Reference times for Tach Meters

[[email protected]]

Relatively new pilot here, trying to optimize flight time/cost for
distance flights.

I belong to a flying club that charges different rates for Tach/
Hobbs. I use the airplane mostly for longer trips (not flight
training anymore), meaning the advantage of the Tach/Hobbs difference
is mostly lost, unless I plan cruise flight very carefully.

So, to plan the flights, I'm trying to optimize flight time with cost
issues. But, to do that, I need to know what speed the Tach Meter is
operating at. Anyone have a potential resource for listing reference
times for Tach Meters? Should I look on the meter itself? In a POH?

Based on my initial research, I'm using a Tach reference of 2400 RPM.
But, if the plane Tach is a 2500, or 2300, or some other number, my
planning for cruise flight BHP, flight times, and costs gets all
thrown off.

Any help would be appreciated!

Thanks,

Jake

[Ol Shy & Bashful]

On May 14, 11:42 am, " <jdkmyspam-
> wrote:
> Relatively new pilot here, trying to optimize flight time/cost for
> distance flights.
>
> I belong to a flying club that charges different rates for Tach/
> Hobbs. I use the airplane mostly for longer trips (not flight
> training anymore), meaning the advantage of the Tach/Hobbs difference
> is mostly lost, unless I plan cruise flight very carefully.
>
> So, to plan the flights, I'm trying to optimize flight time with cost
> issues. But, to do that, I need to know what speed the Tach Meter is
> operating at. Anyone have a potential resource for listing reference
> times for Tach Meters? Should I look on the meter itself? In a POH?
>
> Based on my initial research, I'm using a Tach reference of 2400 RPM.
> But, if the plane Tach is a 2500, or 2300, or some other number, my
> planning for cruise flight BHP, flight times, and costs gets all
> thrown off.
>
> Any help would be appreciated!
>
> Thanks,
>
> Jake

Jake
For the most part, once you get to operating rpm's the recording tach
will match the Hobbs time. The tach time records according to the rpm
and is usually off by about 10% from the Hobbs.
The Hobbs will normally operate off the oil pressure, or the Master
ON. To elaborate, Hobbs is normally used for revenue, and the tach is
used for component/maintenance time. Some aircraft have a micro switch
on the landing gear to record when the gear is UP vs DOWN just as
helicopters will frequently have two Hobbs to record revenue time vs
Collective time (kind of like the landing gear for actual flight
time).
For the club time as you are looking at, figure as soon as you start
the engine you're on the clock for cost and that is the most common
method.

[Jose]

> Based on my initial research, I'm using a Tach reference of 2400 RPM.
> But, if the plane Tach is a 2500, or 2300, or some other number, my
> planning for cruise flight BHP, flight times, and costs gets all
> thrown off.

Is your tach accurate? You may find that differences in the tach
=readings= swamp the fine cost issues you are trying to figure. But to
save the most money, plan for lowest RPM consistant with safe and proper
operation. The speed loss will likely be smaller than the RPM drop.

Make a graph of speed vs RPM (if it's a constant speed prop, use the
same power setting). You will probably find that percentage wise, you
get less additional speed than you put in in additional RPM.

Now headwinds should be factored in too - since changing from a GS of
120 to 100 is a smaller percent change (what you're comparing with the
RPM) than going from 100 to 80 due to a 20 knot headwind.

Jose
--
Quantum Mechanics is like this: God =does= play dice with the universe,
except there's no God, and there's no dice. And maybe there's no universe.
for Email, make the obvious change in the address.

[[email protected]]

Ol Shy & Bashful wrote:

> For the club time as you are looking at, figure as soon as you start
> the engine you're on the clock for cost and that is the most common
> method.

Ol Shy:

Thanks for the response. Are you recommending that I simply plan for
the Tach amount for the flight time? There is a $12 per hour
difference between the Tach and Hobbs rental price (Hobbs is
cheaper). Sounds like a bad deal to me if there is no way to optimize
engine performance to at least 90% of the tach reference RPM.

[[email protected]]

On May 14, 11:58 am, Jose > wrote:
> Is your tach accurate? You may find that differences in the tach
> =readings= swamp the fine cost issues you are trying to figure. But to
> save the most money, plan for lowest RPM consistant with safe and proper
> operation. The speed loss will likely be smaller than the RPM drop.
>
> Make a graph of speed vs RPM (if it's a constant speed prop, use the
> same power setting). You will probably find that percentage wise, you
> get less additional speed than you put in in additional RPM.
>
> Now headwinds should be factored in too - since changing from a GS of
> 120 to 100 is a smaller percent change (what you're comparing with the
> RPM) than going from 100 to 80 due to a 20 knot headwind.
>
> Jose

Jose:

Thanks for the response. I don't know if the Tach is accurate. Do
you have a good suggestion how I might check?

I'm trying to get the information for exactly the kind of graph that
you are suggesting. Headwinds, of course, can't be calculated in the
abstract, but I do add this consideration when I plan actual flights.

[Jose]

> Thanks for the response. I don't know if the Tach is accurate. Do
> you have a good suggestion how I might check?

