KiloNewton/Pound foot of thrust into Horsepower units

  • #1

Main Question or Discussion Point

Hi,
I am currently studying aviation engine outputs and to make my study linear in comparative analysis with standard Automotive engines I need some expert help.
Can anyone help me drive precise equations to convert:
1. Engine thrust in Kilo Newton into Horse Power and Watts
2. Engine thrust in Pound foot into Horse power and Watts
3. Watts into Horse Power.
Seems very basic conversion yet I am struggling to derive precise equations for these statement queries.
Would be great if someone can help me out in this.
Thanks
Arjun
 

Answers and Replies

  • #2
BvU
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  • #3
russ_watters
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Thrust and power are different quantities and can't be directly related. You will need to know more about the propulsion system to find the power at a given thrust. Are we talking jets or propellers?

Also, pound-foot is not a unit of thrust...
 
  • #4
Hello arjun, :welcome:

wattts to horsepower

Did you know you can simply google these things :rolleyes: ?
An obvious observation. Google as an introductory ideation tool is providing extremely complex correlations while I require very basic yet elemental overview. Perhaps I answered your question now can you help solve mine.
 
  • #5
Thrust and power are different quantities and can't be directly related. You will need to know more about the propulsion system to find the power at a given thrust. Are we talking jets or propellers?

Also, pound-foot is not a unit of thrust...
Jets. On wikipedia in design spec of jet engines, the output efficiency is described in pound foot. Is it not the correct denotion? Also aint the horse power and shaft horse power same notions to describe engine output.
 
  • #6
russ_watters
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Jets. On wikipedia in design spec of jet engines, the output efficiency is described in pound foot. Is it not the correct denotion?
I don't see it, can you link/quote the article you are reading please.
Also aint the horse power and shaft horse power same notions to describe engine output.
They are, but jet engines don't have an output shaft....do you mean turboprop/turboshaft engines?
 
  • #7
I don't see it, can you link/quote the article you are reading please.

They are, but jet engines don't have an output shaft....do you mean turboprop/turboshaft engines?
I was just addressing your jet or propeller question, as propellers would be turboprop/turboshaft engines.
 
  • #8
  • #10
russ_watters
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Maybe it would be instructive to look at an engine like the General Electric CF6/LM2500, which has both thrust and shaft producing versions. They aren't identical, but it might get you in the ballpark.
 
  • #11
Maybe it would be instructive to look at an engine like the General Electric CF6/LM2500, which has both thrust and shaft producing versions. They aren't identical, but it might get you in the ballpark.
What is LP? Also can you help me with the equations query posted earliest in this thread.
 
  • #12
russ_watters
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What is LP?
I don't know, where are you seeing that?
Also can you help me with the equations query posted earliest in this thread.
As I said, there is no such equation. These are not comparable units/performance metrics.
 
  • #13
I don't know, where are you seeing that?

As I said, there is no such equation. These are not comparable units/performance metrics.
In the GE engine article you shared, turbine and compressor specifications have HP that is horse power and LP... so what is that?
 
  • #14
cjl
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That's not horsepower in the wiki article for the CF6 - that's RPM. The specification table shows max LP RPM (maximum rotational speed of the low pressure [LP] shaft and components) and max HP RPM (maximum rotational speed of the high pressure [HP] shaft and components). For horsepower, you need to look at the LM2500.
 
  • #15
That's not horsepower in the wiki article for the CF6 - that's RPM. The specification table shows max LP RPM (maximum rotational speed of the low pressure [LP] shaft and components) and max HP RPM (maximum rotational speed of the high pressure [HP] shaft and components). For horsepower, you need to look at the LM2500.
Ok. Thanks. Can you help me with this -
I am currently studying aviation engine outputs and to make my study linear in comparative analysis with standard Automotive engines I need some expert help.
Can anyone help me drive precise equations to convert:
1. Engine thrust in Kilo Newton into Horse Power and Watts
2. Engine thrust in Pound foot into Horse power and Watts
3. Watts into Horse Power.
Seems very basic conversion yet I am struggling to derive precise equations for these statement queries.
 
  • #16
cjl
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As has already been explained to you.

