Charging for tach time in a fixed pitch airplane

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

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

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.

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!!!)

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

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

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.

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.

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