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Old 14th Apr 2007, 21:44   #1 (permalink)
 
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pounds vs horsepower

Is there a formula to convert pounds of thrust in to horse power?
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Old 14th Apr 2007, 23:09   #2 (permalink)
 
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Not really.

You could use the fact that hp is a power measure, and lbf is a force, to use the simple "Power = Force times Velocity" relationship. But if you're trying to convert engine hp into thrust, there are all kinds of issues with efficiencies of various components that make it quite complex.
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Old 15th Apr 2007, 01:57   #3 (permalink)
 
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pounds v horsepower

Pounds of thrust divided by 2.5 equals horsepower.
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Old 15th Apr 2007, 12:35   #4 (permalink)
 
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Quote:
Pounds of thrust divided by 2.5 equals horsepower.
That's very simplistic - but believe it or not, it's not too bad a generalization for static (early takeoff roll) approximation, given a typical prop.

What is missing is any accounting for propeller efficiency, or for vehicle speed. The old rule of thumb was I think HP = F(lb) * V(mph)/375. Note that at zero speed the useful HP is zero; all you're creating is heat and noise.

Do your own units conversion.
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Old 15th Apr 2007, 16:44   #5 (permalink)
 
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The original question was:
Quote:
Is there a formula to convert pounds of thrust into horsepower?
The simple answer is "no", unless you also specify the speed.
Power = force x velocity ( x unit conversion factor).

Several posters have read the question the other way round:
"is there a formula to convert horsepower into pounds of thrust?".
Again, the simple answer is "no".
Thrust = power x propellor efficiency ( x unit conversion factor).
Since propellor efficiency is not something you can put in a formula, out come the graphs.
But as earlier posters have suggested, there are some rule-of-thumb formulae that will give you a rough approximation for a given case.

And sorry, but I'm not getting out my sliderule to do all the unit conversions. It's probably all in wikipedia, anyway.

Kalispera sas, Jimmy !
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Old 15th Apr 2007, 17:07   #6 (permalink)
 
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Kalispera. thanks for all the answeres.

I wanted to know how to convert Ib in to horse power just to ge a feel of how powerful a jet engine is.
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Old 16th Apr 2007, 07:17   #7 (permalink)
 
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For the limited purpose that you have specified you can use:

Thrust Horsepower = (Thrust in lb x TAS in Kts) / 325.658

This is based on:

Thrust Power = Thrust x TAS
1 horsepower = 33000 ft lb/min
1 Kt = 6080 ft/hour

But it cannot be used to estimate BHP of a piston engine or SHP of a turbojet. As others have stated previously this also depends on propeller efficiency which is not constant.
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Old 16th Apr 2007, 10:52   #8 (permalink)
 
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I found it.

At approximately 347kt, one pound of thrust equals one horsepower, but at higher speeds this ratio increases and a pound of thrust is greater than one horsepower. At speeds of less than 347kt, this ratio decreases.
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Old 16th Apr 2007, 11:37   #9 (permalink)
 
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Jimmy,

Your figure of 347 appears to be based on 1 nm = 5706 ft.

(33000 x 60 ) / 347 = 5706

If you use the more accurate figure of 1 nm = 6080 ft you will get closer to my figure of 325.
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Old 16th Apr 2007, 12:06   #10 (permalink)
 
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ok, thank you.
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Old 16th Apr 2007, 14:31   #11 (permalink)

 
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The Allison 501-D13 FAA type certificate contains a formula to calculate the equivalent shaft horsepower provided by the jet thrust as barit1 and Erebus alluded to in their posts.
Equivalent shaft hp = (jet thrust lb)/2.5 + shaft hp

The take off (5 min) rating is 3750 ESHP, 3460 SHP and 726 lb jet thrust
Max continuous 3420 ESHP, 3138 SHP and 705 lb jet thrust
Formula given by Kieth. Williams. is what I learnt near 50 years ago also.
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Old 16th Apr 2007, 15:33   #12 (permalink)
 
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Thanks, Brian,
That explains what at first seemed at first a grossly simplified and arbitrary conversion.
The moment it's only used for take-off, to work out the correction needed for residual jet thrust from a turboprop, it makes sense.
No need to apply rocket science and calculate it to the last 0.1%
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