Aircraft carriers - ramps - makes less than zero sense to me

Discussion in 'Physics & Math' started by Jonathan Ainsley Bain, Apr 11, 2022.

  1. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    Well, and airspeed.

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    Which is why carriers still turn into the wind for takeoff, or at least manoeuvre so as to give their pilots the greatest advantage they can from prevailing winds.
     
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  3. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

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    I would have thought a belt going same direction would act like a weak catapult adding slightly to aircraft forward speed

    Belt going reverse to aircraft direction of travel subtracts from forward speed

    But wrong wrong so so wrongefid



    Time for me to go back to sleep

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    at 2:30AM

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  5. billvon Valued Senior Member

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    Right. What I meant is that nothing the ground is doing matters; it's only what the aircraft is doing relative to the air. (Which is the problem with both the belt question and the downwind-turn question.)
     
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  7. DaveC426913 Valued Senior Member

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    And so do airports. In a manner of speaking.

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  8. DaveC426913 Valued Senior Member

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    If a given aircraft has sufficient thrust/lift to take off when it is not on a conveyor belt, then it can take off when it is on a conveyor belt.
     
  9. James R Just this guy, you know? Staff Member

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    Because the thrust relies on interaction between the plane and the air, and the air is stationary relative to the ground, regardless of how fast the conveyor belt is moving?
     
  10. DaveC426913 Valued Senior Member

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    Yes. But I'll clarify for others.

    Sarkus suggested airspeed is necessary for the plane to get off the ground.
    Sure. That is always true - with or without a conveyor belt.

    IOW, a plane on a conveyor belt has no more trouble getting up enough airspeed to lift off than a plane that is not on a conveyor.

    Because what the wheels are doing is totally and utterly irrelevant to whether the plane can take off. The plane moves because the propeller provides the thrust, acting on the air.
    The wheels are passive and free-spinning and no matter what they do, they do not interfere with the plane's forward acceleration at all.
     
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  11. phyti Registered Senior Member

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    627
    In the example on another forum, the conveyor had a sensor-feedback system that matched the conveyor speed opposite to the wheel speed of the aircraft. That results in zero ground speed and air speed. The plane does not take off.
     
  12. Yazata Valued Senior Member

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    It's all about airflow over the wings.

    1. So if the plane was sitting on the conveyor belt, and the belt was carrying the plane in the direction of takeoff at takeoff velocity, the answer would seem to be 'yes'. It wouldn't even need engines to get off the ground. The conveyor belt would be the equivalent of a catapault. (It would need engines to keep flying.) Of course air resistance would exert force on the body of the plane and slow the plane slower than the belt. The plane would have to be fixed to the belt and released for takeoff. Again, basically the catapault (a very narrow belt in the shape of a cable).

    2. If the plane remained in one spot and the belt just moved under its wheels in the opposite direction to takeoff direction , all that you would accomplish would be spinning the wheels. You wouldn't be creating airflow over the wings.

    3. But if the situation is the reverse of #1 and the conveyor belt was carrying the plane backwards at takeoff velocity, it would probably come off the belt uncontrollably. (Planes aren't designed to fly backwards.) If it waited until it was moving rapidly backwards before it started its engines, it would have more difficulty taking off, since its engine thrust would just cancel out the velocity imparted by the belt and you would have effectively doubled its necessary takeoff velocity.

    4. The Mythbusters situation. The plane starts its engines at rest at the same time the belt starts moving. The engine thrust moves the mass of the plane forward in the direction of takeoff while the belt just spins the wheels and imparts little or no force to the body of the plane. Dave's situation in post #27.
     
    Last edited: Apr 14, 2022
  13. Yazata Valued Senior Member

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    I don't think that would matter very much. The thing is, the engine imparts force to the body of the plane. The wheels just spin. So even if the wheels are spinning, the engine will still pull/push the plane forward in the direction of takeoff. The wheels would just have to spin at twice the usual rpms in the forward direction to make that possible (which may or may not present engineering problems).

    Of course things would be entirely different if the brakes were locked or there were chocks under the wheels. Then you would be pulling the mass of the plane backwards and resisting its engine driven motion forward.
     
  14. DaveC426913 Valued Senior Member

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    Neither of those are the scenario.

    I shouldn't have offered the riddle without explaining it. I realize that has created quite a bit of confusion. Apologies to all.

