Why does airspeed change when I adjust the prop?

Horsepower depends on rpm and MAP. Lowering rpm without
increasing MAP [throttle] will reduce horsepower and that
causes a reduction in speed.



"C J Campbell" wrote in
message
e.com...
| 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
KE5GIX

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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.

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

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

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)

Thomas Borchert wrote:
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.


OK, I stand corrected!! As I indicated in my first post I am a fixed
pitch pilot so I am quite short of knowledge in this area, and I am
always keen to learn more ...

chris wrote:




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

Old wives tale.

"Newps" wrote in message
. ..


chris wrote:



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

Old wives tale.

Quite! Prop and mixture are all you ever need.

On Tue, 16 Jan 2007 01:42:31 -0800, chris wrote
(in article om):


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


Um, well not quite. People get confused about that when learning to use a
constant speed prop. You pull the manifold pressure back to an inch less than
where you finally want it to be simply because when you then reduce the rpm
the manifold pressure has a tendency to increase by that amount. You do the
opposite when increasing rpm. Increase the rpm first and then the manifold
pressure so that you don't have to go back and adjust the manifold pressure
again. Has little to do with being kind to the engine and everything to do
with minimizing your fiddling with the controls.