Charging for tach time in a fixed pitch airplane

[Charlie Axilbund]

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?

[Jose]

> 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.
for Email, make the obvious change in the address.

[Robert M. Gary]

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

[Andrew Sarangan]

On Apr 30, 4:53 pm, Jose > wrote:
> > Because all the time on taxiing and all the time abeam when you pull
> > power are virtually free.
>
> No, it's not "free". It's half price, maybe. (1200 rpm?) The rest of
> it is full price.
>
> Jose
> --

1200 sounds a bit too high for taxiing.

[Brian[_1_]]

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

[Jose]

> 1200 sounds a bit too high for taxiing.

It is. And it's low for windmilling. But it's in the (half price)
ballpark.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

[Paul Tomblin]

In a previous article, Charlie Axilbund > said:
>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

That seems like an excessive difference. Our club charges by tach time,
and the tach is calibrated for 2400 rpm. With an Archer in the pattern,
I'm usually 2450 on climb out, 2000 on downwind, and 1600 on base and
final. When I was training, I got about a 15-25% premium on days when all
I did was pattern work.

Now I fly mostly a Dakota and a Lance, and the rpm/mp chart on the back of
the sun visor shows valid 65% power setting at 2200 or 2300 rpm for the
altitudes I normally fly. That gives a tiny premium, but nowhere near
what I got doing pattern work.

--
Paul Tomblin > http://blog.xcski.com/
My group's mission statement - 'You want *what* ? By *WHEN* ?'
-- Simon Burr

[Dylan Smith]

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.

--
Yes, the Reply-To email address is valid.
Oolite-Linux: an Elite tribute: http://oolite-linux.berlios.de

[Lou]

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

[[email protected]]

On Apr 30, 12:50 pm, Charlie Axilbund > wrote:

>
> 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?


Remember your training: check the POH. The setting you want
is the best range setting, which will give you the best distance for
RPM in a fixed-pitch airplane. If the airplane is really old and
doesn't have a range column in the cruise chart, you can find it by
reducing the power in cruise 100 RPM at a time, then allowing the
airspeed to stabilize and trimming for level flight. Keep track of the
airspeed decrease each time until you get a really large ASI drop,
then go back to the last setting.

Dan

[[email protected]]

On Apr 30, 3:02 pm, Andrew Sarangan > wrote:
> On Apr 30, 4:53 pm, Jose > wrote:
>
> > > Because all the time on taxiing and all the time abeam when you pull
> > > power are virtually free.
>
> > No, it's not "free". It's half price, maybe. (1200 rpm?) The rest of
> > it is full price.
>
> > Jose
> > --
>
> 1200 sounds a bit too high for taxiing.

Naw. I know of lots of pilots who use the brakes to
control speed with RPM way too high. Good business, replacing brakes
and discs. Tires, too, since they get scrubbed.

Dan

[Brian[_1_]]

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

[Bob Noel]

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!!!)

[Ron Natalie]

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.

[Bob Noel]

In article >, Ron Natalie > 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.

argh, I knew better than that too.

:-(

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

[Dylan Smith]

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.

--
Yes, the Reply-To email address is valid.
Oolite-Linux: an Elite tribute: http://oolite-linux.berlios.de

[[email protected]]

On May 2, 3:29 am, Bob Noel >
wrote:
> In article >, Ron Natalie > 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.
>
> argh, I knew better than that too.
>
In Canada, at least, the time used for such things as engines
is Air Time, described in the regs like this:

"air time" - means, with respect to keeping technical records, the
time from the moment an aircraft leaves the surface until it comes
into contact with the surface at the next point of landing.

Technical records include engine logs. So a tach could be way
off if you spend a lot of time taxying or running up or doing other
non-flying stuff.

Dan

[Charlie Axilbund]

On Tue, 01 May 2007 07:20:32 -0700, Dan_Thomas_nospam wrote:

> Remember your training: check the POH. The setting you want
> is the best range setting, which will give you the best distance for RPM
> in a fixed-pitch airplane. If the airplane is really old and doesn't
> have a range column in the cruise chart, you can find it by reducing the
> power in cruise 100 RPM at a time, then allowing the airspeed to
> stabilize and trimming for level flight. Keep track of the airspeed
> decrease each time until you get a really large ASI drop, then go back
> to the last setting.

