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BR715-A1-30
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Thrust VS. Horsepower?

Thu Nov 29, 2007 5:55 pm

Hello all you greasy, skydrol covered mechanics  Smile

This question aimed directly at those who specifically work, or have worked around powerplants.

I am wanting to know the difference between a Jet engine's thrust, and a Turboprop's Shaft Horsepower...

What exactly is the difference..

Thrust = –verb (used with object) 1. to push forcibly; shove; put or drive with force:
And IIRC, A turboprop, spinning, pushes the aircraft forward, forcibly, so how come the unit measured for Turboprops is Thrust? I cant even understand why a Radial is measured in Horsepower, seeing as how the propellers pushed the aircraft forward (or backward) forcibly.

Please help me, as it is giving me a headache

P.S. If a physicist or some other genius wants to chime in, feel free to. All comments are welcome  Smile
Puhdiddle
 
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Jetlagged
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RE: Thrust VS. Horsepower?

Thu Nov 29, 2007 5:59 pm

Thrust is a force. Horsepower is a unit of power (i.e Force times Velocity). A turboprop engine provides thrust just like a jet, it's just that shaft driven engines tend to be rated in horsepower. In other words the power available at the output shaft. When jet engines came along the only way to rate them was in terms of static thrust (i.e the thrust produced on a stationary test rig). Tradition dictates that these methods have continued.

[Edited 2007-11-29 10:01:45]
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BR715-A1-30
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RE: Thrust VS. Horsepower?

Thu Nov 29, 2007 6:01 pm

So if one could convert the two units, How much Thrust would a Connie's engines have?
Puhdiddle
 
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Jetlagged
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RE: Thrust VS. Horsepower?

Thu Nov 29, 2007 6:03 pm

Very roughly: divide the engine power output by forward velocity and if you have your units correct, the result is thrust in pounds.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
futurecaptain
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RE: Thrust VS. Horsepower?

Thu Nov 29, 2007 6:25 pm



Quoting BR715-A1-30 (Reply 2):
So if one could convert the two units, How much Thrust would a Connie's engines have?

In order to convert to thrust you must specify a speed. The thrust being generated will vary with speed.
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2H4
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RE: Thrust VS. Horsepower?

Thu Nov 29, 2007 6:48 pm



Quoting BR715-A1-30 (Thread starter):
I am wanting to know the difference between a Jet engine's thrust, and a Turboprop's Shaft Horsepower...

Also keep in mind that a turboprop's shaft horsepower rating does not necessarily correlate with the thrust produced. For example, you can build a great engine, but if you hang a less-than-efficient propeller on it, the engine's shaft horsepower won't be effectively converted into thrust.

So there's that whole variable (the propeller) stuck between the engine's shaft horsepower and the thrust produced.

Propeller slip is also something to be considered. Propeller slip takes the propeller's inherent inefficiencies into account and compares the result with a theoretically perfect propeller:



If a propeller were to be driven through a solid (like a screw through wood), it would theoretically advance X distance. Through less dense air, however, the propeller won't advance as far. The difference is known as propeller slip, and is an indication of the propeller's efficiency. This, in turn, is directly related to the thrust produced.

On a side note, the propellers used on the Wright Flyer were tested, and found to be about 75% efficient under the conditions of the first flights. It was also found that they actually had a peak efficiency of 82%.

To put that into perspective, the most modern, cutting-edge wooden propellers produced today have a peak efficiency (if I remember correctly) of about 85%.

The lesson? Never underestimate a bicycle mechanic.  Wink

2H4
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Lemmy
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RE: Thrust VS. Horsepower?

Thu Nov 29, 2007 6:55 pm



Quoting 2H4 (Reply 5):
The lesson? Never underestimate a bicycle mechanic.

I'm putting that on my next resume.
I am a patient boy ...
 
CJAContinental
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RE: Thrust VS. Horsepower?

Sun Dec 09, 2007 1:35 pm



Quoting 2H4 (Reply 5):
The lesson? Never underestimate a bicycle mechanic.

Orville and wilbur would certainly agree with that.
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widebodyphotog
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RE: Thrust VS. Horsepower?

Sat Dec 15, 2007 5:22 am

Hey BR715 this is an interesting topic and hopefully I can give you some good information.

First the difference between horsepower and thrust is that horsepower is work over time meaning that HP is comprised of three elements, Force, distance and time. One horsepower equals 33,000lbs moved one foot in one minute or 33,000lb-ft of work in one minute.

