prop rpm question

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

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

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.

Yup you're right, there's more than just volumetric efficiency, but
flame front speed in these slow engines is still around 100 feet per
second, while average piston speed won't be much over 40 or 50 fps with
the midpoint travel being somewhat higher. The intake and exhaust
systems present more drag at higher RPMs and start to affect the
performance, and in many modern auto engines four valves per cylinder
are used to ease breathing.
I wonder if the new direct-drive diesel aircraft engines have much
higher torques in the right places?

Dan

wrote

I wonder if the new direct-drive diesel aircraft engines have much
higher torques in the right places?

Torque out the butt, and the torque stays high for a longer period of time
on the power stroke. In short, it will turn the same size prop of an engine
with nearly twice the HP.
--
Jim in NC

wrote:
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.


Yup you're right, there's more than just volumetric efficiency, but
flame front speed in these slow engines is still around 100 feet per
second, while average piston speed won't be much over 40 or 50 fps with
the midpoint travel being somewhat higher. The intake and exhaust
systems present more drag at higher RPMs and start to affect the
performance, and in many modern auto engines four valves per cylinder
are used to ease breathing.
I wonder if the new direct-drive diesel aircraft engines have much
higher torques in the right places?

Dan


It would seem they would. The high compression ratio gives most diesels
outstanding torque at fairly low RPMS, however, many also have a very
narrow torque peak.

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