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Homebuilt axail turbojet

  1. Jun 21, 2007 #1
    hi,
    ive started drawing up plans for a small axail turbojet and i was wondering(for financial reasons) what the negative effects vs positive effects would be of having a straight cylindrical rod-like shaft rather than the usual shaft shape which is somewhat representative of a series of cones?...

    and would it run very effeciently? its probably going to end up on a gocart or nothing at all so it doesnt need to have much thrust, just so long as it works.

    thanks.
     
  2. jcsd
  3. Jun 21, 2007 #2

    FredGarvin

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    A series of cones? You've lost me on that part.

    By "axial" I am assuming that you mean an axial compressor and turbine.

    To be honest, the only advantage I see to this is the ease in manufacture. There are a ton of down sides to doing this.

    Down sides:
    - Difficult to assemble: With all of the components having the same ID, many components require a press fit on the shaft due to operating loads. It is very difficult to press on a component and have to move it over a long length of shaft. It will mar the shaft.

    - Impossible to balance: You leave no stock for dynamic balancing. You need to adjust the mass distribution around the shaft at various planes to ensure a vibration free operation.

    - You limit the size of bearings you can use by setting one diameter.

    - Very difficult to stretch: In most applications, again due to dynamic loads, you need to stretch the shaft to ensure tight clamp groups. The shaft stretch ensures that groups of components on the shaft stay in contact during operations. With a constant diameter shaft, there is nothing to "force" the shaft to stretch in the particular locations you want it to. You would also have a very hard time ensuring that the stretch is equivalent across the entire length.

    - Stress distributions: There are different stresses induced in a shaft at different times. You may be lacking in cross section for certain areas.
     
  4. Jun 21, 2007 #3
    series of cones is just to describe the appearance of the shaft, i guess its pretty vague but i couldnt think of any other way to describe it.
    i was actually hoping i could thread the shaft where the blades will be and secure them with knuts and washers but then comes vibrations and warping if there is even a slight displacement of the cg of the shaft cross section not to mention how aerodynamically inefficient this would make the engine.

    i guess im just gonna need to build it the usual way then.... oh well, too bad for my wallet....
     
  5. Jun 21, 2007 #4

    FredGarvin

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    It wouldn't necessarily make the engine inefficient just by using that set up. The use of a nut and threaded components on high speed rotating hardware means that you have to have very tight tolerances on the threads themselves as well as on the clamping nuts. The parallelism and flatness is also very much controlled because if the pitch diameter or one of the mating faces is off by just a bit, it will boss around whatever it is clamping and then you will have vibration and run issues.

    When you get into things operating at these speeds, it doesn't take more than a thousandths of an inch discrepancy to cause you issues.

    Do you know what the operating speed range of the engine will be (standard day)?
     
  6. Jun 21, 2007 #5
    as of now i have no idea what my rpm will be as this engine is still only in the early design stages and i havent done any testing on prototypes or components.
    perhaps i should scan what ive drawn up so far and post it here?
     
  7. Jun 21, 2007 #6

    FredGarvin

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    Sure you can post it.
     
  8. Jun 21, 2007 #7
    ok... heres the first general diagram, i would show all of them, i got diagrams for each part but with my laggy com and a scanner that scans the pictures into huge 87mb files(changed the format to upload) it took me an hour and a half to get this onto my com.
    this is just a general overview.
     

    Attached Files:

  9. Jun 21, 2007 #8

    Danger

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    Your explanation was still a bit vague, TC. I assumed from you first post that you wanted to make all of your turbine and compressor wheels the same diameter, rather than tapering the airflow. If that's correct, then you will be losing efficiency.
     
  10. Jun 21, 2007 #9
    ah, yes! it was a bit vague but you understood it perfectly! i do understand there will be a great loss in efficiency but i suppose what im asking is will it still provide a sufficient amount of thrust to get a small vehicle such as a gokart moving at a decent speed?
    i was also thinking of possibly putting it on a model aircraft with about a 2m wingspan and maybe a gross weight of 4 to 10 kg anychance it would provide enough thrust for that?

    my main objective is just to build a turbojet engine which works using only parts i designed myself.(efficiency is not such a big deal here)

    secondary objective is to use the engine on a vehicle(now efficiency maters a bit more)

    the problem i have with a tapered shaft is that to have it made and have the blades fitted for it will cost way more than i can affort...
     
  11. Jun 21, 2007 #10

    FredGarvin

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    There is no way to answer any of these questions. If you are designing these from scratch, I challenge you to even get this thing made let alone get it running.

    I got from your original post that you wanted to keep the shaft a constant diameter. Now it sounds like you are saying that you want the rotor disks to be equal in OD? Which one is it?

    If you are simply talking about keeping the OD of the rotors equal that has no effect on anything. What you would have to do is to taper the inner flowpath to make sure the flow area decreases as you progress through the stages of compression.


    Welcome to the world of jet propulsion. Nothing is ever even close to cheap.
     
    Last edited: Jun 21, 2007
  12. Jun 21, 2007 #11
    here, this diagram should explain what i mean better.

    i will test all those things out once i build it, i just want to be sure before i have the parts made that the loss in efficiency wont cause the engine to stop or not start at all. If the loss in efficiency will cause these problems then i will go back to the tapered shaft
     

    Attached Files:

  13. Jun 21, 2007 #12

    Danger

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    I guess that I didn't understand after all. I thought that you meant...
    [​IMG]
     
  14. Jun 21, 2007 #13
    ok, i guess you did not get the exact design but you still got the concept, both your diagrams and mine demonstrate the same compression effects even though obtained by tapering completely different parts... but yes, your first diagram is exactly what i was thinking of making
     
    Last edited: Jun 21, 2007
  15. Jun 27, 2007 #14
    I think the conical sections help compress the inflowing air and create the expansion chamber for fuel mixture and then compress the mixture for ignition and exhaust. So do you think you are to have anything missing with your altered design?
     
  16. Jun 27, 2007 #15
    yes thats what they do.... well i just want to get a turbojet that spins and gets the air in the front and out the back, not looking for much power or effeciency and yes, altho that would be better, it really costs too much for my budjet.
     
  17. Jun 28, 2007 #16

    Astronuc

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  18. Jun 28, 2007 #17

    FredGarvin

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    Whoever did that Wiki article did a nice job. It's a bit tedious, but they got a lot of information down there.
     
  19. Jun 28, 2007 #18
    Yes the volume has to go down either the shaft has to diverge or the casing has to converge.
    Why not use a old turbocharger from a car dumpsite. That way you have the compressor and turbine on a single shaft with nice bearings. ussually small turbine engines don't use axial compresors (APU's Turboshafts) because the need for a very high compression ratio isn't there. Also you need some flame holding capability (that is more my field) the way you designed your combustor now will not hold the flame!! You need either a dump plane to create recirculation or seperate v-shape flame holders.
    Have you thought about your igniters yet?
     
  20. Jul 7, 2007 #19
    my reason for not using a turbocharger is because i wanted to have every part drawn out and put together on my own, more satisfaction that way... also, i want to keep the engine as light as possible and as straight as possible incase i decide to put it on a rc plane. a turbocharger turbojet is way too heavy and it has a very bulky exterior combustion chamber. i suppose i really dont have much of a choise but to taper either the shaft or engine case. any idea which would be cheaper?
    hey, while on the topic of the turbo charger engine(incase i need one for a future project) what is the maximum possible thrust i could get out of one of those things?
     
  21. Jul 8, 2007 #20

    FredGarvin

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    If I may ask, how, exactly, do you plan on making this from scratch?
     
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