Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Basic problem sorry for posting it.

  1. Nov 3, 2003 #1
    Hi,

    We have been discussing this following problem to the death at a different forum. I figured I will ask the professionally oriented what they have to say about it.

    The problem:

    The airplane (jet or the other one) is on the special surface, which can run backwards - opposite the direction of the tested airplane ready to take off. The speed of the moving surface is equal the speed of spinning wheels of the airplane at any time that is regulated persistently and efficiently by a special device.

    The problem (question): will the airplane be able to run on this surface and then take off?




    My take on it is it would, because the propeller is creating the forward motion not the wheel. But some say the wheel would spin so fast that it would create enough friction to counter the effect of the prop. I can't see that happening.

    Probabaly this is a base school level question here but please chew in.

    Thanks!
     
  2. jcsd
  3. Nov 3, 2003 #2

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I take it you're thinking of a way to make a plane take off without having to travel any linear distance down a runway? Kind of a zero ground-roll take-off?

    Obviously, it's not possible. Airplanes do not require groundspeed to take off, they require airspeed. The airspeed is what generates lift and allows the airplane to take flight. The airspeed is, generally, totally independent of the groundspeed.

    If the airplane is not moving with respect to the air, it will not generate lift, and will not be able to fly -- no matter how fast its wheels are spinning.

    - Warren
     
  4. Nov 3, 2003 #3
    The airplane is stationary at start, so is the surface and the wheel.

    Then we apply power.

    In case the plane will not move my problem is to understand what counters the effect of the jet engine or the propeller. Those move the air backwards even if the plane say would stay at the same point. Where does the energy of tha air hitting the propeller blades go? I think due to action/reaction the prop would make the plane move forward, while it makes the air move backward... while the surface would move also backward increasingly faster.

    But that is just me.
     
  5. Nov 3, 2003 #4

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Into heating the bearings of the wheels, of course.

    - Warren
     
  6. Nov 3, 2003 #5
    So that, and the surface friction would be significant enough to stop the airplane? The next question would be how fast is the surface moving backwards at the moment when the airplane is already at full t/o power.

    It has to be faster than the t/o speed or even the maximum cruise speed or else the opposing energy is just not sufficient IMO.
     
  7. Nov 3, 2003 #6

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Not really surface friction. You mean rolling resistance, which is a dissipative force resulting from the deformation of tires and wheels and bearings and so forth. The (static) surface friction only serves to make the wheels rotate, and is not dependent upon the angular velocity of the wheels. Futhermore, static friction never does any work for a wheel that is not slipping.
    This would depend quite strongly on both the airplane's thrust and its rolling resistance.
    Why? If the engine is producing x horespower of thrust, you just have to make sure the rolling resistance of the wheels is dissipating x horsepower also. Since the power lost to rolling resistance is variable (we're assuming a perfect moving surface that is capable of any speed), this equality will always happen at some velocity. However, it is entirely possible that you'll simply burn the wheels up before you reach the airplane's maximum thrust.

    - Warren
     
  8. Nov 3, 2003 #7

    FZ+

    User Avatar

    If the wheels are in contact with the ground, this isn't possible. Any attempt to increase the speed of the moving surface will also increase the speed of rotation of the wheels by the same amount.

    Try rubbing a roller against a piece of wood, and see what happens. Yep, the roller spins as well.
     
  9. Nov 3, 2003 #8

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    FZ+, what I think he means is simply that the moving surface can

    a) move at any arbitrary velocity
    b) be controlled by some control system to make the airplane remain stationary wrt some non-moving point (i.e. an operator standing off to the side of the moving surface).

    - Warren
     
  10. Nov 3, 2003 #9
    chroot,

    What I meant by the speed of the wheel and the surface:

    When the airplane takes of at under normal conditions (no backward moving surface)... the resistance created by the wheel is not significant. The airplane will take off after the rollout. Say that speed when the airplane takes of is Vx. At Vx the friction and resistance generated by the wheels is a value. This value will not stop the plane from moving and accelerating to its t/o speed or else we would not have airplanes.

    In our example however this force according to you guys (and othwer at the other forum as well) will eventually prohibit the plane from accelerating. In order this be true, the speed of the surface can't be Vx because at Vx we already decided that the wheels don't create enough friction. It must be more than Vx.

    Unless I am missing something.
     
  11. Nov 3, 2003 #10
    upss missed this post. Yes absolutely a, and b, I think is given by the problem although the problem was not created by me.
     
    Last edited: Nov 3, 2003
  12. Nov 3, 2003 #11
    BTW what if we would change our wheels to scates and the surface would be ICE. In theory it is the same. But in reality I have hard time imagining it.
     
  13. Nov 3, 2003 #12

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I agree.

    - Warren
     
  14. Nov 3, 2003 #13

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If the surface were ice (i.e. it were frictionless), then the surface could move at any velocity and impart absolutely no force on the airplane. The airplane will take off as it normally would. Actually, its ground roll would be a little shorter than normal, since it wouldn't have to fight the rolling resistance of the tires.

    - Warren
     
  15. Nov 3, 2003 #14
    So the answer is if there is ANY friction between the wheel and the surface it will not take off. Even the slightest. I think even ICE and water has some friction. But if there is zero friction it won't.

    I still would like to see a few more persons to take their position.
     
  16. Nov 3, 2003 #15

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Right -- assuming, of course, that your moving surface is perfect, and can always go fast enough (ludicrous speed!) to counter the thrust with rolling resistance. In the real world, it would be next to impossible to engineer a surface that could ever move this fast, or accelerate rapidly enough. In addition, the wheels on a real plane would pretty quickly succumb to balance problems and self-destruct.
    It does.
    Yup. In the limit of zero friction, there is no more rolling resistance, and nothing the moving surface could do would be able to stop the airplane. The moving surface exerts no force in the limit of zero friction.
    There's really no position to take -- it either is or it ain't.

    - Warren
     
  17. Nov 3, 2003 #16
    Thanks!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?