Cowl induction for jet aircraft engines , is it possible?

Discussion in 'General Science & Technology' started by cosmictraveler, Jan 17, 2009.

Thread Status:
Not open for further replies.
  1. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    Messages:
    23,198
    I do not recall saying that. If I said that, I want to re-read the contex. What post number or date are you quoting from? *

    I have said that probably plane could not fly at even half the speed of sound is all the air for combustion was forced to turn thru 180 degrees prior to entering the engines. (Or words to that effect). I have also noted that once inside the engines, turning the air need not add to the airplane's drag (and noting: "Nor does a lady seated inside the plane using her fan to turn air 180 degrees add to the drag.")
    ---------------
    *Your "quote" is not from my first post -I just went back and re-read it. I see no need to change even one word of it.
     
    Last edited by a moderator: Feb 9, 2009
  2. Google AdSense Guest Advertisement



    to hide all adverts.
  3. mikenostic Stop pretending you're smart! Registered Senior Member

    Messages:
    4,624

    Whatever. I am already done saying what I had to say. If you're gunna disagree, well there is nothing I can do about that.
    We all know who was trying to start something.
     
  4. Google AdSense Guest Advertisement



    to hide all adverts.
  5. scooter835 Registered Member

    Messages:
    12
    Because.

    Please Register or Log in to view the hidden image!



    Simple version: a (turbine) jet engine works mostly by pulling itself forward, by vacuuming away the air right in front of it. If the air path straight into the engine is obstructed, then the engine loses a lot of efficiency right away. Jet engines don't produce thrust from reactive propulsion (the way a rocket would).

    Non-simple version: in a typical jet engine, the direction of thrust is mostly determined by the entry angle of air into the front of the engine, and so that's why you want that intake as unobstructed as possible. There is some smaller amount of thrust produced at the rear of the engine where the exhaust is expelled--but it is far less than the front. For a modern turbofan, 95% or more of the thrust may be generated by the front-most fan.

    ----

    The front-thrust principle is not an absolute rule: jet engines normally aren't designed to produce very much of their thrust from reactive propulsion (the way a rocket in space would).

    A jet engine can be designed to produce any ratio of its thrust from front-to-rear, and some are designed to produce a higher percentage of exhaust thrust, but the engine's efficiency rises as more of the thrust is produced by the front stage. ....A Harrier jet hovers using only the exhaust thrust--which certainly works--but uses a relatively huge amount of fuel to accomplish. In hover-mode, a Harrier is basically "wasting" the intake-thrust of the engine.

    I have a book around that shows a thrust-diagram of an engine, I'll post a scan if I can find it.
    ~
     
  6. Google AdSense Guest Advertisement



    to hide all adverts.
  7. scooter835 Registered Member

    Messages:
    12
    Well I looked and cannot find the dang thing.

    Please Register or Log in to view the hidden image!


    I know it was this book however:
    The Development of Jet and Turbine Aero Engines by Bill Gunston

    hXXp://wXw.amazon.com/Development-Jet-Turbine-Aero-Engines/dp/1852605863

    [I can't post links yet, so you'll have to figure it out on your own]

    Mine is the second edition, but there is a current fourth edition that I assume would have all the previous material.

    This book (and the accompanying one on aircraft piston engines) are good for anyone with a general interest in the subject. When I bought mine they cost $35 and they weren't very large books, but there was a lot of info and diagrams/photos in them.
    ~
     
  8. Uno Hoo Registered Senior Member

    Messages:
    383

    CHEEZ.

    EVERY combustion engine that acts as a prime mover behaves according to the basic physics Newton principle of action and reaction. There is no Newton Law Of Sucking.

    EVERY combustion engine that acts as a prime mover executes the Newton principle by engaging mass and throwing it back, and, as an equal and opposite reaction, enjoys being reactively moved forward.

    There is no Moving By Sucking in Newton Physics.

    This thread was interesting to some degree earlier, but has been permitted to devolve into faux(?) pseudoscience.

    Like the song says, I've been a long time leaving ( not really ), but I'll be a long time gone (really ).

    Hasta la vista, babies. Suck all you want to.
     
  9. thinking Banned Banned

    Messages:
    1,504
    is not the jet engine based on the air intake , which is large , compared to the exhaust exist ?

    it is

    to have a cowl intake would mean that generally you would have to restrict the air intake

    unless the cowl suction can make up for the the loss of normal jet engine air intake area or greater , then there is no advantage for a cowl air intake
     
  10. weed_eater_guy It ain't broke, don't fix it! Registered Senior Member

    Messages:
    1,516
    Well, if you wanted to get technical, yes, sucking CAN result in a reaction in that direction (and let's all be mature about that statement, lol). But I'm not going to get too mired in this thing anymore...

    Same to you pal! It was an absolute PLEASURE sir!

    Please Register or Log in to view the hidden image!

     
  11. mikenostic Stop pretending you're smart! Registered Senior Member

    Messages:
    4,624
    You promise? Don't tease us and get our hopes up.
     
  12. FP.DBZ Guest

    "Non-simple version: in a typical jet engine, the direction of thrust is mostly determined by the entry angle of air into the front of the engine, and so that's why you want that intake as unobstructed as possible. There is some smaller amount of thrust produced at the rear of the engine where the exhaust is expelled--but it is far less than the front. For a modern turbofan, 95% or more of the thrust may be generated by the front-most fan."

