1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Airplanes and Lift

  1. Nov 27, 2017 #1
    So I have heard this around the internet before but thought is was a question worth exploring if a jet plane is sitting on a treadmill and the treadmill moves backwards at the same rate the plane is moving forward will it take off? I think no because the plane would have to be moving forward for air to go over the wings to provide lift right? unfortunately there is a lot that disagree with me and was wondering if I could get a reply from some more qualified people. (sorry if there is ignorance in this post I still haven't started under grad physics yet)
     

    Attached Files:

  2. jcsd
  3. Nov 27, 2017 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    That is ill-defined.
    Only the speed of the aircraft relative to the air matters.
     
  4. Nov 27, 2017 #3
    basically if the airplane has zero velocity can it still achieve lift
     
  5. Nov 27, 2017 #4

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    To be meaningful, you have to state relative to what the speeds are measured.

    Probably because everyone understands such vague question differently.
     
  6. Nov 27, 2017 #5

    russ_watters

    User Avatar

    Staff: Mentor

    No, it can't.

    Note, this is a very easy question that is made difficult only by the fact that it is worded as a seeming self contradiction and also physically difficult to achieve:

    1. You said "...the plane is moving forward..." (without stating with respect to what) when you really meant the plane is stationary with respect to the ground.

    2. If the plane isn't moving forward with respect to the ground, then something unspecified is holding it in place.

    We have gotten this question a bunch of times, always with similar poor wording, which is what causes the arguments/confusion. Run a search and you'll find somee...
     
  7. Nov 27, 2017 #6

    lekh2003

    User Avatar
    Gold Member

    The plane does not generate lift from speed; it generates lift from the speed of the air it is entering. The fast air hitting it can lift the plane.

    If the plane is simply standstill relative to the air, then it won't move. This is the case in your example, the plane is moving forwards and the treadmill is moving backwards with the same speed. Relative to the moving ground on the treadmill, the plane is moving. BUT remember that to generate lift the plane needs to be moving relative to the air. So it will not lift upwards.
     
  8. Nov 27, 2017 #7

    lekh2003

    User Avatar
    Gold Member

    Not using the correct terminology is resulting in your confusion. The plane's velocity can be 30000 m/s relative to the sun. It could also be 300000000 m/s relative to the light around it. The only thing that matters here is the velocity relative to the air. That velocity is definitely NOT zero.
     
  9. Nov 27, 2017 #8
    but its not moving its essentially fixed to one point because of the backwards movement of the treadmill i just want to know if it will ever lift up off of the treadmill
     
  10. Nov 27, 2017 #9

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    What part of "No" are you not understanding.
     
  11. Nov 27, 2017 #10

    russ_watters

    User Avatar

    Staff: Mentor

    Then it doesn't takeoff! Simple!

    However:
    That is physically impossible. If the airplane's engines are firing, the treadmill alone can't stop the plane from moving with respect to the air.

    I suggest you draw yourself a diagram and label all the motions and forces. If the forces don't sum to zero, the plane has to move.
     
  12. Nov 27, 2017 #11

    jbriggs444

    User Avatar
    Science Advisor

    This partially clarifies your interpretation of the question. The airplane is stationary relative to the ground while a treadmill beneath it moves.

    Further questions:

    Is there any wind? A plane can take off at zero ground speed if there is sufficient wind.

    Are we to neglect the wind induced by the plane's own engine(s)? I suspect that a plane could take off in its own slip-stream if one were to tether it inside a big doughnut-shaped wind tube.

    What is causing the plane to move forward relative to the treadmill? One assumes that its engine is on and the prop spinning.

    What is restraining the plane from moving forward relative to the ground? One assumes that its prop is only spinning fast enough to counter the rolling resistance of the tires.
     
  13. Nov 27, 2017 #12

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Make up your mind.
     
  14. Nov 27, 2017 #13
    it has speed but is not moving from its point due to the movement of the treadmill counteracting its forward movement
     
  15. Nov 27, 2017 #14
    I have updated the post with a diagram
     
  16. Nov 27, 2017 #15

    russ_watters

    User Avatar

    Staff: Mentor

    How? What is the cause of the force in your diagram?
     
  17. Nov 27, 2017 #16
    sorry for any vagueness I added a diagram that can probably explain it better than me
     
  18. Nov 27, 2017 #17

    russ_watters

    User Avatar

    Staff: Mentor

    The diagram shows three forces. The thrust force is obvious. What is the cause of the other two?
     
  19. Nov 27, 2017 #18
    the conveyor turns in the opposite direction of the thrust force of the plane. The plane is sitting on a conveyor belt
     
  20. Nov 27, 2017 #19

    russ_watters

    User Avatar

    Staff: Mentor

    I know the conveyor moves and the wheels spin, but motion is not force. Indeed, in constat speed motion, forces must sum to zero. So what causes the forces? Consider that the spinning wheels don't stop a plane from taking off on a runway, so ordinarily there is essentially no force exchange between runway and wheels. Why does the conveyor change that?
     
  21. Nov 27, 2017 #20
    yeah thats where my question basically lies is if the plane is not moving there is not as much air flowing over the wings therefor less thrust is created so I was wondering if the plane could still take off even though it's wings are not gliding through the air creating that downward thrust
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Airplanes and Lift
  1. Airplane on a string (Replies: 2)

  2. Airplane Wings (Replies: 9)

  3. Paper Airplane (Replies: 2)

  4. Airplane wing . (Replies: 12)

Loading...