Propeller thrust question

Thrust from a propeller drops as speed increases to a theoretical zero. When
this happens, what is providing the thrust to overcome drag? The forward
momentum of the aircraft?

If so, is it a case of the speed very slightly and imperceptibly dipping
below the zero thrust speed, getting a little thrust to bring it back to the
zero-thrust condition and doing this over and over again?

Thanks in advance.

In article ,
says...
Thrust from a propeller drops as speed increases to a theoretical zero. When
this happens, what is providing the thrust to overcome drag? The forward
momentum of the aircraft?

If so, is it a case of the speed very slightly and imperceptibly dipping
below the zero thrust speed, getting a little thrust to bring it back to the
zero-thrust condition and doing this over and over again?

Thanks in advance.

Hang on hang on... Drag *increases* until it equals the Thrust - that's
the max speed of the plane.

Could you re-phrase your questions?

Further... to make planes more efficient/faster - they made these things
called constant velocity props and adjustable pitch props - the idea
being you can have high thrust for takeoff, and as the speed of the
plane goes up, you can coursen the pitch. Regardless though, you'll get
to my first statement, the drag factor - which will rise up to meet the
best thrust the prop can produce and the aircraft just won't go any
faster Capt. Kirk.

--
Duncan

Could you re-phrase your questions?

Ahhh yeah. Just realised you'd never hit the zero thrust speed. Woops!

The thing that STILL puzzles me though is that the thrust created *by the
propeller* goes down (as measured in Newtons for instance) as the velocity
increases.

As detailed here http://www.mh-aerotools.de/airfoils/propuls4.htm , the
thrust at 252 knots for a theoretical plane is 17.2N, whereas at 108 knots
it's 31N.

That's what's got me confused. Higher speed = more drag, yet the measured
thrust from *just the prop* is less. Is the extra thrust to match drag
coming from the momentum of the already moving airframe?

Or am I looking at this all wrong?

In article ,
says...
Could you re-phrase your questions?

Ahhh yeah. Just realised you'd never hit the zero thrust speed. Woops!

The thing that STILL puzzles me though is that the thrust created *by the
propeller* goes down (as measured in Newtons for instance) as the velocity
increases.

As detailed here http://www.mh-aerotools.de/airfoils/propuls4.htm , the
thrust at 252 knots for a theoretical plane is 17.2N, whereas at 108 knots
it's 31N.

That's what's got me confused. Higher speed = more drag, yet the measured
thrust from *just the prop* is less. Is the extra thrust to match drag
coming from the momentum of the already moving airframe?

Or am I looking at this all wrong?

Well *I* think so Yes, the thrust is less at high airspeed - not
much you can do about that (as said, they invented variable pitch and
const. velocity props to help alleviate that) - but you say (admit
that at max airspeed there is still a lot of thrust. Well, whatever
that thrust is, it equals the drag. So starting from velocity = 0, the
aircraft's acceleration goes from high to lower to low to nothing - as
the aircraft goes from 0 to say 'medium' to as fast as it'll go. The
biggest factor isn't so much the lowering thrust though, but the much
increasing drag (someone will have the math but the drag is like,
velocity squared - that sorta graph).

So perhaps think of changing (decreasing) acceleration, rather than just
velocity.

--
Duncan

In article ,
says...
Could you re-phrase your questions?

Ahhh yeah. Just realised you'd never hit the zero thrust speed. Woops!

And PS: yep, all of us light a/c folk have hit that. I think we've all
flown flat out, level flight. Quite a few light a/c are approved to run
continuous full throttle - doesn't do much on yer endurance or fuel
economy though.

--
Duncan

xerj wrote:
Thrust from a propeller drops as speed increases to a theoretical zero. When
this happens, what is providing the thrust to overcome drag? The forward
momentum of the aircraft?

If so, is it a case of the speed very slightly and imperceptibly dipping
below the zero thrust speed, getting a little thrust to bring it back to the
zero-thrust condition and doing this over and over again?

Thanks in advance.



The operative word above is theoretical. The prop can only get to this
speed if it is being pushed by another force. It can't pull itself to
this speed. The other option would be to place the prop in a wind
tunnel and blow air at it at a rate faster than what it can pull the
air. You could them reach a zero thrust point. However, you might
overrev and blow the engine shortly thereafter. :-)

Matt

Thrust from a propeller drops as speed increases to a theoretical zero. When this happens, what is providing the thrust to overcome drag? The forward momentum of the aircraft?

Think of it this way - the propeller is attempting to smoothly slice its
way through the air, but the air gets in the way. If the propeller
blades were in completely flat pitch, it could do so standing still (in
zero wind), but would provide no thrust (I'm leaving out the fine points
of the airfoil, not important to this view).

If there's a wind on the nose however, this superflat pitch propeller
would not slice through the air; you'd have to change the pitch to let
that air slide through the prop as it's turning. The stronger the wind
on the nose, the more you have to unflatten the pitch. Don't do it
enough, and the plane will be blown backwards by the wind. Do it too
much and the plane will move forward due to thrust.

The same thing happens in the air, except that since we depend on the
prop to pull the plane, we never get to "super-flat" pitch. And in a
dive, I suppose you could get the plane to go faster than the propeller
wants to drag it, but gravity is helping in that case.