You are right in the real world, but my purpose was to question whether
fixed pitch airplanes should be charged by tach time. From my
understanding, though, best range is defined in terms of fuel consumed
not tach time; i.e. given 38 gallons of usable fuel, the best range speed
is the one that will get you the farthest. My somewhat whimsical scenario
was to propose a method, given that the charge for the airplane is wet,
that would minimize the cost to the renter. I was trying to think of how
I could get from point a to point b with minimum time on the tach, not
with minimum fuel. I can think of alternatives such as doing the whole
distance at Vx, but I was trying to think how I could keep the rpms at a
minimum. In the end, the best method to minimize rental charges would be
to minimize average rpms x time in flight. Since I have absolutely no
intention to try my method (or a flight at Vx), I have no idea what would
really work best.

[Jim Macklin]

Wait until you have a tailwind.



"Charlie Axilbund" > wrote in message
...
| On Tue, 01 May 2007 07:20:32 -0700, Dan_Thomas_nospam
wrote:
|
| > Remember your training: check the POH. The setting you
want
| > is the best range setting, which will give you the best
distance for RPM
| > in a fixed-pitch airplane. If the airplane is really old
and doesn't
| > have a range column in the cruise chart, you can find it
by reducing the
| > power in cruise 100 RPM at a time, then allowing the
airspeed to
| > stabilize and trimming for level flight. Keep track of
the airspeed
| > decrease each time until you get a really large ASI
drop, then go back
| > to the last setting.
|
| You are right in the real world, but my purpose was to
question whether
| fixed pitch airplanes should be charged by tach time. From
my
| understanding, though, best range is defined in terms of
fuel consumed
| not tach time; i.e. given 38 gallons of usable fuel, the
best range speed
| is the one that will get you the farthest. My somewhat
whimsical scenario
| was to propose a method, given that the charge for the
airplane is wet,
| that would minimize the cost to the renter. I was trying
to think of how
| I could get from point a to point b with minimum time on
the tach, not
| with minimum fuel. I can think of alternatives such as
doing the whole
| distance at Vx, but I was trying to think how I could keep
the rpms at a
| minimum. In the end, the best method to minimize rental
charges would be
| to minimize average rpms x time in flight. Since I have
absolutely no
| intention to try my method (or a flight at Vx), I have no
idea what would
| really work best.

[Denny]

There is a reason that most rental airplanes have a Hobbs Meter, and
this discussion is it,..
Now, in the real world with a fixed pitch prop it almost doesn't
matter..

RPM determines tach time, fuel burn, and wear on the engine - which
are the greater items of cost to the FBO than the clock... So, tach
time is merely the substitute for measuring fuel burn and engine
wear... Lets think about a nominal 1 hour tach run for an engine
calibrated for 2400 RPM cruise...

#1 - If you run the intended 2400 rpm the tach time and the clock time
will be about the same, and like wise the fuel burn and wear on the
engine will be the nominal value for an hour.. You will land at 1 hour
on both that tach and the clock, with a nominal 1 hour of both fuel
burn and engine wear...

#2 - If you run 2700 RPM your tach time will grow faster than clock
time, but so will fuel burn and wear rate... In the end you bring the
plane back at 1 hour of tach time, it will sooner than the nominal 1
hour of clock time, which reduces the fuel burn and the engine wear
compared to having run the engine at 2700 for a full 1 hour of clock
time...

#3 - If you run the engine at 2100 RPM the tach runs slower than the
clock, but you are burning less fuel and creating less wear on the
engine.. You will bring the plane back at 1 hour of tach time, but
longer than that on the clock... The lower RPM over the longer time
period will make the fuel burn and the engine wear roughly the same as
case #1...

TANSTAAFL

denny

[Orval Fairbairn]

In article om>,
Denny > wrote:

> There is a reason that most rental airplanes have a Hobbs Meter, and
> this discussion is it,..
> Now, in the real world with a fixed pitch prop it almost doesn't
> matter..
>

I always hated Hobbs meters, because you got charged the full rate,
whether you were on the ground, 16th in line for takeoff or at cruise.