# Turbo jet thrust question

1. Feb 20, 2008

### monkeyc

Can anyone tell me some formulae that i can use for finding the thrust in a remote control turbojet engine if the area of the intake is 196.35cm square and the exhaust is 78.5cm square and the pressure increases by 31 times?

2. Feb 20, 2008

### monkeyc

This is just an add on, if i was to make an A/C generator and attach it to the turbine shaft, is it possible that i can run the whole thing without combustion off the gen? I also was wondering if i could use ram air to increase pressure in the middle to replace the combustion chamber and also cool the electrical system to reduce resistance. Could any of this work???

3. Feb 20, 2008

### Danger

Welcome to PF, Monkey.
I can't help you with this, but the main problem that I see is that your question is not worded in such a way that anyone could properly respond. We seem to have a serious language gap. Can you please back it up and restate the situation one step at a time so that we might understand what you need?

4. Feb 20, 2008

### monkeyc

hey thanks, umm, im am designing an R/C plane with a turbofan engine, however i was hoping to make the engine completely electronic so that i don't waste all my money on fuel. The problem is that i dont know the formulae to find out if it can produce enough thrust to move.

5. Feb 20, 2008

### FredGarvin

Unfortunately, the information you have is not sufficient to calculate the thrust. The "easiest" way is to know the mass flow rate of the air and the inlet and exit velocities. If you can work those out, the basic physics is the change in momentum of the air stream which is (assuming no bleeds and that the mass flow of the fuel is negligible compared to the airflow):

$$T = \dot{m} \left[u_e - u_i \right]$$

where

$$T =$$ thrust
$$\dot{m} =$$ mass flow rate of air
$$u_e =$$ exit velocity of the airstream
$$u_i =$$ inlet velocity or forward velocity of the aircraft (static condition = 0)

If you can't get a handle on those figures, then it gets harrier because then we'd have to basically walk from the front of the engine to the end doing the cycle calculations to arrive at the final answer. It's not horrible, but that just means that there are a lot more variables that you probably will not know so you'll have to guess.

Last edited: Feb 20, 2008
6. Feb 20, 2008

### FredGarvin

Yes. If you put it up in the very front of the engine where the temperatures are low. However, you will have a very fast shaft speed which will mean you will have to do some very precise shaft dynamics work to make sure you keep everything properly balanced and don't throw in anything like unexpected shaft bending modes.

Ram air can not replace the combustion chamber. Ram air, in a ramjet, replaces the compressor section of a turbojet, but there is still a burner. I am not certain on what you are getting at for doing that. If you can explain what you're thinking that may help here.

Cooling the electrical system to reduce resistance is doable, but is it really worth the headaches? I doubt the return on the penalty for pulling air out of the engine cycle would be worth the gains in lowered resistance.

7. Feb 20, 2008

### Danger

Spoken like a true pilot. :rofl:
Did you do that on purpose, or did you mean to type 'hairier'?

8. Feb 20, 2008

### RonL

Sound a little like one of my wild Arse ideas involving two or more electric motors, counter rotation of the moving parts, air flow diverted thru a turbine, and reduced in thermal value, exausted out the back in a very cold, hi pressure condition.

I'm trying to piece together a small wind tunnel, for some experimenting.
Hope to see some comments to the OP, and see if were in tune.

________

9. Feb 20, 2008

### FredGarvin

Whoops. I did do that, didn't I. I am a horrible speller at times. It gets worse with age.

10. Feb 20, 2008

### FredGarvin

How can you get high pressure after going through a turbine and expect a temperature drop? I'd say you were on a good track if you said send high pressure air through a turbine and come out expanded and thus much lower temperature. That's exactly how a air cycle machines work on major aircraft. I remember testing a unit used on an F-16 that kicked serious butt. It would freeze up if you let it.

11. Feb 20, 2008

### RonL

My thoughts are along the design of a pair of Tesla units, counter rotating, and having a high speed ducted fan system pulling a low pressure (vacuum?) at the central exaust of the Tesla units, and excelerating it out the discharge as a cold thrust.
The electric motors act as both motor/generator and are aided by the turbine units, one for the field group, and one for the armature group. Fast cycle times from motor to generator keep the efficiency at a high level.
The faster it flys the more energy transformed thru the turbines.

I might not have said it right, but it all boils down to how much thermal energy can be extracted from a high volume of air moving thru the system. (A lot of leverage both electric, and thermal) did i just say all that

<<Looking for a place to hide>>

12. Feb 20, 2008

### Danger

As typos go, that one was brilliantly appropriate.

13. Feb 20, 2008

### monkeyc

thank you heaps Fred.