Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Propeller torque & engine torque

Tech Log The very best in practical technical discussion on the web

Propeller torque & engine torque

Old 11th Apr 2012, 14:09
  #121 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 730
Likes: 0
Received 0 Likes on 0 Posts
barit

I said 'THP is the power available'. It wasn't meant as a definitive definition of THP, merely an attempt to use some words to give context to the paragraph.


Nope. THP is the power DELIVERED TO THE VEHICLE.

Now, are you trying to tell me that THP is not the measure of power available? If so, which of the other definitions of power that are out there, that one might call commonplace, eg IHP, BHP, SHP, ESHP do you believe is the right one to use in the context of power output available from a prop? Or do you have one of your own?

Last edited by oggers; 11th Apr 2012 at 14:21.
oggers is offline  
Old 12th Apr 2012, 01:52
  #122 (permalink)  
 
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes on 0 Posts
To spell it out, using a 10000# thrust engine for example:

At zero TAS, THP = 0

At 375 MPH (i.e. 550 fps) TAS, THP = 10000

At 750 MPH (i.e. 1100 fps) TAS, THP = 20000

...and so on. As you see, the propulsive work delivered to the aeroplane increases proportional to velocity.

In real life, things aren't so tidy, simply because the engine thrust varies a bit with different inlet conditions; but if the machine were adjusted to hold constant thrust, the above would indeed apply.

HINT: Note that the above cases are based on the definition of 1 HP, namely 33000 ft-lb/min, or 550 ft-lb/sec. It helps to think of horsepower in terms of its definition!
barit1 is offline  
Old 12th Apr 2012, 07:59
  #123 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 730
Likes: 0
Received 0 Likes on 0 Posts
barit1

As you see, the propulsive work delivered to the aeroplane increases proportional to velocity
"Propulsive work". Clearly, everyone can see that.

But the question I asked of you was: is THP the measure of power available from the prop or not? Because when I said it was, you responded quite clearly:

Nope. THP is the power DELIVERED TO THE VEHICLE.
I'm asking what you believe is the measure of power available?
oggers is offline  
Old 12th Apr 2012, 10:48
  #124 (permalink)  
 
Join Date: Jan 2011
Location: England
Posts: 660
Received 18 Likes on 11 Posts
Oggers you are overlooking the power that is being wasted in doing work on the propwash.

When the aircraft is stationary on the ground with engine running all of the propeller power output is being wasted in accelerating the propwash rearwards. So none of the propeller power output is being delivered to the airframe. That means that there is no THP, because as Barit has said "THP is the power delivered to the airframe".

In theory if the aircraft were to fly at its own propwash speed no power would be wasted in the propwash, so all of the propeller power output would be being delivered to the airframe. So it would all be THP. This is of course only true in theory because if the aircraft was flying at its own propwash speed, no air would be being accelerated rearwards by the propeller, so there would be no thrust.

At any speed between the zero and the propwash speed some of the propeller power output would be being wasted in the propwash and the rest would be being delivered to the airframe as THP.

If you think about it this means that the propeller power output is the THP plus the power wasted in the propwash. So we cannot say that propeller power output is THP.

BUT. If you look at any fixed wing diagram of power available against power required you would see that the power available is the THP. It would perhaps be better to say that this is the USEFUL power available.

Your mistake throughout this entire thread has been in assuming that THP is thrust x propwash speed. It is not. Beacuse THP is the power being delivered to the airframe, it is thrust x aircraft speed.
keith williams is offline  
Old 12th Apr 2012, 11:21
  #125 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 730
Likes: 0
Received 0 Likes on 0 Posts
Interesting post Keith. I will give it my full consideration. For now I'll just point out that I most certainly haven't overlooked the propwash, having specifically referred to it as being the place the power went - in my very first post on the topic.

