Converting HP to thrust with a jet engine.... A motor jet estimation?

[vonsteimel]

I'm looking to verify some a figure on the conversion of HP to Thrust with the use of a jet engine. Basically a motor-jet

I've been told 7lb of static-thrust roughly for every 1HP fed into the compressor. That sounds a bit high to me.

Obviously there is a lot to it but I just want ball-park figures. Making whatever assumptions need to be made, what is a rough approximation? Say i'm going to build a motor-jet and I want to use a 100hp motor. What can I roughly expect to get in return for Thrust?

 

[GregLocock]

You really need to ask a better question, but it depends fundamentally on the diameter of the prop.

Cheers

Greg Locock


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[vonsteimel]

It has nothing to do with a prop....? A motor jet, not a turbo prop.... I known there are a lot of variables, but for a ballpark approximation the ideal variables can be assumed or guessed or pulled out of your bum... Thats what ballpark aproximations are all about... To re-word the question; pick any turbo-jet engine you want, roughly how many Pounds of thrust are generated for every HP consumed by the compressor, at any specific speed/Altitude/pressure.etc.

 

[GregLocock]

Sorry, my crappy-technology recognition circuit got that one wrong. OK, so a motor-jet is an internal combustion engine driven compressor feeding air to a combustion chamber which then leads to an exhaust nozzle.

Your second question is much better than the first.

However a moments googling takes us to

http://www.angelfire.com/art/jetengine/

which has a whole bunch of numbers.



Cheers

Greg Locock


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[rb1957]

1) do you really mean "motor jet", in which case greg's link seems to tell you lots about (and yet another question answered by a quick google).

if you mean a typical turbo-jet, i'm not sure there is a relationship. consider jet engines have pwoered described as thrust, and propeller engines have their pwoer described by hp.

 

[drawoh]

vonsteimel,

Power is force times velocity. Horsepower does not convert directly into thrust, unless you keep speed constant.

I remember visiting a marine museum and looking at a 6[ ]foot diameter propeller from a fishing boat with a 17HP engine. On my uncle's 200HP boat, the propeller was something around 1[ ]foot diameter.

If the fishing boat goes 15mph, there is a little over 400lb thrust. This is believable to me.

--
JHG

 

[IRstuff]

Alternately, one can consider the specific fuel consumption, which is usually given in lb/lbf*hr, and using NASA's value of 42.8MJ/kg for fuel energy, we get 5.3hp/lbf. This is specifically for a TF34 high-bypass turbofan. However, if you have the specific fuel consumption for your engine, you can do the same calculation. I don't know whether the values are correct or even plausible. I just happened upon the calculation while looking for something else.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720026093_1972026093.pdf

TTFN
faq731-376

 

[vonsteimel]

rb1957, my question is directed toward a jet-engine. Whether the compressor of this jet engine is powered by a separate piston engine(motor-jet) or powered by an internal turbine(Turbo-jet) is irrelevant. In any event, a compressor is consuming power and the engine is putting out thrust. I'm interested in the ratio of compressor-power-consumption OVER total-thrust-output. Fuel is not a concern.

Mathematically, you cannot convert HP straight to Thrust... But that is not what I'm trying to do...
However, you CAN convert HP to thrust through thermodynamic/mechanical means which is what the question is about. When using a purely "jet" engine (regardless of how the compressor gets its power), what type of ratio can be achieved.

IRstuff this method will give the total power consumption of the engine but will not provide the ratio of how much of that power/fuel is used to turn the compressor, as some fuel will be used to spin the compressor and some will be to produce engine thrust.

I didn't think this would be so hard to understand? I've wore myself out on google and cannot find this anywhere.

 

[IRstuff]

I understand all that, but nonetheless, it puts a bound on that value. Moreover, one would expect that the bulk of the available power ought to go into thrust, since that's were you get the highest efficiency of conversion. Therefore, one could postulate that the compression consumption is on the order of 10% of the calculated value.

TTFN
faq731-376

 

[GregLocock]

So what's wrong with the numbers on the page i found for you?

Cheers

Greg Locock


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[IRstuff]

Using the numbers from Greg's cite and the specific fuel consumption for the TF34, the compressor power is about 12% of the total energy flow.

TTFN
faq731-376

 

[vonsteimel]

These numbers are way off from normal guidelines. Typically the compressors of turbo-jet engines consume about 80% of the power... But this is just a rough figure. I'm looking for a little more specific information than this. Some figures or calculations validating the 80% figure.

Or any documentation for existing motor-jet engines would be good.
Thanks,

VS

 

[DeltaWing]

You're asking for the ratio of thrust to the power required to run the compressor, right? The relevant equations for an ideal turbojet can be found on pages 279-281 of Mattingly's propulsion book. You need to do a cycle analysis for whatever engine design you're interested in. Because the turbine power and compressor power are matched, you can express the power in terms of either turbine or compressor parameters. It isn't a simple thermodynamic relationship.