Why does airspeed change when I adjust the prop?

It's a simulator. The sim probably isn't all that accurate and
may be translating the lower RPM into a lower airspeed, as a
fixed-pitch prop would.
It's MSFS, not some certified IFR simulator where things have
to closely approximate the real thing. We're arguing with a bogeyman.

Dan

When I change the prop setting on my (simulated) Baron 58, lowering
the prop RPM, my airspeed drops. I thought that for a given throttle
setting, the actual thrust produced by the powerplant was supposed to
remain the same for a wide range of prop settings, because of
automatic pitch changes made when I change the prop RPM. However,
that doesn't seem to be the case. A lowering of the prop RPM also
lowers airspeed, which implies a change in thrust. The fuel flow also
diminishes, which implies a change in power (?).

So, exactly what do I gain or lose by adjusting prop RPM when I'm
cruising along? Why would I want to change it? Some sources I've
read say that the prop makes less noise, which is surely true, but it
seems that I can't lower the RPM without losing airspeed (and thus I
must be losing power, right?).

I can hardly believe that I am reading this thread, much less responding to
it!

However, someone reading all this may very well work their way up to
captain, and I may very well be one of their passengers...

As of 9:50am EST on Jan 16, most of the responses seem to pertain to the
first power reduction after take-off, even though the question was specific
to cruise, and most of the remainder appear to presume a very radical change
in RPM. OTOH, none have mentioned that many aircraft--especdially
twins--have an automatic manifold pressure control (as contrasted to a plane
old throttle plate) connected to the throttle levers. This is (or should
be) universal for turbo-supercharged engines equipped with automatic
waste-gates.

None of these systems function perfectly, but to the extent that they
approxamate a constant manifold pressure, horsepower will thus be directly
proportional to RPM.

In addition; when operating above the critical altitude a normally aspirated
engine will typically operate as though the manifold pressure is
regfulated--but a turbo-supercharged engine (or any engine with a
centrifugal supercharger) will work in reverse of what might be intuitive:
an increase in RPM will result in an increase in manifold pressure and a
decrease in RPM will result in a decrease in Manifold pressure.

The exception to the above occurs when operating an "entry level" conplex
aircraft at low altitude and reduced power. In that case, a reduction in
RPM by means of the prop control will result in an increase in manifold
pressure. There will still be a reduction in horsepower, but not nearly as
much.

Peter

Peter,

I can hardly believe that I am reading this thread, much less responding to
it!

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.

There will still be a reduction in horsepower, but not nearly as
much.

You're right with all you say, of course. In "real life", while one needs to
know what's going on behind the scenes, it still comes down to setting both
parameters (MP and RPM) to values that are "by the book".


--
Thomas Borchert (EDDH)

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

Kev,

The extra
questioning


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

--
Thomas Borchert (EDDH)

In article ,
Thomas Borchert wrote:

The extra
questioning

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

ouch

--
Bob Noel
Looking for a sig the
lawyers will hate

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

Paul,

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


Wanna bet several of those horses have left the building over the
years?

--
Thomas Borchert (EDDH)

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


What?

Thomas Borchert wrote:
Paul,

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


Wanna bet several of those horses have left the building over the
years?


At 1200SMOH, no argument, but the principle still holds :-)