prop rpm question

Bob Fry wrote:

I wish to leave the engine out of the discussion, but let's
continue...


"KB" == Kyle Boatright writes:


KB If we assume the plane in question is a C-152,

Close enough, it's an Aircoupe with a C90.

But let's look just at the prop. Why does a prop produce so much more
thrust, much more than double, when it's turned at only twice the
rate?

KB Another way to look at it is that your prop has an advance
KB rate. Let's say it the advance rate is 4 feet per
KB revolution.

Yep, 48" pitch.

KB At 1,000 rpm, and no drag on the airplane (rolling
KB or aerodynamic), the airplane would have a terminal velocity
KB of 4,000 fpm, or about 48 mph. Of course, there is rolling and
KB aerodynamic drag, and there is prop drag too, so the engine
KB can only drag the plane along at, say, 30 mph, assuming a flat
KB smooth runway.

KB At 2,000 rpm, with no drag, the terminal velocity would be
KB 8,000 fpm, or about 85 mph.

Hmmmm...so prop thrust is indeed only twice at double the
rpm?...ideally speaking of course.

The idealized (no viscosity etc.) math seems to say that it is linear,
but intuitive feel says not.

Is aerodynamic drag of an airfoil linear with speed? Is airfoil lift
linear with speed? Since a prop is just an airfoil going in a circle,
why would you expect it to be linear?

Matt

Kershner states that the maximum thrust force occurs when the plane is
standing still (at a fixed throttle setting, I guess), and decreases as you
go faster.

There are some fast homebuilts for which this won't be true.
They have fixed-pitch props with very high pitches, and the blade is
largely stalled at the start of the takeoff roll, making accelleration
dismal indeed. The pilots report that the airplane seems to come alive
at some point before liftoff when the prop blades finally get to work.
My own Jodel has an efficient wooden prop and I often note a small RPM
drop as the airplane accellerates through about 20 MPH. There's
something happening with the airflow through the blades, probably to do
with unstalling, or, perhaps (less likely) with leaving behind the
larger prop vortex generated in the static condition.

Dan

If you can find the engine performance plots you will see that the
percent of RPM and percent of power (HP or torque) are not at all
the same thing.

And it's torque that turns the propeller (not RPM).

1000 rpm might be near 1/2 RPM, but barely 10-20 percent max torque.

At full power (torque), the prop can deliver x number of pounds
thrust for any given airspeed. That's the most you'll get.

Rolling off RPM also rolls one down the torque curve.

And you are right, it's a very non-linear curve.


Richard

ps:

also on the torque curve, note that max torque and max HP are usually
NOT found at the same RPM...

ta

Richard Lamb wrote:

If you can find the engine performance plots you will see that the
percent of RPM and percent of power (HP or torque) are not at all
the same thing.

And it's torque that turns the propeller (not RPM).

1000 rpm might be near 1/2 RPM, but barely 10-20 percent max torque.

At full power (torque), the prop can deliver x number of pounds
thrust for any given airspeed. That's the most you'll get.

Rolling off RPM also rolls one down the torque curve.

And you are right, it's a very non-linear curve.


Richard

ps:

also on the torque curve, note that max torque and max HP are usually
NOT found at the same RPM...

ta

It's been a while since I saw so many errors in so little text.

Matt

I think it has to do with how fast the prop can 'push' air versus how fast
the plane is moving through the air.
Imagine you are in a small boat with an outboard motor. The motor is moving
the boat at 15 mph. You decide you want to go faster so you pull out an oar.
You can only move the oar at a speed of 15 mph so the net force you are
applying to the water is zero. If you were going slower, you could apply
force to the water, thus making the boat inch ahead a little faster.

wrote in message
...

I'm not an engineer, nor do I play one on TV. I do find it interesting
that the speed or velocity of the aircraft is a factor in figuring
thrust in both the propeller and the jet propulsion formulas.

Perhaps someone else can explain in it terms that you and I can
understand.

