Piper Power Settings?

[Journeyman]

So, I'm going through the performance charts for my new ride (in
case anyone missed my long postings), and there's a graph that
lists fuel flow for a given power setting (eg. 9.16 GPH @ 65%).
Another charts lists MP at altitude for each power setting.

The weird thing is the MP drops with altitude. ISTM, at constant
power, MP should increase to compensate for reduced air density.
I.e. at a given fuel flow, which translate to a given power, you
need a bigger volume of air to maintain the proper mixture.

We're talking small changes in MP, few 1/10th of inches per 1000',
but they're definitely decreasing with altitude.

Am I wrong or are the tables bogus?


Morris (so why didn't Jim Fisher comment on my transition to low wings?)

[Friedrich Ostertag]

Hi there,

> The weird thing is the MP drops with altitude. ISTM, at constant
> power, MP should increase to compensate for reduced air density.
> I.e. at a given fuel flow, which translate to a given power, you
> need a bigger volume of air to maintain the proper mixture.

basically, MP should not change with altitude at all, at least as long
as we are talking absolute pressure. To maintain a certain power at a
given engine speed, you need to have the same amount of air mass going
into the engine. As the volume of air the engine sucks in is determined
by the displacement and does not change, the density needs to remain
the same also, hence the pressure in the manifold stays the same.

(Of course, relative pressure will increase, as ambient pressure
reduces! However MP is always absolute, as relative pressure doesn't
tell you anything really. On a non-turbocharged engine relative
manifold pressure is always negative.)

> We're talking small changes in MP, few 1/10th of inches per 1000',
> but they're definitely decreasing with altitude.

that said there is an influence by temperature also. As at altitude you
have lower temperature in general, for a given density you need
slightly less pressure.

Also, with lower ambient pressure there is less backpressure on the
engine exhaust, leading to a slight power increase as the work to push
the exhaust gas out reduces. The volumetric efficiency of the engine
increases slightly (more air can be sucked in for given MP), again
increasing power for given MP.

-> slightly increased power for given MP at altitude, resp. slightly
reduced MP for given power.

> Am I wrong or are the tables bogus?

The tables are correct.

regards,
Friedrich

--
for personal email please remove "entfernen" from my adress

[Dave]

Journeyman > wrote in message >...
charts lists MP at altitude for each power setting.
>
> The weird thing is the MP drops with altitude. ISTM, at constant
> power, MP should increase to compensate for reduced air density.
> I.e. at a given fuel flow, which translate to a given power, you
> need a bigger volume of air to maintain the proper mixture.
>
> We're talking small changes in MP, few 1/10th of inches per 1000',
> but they're definitely decreasing with altitude.
>
> Am I wrong or are the tables bogus?

MP should drop about 1" per 1000 feet, given standard conditions. So,
to maintain a desired % power, you have to continually increase MP as
you climb. Of course, with a normally aspirated engine, there will
come a point that you can't achieve full MP. Here is where a turbo
charged engine comes in handy....

[Jeff]

Unless you have a turbo charged airplane, as your altitude increases your
engine power decreases thus your MP will drop.
in a turbo charged airplane you can just increase MP untill you reach the
critical altitude for the turbo charger (12000 ft for the turbo arrow)

when you have reached the critical altitude for your engine, you should be
at full throttle, then if you have not done it already, you can increase the
RPM which will give you another inch or so of MP, but after that, your MP
will just decrease as you go higher.

what kind of plane did you get ?


Journeyman wrote:

> So, I'm going through the performance charts for my new ride (in
> case anyone missed my long postings), and there's a graph that
> lists fuel flow for a given power setting (eg. 9.16 GPH @ 65%).
> Another charts lists MP at altitude for each power setting.
>
> The weird thing is the MP drops with altitude. ISTM, at constant
> power, MP should increase to compensate for reduced air density.
> I.e. at a given fuel flow, which translate to a given power, you
> need a bigger volume of air to maintain the proper mixture.
>
> We're talking small changes in MP, few 1/10th of inches per 1000',
> but they're definitely decreasing with altitude.
>
> Am I wrong or are the tables bogus?
>
> Morris (so why didn't Jim Fisher comment on my transition to low wings?)

