Charging for tach time in a fixed pitch airplane

My flying club has recently started charging out its 172's based on tach
time rather than Hobbes time. In my experience fixed pitched airplanes
I've rented have been charged out based on Hobbes, while constant speed
airplanes have been charged out based on tach.

So recently, a friend and I took a 172 to do some pattern work. We were
out a little over an hour according to the Hobbes meter and our
wris****ches, but the tach registered about half that. Now I've flown a
182 and noticed about a 10% to 15% difference between Hobbes and tach
time depending on what I was doing. Especially in the pattern, I never
noticed anything like a 50% difference. So I began to think about it.
With a fixed pitch prop, at a given throttle setting, rpms will drop in a
climb and increase in a descent. They also drop when the throttle is
pulled back. Hence our low tach time in the pattern -- lower than peak
rpms in the climb, then low rpms on the downwind and descent because of
the reduced throttle. In the 182, however, you have near or maximum rpms
in the climb and, with the propeller fully forward, higher rpms in the
pattern and descent than you would in a 172 with a similar throttle
setting. Hence the tach time should be higher in the 182 than in the 172
despite similar wear and tear on the airplane.

So, this got me thinking. Given a 172 charged out based on tach time,
what would be the most expensive way and the cheapest way to get from
point a to point b? Most expensive -- climb at Vy+5 knots, then cruise at
a TAS of 110 knots. Cheapest -- do a Vx climb to altitude (to keep the
rpms low); when at altitude, chop the power completely and start a
descent. After descending for a while, climb back to altitude at Vx.
Repeat until reaching destination (or sick, whichever comes first). Am I
crazy? Isn't this why fixed pitch equipped airplanes are usually charged
out based on Hobbes time?

So recently, a friend and I took a 172 to do some pattern work. We were
out a little over an hour according to the Hobbes meter and our
wris****ches, but the tach registered about half that.

Something is probably wrong. We have two Archers (fixed pitch) and a
Dakota (constant speed), and I've never gotten that much of a difference.

Taxiing time is far less expensive, because I'm taxiing at less than
half max rpm. But simple pattern work? Check the meters.

Jose
--
Get high on gasoline: fly an airplane.
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On Apr 30, 11:50 am, Charlie Axilbund wrote:
My flying club has recently started charging out its 172's based on tach
time rather than Hobbes time. In my experience fixed pitched airplanes
I've rented have been charged out based on Hobbes, while constant speed
airplanes have been charged out based on tach.

So recently, a friend and I took a 172 to do some pattern work. We were
out a little over an hour according to the Hobbes meter and our
wris****ches, but the tach registered about half that.

Because all the time on taxiing and all the time abeam when you pull
power are virtually free.

-Robert, CFII

Most places I am aware of charge simply by Hobbs Time. The general
rule of thumb arround here is that on an average the Tach time will be
80% of the hobbs time. Most businesses do it this way because Hobbs
time generates more revenue for the airplane per maintenanence hour.

Example: If they charge $100 Hobbs for the airplane.
They get about $120 for every Tach Hour. Since they do maintenance off
of Tach time they get an extra $2000 for every 100 hours put on the
airplane over charging Tach time.

I do understand why clubs might prefer to charge by Tach, but they
usually do so by charging a higher rate than if they were charging by
hobbs. Charging by Tach encourages the pilots to throttle back some
which can make the engine last longer.

It makes sense to me to charge by Hobbs on constant speed props
because charging by Tach encourages running High MFP and Low RPM. Low
RPM is ok but need to keep the MFP in line with it as well.

With the fixed pitch prop, the roller coaster approach is probably the
least expensive. This is simlar to what motor gliders do for cross
country. 2nd would probably be to simply run at the best economy
cruise.

Brian

On 2007-04-30, Brian wrote:
It makes sense to me to charge by Hobbs on constant speed props
because charging by Tach encourages running High MFP and Low RPM. Low
RPM is ok but need to keep the MFP in line with it as well.

That's an old wives' tale, I'm afraid. The best regime to operate an
engine (most efficient and least maintenance) is the lowest RPM and
highest MP for the desired power setting. There are some caveats (some
aircraft have a range of RPMs which you should not continuously operate
the engine), but generally speaking, using the lowest RPM possible for
the desired power setting results in less noise, less vibration and less
wear on the engine.

The thing about operating engines (specifically normally aspirated flat
engines like most of us use) 'above square' is hogwash, I'm afraid.

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Doesn't this also bring up the question of when is an overhaul due
(according to hours)?
At 2000 hobbs hours or 2000 tach hours?
Lou

In article .com,
Lou wrote:

Doesn't this also bring up the question of when is an overhaul due
(according to hours)?
At 2000 hobbs hours or 2000 tach hours?
Lou

Tach

--
Bob Noel
(goodness, please trim replies!!!)

Bob Noel wrote:
In article .com,
Lou wrote:

Doesn't this also bring up the question of when is an overhaul due
(according to hours)?
At 2000 hobbs hours or 2000 tach hours?
Lou

Tach

BZZT Wrong answer. Time in service. Tach is acceptable,
so are elapsed time meters. I don't even have a recording
tach in my plane.

On May 1, 7:18 am, Dylan Smith wrote:
On 2007-04-30, Brian wrote:

It makes sense to me to charge by Hobbs on constant speed props
because charging by Tach encourages running High MFP and Low RPM. Low
RPM is ok but need to keep the MFP in line with it as well.

That's an old wives' tale, I'm afraid. The best regime to operate an
engine (most efficient and least maintenance) is the lowest RPM and
highest MP for the desired power setting. There are some caveats (some
aircraft have a range of RPMs which you should not continuously operate
the engine), but generally speaking, using the lowest RPM possible for
the desired power setting results in less noise, less vibration and less
wear on the engine.

The thing about operating engines (specifically normally aspirated flat
engines like most of us use) 'above square' is hogwash, I'm afraid.


Agreed, But how do you make a simple rule of thumb for this for pilots
with less then 100 hours or like my latest student that is starting
his primary instruction an an airplane with a constant speed prop.

The problem with your statement is that with new, or low time pilots,
as is often typical in clubs, that are going to take it literally and
is you will end up with a few pilots running around WOT and 1500 RPM.
I doubt that is what you ment and the cyinder pressures have got be
quite high in this configuration, especially if they don't lean
properly either.

The "above square rule" is a great simplifcation of how you "can"
operate constant speed props. It certainly is not a hard and fast rule
but you have to understand what you doing operate otherwise safetly.
For a pilot that barely knows how to operate the throttle and mixture
properly they probably ought to just stick with the "over square rule"

Brian
CFIIG/ASEL

On 2007-05-02, Brian wrote:
Agreed, But how do you make a simple rule of thumb for this for pilots
with less then 100 hours or like my latest student that is starting
his primary instruction an an airplane with a constant speed prop.

You don't. You tell them to use the power setting tables in the POH. If
they are incapable of this, they aren't really cut out for aviation.

The problem with your statement is that with new, or low time pilots,
as is often typical in clubs, that are going to take it literally and
is you will end up with a few pilots running around WOT and 1500 RPM.

You won't because they won't make enough power to maintain altitude.

I doubt that is what you ment and the cyinder pressures have got be
quite high in this configuration, especially if they don't lean
properly either.

Cylinder pressures will be in the same order of magnitude as 'normal'
operation. You will neither climb nor go fast though in this regime.

For a pilot that barely knows how to operate the throttle and mixture
properly they probably ought to just stick with the "over square rule"

They probably ought to have some remedial training if they can't set the
throttle and mixture correctly! It's not even hard to do these things.

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