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?

Mxsmanic wrote:
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?).



Of course you are loosing power. All other things being equal, the
slower an engine spins the less power it can put out, as you have also
noticed, the less fuel it needs. The reason you lower your rpm is to
reduce stress on the engine. In general, engines aren't rated to run
continuously at take off rpm.

--
Chris W
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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 wrote:
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?).

As a pilot who has never flown a a/c with a CSU I might have this all
wrong, but this is AFAIK...

With a fixed pitch prop you can fit a climb prop which gives good climb
but poor cruise, or a cruise prop which is the other way around. A
variable pitch prop allows us to vary the pitch to suit the conditions.
Are you perhaps pulling the RPM back too much and seeing a decrease in
airspeed?
The only firm example I have here is for the Cherokee 6, which has
climb settings of 25" and 2500rpm, and cruise setting of 23" and
2400RPM, so not a huge difference.

Takeoff and landing of course are with the prop set to full fine

Hope I didn't stuff that up and hope that helps

chris writes:

Are you perhaps pulling the RPM back too much and seeing a decrease in
airspeed?

The redline RPM is about 2500, and I pull it back to about 2000-2200.
My IAS then drops by 10 kts or so. The engine is less noisy. I don't
adjust the throttle (which in most cases is fully forward) or mixture
(adjusted for slightly ROP). The fuel rate drops a lot with the RPM,
but the manifold pressure doesn't seem to move much, or I haven't
looked at it closely enough.

Takeoff and landing of course are with the prop set to full fine

Yes, for take-off and landing I make sure the props and mixture are
set fully forward again.

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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)

Mxsmanic wrote:
chris writes:

Are you perhaps pulling the RPM back too much and seeing a decrease in
airspeed?

The redline RPM is about 2500, and I pull it back to about 2000-2200.
My IAS then drops by 10 kts or so. The engine is less noisy. I don't
adjust the throttle (which in most cases is fully forward) or mixture
(adjusted for slightly ROP). The fuel rate drops a lot with the RPM,
but the manifold pressure doesn't seem to move much, or I haven't
looked at it closely enough.

Takeoff and landing of course are with the prop set to full fine

Yes, for take-off and landing I make sure the props and mixture are
set fully forward again.

Manifold pressure is set by the throttle.. And you need to pull the
throttle back before pulling the prop control back, and vice versa for
increasing RPM

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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Chris,

And you need to pull the
throttle back before pulling the prop control back, and vice versa for
increasing RPM


Uhm, well, good in principle, but not necessarily true.

Example: You're in the climb after take-off, passing what you consider a
safe altitude to start reducing power (1000 AGL for me). Many people are
taught to pull back to 25/25 (IOW 25 MP and 2500 RPM). That is actually
very hard on the engine. The reason is that you're in a high power, bad
cooling situation (less cooling airflow in the climb at low airspeed).
At full throttle, extra fuel is added to provide extra cooling. By
pulling back to 25, you lose that. And most POHs allow you to pull back
the RPM to 2500 and remain at full power. In the Tobago we fly, we pull
back to 2450 (top of the green) while leaving the throttle at full.

--
Thomas Borchert (EDDH)