    1. Plane is sitting on a conveyor belt.
    2. Plane powers up and ostensibly starts to move.
    3. Conveyor belt is controlled so that it always moves backward at the exact speed the wheels are moving forward. (So: If this were a car, it could be showing 100mph on the speedometer. But the conveyor belt is now moving backwards at 100mph. Thus, the car remains stationary wrt the ground.)
    The riddle then, is: does it work the same way for an airplane? To-wit: If the "runway" is moving backward at 100mph, can the plane ever take off?

    The answer should now be obvious. Of course the plane takes off. Regardless of what the wheels are doing, the plane moves forward under the thrust of the propeller. The conveyor could be moving backward at 500mph, with the wheels spinning madly and smoking, but the plane will happily start to accelerate from zero (wrt to the ground - and more critically - the air) to takeoff speed.
     
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  15. DaveC426913 Valued Senior Member

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    16,951
    Correct.

    Incorrect!

    Because:
     
  16. DaveC426913 Valued Senior Member

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    Sure, but this challenge is not about nitpicking - adding a little or subtracting a little.
    This challenge is binary: does the plane take off, yes or no?
     
  17. DaveC426913 Valued Senior Member

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    And circling back around to the topic, even seasoned pilots get aviation theory wrong occasionally.
     
  18. Yazata Valued Senior Member

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    That does create a bit of a conundrum. The airplane's wheels would have to be turning faster than the belt in order to take off (belt speed + takeoff speed). So what if the belt speed kept increasing to match that sum?

    I think that it would take off because of the reason given above, the engines are imparting force to the plane but the spinning wheels aren't (or just minimally). But if the wheels are turning at belt-speed plus take-off velocity and the belt is always increasing in hopes of matching that sum, you would likely burn up the wheels (or more likely the belt) and something would seize up.
     
  19. DaveC426913 Valued Senior Member

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    Yes. Discussions of this usually bring up this conundrum at some point.

    It's not really a problem once it is recognized that the problem lies in an invalid assumption in the statement of the riddle., to-wit:
    " a sensor-feedback system that matched the conveyor speed opposite to the wheel speed of the aircraft"
    This cannot happen.
     
  20. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    Was just correcting whoever said "the only thing that matters..."

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    An alternative answer exists that can actually happen, at least in a specific scenario... Imagine, for a moment, that the tyres are rigid, and don't turn at all - wheel speed of zero. Therefore the conveyor belt matches that speed and doesn't turn. In this scenario, the aircraft will eventually create thrust to overcome the inertia and friction, and start to slide over the conveyor belt, tyres still not turning, conveyor belt not turning, and the aircraft should eventually take off (and probably with shredded tyres!

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    )
    I guess it's arguable whether you could call that a "wheel", though, if it doesn't turn.

    This also helps explain the difference between a car and aircraft... if you stop the car tyres from turning you will definitely not get any forward motion from the engine.
     
    Last edited: Apr 14, 2022
  21. billvon Valued Senior Member

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    No it doesn't. Again, as long as the wheels are free turning (i.e. you're not standing on the brakes) it doesn't really matter what the wheels are doing. Three simpler examples:

    1) The wheels are not turning. The aircraft is stationary. A wind comes up that exceeds the stall speed of the aircraft, and the plane is oriented into the wind. The plane will take off, even if the wheels aren't turning.

    2) You put the wheels on dollies. The aircraft begins acceleraing down the runway. At takeoff airspeed, the aircraft will take off - even if the wheels aren't turning.

    3) You put the wheels on a powered dynamometer (used for testing brakes) and spin the wheels faster than takeoff speed. The plane will just sit there; it cannot take off without airspeed.
     
  22. DaveC426913 Valued Senior Member

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    16,951
    Exactly!.
    I've often been tempted to pose just such an answer:
    "...what happens below the fuselage is irrelevant. The wheels could be square concrete blocks, and the plane would still take off."
    Unfortunately, I delete it every time because, rather than illuminate the challenge, I think it obfuscates it.
    "How can the plane move if it has to drag those concrete blocks?" etc.
     
  23. RainbowSingularity Valued Senior Member

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    jet engine has increasing thrust power
    ramp points upward so increased thrust gets increased elevation
    over all power gain on ramp pressure and engine increasing thrust power gives extra thrust
    extra thrust in upward direction

    seems fairly simple
     

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