I hope barit1 will answer the simple question of what power is the measure of power available for himself. I think that is totally fair as he insisted in his response to to me that it isn't THP. In capitals no less!
oggers is offline  
Old 12th Apr 2012, 11:38
  #126 (permalink)  
 
Join Date: Oct 2010
Location: Germany
Age: 47
Posts: 402
Likes: 0
Received 0 Likes on 0 Posts
you really beat it to death gentlemen .

like written above in this particular case you have to split the airframe and the engine. power by definition cannot exist without movement- without movement you can only have a force but not power. so the airframe does not develop any power when the aircraft stands still and no power but anly a force ( e.g the force on the brakes ) is delivered to the airframe by the engine.

but the engine by itself moves ( spins ) and so it of course develops power.

its pretty the same when a diesel locomotive starts to pull a train but its to heavy that it is able to make it move. in this situation the locomotive develops only a force ( on the couplings) but not power.

the diesel engine of the locomotive on the other hand develops power because it spins at a given rpm developing a given torque.

and thats basicly all about this purely theoretic discussion ...

cheers !
aerobat77 is offline  
Old 12th Apr 2012, 12:16
  #127 (permalink)  
 
Join Date: Mar 2002
Location: Florida
Posts: 4,569
Likes: 0
Received 1 Like on 1 Post
you really beat it to death gentlemen
You hit the nail on the head

Most of what I read is intuition and reason and a lot of that was correct even though there was little agreement

It really is all about definitions that allow one to communicate in a common language.

It was fun and we really should all go away satisfied
lomapaseo is offline  
Old 12th Apr 2012, 12:27
  #128 (permalink)  
 
Join Date: Jan 2011
Location: England
Posts: 660
Received 18 Likes on 11 Posts
Oggers, it was actually your second post in which you said the following:

in your 'brakes on' scenario the aircraft is still producing 200 THP as well as 200 BHP because it is accelerating a mass of air rearwards
But you were wrong, because you were confusing THP with the power that is being wasted in the propwash. You have then gone on to repeat this mistake thorughout the majority of your posts in this thread.

In the brakes on condition we have:

Propeller power output = power being wasted in the propwash.

In flight we have:

Propeller power output = THP + power being wasted in propwash.

As we accelerate from zero, the THP gradually increases while the power being wasted in the exhaust gradually decreases.
keith williams is offline  
Old 12th Apr 2012, 12:59
  #129 (permalink)  
 
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes on 0 Posts
oggers:
I'm asking what you believe is the measure of power available?
Power AVAILABLE (not power delivered) is very nicely defined by KW:
In the brakes on condition we have:

Propeller power output = power being wasted in the propwash.

In flight we have:

Propeller power output = THP + power being wasted in propwash.
And I will add: The wasted propwash power is dissipated in heat (friction between air molecules in the turbulent flow).
barit1 is offline  
Old 12th Apr 2012, 14:32
  #130 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 730
Likes: 0
Received 0 Likes on 0 Posts
Okay barit1. I can see you don't want to answer the question. THP is the unit of measurement for power available. I think that is about as close to standard use of terminology as one is going get in this thread, even though you objected to it for reasons that are not clear.
oggers is offline  
Old 12th Apr 2012, 14:39
  #131 (permalink)  
 
Join Date: Jan 2011
Location: England
Posts: 660
Received 18 Likes on 11 Posts
Oggers you really must try reading instead of just writing!

The POWER AVAILABLE from the PROPELLER is turned into two different types of power.

The first type is THP which is used to push the aircraft forward against the drag.

The second type is the power that is wasted in propelling the propwash rearwards.

So we cannot say that power available is THP.

