Variable Pitch Prop on T/o Roll

[Aviator_Prat]

Hi,
I was going through propellers in Oxford Principles of Flight and it says on Take off roll:-
As the aircraft continues to accelerate the TAS will increase, which decreases the angle of attack of the blades. Less thrust will be generated and less
propeller torque. This gives less resistance for the engine to overcome and RPM would tend to increase.

I get it up till less thrust but why will the prop torque lessen??
And if the torque lessens then why does the RPM Increase? is it because of lesser torque opposing the engines torque?

Thanks for the Patience guys!!

[john_tullamarine]

The wordiology is a bit loose but, basically, OK. Probably more appropriate for a constant pitch prop as the VP/CSU will vary the blade angle better to maintain a useful blade angle and match required to available torque - ie maintaining RPM.

Consider a wing - high alpha, high lift, high drag. The drag is looked after by engine thrust or descending.

Consider a prop at brakes release - high blade angle, high blade alpha, high blade thrust, high blade drag. The drag is looked after by engine torque capability ie the stuff which is trying to keep the prop turning.

Bit like driving a motor car -

(a) driving along on the flat, constant throttle setting, constant speed.

(b) into the downhill stretch, gravity helps, don't need as much power but, with the original throttle setting it's there so we speed up

(c) into the uphill stretch, gravity is the enemy, need more power but, with the original throttle setting it's not there so we slow down.

If we recast the story a little ..

As the aircraft continues to accelerate the TAS will increase, which decreases the angle of attack of the blades. Less thrust will be generated which results in less drag and requires less propeller torque to maintain the desired RPM. If the throttle setting is maintained, we have more torque than required to maintain the desired RPM and RPM would tend to increase until the increased drag level balances the torque available to drive the propeller.

[Aviator_Prat]

The Drag caused by a running propeller will be the Torque that is shown by the resolution of the total reaction vector (Labelled Lift in Img) of a propeller ?

http://content.answcdn.com/main/content/img/McGrawHill/Aviation/f0017-02.gif

How does this Propeller Reaction Torque reduce the engine torque required to rotate the propeller thereby increasing the RPM?

I was thinking of finishing propellers today looks like all my Basics are messed up!!
Appreciate your help.

[bcgallacher]

The fact is that the rpm will not increase - the rpm will remain constant,that is what a vp prop is meant to do. As the load on the prop decreases on the t/o roll the csu will control the rpm to the set figure by increasing the pitch and maintaining the selected rpm.

[barit1]

Just remember - airfoil drag is equivalent to torque. An unloaded engine creates NO torque. It's only when the prop is biting into the air that torque is created, and at a constant power setting (fixed throttle), more torque => less RPM, and less torque => more rpm.

The whole idea behind a constant speed prop is to control pitch to maintain a constant torque load on the engine, and that results in constant RPM.

[keith williams]

Engine torque is the torque that the engine applies to the drive shaft to turn the propeller.

Propeller torque is the torque that is caused by the rotational drag as the propeller turns. The propeller torque opposes the rotation of the propeller.

As the TAS increases during the take-off run the angle of attack of the blades decreases. This decreases both the thrust and the propeller torque. If we assume that the engine torque has not changed, then the engine torque will be greater than the propeller torque. So the RPM will tend to increase. If it is also a constant speed prop the CSU will sense this tendency and will increase pitch angle to prevent it.

[bearfoil]

Constant speed is not variable Pitch, necessarily. Two different animals. One is Automatic. One wants pilot input.

[barit1]

Engine torque is the torque that the engine applies to the drive shaft to turn the propeller.

Propeller torque is the torque that is caused by the rotational drag as the propeller turns. The propeller torque opposes the rotation of the propeller.

In steady state operation, the two are equal. Whenever they are not ("unbalanced torque") then engine speed will change in the direction of unbalance, until they again match. For example, a fixed-pitch prop.

[Aviator_Prat]

Ok. So i get it now , correct me if I am wrong.
Engine torque rotates the propeller & propeller torque opposes this rotation and creates a torque in the opposite direction. I.e. If the engine torque rotates the propeller clockwise the propeller torque will tend to rotate it anticlockwise.
Both these torques are balanced out for normal operating conditions.
A 3D perspective of how the torque's are acting on all blades of the propeller, is kind of hard to decipher.
But thanks I get it now.

[john_tullamarine]

The devil's in the wordiology and detail.

Prop torque is a bit of a misnomer but, for those who like to use the term, fine. This concept is only an effort to relate instantaneous blade drag loads to a rotating bit of machinery. That is, we are still only talking about drag acting on the disc. Whatever you might want to call the animal, it only is a measure of that which is endeavouring to slow the disc rotation - it can't cause a reverse rotation.

Both these torques are balanced out for normal operating conditions for constant RPM operation.

[Aviator_Prat]

Ok. The drag caused by the rotating propeller (or the propeller torque) acts opposite to the engine torque i.e. for the point of understanding if the engine torque rotates the prop clockwise the drag acts counter clock.
And like you said the prop drag will be much lesser than the engine torque, but it reduces the thrust power as compare to the shaft power, thereby reducing the prop efficiency.

[john_tullamarine]

The drag caused by the rotating propeller (or the propeller torque) acts opposite to the engine torque i.e. for the point of understanding if the engine torque rotates the prop clockwise the drag acts counter clock.

That's OK

The next bit's not quite right ..

the prop drag will be much lesser than the engine torque

We can't equate the two as they are are different - drag is a force while torque is like a moment. Better to say that the engine torque can provide the means to

(a) allow the prop to slow down (engine torque less than what is required to balance the prop disc drag loads so the prop decelerates until the drag loads are balanced by the engine torque)

(b) maintain an RPM (engine torque equals what is required to balance the prop disc drag loads)

(c) cause the prop to accelerate (engine torque greater than what is required to balance the prop disc drag loads so the prop accelerates until the drag loads are balanced by the engine torque)

but it reduces the thrust power as compare to the shaft power thereby reducing the prop efficiency.

I'd leave this bit out of consideration.

[SpanWise]

john_tullamarine has answered it pretty well in his first post.

As the TAS increases, AoA decreases, prop drag decreases.

Less torque IS NEEDED to maintain the RPM. Thus RPM Rises.

RPM rises under the affects of excess torque.

[Aviator_Prat]

The next bit's not quite right ..

the prop drag will be much lesser than the engine torque

We can't equate the two as they are are different - drag is a force while torque is like a moment. Better to say that the engine torque can provide the means to

(a) allow the prop to slow down (engine torque less than what is required to balance the prop disc drag loads so the prop decelerates until the drag loads are balanced by the engine torque)

(b) maintain an RPM (engine torque equals what is required to balance the prop disc drag loads)

(c) cause the prop to accelerate (engine torque greater than what is required to balance the prop disc drag loads so the prop accelerates until the drag loads are balanced by the engine torque)


Oh..That makes a lot of sense.
Thank you!
Sorry for the amateur questions and conclusions!!

[john_tullamarine]

As many of us have observed here "the only dumb question is the one not asked".

For every apparently simple question, there will be those lurking who will benefit from the discussion.

Aviator_Prat, you are most welcome in this fraternity.

[Volume]

For those with a more graphic mind, just imagine the torque curve of the engine and the torque curve of the propeller. Where they intersect, the system is in stable equilibrum (as the propeller curve will always be steeper than the engine curve above idle). The propeller curve is parabolic, torque increases with speed to the power of 2. Increasing TAS (roughly) shifts the curve to the negative ("downwards"), the new intersection therefore is at higher rpm.