Ask your maintanance guy. A tach check may be part of the annual or the
100 hour. If you have access to a strobe and an accurate flash rate,
you could use that on the prop too. Use a partner, for obvious reasons.
"Kids, don't try this at home!"

> Headwinds, of course, can't be calculated in the abstract, but
> I do add this consideration when I plan actual flights.

You can figure a break-even point for winds.

You also need to include taxi time in your figuring. On the hobbs, you
taxi at full price. On the tach, it's less than half price. An
unexpected ground delay or early release could also swamp your calculations.

Now, do you actually have a choice, per flight, as to what method you'll
be using (tach or hobbs)? Can you decide after the flight?

Jose
--
Quantum Mechanics is like this: God =does= play dice with the universe,
except there's no God, and there's no dice. And maybe there's no universe.
for Email, make the obvious change in the address.

[JDK]

On May 14, 12:35 pm, Jose > wrote:

> Ask your maintanance guy. A tach check may be part of the annual or the
> 100 hour. If you have access to a strobe and an accurate flash rate,
> you could use that on the prop too. Use a partner, for obvious reasons.
> "Kids, don't try this at home!"

A good question to ask. I called the flight club and spoke with a
young CFI, but knew I wasn't going to get a good answer when he asked
me what I meant by the tach reference speed/time. I'll have to follow
up in person.

> Now, do you actually have a choice, per flight, as to what method you'll
> be using (tach or hobbs)? Can you decide after the flight?

Unfortunately, no. That would be great, though, wouldn't it?

[Blueskies]

> wrote in message ups.com...
> Relatively new pilot here, trying to optimize flight time/cost for
> distance flights.
>
> I belong to a flying club that charges different rates for Tach/
> Hobbs. I use the airplane mostly for longer trips (not flight
> training anymore), meaning the advantage of the Tach/Hobbs difference
> is mostly lost, unless I plan cruise flight very carefully.
>
> So, to plan the flights, I'm trying to optimize flight time with cost
> issues. But, to do that, I need to know what speed the Tach Meter is
> operating at. Anyone have a potential resource for listing reference
> times for Tach Meters? Should I look on the meter itself? In a POH?
>
> Based on my initial research, I'm using a Tach reference of 2400 RPM.
> But, if the plane Tach is a 2500, or 2300, or some other number, my
> planning for cruise flight BHP, flight times, and costs gets all
> thrown off.
>
> Any help would be appreciated!
>
> Thanks,
>
> Jake
>

I've always assumed (dangerous I know) that 100% rpm = 1 hour on the tach meter. As prop speed went down so does the
tach hour. So if 2700 is redline = 1 hour, then 2500 rpm = .92 per hour, 2200 = .81, etc...

[JGalban via AviationKB.com]

Blueskies wrote:
>
>I've always assumed (dangerous I know) that 100% rpm = 1 hour on the tach meter. As prop speed went down so does the
>tach hour. So if 2700 is redline = 1 hour, then 2500 rpm = .92 per hour, 2200 = .81, etc...

That would not be a good assumption. Tachs are calibrated to record 1 hr.
at various rpms, depending on the installation. Check the Mitchell tachs
on the link below. They'll record an hour anywhere from 1800 rpm to 2566 rpm.


http://www.aircraftspruce.com/catalog/inpages/mech_tachs.php

John Galban=====>N4BQ (PA28-180)

--
Message posted via http://www.aviationkb.com

[Bob Moore]

Blueskies wrote
> I've always assumed (dangerous I know) that 100% rpm = 1 hour on the
> tach meter. As prop speed went down so does the tach hour. So if 2700
> is redline = 1 hour, then 2500 rpm = .92 per hour, 2200 = .81, etc...

Bad assumption....from two web sites:

happylanding.com
3/ Tachometer and time
Hands up everyone who uses the hour meter on the aircraft tachometer to
record air time. You are ripping yourself off.
The tach time is only accurate at one power setting and that’s usually
around 2300 rpm. If you cruise at a lower power, the meter ticks over at
a slower rate than real time. That would save on aircraft inspections
which are based on air time but who flies below 2300 rpm? Nobody does on
takeoff or in a climb and few pilots do in cruise. At higher power
settings, the hour recorder on the tach runs faster than the clock.
Aircraft inspections come up sooner if the tach is used for air time.
The meter also works on the ground. It’s clocking slowly at idle but
it’s turning and adding to the air time which doesn’t have to start
until liftoff.
------------------------------------------------------------------------
-----------------------------------------------
foxflying.com
What are TACH and HOBBS times?
Airplanes have timers in them that are used to track operating time for
maintenance and billing. For example, most small aircraft engines are
overhauled after reaching 2000 tach hours. The Tach timer is connected
to the tachometer, which indicates how fast the engine and propeller are
turning in revolutions per minute, just like your car. This timer is set
by the factory to be equal to normal clock time when the engine is at
2300 rpm (Piper) or 2400 rpm (Cessna). Therefore, when the engine is
operating at low power levels, common during training, the Tach timer
operates slower than clock time. The Hobbs timer records clock time
whenever the engine is operating.
Generally, Tach time is 85% to 90% of Hobbs (clock) time. So a billing
rate of $75 per Tach hour is equivalent to $64 to $68 per Hobbs hour.
So, the difference is very important when comparing the cost of flying!