Why does airspeed change when I adjust the prop?

Mxsmanic wrote:
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.

Because the RPM drops. Ceteris paribus, the same amount of fuel/air mixture
enters each cylinder with each intake stroke, and each cylinder imparts the
same amount of torque with each power stroke, but because of the lower rpm,
you have less power strokes per time, thus, the engine yields less power.
If you reduce the RPMs by 20%, c.p.* the engine power will drop by 20%,
too. Hence, you loose airspeed.

And the gearbox-metaphor is not very well suited because most cars don't
have a contiously-variable transmission but rather distinct gears, also,
you don't specify the desired engine rpm but rather the desired gear ratio.
That is not the case with the constant-speed prop, you select a desired
rpm, and the prop governor adjusts the load on the engine (by varying the
prop pitch) to maintain that rpm, regardless of the actual power output of
the engine (within the limits of the prop's abilities, of course).

Anno.

*) and that's a pretty strong c.p.

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,

OK, but why does the airspeed drop?

Because your power output is lower.

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.

Why?

So you're saying that lowering the RPM necessarily means a drop in net
thrust unless the manifold pressure is increased (a throttle
increase)?

Yes.

--
Thomas Borchert (EDDH)

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.

What happens when, in a car, you go from second gear to fifth gear?
Unless you are going fast enough for fifth gear to be appropriate, the
car will lug, and slow down. The point of car gearing is to keep the
engine at its most efficient RPM range. While this is an extreme case,
something similar happens with an airplane engine - if you lower the
RPM, the pitch will become coarser (all other things being equal) and
the engine will have a harder time (do more work) for each revolution.
Each revolution pulls you through more air.

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

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?).

I can hardly believe that I am reading this thread, much less responding to
it!

However, someone reading all this may very well work their way up to
captain, and I may very well be one of their passengers...

As of 9:50am EST on Jan 16, most of the responses seem to pertain to the
first power reduction after take-off, even though the question was specific
to cruise, and most of the remainder appear to presume a very radical change
in RPM. OTOH, none have mentioned that many aircraft--especdially
twins--have an automatic manifold pressure control (as contrasted to a plane
old throttle plate) connected to the throttle levers. This is (or should
be) universal for turbo-supercharged engines equipped with automatic
waste-gates.

None of these systems function perfectly, but to the extent that they
approxamate a constant manifold pressure, horsepower will thus be directly
proportional to RPM.

In addition; when operating above the critical altitude a normally aspirated
engine will typically operate as though the manifold pressure is
regfulated--but a turbo-supercharged engine (or any engine with a
centrifugal supercharger) will work in reverse of what might be intuitive:
an increase in RPM will result in an increase in manifold pressure and a
decrease in RPM will result in a decrease in Manifold pressure.

The exception to the above occurs when operating an "entry level" conplex
aircraft at low altitude and reduced power. In that case, a reduction in
RPM by means of the prop control will result in an increase in manifold
pressure. There will still be a reduction in horsepower, but not nearly as
much.

Peter

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

Peter,

I can hardly believe that I am reading this thread, much less responding to
it!

As usual, the questions are worthwhile. It's the answers to the answers. But
my reasoning was that a lot of pilots transitioning to complex airplanes have
these questions.

There will still be a reduction in horsepower, but not nearly as
much.

You're right with all you say, of course. In "real life", while one needs to
know what's going on behind the scenes, it still comes down to setting both
parameters (MP and RPM) to values that are "by the book".


--
Thomas Borchert (EDDH)

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.

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?

--
Thomas Borchert (EDDH)