Thrust on the other hand is just a force in one direction. A jet engines thrust is just how hard it can push forward under a given set of conditions. Now what you need to relate the two, horsepower and thrust is to put thrust in the context of doing useful work and thus you can convert thrust to horsepower. Here is an example:

A given 747 needs a total of 45,000lbs of thrust to cruise at 564 Mph or 49,632 Fpm. The work done in one minute is a force of 45,000lbs over 49,632 feet in one minute or 2,233,440,000 lb-ft per minute. We know that one horsepower equals 33,000 lb-ft of work per minute so simple division 2,233,440,000/33,000 gives us an even 67,680 HP for the equivalent horsepower to move a 747 at cruise.

Typically an aero engineer calculates the power to move an airplane through the air in units of force for a given set of conditions. Using the method above will convert that force to work and HP. This will not however correlate to engine power required to turn a propeller to generate that thrust as has been said already. 2H4's nifty post tells you the how and why relative to propellers.

Hope that helps some

-widebodyphotog
If you know what's really going on then you'll know what to do
 
WingedMigrator
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RE: Thrust VS. Horsepower?

Sat Dec 15, 2007 6:22 am

Great explanation from you and 2H4. One minor nit:

Quoting Widebodyphotog (Reply 8):
One horsepower equals 33,000lbs moved one foot in one minute

33,000lbs lifted one foot in one minute.
 
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Starlionblue
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RE: Thrust VS. Horsepower?

Sat Dec 15, 2007 6:49 am



Quoting WingedMigrator (Reply 9):
33,000lbs lifted one foot in one minute.

In a standard gravity?
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Buzz
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RE: Thrust VS. Horsepower?

Sat Dec 15, 2007 1:45 pm

Hi BR715-A1-30, Buzz here. Those guys have given an excellent detailed answer. A shorter, rule of thumb would be that pounds of thrust times 60% is approximate horsepower ... well, at static thrust anyway - as though you were sitting at the end of a runway and hoping you had enough to get over the fence on the far end (grin). That's one reason a little Fouga Magister with a couple 1200 lb thrust engines shouldn't use a short runway like a DC-3 with a couple 1200 horsepower engines. (OK, so the wing is quite different... I'm thinking of the distance it takes to accelerate to flying speed)

Propellers are good at accelerating a wide swath of air at low speed. Little turbojet / turbofan engines aren't as good at accelerating from a standing start. Ever seen a loaded KC-135 take off? Sure... the engines are 10,000 lbs thrust each. But on a hot day it seemed that the curvature of the earth dropped out from under the wheels (instead of the airplane climbing away).

Who's Lockheed Connie are you thinking of? They are pretty airplanes. I used to be on a crew with some WW2 airplanes, we had a Lockheed PV-2D Harpoon. The props were too short to absorb all the horsepower the engines would produce, so it came with paddle blade props like the high altitude airplanes had. It's a patrol plane... down looking at ships (and a few other things). Anyway, I liked the way Lockheed designed systems back then: practical and fairly easy to build and repair.

So to estimate the takeoff thrust of the Connie, try 1.4 times the horsepower. It's sort of comparing apples and oranges, not very exact.

g'day
 
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jetmech
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RE: Thrust VS. Horsepower?

Sat Dec 15, 2007 5:25 pm



Quoting Widebodyphotog (Reply 8):
One horsepower equals 33,000lbs moved one foot in one minute or 33,000lb-ft of work in one minute.



Quoting WingedMigrator (Reply 9):
33,000lbs lifted one foot in one minute.

Wouldn't the best description be applying a force of 33,000 lbf over a distance of one foot in one minute?

Quoting Starlionblue (Reply 10):
In a standard gravity?

I think in general, that a value of around 9.8 m/s/s, or Earth's normal gravitational acceleration would be used.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
WingedMigrator
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RE: Thrust VS. Horsepower?

Sun Dec 16, 2007 5:13 am



Quoting JetMech (Reply 12):
Wouldn't the best description be applying a force of 33,000 lbf over a distance of one foot in one minute?

Yes. The confusion arises from pounds being loosely used as a unit of mass. Give me slugs any day  crazy 
 
flipdewaf
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RE: Thrust VS. Horsepower?

Mon Dec 17, 2007 5:44 pm



Quoting WingedMigrator (Reply 13):
Yes. The confusion arises from pounds being loosely used as a unit of mass. Give me slugs any day crazy

Newtons FTW!
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