Why does airspeed change when I adjust the prop?

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?).

--
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On Tue, 16 Jan 2007 00:14:47 -0800, Mxsmanic wrote
(in article ):

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 (?).

Think about it. What happens to thrust and airspeed if you reduce RPM to 0?

C J Campbell writes:

Think about it. What happens to thrust and airspeed if you reduce RPM to 0?

Yes, I was thinking about that. But supposedly reducing the RPM
slightly just causes the CS prop to change pitch, which means that it
should still be producing the same thrust (or at least what I've read
seems to assert this). This doesn't hold for very low RPMs because at
some point the limits of practical pitch adjustment are reached.

--
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Mxsmanic,

But supposedly reducing the RPM
slightly just causes the CS prop to change pitch,


Who supposes that?

We need to be clear about definitions from the start:

There are prop installations (though uncommon) where the pilot can set
prop pitch directly. This is not a constant speed prop. If, for
example, the pilot pitches the aircraft down, that will lead to an
increase in RPM with such a constant pitch prop.

A Constant Speed prop, OTOH, hold RPM (its "speed", hence the name)
constant. If you pull into a climb, the prop RPM will not slow, it will
stay the same. If you push into a descent, RPM will not increase, it
will stay the same. In order to achieve that, the prop will change its
blade pitch continually. With the prop lever, you don't change pitch
(directly), you set an RPM which the CS mechanism will then maintain by
adjusting pitch.

Ok, once this is understood, it becomes much clearer (I hope) that RPM
does of course influence engine power. Think about less air-fuel mix
being burned per minute if the engine does fewer revolutions in that
minute.

What you get with CS, however, is something that is best likened to a
gear-box in a car. You can optimize the engine RPM and the "load" on
the prop to what you are doing. High RPM means the prop is taking
"small bites out of the air" per revolution, low rpm means it's taking
big bites. Hence, high RPM is good for take-off, lower RPM is good for
cruise.

As per the cruise power tables in the POH (you really need to start to
read these things), various combinations of manifold pressure and RPM
will give you the same amount of engine power (often, tables are for 65
and 75 percent of engine power). Which to chose? Well, many people
prefer a low noise setting, that means a combination of low RPM and
high MP. You can't use any combination if you don't want to harm your
engine, but you can use all that are in the POH table.

I know you don't read the stuff you're pointed to here, but for
lurkers: This is all very well explained by John Deakin in his columns
on engine management at www.avweb.com.

--
Thomas Borchert (EDDH)

Thomas Borchert writes:

Who supposes that?

The sources I've read. One of them compares pitch to the gearing in a
car, with fine pitch being like a low gear, and coarse pitch being
like a high gear. The implication is that you can go faster in cruise
with coarse pitch (just as you can go faster in overdrive in a car
when cruising), but it doesn't seem to work that way. Decreasing the
RPM supposedly compels the pitch to become coarser, like upshifting to
a higher gear.

There are prop installations (though uncommon) where the pilot can set
prop pitch directly. This is not a constant speed prop. If, for
example, the pilot pitches the aircraft down, that will lead to an
increase in RPM with such a constant pitch prop.

The Baron has a CS prop, I know that.

A Constant Speed prop, OTOH, hold RPM (its "speed", hence the name)
constant. If you pull into a climb, the prop RPM will not slow, it will
stay the same. If you push into a descent, RPM will not increase, it
will stay the same. In order to achieve that, the prop will change its
blade pitch continually. With the prop lever, you don't change pitch
(directly), you set an RPM which the CS mechanism will then maintain by
adjusting pitch.

Ok, once this is understood, it becomes much clearer (I hope) that RPM
does of course influence engine power. Think about less air-fuel mix
being burned per minute if the engine does fewer revolutions in that
minute.

What you get with CS, however, is something that is best likened to a
gear-box in a car. You can optimize the engine RPM and the "load" on
the prop to what you are doing. High RPM means the prop is taking
"small bites out of the air" per revolution, low rpm means it's taking
big bites. Hence, high RPM is good for take-off, lower RPM is good for
cruise.

OK, but why does the airspeed drop? In a car, you use the highest
gears (coarsest pitch, hence lowest prop RPM) for high-speed cruise.

It seems that high speed and fine pitch should produce exactly the
same thrust as low speed and coarse pitch, as long as the prop blades
don't stall or reach transonic speeds.

As per the cruise power tables in the POH (you really need to start to
read these things), various combinations of manifold pressure and RPM
will give you the same amount of engine power (often, tables are for 65
and 75 percent of engine power). Which to chose? Well, many people
prefer a low noise setting, that means a combination of low RPM and
high MP. You can't use any combination if you don't want to harm your
engine, but you can use all that are in the POH table.

So you're saying that lowering the RPM necessarily means a drop in net
thrust unless the manifold pressure is increased (a throttle
increase)?

I'm reminded of diesel-electric locomotives. These locomotives have a
large diesel engine that drives a generator or alternator, which
provides electricity for traction motors. When you control speed on
these locomotives, you don't adjust the diesel engine speed directly;
instead you adjust the power demand of the traction motors, and a
governor adjusts the actual diesel engine throttle to provide the
necessary power.

--
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Mxsmanic wrote:
OK, but why does the airspeed drop? In a car, you use the highest
gears (coarsest pitch, hence lowest prop RPM) for high-speed cruise.

Because the RPM drops. Ceteris paribus, the same amount of fuel/air mixture
enters each cylinder with each intake stroke, and each cylinder imparts the
same amount of torque with each power stroke, but because of the lower rpm,
you have less power strokes per time, thus, the engine yields less power.
If you reduce the RPMs by 20%, c.p.* the engine power will drop by 20%,
too. Hence, you loose airspeed.

And the gearbox-metaphor is not very well suited because most cars don't
have a contiously-variable transmission but rather distinct gears, also,
you don't specify the desired engine rpm but rather the desired gear ratio.
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).

Anno.

*) and that's a pretty strong c.p.

Mxsmanic,

OK, but why does the airspeed drop?

Because your power output is lower.

It seems that high speed and fine pitch should produce exactly the
same thrust as low speed and coarse pitch, as long as the prop blades
don't stall or reach transonic speeds.

Why?

So you're saying that lowering the RPM necessarily means a drop in net
thrust unless the manifold pressure is increased (a throttle
increase)?

Yes.

--
Thomas Borchert (EDDH)

The implication is that you can go faster in cruise
with coarse pitch (just as you can go faster in overdrive in a car
when cruising), but it doesn't seem to work that way.

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.

Jose
--
He who laughs, lasts.
for Email, make the obvious change in the address.

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

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)