

[John Roncallo]

Ron Natalie wrote:
> "John Roncallo" > wrote in message . com...
>
>
>>I guess you are saying the street light flickers at 60Hz.
>
>
> It flickers at 120 Hz.
>

I guess this is due to voltage passing through zero 2 time per cycle?
using 60 Hz. AC. Looks like I will only pick up the blade looking like a
4 bladed prop at 1800 RPM.

John Roncallo

[John Roncallo]

Dan Raneri wrote:

> All those posts, and no one answered the question...
>
>
>>"John Roncallo" > wrote
>>
>>I guess this is the question I'm asking. Where does 1:1 occur. On the
>>particular trip I was running 65% which at that altitude was 2425 RPM.
>>
>>John Roncallo
>
>
> It depends on the tach. Typically, you can buy one that's designed for 1:1
> at 2300 or 2500... If it's not marked on the faceplate, check the model
> number or s/n of the tach, and go to the manufacturer. That's the best way
> to tell how the tach is calibrated.
>
> If the tach is calibrated to run at 2300 and you're cruising at 2425, that's
> a 5.4% increase, so a trip that took 3.9 would read 4.1, assuming no
> takeoff and climb...
>
>
> "John Roncallo" > wrote in message
> . com...
>
>>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?
>>
>
>
>
Thanks, this makes sense. Is there anyway I can tell what is supposed to
be installed in this particular aircraft?

John Roncallo

[Dylan Smith]

In article >, John Roncallo
wrote:
>> Why? Do the Archer's engine components wear less at high RPM than an
>> engine on an Arrow?
>
> Yes they probably do wear less. They are under less load for the given
> circumstance. Cylinder pressures are much higher in an engine at 65%
> power 2300 RPM than they are at 65% 2480 RPM.

But a far greater effect will be the additional wear from friction of
running at 2480 RPM instead of 2300 RPM - a significant increase in the
number of friction cycles per hour. I'd say the Archer's engine
will wear more quickly.

--
Dylan Smith, Castletown, Isle of Man
Flying: http://www.dylansmith.net
Frontier Elite Universe: http://www.alioth.net
"Maintain thine airspeed, lest the ground come up and smite thee"

[Ron Natalie]

"Dylan Smith" > wrote in message ...

>
> But a far greater effect will be the additional wear from friction of
> running at 2480 RPM instead of 2300 RPM - a significant increase in the
> number of friction cycles per hour. I'd say the Archer's engine
> will wear more quickly.

The number of revolutions acquired isn't the driving factor in engine
life. Flight school aircraft that are flown frequently and full throttle
tend to far exceed TBO where as babied, infrequently flown single
owner birds tend not to get anywhere near TBO.

[Paul Sengupta]

Or 50 here in Europe.

Paul

"G.R. Patterson III" > wrote in message
...
> Any multiple of 60.

[Paul Sengupta]

To check, you can get one of these:
http://www.aircraftspruce.com/catalog/inpages/trutach.php

As pointed out, can get get various 1:1 tach RPM settings.
You specify it when you buy one.

Go to http://www.gulf-coast-avionics.com/
and type "tachometer" in the search box. You'll see the
1:1 RPM listed. Maybe a number in the model number
gives it away (like the 3 or 5 at the end in the ones listed).

Paul

"John Roncallo" > wrote in message
om...
> Thanks, this makes sense. Is there anyway I can tell what is supposed to
> be installed in this particular aircraft?

[John Roncallo]

Paul Sengupta wrote:
> To check, you can get one of these:
> http://www.aircraftspruce.com/catalog/inpages/trutach.php
>
> As pointed out, can get get various 1:1 tach RPM settings.
> You specify it when you buy one.
>
> Go to http://www.gulf-coast-avionics.com/
> and type "tachometer" in the search box. You'll see the
> 1:1 RPM listed. Maybe a number in the model number
> gives it away (like the 3 or 5 at the end in the ones listed).
>
> Paul
>
> "John Roncallo" > wrote in message
> om...
>
>>Thanks, this makes sense. Is there anyway I can tell what is supposed to
>>be installed in this particular aircraft?
>
>
>
>

Thanks

Our annual is comming up. I will be looking into this.

