prop rpm question

This won't be a complete ( or maybe even correct answer) but ...

The prop is an airfoil. Here *lift* will be *thrust*. The thrust force
generated will be

T = C A d/2 V^2. V is the speed of the airfoil (prop). So the force is
nonlinear, it goes as the square.
For the rest of the parameters: C is coeff of lift (depends on angle of
attack), A is airfoil area, d= air density.

The prop will have induced and parasitic drag. The relative wind angle of
attack on the prop foil will depend on the aircraft speed. At low speed it
is at high angle of attack. The induced drag is large and the engine can't
get up to full rpm. As the aircraft speed increases, the AOA decreases, the
induced drag decreases and the can rev up to higher rpm (as constnt
throttle). There will be an AOA where the drag is minimum, This is where the
prop is most efficient. It is a narrow range, because, as you go faster,
parasitic drag on the prop kicks in. Constant speed props are used to adjust
the pitch to remian efficient over a wider raneg of airspeeds.

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. I do not understand this. Is it beacese AOA is largest? I am
trying to see how this relates to power. Power would be force*distance/time
or force*velocity. Maybe the thrust decreases slowly with airspeed, but the
power still goes up as you go faster.

This is just a hand waving argument. Please, anyone who knows more, feel
free to correct this picture.

Dave






"Bob Fry" wrote in message
...
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.

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

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

On Tue, 17 Jan 2006 22:48:09 -0800, "skyloon"
wrote:


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. I do not understand this. Is it beacese AOA is largest? I am
trying to see how this relates to power. Power would be force*distance/time
or force*velocity. Maybe the thrust decreases slowly with airspeed, but the
power still goes up as you go faster.

This is just a hand waving argument. Please, anyone who knows more, feel
free to correct this picture.

Dave

I'll pick up on this one. There's a mechanics equation which is
specially straight-forward. It says if you apply a constant force to
an object,
and it moves in the direction of the force, then the work done is the
product of force times distance.

As expressed in the SI system, it's specially simple: F X D = W
gets the units of
F in Newtons times Distance in Meters equals work in joules
Even more interesting: F X V = P
force times speed = power.
In SI units again:
force in Newtons times speed in meters per second = power in Watts

OK that was the engineering/physics.

Now the application:
An airplane with a constant power recip prop engine.
Lets say the engine is putting out 90 HP say a C-152
90 HP = 90 X 746 watts = 67kW

Lets check the numbers at 10 mph, 50 mph and 100 mph
10 mph = 4.5 meters/sec
50 mph = 22.4 meters/sec
100 mph = 44.7 meters/sec

The unknown in the following equation is F
F X V = P or F = P/V

Now force is the same measure as thrust, so now we can
check available thust at these three speeds:

10 mph
F = 67000 W/4.5 M/Sec = 14890 Newtons
A newton, like a small apple weighs a quarter pound about.
So 14890 Newtons = 3340 pound That's a lot of thrust!

Now 50 mph
F = 67000/22.4 = 2990 Newtons or 671 lb.

Now 100 mph
F = 67000/44.7 = 1500 Newtons or 336 lb.

Or the general rule: the faster you go with constant power, the less
the thrust available.
Same applies to boats.

But think about planes with (some) jet engines,
these can be constant THRUST.

That means, the faster they go, the more HP they put out!
(A reason why jets on slow planes is not a great idea)

Brian Whatcott Altus OK

On Sun, 22 Jan 2006 at 20:18:32 in message
, Brian Whatcott
wrote:

But think about planes with (some) jet engines,
these can be constant THRUST.

That means, the faster they go, the more HP they put out!
(A reason why jets on slow planes is not a great idea)

No, is not quite that easy. Another way of looking at force is that it
is rate of change of momentum.

In a simplified way the thrust of a jet engine comes from the change of
momentum from the air captured by the engine to the momentum of the air
that leaves the back of the engine. If you think of a Turbofan engine
then the fan is not so different from a propeller. The internal
efficiency of the two types of engine is somewhat different.

In any case the greatest propulsive efficiency comes from a momentum
change of a large mass of air with a very small velocity change.
--
David CL Francis

On Tue, 24 Jan 2006 22:48:26 GMT, David CL Francis
wrote:

On Sun, 22 Jan 2006 at 20:18:32 in message
, Brian Whatcott
wrote:

But think about planes with (some) jet engines,
these can be constant THRUST.

That means, the faster they go, the more HP they put out!
(A reason why jets on slow planes is not a great idea)

No, is not quite that easy. Another way of looking at force is that it
is rate of change of momentum.

In a simplified way the thrust of a jet engine comes from the change of
momentum from the air captured by the engine to the momentum of the air
that leaves the back of the engine. If you think of a Turbofan engine
then the fan is not so different from a propeller. The internal
efficiency of the two types of engine is somewhat different.

In any case the greatest propulsive efficiency comes from a momentum
change of a large mass of air with a very small velocity change.


Hi David,

let's forget about jets and recips. Let's imagine a vehicle that is
provided with a constant thrust device.
Then, the faster it goes, the more hose power it provides.
You can take it to the bank

Brian

On Thu, 26 Jan 2006 at 19:29:21 in message
, Brian Whatcott
wrote:

Hi Brian,

Hi David,

let's forget about jets and recips. Let's imagine a vehicle that is
provided with a constant thrust device.
Then, the faster it goes, the more hose power it provides.
You can take it to the bank


No don't let us forget the basic ideas of propulsion.

A constant thrust device is doing one of two things;

1. Accelerating. In which case it is adding to its kinetic energy
and its power is going into that or

2. It reaches a constant speed against a constant drag and a steady
state occurs..

I presume you are not claiming that a constant thrust motor can generate
infinite power? In that case you would be right!

What do you have in mind as a constant thrust device? Newton's laws are
pretty good and I am not aware of any means of getting around them. They
only need adjustments at velocities and masses far beyond normal
terrestrial transport activities.

It is of course true that the efficiency of propulsion devices does vary
with speed and many other conditions.

--
David CL Francis