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To push or pull an airplane

  1. Nov 17, 2015 #1
    Hi, I'll start by saying I'm new here. I'm in my first year at college and I want to be an inventor.
    My question is this - if there was an airplane propeller that could pull on air instead of pushing against it to create thrust, would it be more effective at 1. creating thrust and 2. avoiding turbulence?
    This question is mostly theoretical right now. I'm just looking for some thoughtful input.
     
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  3. Nov 17, 2015 #2

    Nugatory

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    How do you pull on air? You move a gas by creating a pressure difference, and then the pressure difference pushes the gas around. Even a vacuum cleaner, which appears to pull air in through its nozzle, is actually pushing air out the other end to create a pressure difference that pushes air unto the nozzle.
     
  4. Nov 17, 2015 #3

    russ_watters

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    Since a propeller is just a spinning wing, it is both pulling and pushing on the air.

    It isn't clear to me what you are actually after here, but a lot of work has already been done to maximize propeller efficiency...
     
  5. Nov 17, 2015 #4

    DrClaude

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  6. Nov 17, 2015 #5

    A.T.

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    This is just confusing semantics. The forces between the blades and the air molecules are always repulsive, so there is no pulling.
     
  7. Nov 17, 2015 #6

    russ_watters

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    This whole thread is probably just an issue of semantics, but I disagree that my formulation would be confusing to most people. Most people think vacuums suck, and there is nothing wrong with that for most purposes. I'll wait for a clarification from the OP on what exactly he means to go further though, since it's his question and the framing is up to him.
     
  8. Nov 17, 2015 #7

    sophiecentaur

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    I can only think that pusher propulsion for aircraft is not used is that the air flow onto the front of the airscrew is less disturbed when the first thing it sees is the leading edge of the screw. With a pusher, the air has to go past the engine and wing before it gets to the screw, which (on the principle that you can't suck air) would mean that the air can't get into the propellor as well as when it's at the front.
    What happens to the air that's actually being pushed out of the back of the screw is easier to control and to be optimised.
     
  9. Nov 17, 2015 #8

    jbriggs444

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    Lots of discussion on the web about this. One such reference is http://djaerotech.com/dj_askjd/dj_questions/pushtractor.html [Broken]. Good reading. Of the multiple points made in that article, one extract is:

    "
    A prop is more than a "box that deflects air". It is a set of rotating wings that fly through a very complex, helical, non-symmetrical flow field. Their efficiency depends a great deal on just how non axisymmetrical that flowfield is. A pusher prop has to fly through all the disturbed flow coming off of the airframe in front of it, and as a result tends to have significantly less efficiency (usually at least a couple percent less, typically quite a bit worse than that, and in some cases more than 15% less efficient) than an equivalent tractor installation. Yes, there can be some gains in airframe efficiency from not immersing as much of the airframe in the higher speed flow behind the prop, but it is usually a far smaller gain than the losses due to the airframe's detrimental effects on the prop. Only a small portion of the total airframe drag tends to be influenced by the flow of the prop, while all of the thrust from the prop can be influenced by the flow coming off the airframe. Attempts to help the airframe by putting the prop in a bad situation almost inevitably end up being "penny wise and pound foolish."
    "
     
    Last edited by a moderator: May 7, 2017
  10. Nov 17, 2015 #9

    A.T.

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    Although in principle it makes sense to place the engines in the air accelerated forwards by the airframe, to recover some of the KE from it.
    (around 6:20):



    But not with propellers much bigger than the boundary layer of the airframe.
     
    Last edited by a moderator: May 7, 2017
  11. Nov 17, 2015 #10

    sophiecentaur

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    Could you explain that a bit? It is counter-intuitive to me. (But I don't trust intuition so I would easily be swayed.)
     
  12. Nov 17, 2015 #11

    jbriggs444

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    Seems straightforward enough. All other things being equal, if given a choice of pushing on an airstream that is at rest or on one that is moving in the same direction as the aircraft, you are better off pushing on the one that is moving the same way the aircraft is going. The airframe creates such a stream in its wake, so you are, naively at least, well served to use it.

    How well that theory holds up in practice is not immediately obvious.
     
  13. Nov 17, 2015 #12

    SteamKing

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    One of the largest aircraft ever built in series production used the pusher prop layout. I'm talking the 'Aluminum Overcast', the B-36 bomber:

    b36cutaway1.jpg
    This plane featured an unusually thick wing, so thick in fact that a crawl space in the wing allowed the crew to crawl out to the reciprocating engines and service them during flight. This unusual propulsion arrangement was chosen because if the propellers were fitted in the more common tractor configuration, it would have upset the airflow over the wing too much. Early models of this aircraft flew with only the 6 propeller arrangement; later models had four jet engines fitted to provide additional speed over the target and to help get this beast off the ground. When everything got going, the B-36 had 'six turning and four burning' when it started its takeoff roll.
     
  14. Nov 17, 2015 #13
    Wow, great input! Lots of great resources. The question I had originally was more theoretical though. I want to create an area of low pressure behind or at the back of a tube which would pull air into and through it. This is what I meant by pulling instead of pushing with propellers. I have an idea of how to do this, but I want to build a proof of concept before I post it and sound foolish. Do you need something to push the air around to get thrust, or could an extremely powerful vacuum pull you forwards?
     
  15. Nov 17, 2015 #14

    jbriggs444

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    Google for Feynman reverse sprinkler.
     
  16. Nov 17, 2015 #15

    CWatters

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    Ok imagine you have two options...

    1) You can create a low pressure region in front of the plane and atmospheric pressure behind.
    2) You can create a high pressure region behind the plane and atmospheric pressure in front.

    In case 1 the maximum pressure difference front to back is limited to atmospheric pressure (eg zero in front and atmospheric behind).

    In case 2 the maximum pressure difference front to back isn't limited in the same way. You could have 2,10, or 100 atmospheres behind and one in front if you can solve the engineering issues.

    Aside: Some early steam engines relied on steam condensing to form a partial vacuum. They used atmospheric pressure to push the piston. They had same limitation as above and were superseded by engines that used high pressure steam.
     
  17. Nov 17, 2015 #16

    sophiecentaur

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    I did consider that but the air needs to be accelerated more, in order to give it momentum backwards (to obtain the forwards reaction force.
     
  18. Nov 17, 2015 #17

    jbriggs444

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    It needs to be accelerated the same amount. Force is given by mass flow rate times delta v in either case.

    But the energy requirement is lowered when the inlet flow has a lower rearward velocity. [Hence sails]

    Edit: It appears that this correctly echoes A.T.'s thoughts on the matter.
     
  19. Nov 17, 2015 #18

    A.T.

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    For the same thrust, you need the same change of momentum, not the same final momentum. But the work done on the air for the same thrust will be less, it the air moves forward.

    Alternatively consider the frame of the plane, where the airspeed the engine operates in is reduced, allowing it to generate the same thrust with less power.
     
  20. Nov 17, 2015 #19

    russ_watters

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    Fans and propellers generate airflow by creating a pressure rise. If you duct propeller with the propeller in the front, the duct will be positively pressurized and if you put the propeller at the back, the duct will be negatively pressurized/pulled to a vacuum (gauge pressure of course).
     
  21. Nov 17, 2015 #20

    Nugatory

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    There's only so powerful that a vacuum can be - about fourteen pounds per square inch at sea level is as good as it gets.

    However, you're asking the wrong question when you ask whether a vacuum could pull you forward without pushing. It can, but you have to ask where the vacuum comes from... and it comes from pushing air out of the space begin evacuated. So if you're getting thrust, then one way or another you've been pushing air.
     
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