1) There is no direct thrust to horsepower conversion. They're fundamentally different kinds of units.
2) Pound foot isn't a thing. When you see lbf in a specification for a jet engine, that's pounds force.
3) Watts to horsepower is easy. It's 746 (if I remember right) watts per horsepower. You can look this up on google in 10 seconds.
 
  • #17
As has already been explained to you.

1) There is no direct thrust to horsepower conversion. They're fundamentally different kinds of units.
2) Pound foot isn't a thing. When you see lbf in a specification for a jet engine, that's pounds force.
3) Watts to horsepower is easy. It's 746 (if I remember right) watts per horsepower. You can look this up on google in 10 seconds.
Thanks. I had seen the watts to horsepower conversion. Just wanted to confirm.
 
  • #18
CWatters
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Power = thrust * velocity

So you need to know how fast the engine/aircraft is moving when it makes the specified thrust. If it not moving then technically it's not producing any useful output power, although it is producing a lot of power in the form of waste heat and moving air.
 
  • #19
As has already been explained to you.

1) There is no direct thrust to horsepower conversion. They're fundamentally different kinds of units.
2) Pound foot isn't a thing. When you see lbf in a specification for a jet engine, that's pounds force.
3) Watts to horsepower is easy. It's 746 (if I remember right) watts per horsepower. You can look this up on google in 10 seconds.
One more
Power = thrust * velocity

So you need to know how fast the engine/aircraft is moving when it makes the specified thrust. If it not moving then technically it's not producing any useful output power, although it is producing a lot of power in the form of waste heat and moving air.
Yes. I was trying to understand various engine output comparisons between various aircrafts in an attempt to understand what factors affect the type of an engine selected for a specific aircraft. Also how an aviation jet engine output is comparable to a scale down piston engine or electric motor simply in terms of generated speed of the vehicle. So my logic was that thrust is basically force and force=weight x acceleration. And power (watts) = force x velocity. So was trying to derive a standard watt equation using thrust but couldn't. Also, some engines do have HP in their engine spec, so was trying to work out a logic for these two different specifications. One observation that I made is that turbo shaft and turbo prop engines in many articles do have HP as their output unit and almost all high output turbojet and turbofan engines have thrust as the unit. Is it because of the high output of the later mentioned engines? Also thrust beyond an output denotion didn't seem to have any relative values for standard unit conversions. My queries may seem elementary level so pardon the indulgence.
 
  • #20
russ_watters
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Also, some engines do have HP in their engine spec, so was trying to work out a logic for these two different specifications. One observation that I made is that turbo shaft and turbo prop engines in many articles do have HP as their output unit and almost all high output turbojet and turbofan engines have thrust as the unit. Is it because of the high output of the later mentioned engines?
No, it's because turboshaft and turboprop engines have output shafts and jet engines just move air.
 
  • #21
No, it's because turboshaft and turboprop engines have output shafts and jet engines just move air.
OK. Thanks for your help guys. Appreciated.
 
  • #22
cjl
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Note that you can get to a level of detail on modern jet engines where horspower is useful - specifically if you look at the power required to drive the compressor, front fan, etc, but that's all internal to the engine. Just as a point of comparison though, the power required to drive the front fan on a large modern jet engine can be in the ballpark of 100,000 horsepower.
 
  • #23
Note that you can get to a level of detail on modern jet engines where horspower is useful - specifically if you look at the power required to drive the compressor, front fan, etc, but that's all internal to the engine. Just as a point of comparison though, the power required to drive the front fan on a large modern jet engine can be in the ballpark of 100,000 horsepower.
That's a very ambitious number. I was thinking in the range of 20,000 to 25000 HP. Also is the thrust produced by the entire engine (combination of this fan and the resultant exhaust from the compressor and combustion chamber) has any mathematical relativity to the shaft HP of the fan it is powering.
 
  • #24
cjl
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I'll admit that I'm just going off memory, but I think that number is about right. That having been said, it doesn't really have any relevance to the performance of the jet engine, it's just part of the internal design. As for your question, no there's not really any direct relation between thrust and fan power. You might be able to construct some relation if you also account for bypass ratio and design cruise speed or something, but it's certainly not just a direct HP to thrust relation.
 
  • #25
CWatters
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If it helps... The RR Trent 60 gas turbine for power generation is around 60MW.
 

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