In the most favourable conditions, that is in forward flight we have:

Power available from propeller = THP + Power wasted in propwash

But on the ground with the brakes on we have:

Power available from the propeller = Power being wasted in propwash


It isn't a difficlut concept, but to get it you need to READ what has been written. Simply waiting a few seconds then writing another post will not do it.
keith williams is offline  
Old 12th Apr 2012, 15:57
  #132 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 730
Likes: 0
Received 0 Likes on 0 Posts
keith, don't worry. I'm reading your last three posts very carefully, and I can see a couple or three things that deserve a considered response

[edit: it may take til after the weekend though]
oggers is offline  
Old 12th Apr 2012, 16:25
  #133 (permalink)  
 
Join Date: Oct 2009
Location: UK
Posts: 1,270
Likes: 0
Received 0 Likes on 0 Posts
Hi Keith,

Please could you clear up some doubts in my mind?
The first type is THP which is used to push the aircraft forward against the drag.
I thought we simply called that "thrust"?
The second type is the power that is wasted in propelling the propwash rearwards.
I thought that is how we generate thrust - by accelerating a mass of air backwards - why would it be considered wasted if it generates something useful?

The only waste I can see is considering all of this with the brakes on, so there is no useful acceleration of the aircraft.
rudderrudderrat is offline  
Old 12th Apr 2012, 18:57
  #134 (permalink)  
 
Join Date: Jan 2011
Location: England
Posts: 660
Received 18 Likes on 11 Posts
Hi Keith,

Please could you clear up some doubts in my mind?

Quote:
The first type is THP which is used to push the aircraft forward against the drag.
I thought we simply called that "thrust"?

OK, then I will refine the statement a little. THP is the power that we use to carry out the work of pushing the aircraft forward against the drag at our chosen aircraft speed.The second type is the power that is wasted in propelling the propwash rearwards.

Quote:
The second type is the power that is wasted in propelling the propwash rearwards.
I thought that is how we generate thrust - by accelerating a mass of air backwards - why would it be considered wasted if it generates something useful?

The only waste I can see is considering all of this with the brakes on, so there is no useful acceleration of the aircraft.

We often say that we create thrust by accelerating air rearwards. It would be more true to say that we create thrust by exerting a rearward force on the air. The air then exerts a forward force (the thrust) on our propeller in accordance with Newton's thrid law.

Unfortunately the air is a fluid, so it is not rigid enough to resist the force that we apply to it. This causes it to be accelerated rearwards. This in turn gives it rearward velocity. In giving it rearward velocity we have given it kinetic energy. This represents a waste of energy in that we are never going to get it back nor are we getting any benefit from it.

If you cannot see the difference then let's consider a man in a boat in a shallow canal. He has a long oar with which he can row the boat along or push it along by pushing against the bottom of the canal. The canal bottom and walls are lined with concrete.

If he pushes against the bottom of the canal, the canal bottom will resist this force so it will not move. Only the boat will move, so all of the energy that he uses goes into the boat to give it velocity. None of the energy is being wasted in moving the canal bottom rearward.

But if he chooses to row, then some of the energy will be wasted in accelerating water rearwards and some will go into moving the boat forward. This wasted energy is the equivalent of the energy that we waste when our propeller accelerates air rearwards.

Pushing against the bottom will clearly require less energy (and hence less power) than rowing.

Last edited by keith williams; 12th Apr 2012 at 20:06.
keith williams is offline  
Old 13th Apr 2012, 12:30
  #135 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 730
Likes: 0
Received 0 Likes on 0 Posts
Keith Williams....




....any thrust horsepower or not? Propwash wasted? And propeller efficiency - more than zero or not? Same questions to barit1.

[BTW haven't forgotten to respond to your other stuff ]

Last edited by oggers; 13th Apr 2012 at 12:52.
oggers is offline  
Old 13th Apr 2012, 12:48
  #136 (permalink)  
 
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes on 0 Posts
The answer is found in the question "how much aerodynamic drag (due to vehicle speed) is there?"

If there's no velocity, and no drag, then there's no work being done to move the plane. And thus there is no thrust horsepower.

No THP = v(ft/sec) x f (thrust) / 550

IF THE ENGINE AND PROP CAN BE REPLACED BY A STATIC BRACKET, THERE IS NO THP.

Yes, the engine is producing rpm and torque (= mechanical hp), and the prop is stirring up the air, but no useful work is being done. Efficiency = zero.

