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Propeller torque & engine torque

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Propeller torque & engine torque

Old 5th Apr 2012, 17:07
  #101 (permalink)  
 
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Keith, I'm not interested in your purpose for contributing. You did contribute.

If you believe that propeller efficiency cannot be other than zero when the aircraft is stationary just ask yourself next time you start the take off run where the aircraft finds the thrust and power to even begin moving.
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Old 5th Apr 2012, 22:31
  #102 (permalink)  
 
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Induced Power

Hi oggers,

Is this what you are searching for?

"Induced Power
Induced Power is that portion of the power required to produce lift. It is the power required to overcome the portion of rotor drag which is caused by the induced flow tilting the total reaction rearwards.
Induced power is the force required to move a mass of air through the disk at the induced velocity.
If T is the rotor thrust (in a hover equal to weight (Mass * G)), which is a force, and this force moves the air at a velocity Vi , Pi = TVi .

Helicopter Power Required
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Old 6th Apr 2012, 09:54
  #103 (permalink)  
 
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RRT,

Thanks that's a nice clear reference.

Pi = T x Vi

THP = T x V

The principle of THP extended to the hover. No need for the aircraft to move.
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Old 7th Apr 2012, 02:34
  #104 (permalink)  
 
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A few thoughts:

At zero airspeed, how much induced drag does the aeroplane see? How much profile drag?

How much horsepower does it then take to (ahem) "move" it at zero airspeed?

There may be a clue here...
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Old 7th Apr 2012, 10:09
  #105 (permalink)  
 
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Perhaps it would help to remember that aircraft propulsion systems are not "lossless". Not all the power goes into moving the aircraft so you can't simply say the power generated by the engine is

Power = force x velocity

You have to write something like...

Power = (force x velocity) + "losses"

You can define:

force = thrust
velocity = aircraft airspeed

but best leave the definition of "losses" vague for the moment as there are too many things to list and they are NOT constant. Some of the losses are also dependant on velocity.

Now it's clear from the modified equation that the power output is not necessarily zero when the aircraft is stationary. In such a case "Losses" includes throwing a lot of air backwards for no purpose.

In the case of the car on the dyno, "losses" include the load the dyno places on the wheels (eg heating up the brake unit).

Interesting to consider what happens at take off. Lets say you have brakes on, run up the engines and then release the brakes... as the aircraft starts moving through the air the sum of all the "losses" must reduce to balance the equation. (I'll ignore the fact that some engines produce more power when moving).
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Old 7th Apr 2012, 11:57
  #106 (permalink)  
 
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Hi barit1,
How much horsepower does it then take to (ahem) "move" it at zero airspeed?
There may be a clue here...
That depends on the acceleration which the power unit is giving to the aircraft at that very instant.

Last edited by rudderrudderrat; 7th Apr 2012 at 13:20. Reason: typo
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Old 7th Apr 2012, 16:05
  #107 (permalink)  
 
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True, but I believe the unstated assumption here is a steady-state (unaccelerated) vehicle.
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Old 7th Apr 2012, 16:23
  #108 (permalink)  
 
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Are you only considering a shed then?
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Old 7th Apr 2012, 20:13
  #109 (permalink)  
 
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The conditions I propose:

o Parked, brakes locked (zero GS)

o Zero wind (thus zero IAS)

The questions: How much induced AND profile drag?

and...

From a THP standpoint, how is this any different from running the engine on a test bench?

Keeping in mind the definition of one horsepower (i.e. 33,000 foot-pounds of work per minute), is there any useful work being done?
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Old 11th Apr 2012, 11:17
  #110 (permalink)  
 
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A few thoughts:

At zero airspeed, how much induced drag does the aeroplane see? How much profile drag?

How much horsepower does it then take to (ahem) "move" it at zero airspeed?

There may be a clue here...
Sure, no power is required if the aircraft is at standstill but that does not mean no power is available. OTOH power is required to accelerate the mass of the vehicle from standstill with or without aerodynamic drag. No power available = no acceleration. THP is the power available. If the aircraft has to move before you get THP then you are in a chicken and egg situation - you can't accelerate the aircraft without power but you can't have power until you move the aircraft.

The question is: if the engine is producing 200 BHP and the prop is providing thrust can we say that the prop is providing THP, even whilst the aircraft is stationary?

THP = BHP x PE

If there is thrust PE is not zero. If PE is not zero THP cannot be zero. THP is not the same as power required, though it may have the same value.

-------------------

RRT,

As an aside, the power required curve in the link you provided should be very familiar to helicopter pilots: the high power required to hover and the ‘hump’ near zero IAS as one transitions to forward flight are important characteristics for rotorheads to consider.

Last edited by oggers; 11th Apr 2012 at 11:43.
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Old 11th Apr 2012, 12:23
  #111 (permalink)  
 
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THP is the power available.
Nope. THP is the power DELIVERED TO THE VEHICLE.

THP is not the same as power required, though it may have the same value.
See above.
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Old 11th Apr 2012, 14:09
  #112 (permalink)  
 
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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.
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Old 12th Apr 2012, 01:52
  #113 (permalink)  
 
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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!
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Old 12th Apr 2012, 07:59
  #114 (permalink)  
 
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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?
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Old 12th Apr 2012, 10:48
  #115 (permalink)  
 
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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.
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Old 12th Apr 2012, 11:21
  #116 (permalink)  
 
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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!
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Old 12th Apr 2012, 11:38
  #117 (permalink)  
 
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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 !
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Old 12th Apr 2012, 12:16
  #118 (permalink)  
 
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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
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Old 12th Apr 2012, 12:27
  #119 (permalink)  
 
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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.
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Old 12th Apr 2012, 12:59
  #120 (permalink)  
 
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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).
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