John Roncallo

[John Roncallo]

Dylan Smith wrote:

> In article >, John Roncallo
> wrote:
>
>>>Why? Do the Archer's engine components wear less at high RPM than an
>>>engine on an Arrow?
>>
>>Yes they probably do wear less. They are under less load for the given
>>circumstance. Cylinder pressures are much higher in an engine at 65%
>>power 2300 RPM than they are at 65% 2480 RPM.
>
>
> But a far greater effect will be the additional wear from friction of
> running at 2480 RPM instead of 2300 RPM - a significant increase in the
> number of friction cycles per hour. I'd say the Archer's engine
> will wear more quickly.
>

Wear is a function of pressure and velocity. Mechanical dynamic friction
is a function of mostly pressure. High cylinder pressures cause high
mechanical pressures everywhere , journal bearings, piston side forces
etc. With both cases of high and Low RPM at equal power output the wear
should for the most part be relatively equal.

The absolute which case is better argument and where is the exact equal
wear points as far as RPM goes will probably never be much more than an
educated guess but it should be clear that the lower loaded Archer
engine should be allowed more RPM than the higher loaded Arrow engine
even if they were the same exact engine, which they are not.

John Roncallo

[John Roncallo]

Ron Natalie wrote:

> "Dylan Smith" > wrote in message ...
>
>
>>But a far greater effect will be the additional wear from friction of
>>running at 2480 RPM instead of 2300 RPM - a significant increase in the
>>number of friction cycles per hour. I'd say the Archer's engine
>>will wear more quickly.
>
>
> The number of revolutions acquired isn't the driving factor in engine
> life. Flight school aircraft that are flown frequently and full throttle
> tend to far exceed TBO where as babied, infrequently flown single
> owner birds tend not to get anywhere near TBO.
>

This is true but the tach doesn't know any more than RPM. The rest of
the story is all assumed. What I'm wondering is did we replace our clubs
perfectly good running engine last year at 2000 hr or was it more like
(2300RPM/2566RPM)* 2000hr = 1793 hr. My most recent flying and some of
the posts in this group are telling me we may have replaced it at 1793 hr.

Thanks to all who posted

J. Roncallo

[EDR]

In article >, John
Roncallo > wrote:

> This is true but the tach doesn't know any more than RPM. The rest of
> the story is all assumed. What I'm wondering is did we replace our clubs
> perfectly good running engine last year at 2000 hr or was it more like
> (2300RPM/2566RPM)* 2000hr = 1793 hr. My most recent flying and some of
> the posts in this group are telling me we may have replaced it at 1793 hr.

It doesn't matter how many hours are on the engine, it's why you had to
replace it?
Was it making metal?
Were parts worn out of spec that they could not be replaced?
Many things affect TBO.
Some engines go significantly beyond TBO, some significantly less.
Operation and environment make a difference.

[John Roncallo]

EDR wrote:
> In article >, John
> Roncallo > wrote:
>
>
>>This is true but the tach doesn't know any more than RPM. The rest of
>>the story is all assumed. What I'm wondering is did we replace our clubs
>>perfectly good running engine last year at 2000 hr or was it more like
>>(2300RPM/2566RPM)* 2000hr = 1793 hr. My most recent flying and some of
>>the posts in this group are telling me we may have replaced it at 1793 hr.
>
>
> It doesn't matter how many hours are on the engine, it's why you had to
> replace it?
> Was it making metal?
> Were parts worn out of spec that they could not be replaced?
> Many things affect TBO.
> Some engines go significantly beyond TBO, some significantly less.
> Operation and environment make a difference.

No it was running just fine. But our club feels that Lawyers will eat us
alive if an engine fails and we let it go beyond TBO. I tend to agree,
but I would like to see our rules changed to as required or first annual
past TBO which ever comes first.

John Roncallo