Why does airspeed change when I adjust the prop?

Paul,

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


Indeed it does ;-)

--
Thomas Borchert (EDDH)

On Jan 16, 9:52 am, "Paul kgyy" wrote:
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

hmmm my Mooney's POH lists

5000ft / 25" / 2400 RPM = 154 HP / 77% power for a 200hp Lycoming IO -
360-A1A.

and it explicitly states to increase engine speed (RPM) before manifold
pressure and
conversely reduce manifold pressure before RPM. No wife tales in my POH
grin

Jd,

No wife tales in my POH


Ah, but there are many OWTs in POHs. They get at least half written by
company lawyers...

--
Thomas Borchert (EDDH)

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)

Tony writes:

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?

Can anyone offer a reason to do it this way? The fact that it is part
of legend is not a reason.

Old wives tale or not, why do it any other way except to prove that you
can.

Because old wives' tales are frequently in accurate and sometimes do
exactly the opposite of what they are held to do. Science is a better
source of guidance.

Is there a circumstance where it would be a better way to manage
the engine?

Is doing it this way any better than doing it another way?

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

Tony,

Yes, the principle is good. But...

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?


I described one in an earlier post: take-off. Also, I've seen CFIs insist
to reduce MP only to return it to the same value, just for an RPM
decrease of 100 or 200, with both RPM settings allowed for that MP.
That's ridiculous.


--
Thomas Borchert (EDDH)

Tony wrote:

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?



Can you offer a good reason TO do it that way?

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?).

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

Once again the "maniac" posts a well thought question, and learned
pilots answer.

"He" probes for a deeper understanding, bringing out the best (this
time) or the worst in the group, and some of us learn a whole bunch.
(I will speak for myself here)

Thanks MX, Tom and others!

This is why I "check in" here almost every night...

Dave





On Tue, 16 Jan 2007 16:53:59 -0800, "BT" wrote:

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?).

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

BT writes:

ohh... you don't have a simulated POH for your simulated Be58

Actually I do. Part of it is written especially for the simulation,
part of it comes from the POH of the real aircraft.

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