BTW - don't confuse yourself by thinking of the aero drag created by the propwash as useful work!
barit1 is offline  
Old 13th Apr 2012, 13:08
  #137 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 730
Likes: 0
Received 0 Likes on 0 Posts
BTW - don't confuse yourself by thinking of the aero drag created by the propwash as useful work!
That's an odd thing to say barit1. I'm interested, has anybody on this thread suggested such a thing or did the idea arise in your own domain?

Efficiency = zero.
Sorry, is that propeller efficiency = zero? I specifically asked about prop efficiency. Just trying to get clarity, thanks.

Last edited by oggers; 13th Apr 2012 at 13:36.
oggers is offline  
Old 13th Apr 2012, 14:02
  #138 (permalink)  
 
Join Date: Jan 2011
Location: England
Posts: 660
Received 18 Likes on 11 Posts
Oggers, some of your posts appear to indicate that you think that if thrust is not zero then propeller efficiency cannot be zero. If you do believe this, then you are wrong (once again).

If you do a search for the definition of propeller efficiency (I've just done one) you will find that the most commonly used definition is as follows:

Propeller Efficiency = (Thrust x Aircraft Velocity) / Propeller power input

If you look at that equation you will see that there are two possible conditions that will give zero propeller efficiency.

These two conditions are, when thrust = zero, and when aircraft velocity = zero.

If either one of these conditions exist then the propeller efficiency will be zero.

In your hovering aeroplane picture the aircraft velocity is zero, so the propeller efficiency is zero.

A number of your arguments appear to be based on the belief that the aircraft velocity is the same as the propwash velocity. It is not.
keith williams is offline  
Old 13th Apr 2012, 14:51
  #139 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 730
Likes: 0
Received 0 Likes on 0 Posts
Keith:

appear to indicate that you think that if thrust is not zero then propeller efficiency cannot be zero. If you do believe this, then you are wrong (once again).
Just to be clear in case I left you in any doubt whatsoever. Yes I DO believe that if thrust is not zero then propeller efficiency is not zero. Thanks for clarifying that you don't though.

I am aware of the equation you mention. I am intensely relaxed about it. It is the obvious equation to use for an aircraft in flight. Earlier in the thread -page 4 #62 - I actually linked to a page from a prop designer who uses it . But, he is careful to point out that it cannot be used to determine the efficiency of a prop in the static thrust situation. It is - according to him - "invalid" in that situation. So, he uses a different equation for that.
oggers is offline  
Old 13th Apr 2012, 15:44
  #140 (permalink)  
 
Join Date: Jan 2011
Location: England
Posts: 660
Received 18 Likes on 11 Posts
Oggers,

The first point that we should note is that the author of that paper is not a universal authority on these matters. He has his point of view, but this cannot be used as an argument for rewriting all of the texts on these subjects.

The second point to note is that he was using a different equation because he wanted to know how much thrust the prop was producing when the aircraft velocity was zero. The final part of the essay is an equation for thrust produced.

But his equation cannot work out the efficiency of the propeller in converting power input into useful power output. And that is what the definition of propeller efficiency is all about. That is why his equation did not start with the words "Propeller Efficiency = "

But for some bizarre reason you have decided to argue that his equation is the one that should be used to calculate propeller efficiency.

Now if you wish to ignore all of the above then look at the essay a bit more closely. It includes the following equation

Substitute this into the equation for efficiency (1), and simplify to get:
The author had earlier defined VA as the aircraft velocity.

When the aircraft velocity is zero, the above equation shows that propeller efficiency is zero.


This entire thread has been about what is and what is not thrust horsepower. So there is nothing to be gained by introducing an equation that works out how much thrust a propeller produces.

If we look again at the basics:

Work is done when a force moves its point of application in the direction of the force.

Work = force x distance.

Power is the rate of doing work

Power = Force x distance / time

Power = force x velocity

From the above we can see that THP = Thrust x aircraft velocity.

Now look at your hovering model aeroplane and give us a logical explanation of why you believe that some THP is being produced.

Last edited by keith williams; 13th Apr 2012 at 16:30.
keith williams is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.