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

In article ,
"xerj" wrote:

Could you re-phrase your questions?

Ahhh yeah. Just realised you'd never hit the zero thrust speed. Woops!

The thing that STILL puzzles me though is that the thrust created *by the
propeller* goes down (as measured in Newtons for instance) as the velocity
increases.

As detailed here http://www.mh-aerotools.de/airfoils/propuls4.htm , the
thrust at 252 knots for a theoretical plane is 17.2N, whereas at 108 knots
it's 31N.

That's what's got me confused. Higher speed = more drag, yet the measured
thrust from *just the prop* is less. Is the extra thrust to match drag
coming from the momentum of the already moving airframe?

Not quite.

The plane accelerates (and the drag increases and the thrust decreases
just as you observe) until the thrust and drag are exactly the same. At
that point the airplane stops accelerating, that is, its velocity
becomes constant. The momentum of the plane doesn't provide any extra
*thrust*, but the momentum is (more or less) what keeps the plane moving
at that point. (The reason I say "more or less" is that what really
keeps the plane moving is the need to maintain a particular speed in
order to balance thrust and drag. If the plane slows down then thrust
exceeds drag and the plane speeds up again, and vice versa. This would
happen even if the plane's momentum (i.e. its mass) were zero. In this
case, the plane would reach its final velocity instantly once the prop
was turning, and would stop instantly as soon as the prop stopped
turning.)

(Note that it doesn't actually matter that the thrust decreases with
velocity, only that the drag increases faster than the thrust as
velocity increases so that at some speed they become the same.)

rg

xerj wrote:

Thrust from a propeller drops as speed increases to a theoretical zero. When
this happens, what is providing the thrust to overcome drag? The forward
momentum of the aircraft?

If so, is it a case of the speed very slightly and imperceptibly dipping
below the zero thrust speed, getting a little thrust to bring it back to the
zero-thrust condition and doing this over and over again?

Thanks in advance.

Its hard to separate the thrust forces created by the prop from the
torque / hp of the motor exerting the shaft turning force to spin the
prop. Its also necessary to consider that as the speed of the plane
increases, so does its drag (parasitic drag from the plane itself).
At slow speeds, the angle of attack can also create higher drag against
which the prop must work. See:
http://www.petester.com/html/bachap04.html for more information. That
said, I think there are four states we can look at:

The first state is when the plane is accelerating. In this state,
thrust is greater than drag, the prop is creating more forward force
than the drag of our airplane. This is at lower speeds when the drag of
the plane is lower. At low speeds / higher angles of attack as in a
climb, equilibrium is reached at a lower speed due to higher induced drag.

As the speed of our plane increases, so does the drag (so does the
lift). At some point drag will equal thrust. If we throttle to 75%
power, the speed will settle in at a certain point when drag = thrust,
we are at 75% power cruise speed. If we throttle back to 55% power,
then the thrust will decrease (less power to spin the prop) and the
speed will decrease (and the plane's drag, as a factor of speed, will
decrease along with it) until the that equalibrium (drag = trhrust) is
reached. The resulting speed is the cruise at 55% power. This
"equilibrium" is the second state (cruise).

At higher angles of attack as in a climb or low speed flight,
equilibrium is reached at a lower speed due to higher induced drag.
When we throw out some flaps, we are increasing parasitic drag (not very
streamlined) and induced drag (higher angle of attack / aerodynamic
drag). Consider the thrust required to fly at minimum controllable
airspeed without losing altitude.

The third state is when the the prop creates less thrust than the plane.
Consider throttling back and accelerating in a dive. The speed
(pressure) of the air rushing past the prop is causing the prop to spin
at a higher rate than the speed it would turn under the same power at
equilibrium. So thrust is less than drag.

The forth state is when the aircraft is parked and the engine off.
There is 0 thrust and 0 drag. It is a kind of equilibrium, but one that
doesn't really do much for us.

For a given power setting, as the plane flies faster through the air,
the prop takes less "bite" and the drag against the prop decreases. But
it doesn't reach 0 (or 0) drag unless its in the third state. That's
when we want to throttle back so that we don't over-rev the engine. A
constant speed prop simply changes the angle of attack of the prop to
provide higher thrust at higher speeds as the prop "bite" decreases with
speed.



There may be some fine details that don't quite fit the above, but I
think this is generally the prinicpal of thrust (prop + power) vs. drag.

In article ,
"xerj" wrote:

Thrust from a propeller drops as speed increases to a theoretical zero. When
this happens, what is providing the thrust to overcome drag? The forward
momentum of the aircraft?

If so, is it a case of the speed very slightly and imperceptibly dipping
below the zero thrust speed, getting a little thrust to bring it back to the
zero-thrust condition and doing this over and over again?

Thanks in advance.



On a reciprocating engine, you have only so much power available. In
level flight:

Power = Thrust * Velocity. Or: Thrust = Power/Velocity.

If you go faster, thrust decreases to meet power. Thrust, theoretically,
will not go to zero, but will equal drag, which increases with the
square of velocity.

If you dive, thrust is added by the weight of the aircraft * sine of the
dive angle.

--
Remve "_" from email to reply to me personally.