TC

Matt Whiting wrote:
Richard Lamb wrote:

If you can find the engine performance plots you will see that the
percent of RPM and percent of power (HP or torque) are not at all
the same thing.

And it's torque that turns the propeller (not RPM).

1000 rpm might be near 1/2 RPM, but barely 10-20 percent max torque.

At full power (torque), the prop can deliver x number of pounds
thrust for any given airspeed. That's the most you'll get.

Rolling off RPM also rolls one down the torque curve.

And you are right, it's a very non-linear curve.


Richard

ps:

also on the torque curve, note that max torque and max HP are usually
NOT found at the same RPM...

ta


It's been a while since I saw so many errors in so little text.

Matt


That RPM and torque are NOT the same thing?

Or that at full power will give deliver full thrust?

Ot that the thrust delivered changes with airspeed?

Or that it's very non linear?

Very oversimplified, but go ahead and straighten me out, Matt.

Richard

Richard Lamb wrote:
Matt Whiting wrote:

Richard Lamb wrote:

If you can find the engine performance plots you will see that the
percent of RPM and percent of power (HP or torque) are not at all
the same thing.

And it's torque that turns the propeller (not RPM).

1000 rpm might be near 1/2 RPM, but barely 10-20 percent max torque.

At full power (torque), the prop can deliver x number of pounds
thrust for any given airspeed. That's the most you'll get.

Rolling off RPM also rolls one down the torque curve.

And you are right, it's a very non-linear curve.


Richard

ps:

also on the torque curve, note that max torque and max HP are usually
NOT found at the same RPM...

ta



It's been a while since I saw so many errors in so little text.

Matt



That RPM and torque are NOT the same thing?

Or that at full power will give deliver full thrust?

Ot that the thrust delivered changes with airspeed?

Or that it's very non linear?

Very oversimplified, but go ahead and straighten me out, Matt.

Richard

Yes, horsepower and torque are absolutely not the same thing. The
following suggests that they are "At full power (torque)..."

Rolling off RPM may or may not roll you down the torque curve. If you
are running at an RPM above the torque peak, reducing RPM might actually
increase the torque available.

1000 RPM isn't 1/2 RPM. It may be close to 1/2 of the maximum allowable
RPM, which is what you hopefully intended to say.

Matt

Do you really think including factual data is likely to resolve the
question?

And do you know Dr Dan?

Thrust is a direct relation of diameter x pitch x rpm.
Get a copy of H.Glauert's book: The elements of aerofoil and airscrew
theory.
isbn 052127494
Also, thrust is a function of the third power of the prop diameter so
changing the prop diameter on inch can have a major effect on thrust
and vice versa.

wrote:

Rolling off RPM may or may not roll you down the torque curve. If you
are running at an RPM above the torque peak, reducing RPM might actually
increase the torque available.


Reducing RPM on a fixed-pitch prop will reduce the torque. Reducing
RPM using the prop control on a constant-speed prop could increase
torque as the RPM drops, depending on the engine's torque curve.

Not in all conditions. In SS level flight, yes.

The torque curve is affected by volumetric efficiency, which is
a bigger factor in high-RPM engines, less so in slow-turning aircraft
engines. As RPM rises, the cylinder can't achieve anything near
atmospheric pressure at the bottom of the intake stroke so that the
amount of fuel/air mix is progressively reduced, and any horsepower
increase with rising RPM is due to RPM only. The turbo or supercharger
is the solution to the probem. In light airplanes, the turbo is more
normally employed to alleviate altitude losses.

It's been a while since I studied engines in any detail, but I believe
there is more to it than just VE. I don't believe that bearing friction
is linear with RPM for example. Also, speed of the flame front becomes
and issue at higher RPM. I believe the drop-off in torque with RPM is a
function of a number of factors. VE is dominant, but not the only one.
Even turbocharged engines have a torque peak with a drop-off at some
point.

Matt


Matt