[MikeM]

A MP gauge is nothing but an absolute pressure gauge. If you
disconnected it from the induction manifold and took it aloft,
it would read the same as a manometer; absolute air pressure
decreases with increasing altitude.

When connected to the induction, all it shows is the air pressure
on the downstream side of the throttle plate; it is NOT a direct
measure of engine power! The tables are just attempting to
explain how ****ty a MP is at measuring engine power...

MikeM
Skylane '1MM




Journeyman wrote:
> So, I'm going through the performance charts for my new ride (in
> case anyone missed my long postings), and there's a graph that
> lists fuel flow for a given power setting (eg. 9.16 GPH @ 65%).
> Another charts lists MP at altitude for each power setting.
>
> The weird thing is the MP drops with altitude. ISTM, at constant
> power, MP should increase to compensate for reduced air density.
> I.e. at a given fuel flow, which translate to a given power, you
> need a bigger volume of air to maintain the proper mixture.
>
> We're talking small changes in MP, few 1/10th of inches per 1000',
> but they're definitely decreasing with altitude.
>
> Am I wrong or are the tables bogus?
>
>
> Morris (so why didn't Jim Fisher comment on my transition to low wings?)

[Journeyman]

In article >, Friedrich Ostertag wrote:
>
> basically, MP should not change with altitude at all, at least as long
> as we are talking absolute pressure. To maintain a certain power at a
> given engine speed, you need to have the same amount of air mass going
> into the engine. As the volume of air the engine sucks in is determined
> by the displacement and does not change, the density needs to remain
> the same also, hence the pressure in the manifold stays the same.

Ah. Got it now. Thanks.

>> Am I wrong or are the tables bogus?
>
> The tables are correct.

I figured the tables would've been corrected by now if they were wrong.


Morris (writing down the ideal gas law on the blackboard 100 times)

[Journeyman]

In article >, Jeff wrote:
>
> what kind of plane did you get ?

1974 Arrow II


Morris

[Journeyman]

In article >, MikeM wrote:
> A MP gauge is nothing but an absolute pressure gauge. If you
> disconnected it from the induction manifold and took it aloft,
> it would read the same as a manometer; absolute air pressure
> decreases with increasing altitude.

Right. And when you turn the engine off on the ground, it reads
current atmospheric pressure.

> When connected to the induction, all it shows is the air pressure
> on the downstream side of the throttle plate; it is NOT a direct

Yeah. I forgot the ideal gas law for a moment. I should've thought
it through better before posting the question.

> measure of engine power! The tables are just attempting to
> explain how ****ty a MP is at measuring engine power...

It may not be linear but it is monotonic. Increase the MP (at a fixed
RPM) and you get increased power.


Morris

[MikeM]

Journeyman wrote:

> It may not be linear but it is monotonic. Increase the MP (at a fixed
> RPM) and you get increased power.

Very indirectly! Change the angle of the throttle plate, thereby causing more
or less air pressure drop across the restriction, thereby causing the MP gauge
to see a different absolute pressure.

Change the rpm, and the volume of air flow past the throttle plate changes,
thereby causing the MP gauge to see a different absolute pressure.

Take the airplane to a higher altitude, thereby causing the MP gauge to see a
different absolute pressure.

Let the air induction filter get dirty, causing a flow restriction, thereby
causing the MP gauge to see a different absolute pressure.

Let the temperature/humidity change, thereby causing a change in air density,
thereby causing the MP gauge to see a different absolute pressure.

What you really want is a Mass Flow sensor. Every modern automobile has one,
but our 1930's technology aircraft dont...

MikeM