1. hi,

I found this image for how a harrier jet engine works:

I want to know why does the engine spit out cold air on the front thrusters, but hot air (+combusted fuel) on the rear thrusters? Why not just hot air on both?

Does anyone have any good resources about how this engine works? I couldn't really find any good ones - I realise, being a military aircraft, that there will be alot of information withheld about the precise engineering, but is there a good website showing the basic physics of it?

Thanks,
bit4bit

2.

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4. Thanks I've found some quite good resources now anyway, and the answer to my question too.

5. My dad worked for Macdonald Douglas on the AV-8B..the american version of the Harrier. One day I dropped him off at work and one was flying around....thrusters down. It was the loudest thing I ever heard. I had a 1000 watt car stereo...and when it hovered over the top of my car, It COMPLETELY drowned out the music...it was amazing.

6. I know certain members of my family whose rear ends could do the same lol

7. Originally Posted by leohopkins
I know certain members of my family whose rear ends could do the same lol
Heheh me too!!

I have a couple more general questions about jet turbine engines:

1.) The harrier engine is supposed to have 3 low pressure (LP) compressor stages and 8 high pressure (HP) compressor stages. I am no expert but this is my understanding:

*Air entering the front of the engine (through the intake fan) is at normal atmospheric pressure.
*The LP fan stage turns this air into a lower pressure.
*Some of this LP air is bypassed to the front thrusters, and the rest goes through the engine to the HP stage.
*HP stage turns this LP air into HP air (higher than original atmospheric pressure) to be combusted.

I have drawn a diagram to illustrate my point:

It seems stupid to me to convert the (normal atmospheric pressure) intake air into a LP at all, because it must be converted to a HP anyway in order to be combusted. The only reason I can think of to have a LP stage at all, is because the air expelled through the forward thrusters needs to be LP. But why is this? Why would LP air be more effective?
Otherwise, why is my understanding wrong?

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2.) Also referring to the diagram again, is my configuration of the fans correct?
My reasoning:

A fan will either push from left to right, or push from right to left. I have shown the LP fans pushing against the nett flow of air through the engine, and the HP fans pushing with the nett flow, to achieve the different pressures in each respective cavity of the engine.
Have I got this the right way round or is it the other way?
Is there actually more to these fans than simply pushing from 'right to left' or 'left to right'?

Much respect to anyone who can answer my questions.
Thanks again,
bit4bit

8. *Air entering the front of the engine (through the intake fan) is at normal atmospheric pressure.
Yes, mainly a function of altitude.

*The LP fan stage turns this air into a lower pressure.
No. Be careful with acronyms, they usually don't provide an explanation. To be complete, it should actually be LPC, which stands for low-pressure compressor. It's a compressor, hence it *increases* air pressure. The reason why it's called "low" pressure compressor is because in this (front) part of the engine the pressure is still relatively low (though much higher than atmospheric) as compared to the HPC = high-pressure compressor. In most jet engines, the whole compressor = LPC+HPC is separated into these two parts so they can run at different speeds for higher efficiency and operability.

*Some of this LP air is bypassed to the front thrusters, and the rest goes through the engine to the HP stage.
Makes sense if the LPC provides sufficient pressure.

It seems stupid to me to convert the (normal atmospheric pressure) intake air into a LP at all, because it must be converted to a HP anyway in order to be combusted.
Your idea of an "LP" wouldn't work. If LP really stood for lower-than-atmospheric pressure, air would be sucked into the engine, not blown out! The front thruster would then be a front sucker! That's not the case, as explained above.

I am not commenting on your diagram because I can't access it. In the colored picture of your original post, all air flow is from left to right (and radially through the thrusters).

9. Lol yes I see your point about having a Lower-than-atmospheric pressure in the engine, I can't believe I didn't realize that before. Thanks.

10. So the LPC will keep that cavity of the engine (between the LPC and the HPC) at a constant high pressure, so that there is a constant thrust out of the front thrusters of the engine while it is active? This thrust at any given moment would be calculated by:

F=ma

Where,
F = thrust (N)
m = mass of air being expelled from the thruster (kg)
a = average acceleration of this mass (ms^-2)

What I want to know is how would I calculate the mass of air, and its acceleration, as it travels from the higher to lower pressure? (Given P1, internal pressure of the cavity, and P2, atmospheric pressure outside)
-Or even, a way to calculate the Force directly?

I realize that the volume and shape of the cavity and the ducts will also come into play, but what is the simple model/equation for such a situation?

The strange thing about this scenario is that both P1 and P2 will be constant, such that there will be a mass transferred from one area to the other, but P1 and P2 will not change their values (reach an equilibrium) meaning this mass transfer itself is constant and so there will be a constant force too. (of course in practice this is only approximate, but I'll worry about that later. For now I'm looking for the theory)

Thanks

11. Unfortunately, there is no simple formula for the thrust, just knowing inlet and exit pressures. To get an accurate result, the flow equations (Navier-Stokes equations) are solved. Those are a system of nonlinear differential equations (in other words, this is no hand calculation!). Even to get an order of magnitude approximation, you should at least know the mass flow (air flow in kg/s), the inlet and exit flow directions, and the average velocity at inlet and exit. Then you can get the force from a momentum balance (Newton's law applied on a fluid control volume). If you are completely new to this I suggest to look at a fluid dynamics textbook (introductory level will suffice for now).

12. can you recommend some reading material?
I already know the basic calculus, as far as the maths is concerned.

13. On the top of my head I can only think of a textbook I once used for a dynamics class:

"Vector Mechanics for Engineers: Dynamics", F.P.Beer et al., McGraw-Hill

It has a chapter (or sub-chapter) on fluid mechanics, but I wouldn't buy it just for that. Maybe you can take a look at it in your science book store or library.

For a more specialized and detailed treatise check out this (updated) classic:

"Gas Turbine Theory", H. Saravanamuttoo et al., Prentice Hall

Again, I wouldn't buy it (very pricey!) just for the limited information you need. See if you can find it in a local university library.

14. Thanks I'll look them up in my local library then - I dont want to buy anything expensive, and their are no e-books out there either.

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