Why does airspeed change when I adjust the prop?

For my Arrow, takeoff numbers 30" 2700 RPM, product 81000,


What?

On Jan 16, 9:52 am, "Paul kgyy" wrote:
Power is a linear function of the amount of air processed through the
engine. The amount of air is a function of the product of manifold
pressure and RPM.

For my Arrow, takeoff numbers 30" 2700 RPM, product 81000, 200 hp.

At 5000 ft, 25", 2400 RPM, product 60000. 60/81 = 74% power, 150 hp.

If I reduce to 2100 RPM, product is 52500. 52.5/81 = 65% power, 130 hp

hmmm my Mooney's POH lists

5000ft / 25" / 2400 RPM = 154 HP / 77% power for a 200hp Lycoming IO -
360-A1A.

and it explicitly states to increase engine speed (RPM) before manifold
pressure and
conversely reduce manifold pressure before RPM. No wife tales in my POH
grin

Thomas!

What gives? You have been a leader of the don't answer HIM group. Now
this?

I don't get it, but I don't seem to get a lot of what goes on around here,
lately.

Just curious about the change of heart.
--
Jim in NC

Thomas Borchert wrote:
As usual, the questions are worthwhile. It's the answers to the answers. But
my reasoning was that a lot of pilots transitioning to complex airplanes have
these questions.

I think everyone appreciates the calm, thoughtful and useful responses.

Some people would simply ask, get one answer, and go away. The extra
questioning that Mx gives to some answers can be frustrating, but also
interesting, because it's the in-depth replies that really bring out
the piloting experiences we all like to hear about.

Just saying something is true isn't enough sometimes. As you yourself
recently wrote: "Argument by authority isn't really a good way to try
to convince intelligent people".

Regards and thanks,
Kev

Anno v. Heimburg writes:

And the gearbox-metaphor is not very well suited because most cars don't
have a contiously-variable transmission ...

FWIW, I did have a scooter with this type of transmission. It was a
bit bizarre, because if you opened the throttle wide open from a
standing start, the engine would speed up to its optimal RPM, and then
the transmission would continuously change ratios to keep the engine
at the same RPM as the scooter accelerated. You'd hear a constant
whirr from the engine, even though you were accelerating very rapidly.
It also provided good acceleration because of this, since the
(relatively small) engine was held at a single optimum speed.

That is not the case with the constant-speed prop, you select a desired
rpm, and the prop governor adjusts the load on the engine (by varying the
prop pitch) to maintain that rpm, regardless of the actual power output of
the engine (within the limits of the prop's abilities, of course).

So what do you gain with the prop adjustment? Just a reduction in
wear and tear and/or noise?

--
Transpose mxsmanic and gmail to reach me by e-mail.

Thomas Borchert writes:

Why?

If you make the pitch twice as coarse and run it at half the speed, it
should still move the same amount of air.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Jose writes:

What happens when, in a car, you go from second gear to fifth gear?
Unless you are going fast enough for fifth gear to be appropriate, the
car will lug, and slow down. The point of car gearing is to keep the
engine at its most efficient RPM range. While this is an extreme case,
something similar happens with an airplane engine - if you lower the
RPM, the pitch will become coarser (all other things being equal) and
the engine will have a harder time (do more work) for each revolution.
Each revolution pulls you through more air.

So ultimately why do you change the prop adjustment? At full power,
slowing the RPM just slows the aircraft down. So does reducing the
throttle. Why would I slow the RPM rather than reduce the throttle?

--
Transpose mxsmanic and gmail to reach me by e-mail.

Morgans,

What gives? You have been a leader of the don't answer HIM group. Now
this?


I haven't, really. In fact, I have answered him way too much. As I said,
this question is of great interest to many readers, I think. So it
deserves an answer. His usual bickering and "I know better" stuff I'll
ignore.

--
Thomas Borchert (EDDH)

Kev,

The extra
questioning


There's that - and then there'S MX.

--
Thomas Borchert (EDDH)

Jd,

No wife tales in my POH


Ah, but there are many OWTs in POHs. They get at least half written by
company lawyers...

--
Thomas Borchert (EDDH)