Why does airspeed change when I adjust the prop?

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?).

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On Tue, 16 Jan 2007 11:39:01 -0800, wrote
(in article . com):



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.

Okay, looking at 22 inches MP for the Cessna 206H, standard day, 8000 feet
straight from the manual (from the days when I owned a 206):

RPM KTAS GPH

2500 139 15.6
2400 137 15.0
2300 133 14.5
2200 130 13.9
2100 126 13.2

Same pattern at all other altitudes, temperatures, and MPs. If you decrease
your RPM, your KTAS and fuel consumption decrease.

Actually, the simulator sounds pretty accurate to me.

On Tue, 16 Jan 2007 10:29:50 -0800, Mxsmanic wrote
(in article ):



So what do you gain with the prop adjustment? Just a reduction in
wear and tear and/or noise?

No, you get a lot more than that. If you had the manual for a Baron you would
see. But let us look at the Cessna 206H, a single engine plane with a
constant speed prop.

At 8000 feet, with 22 inches MP and 2500 RPM, you generate 69BPH and have an
airspeed of 139 KTAS and burn 15.6 GPH. At 2100 RPM you generate only 57 BPH
and have an airspeed of 126 KTAS and burn 13.2 GPH. This is interesting, in
that you get exactly these same numbers (57 BPH, 126 KTAS, and 13.2 GPH) if
you have 2200 RPM but only 21 inches MP. And it is only slightly different at
2300 RPM and 20 inches MP.

Obviously, a reduction in either MP or RPM reduces your horsepower, airspeed,
and fuel consumption.

If you were in a fixed prop Cessna 172, you would have your throttle full
forward on takeoff in order to generate maximum horsepower. You want to get
away from the ground as quickly as possible, both for safety and noise
abatement (the sound of crumpling metal annoys the neighbors). But because
the prop has a fixed pitch, you cannot increase RPM. You cannot increase MP
because you already gave it full throttle. As the airplane pitches up the
prop will slow down. So you no longer have full power. If only you could
reduce the pitch of the prop to keep it at 2700 rpm. Well, in a CSP plane,
you can do exactly that. A constant speed prop is a performance enhancement.
It allows you to use all the power the engine has, at least until you have
climbed high enough that you need to turbocharge the engine, anyway.

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?

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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.

--
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Chris,

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 ...


Go for the avweb.com columns of John Deakin on engine management.

--
Thomas Borchert (EDDH)

Paul,

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


Indeed it does ;-)

--
Thomas Borchert (EDDH)

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)

Mxsmanic,

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

Actually I do.


So read it.

--
Thomas Borchert (EDDH)

Thomas, I made no claim that one should not fly 'over square' as you
mentioned in the post you cited -- I did it routinely with the IO 360
pulling an M20 around. In fact, internal engine frictional losses in
the normal operating range vary as some positive function of RPM --
probably much worse than linear. Knowing that, the prudent pilot
cruises with the lowest RPM and highest MP that seems reasonable --
even if MP measured in inches of Hg exeeds the number produced by
dividing engine RPM by 100.

I suspect you will agree with me that when one pays for fuel and
repair, being kind to the hardware is a good idea. If one is going to
adjust both throttle and engine speed, I cannot think of a cirumstance
when following the commonly taught and used procedure of first reducing
throttle then rpm, or first advancing rpm then throttle, is not the
prudent technique. A PIC can do it differently, but why?

I am reminded, when techniques like this are called old wives tales, of
people who ignore the fine print in contracts, calling it 'boiler
plate." It's there because it's been shown to protect the interests of
someone, and that someone is very often not the person being asked to
sign the document.

Old wives are often right.

When one's 'flying' only distrubes large numbers of electrons in a
computer and a much smaller number of people in this newsgroup these
issues are not especially important, but as someone else suggested,
there are wannabe and low time pilot lurkers here. The pilots with real
world experience are doing them and low time pilots a service by not
only reciting conventional dogma but discussing its logic as well.

And besides that, it's fun.

Do you remember those commercials in the US where someone is either
piloting a swing wing, or doing an emergency operation and when asked
"Are you a pilot (or a doctor)" the answer is "No, but I stayed in a
Holiday Inn Express last night"?

Around here, we get arguements from someone who if asked "Are you a
pilot?" responds with "No, but I played MSFS last night."



On Jan 17, 3:43 am, Thomas Borchert
wrote:
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)