    Surely then by this logic "reverse thrust" wouldn't work as well as it does, and the 'being pushed forward' feeling I get when the plane lands is just my imagination? reverse thrust works by redirecting what comes out the back of the engine as can be seen on many youtube amateur aviation videos.

    i call bs on that above highlighted statement.
     
  13. scooter835 Registered Member

    Messages:
    12
    I hate when I say I've got it, and then can't find it......

    Okay---I did not find the book, but found the scans of the page on another computer of mine. Somebody else asked a similar question on some other forum ages ago, and so I had scanned the thing back then.

    Try this:
    hXXp://wXw.norcom2000.com/users/dcimper/assorted/inanities/general_wtf/jet.html

    (fix the link of course)

    .....
    The question of how thrust reversers work is one I wonder about myself.
    I have not seen any good explanation of it, considering the above diagram.
    ~
     
    Last edited: Mar 12, 2009
  14. scooter835 Registered Member

    Messages:
    12
    These two things are true--but a {typical} jet engine does not produce {much} thrust simply by throwing gas out the exhaust nozzle.

    Think of it as this: a jet engine is a rotating shaft with a bunch of wings on it. The wings at the front are the compressor, and pull air into the engine by principles of aerodynamic lift. The wings at the back are the turbines, and are oriented to spin the shaft.

    The gas pressures in the rear end of the engine can never exceed the maximum pressure capability that the front compressor can deliver--because if that happened, gas would begin to flow backwards through the engine.

    And if you've seen many diagrams of modern jet engines, you have noticed that they always have much more compressor stages than turbine stages.
    ~
     
  15. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    Messages:
    23,198
    That certainly sounds like it would be true. Perhaps it even is true for flow that is everywhere subsonic. I am not sure it is for supersonic flows.

    For example, one design for hypersonic air craft uses external combustion and NO compressors. The cross sections of the aircraft narrows about half way back and the fuel is released there, EXTERNALLY (but I think not into the supersonic stream). As it burns the pressure increase and when trying to expand forward it finds a "fatter" body of the air craft in its way. When trying to expand to the side, if runs into the supersonic air stream. Thus the combustion pressure makes it expand to the rear.

    The fuel was already going supersonically when released, say at Mach X, so with rearward expansion it is now going more slowly. This means it has less momentum wrt the distant air. Perhaps it is even at rest wrt the distant air. (As if it were a rocket exhaust from rocket flying with the exhaust velocity.)* WRT the air craft, the fuel exhaust has been thrown to the rear after combustion and that means it is thrusting the air craft body forward (only to overcome the friction of hypersonic flight, if the air craft body is not still accelerating)

    I am not sure of the details, but think that the oxygen for combustion comes for the expansion of the supersonic air stream slipping by the air craft body into the aft region where the cross section is reduced. It is very attractive idea as practically no moving parts to keep cool etc. – just the shock heating of the typical compressor blades might melt them in hypersonic flight. I think the fuel circulate first thru the nose to keep it cool. You may not find much about this in the open literature. It is all DOD work. I don’t know any facts.

    I also do not know anything about the gas dynamics inside a jet engine, but there sure are a lot of shock wave reflections visible in the exhaust stream of a modern fighter jet. Thus, I think it possible your statement, which on the surface seems true, may not always be true.
    ---------------
    *Leaving the exhaust with zero kinetic energy is the most efficient operation.
     
    Last edited by a moderator: Mar 12, 2009
  16. mikenostic Stop pretending you're smart! Registered Senior Member

    Messages:
    4,624
    It's the same principle as a recip engine with a turbocharger.

    I'm no physicist, but that just doesn't sound right. What I do know is that compressed gas, much like electricity, is going to follow the path of least resistance, or rather gas in an area of high pressure is going to always seek areas of lower pressure...ala Bernoulli's Principle.
    Now in theory, IF the gas pressure between the burner can and turbine blades do happen to achieve a higher pressure than in the compressor stage, it's not going to go backwards against the igniting gas from the burner can going the opposite way, nor will it go back through the compressor stage with fans spinning the opposite way.
    The quickest way for that ignited, pressurized gas to get out, is through the rear turbine blades/fans.

    Uhhhh, that's pretty much how a jet turbine engine operates.
    Now, turboprop and turboshaft turbine engines may work the way you describe, but jet engines operate on exhaust gas thrust, not a mechanical link to props or blades, one exception would be the high-bypass turbofan, where a couple of the compressor blades are way oversized and act as giant propellors.

    Please Register or Log in to view the hidden image!

     
  17. thinking Banned Banned

    Messages:
    1,504

    hmm.. but what of before lift ?

    actually no

    notice that the intake opening of a jet engine is larger than the exhaust openings

    for a reason , obviously

    hence my above point
     
    Last edited: Mar 14, 2009
  18. scooter835 Registered Member

    Messages:
    12
    There is nothing "before" lift.

    But not the reason you think--because if that were so, then the jet engine would not need compressor rotors at all.

    -------

    I found the book, put up a couple more diagrams from it on another page (also linked off the original page).

    hXXp://wXw.norcom2000.com/users/dcimper/assorted/inanities/general_wtf/jet/jet02/jet2.html

    In images 1 and 2 (both pressure diagrams) the highest pressures in the engine are reached right behind the compressor, and the lowest pressure is in the exhaust nozzle.


    [end]
     
Thread Status:
Not open for further replies.

Share This Page