Piper Power Settings?

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?)

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

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