Why does airspeed change when I adjust the prop?

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.

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

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 ...

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?

For better efficiency.

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

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 :-)

"Mxsmanic" wrote in message
...

snip

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?


A fixed prop optimized for takeoff would overspeed the engine at cruise
speed. With variable pitch, the prop can be flat for take and higher pitch
for cruise.

Danny Deger

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

The habit of backing off throttle first, then reducing prop RPM, and
the habit of increasing RPM first, then advancing the throttle, is one
of those things that can save wear and tear, and maybe an engine. Can
anyone offer a good logical reason to do it any other way?

Old wives tale or not, why do it any other way except to prove that you
can. Is there a circumstance where it would be a better way to manage
the engine?

On Jan 16, 2:26 pm, Thomas Borchert
wrote:
Jd,

No wife tales in my POHAh, but there are many OWTs in POHs. They get at least half written by
company lawyers...

--
Thomas Borchert (EDDH)

look at the POH for various MP and RPM settings and review the resulting TAS
and Fuel Flow.
ohh... you don't have a simulated POH for your simulated Be58
BT

"Mxsmanic" wrote in message
...
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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No, it looses efficiency and you need more power to move a prop with coarser
pitch.

"Mxsmanic" wrote in message
...
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.

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