Physics Question for you.....

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  • TK421
    Forum Newbie
    • Aug 2006
    • 25

    #76
    Yes it will fly.

    I hope this hope this clears it up.

    Lets say that the plane in question is actually a glider plane modified with a typical tricycle landing gear setup. The plane is on the conveyor belt runway attached via cable to a truck on a road running parallel to the runway. As the truck starts moving the down the road the plane starts moving as well. It has to! They are attached! It doesn't matter how fast the conveyor belt going or how fast the wheels are spining. Thats all the wheels are doing - spining.
    The truck is providing the "thrust" that a real plane would get from its engine.

    or

    Lets say you had a real jet on the conveyor belt runway and it was tied down via tie down chains. With the engines at idle you bring the the conveyor belt up to speed (lets say 200 mph ~ 300 ft/sec). The plane is tied down its not moving, the wheels are just spining. But if one were careful you could attach the chains to a tow truck and pull it down the runway. It wouldn't matter how fast the conveyor was moving because once again, the wheels are free spining.

    Thats the key. The wheels provide a 'frictionless' surface on which the plane rests while on the conveyor belt.

    Comment

    • leehljp
      Just me
      • Dec 2002
      • 8429
      • Tunica, MS
      • BT3000/3100

      #77
      TK421,
      Your premis about it being a glider wasn't the original statement.

      A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

      The question is:

      Will the plane take off or not? Will it be able to run up and take off?


      "Will it be able" implies directly to its self. Gliders are referred to as gliders. Powered flying wing vehicles are referred to a planes.
      Hank Lee

      Experience is what you get when you don't get what you wanted!

      Comment

      • Stytooner
        Roll Tide RIP Lee
        • Dec 2002
        • 4301
        • Robertsdale, AL, USA.
        • BT3100

        #78
        Until I separated the action of the tires on the plane, I thought the conveyor was canceling out forward motion of the plane. The only thing the conveyor acts on is the tires. The plane still moves forward regardless of the direction or speed of it's tires.


        Think about this. Change the direction of the conveyor and now match the speed of the plane. The tires aren't even turning now, but the plane is moving down the conveyor and will take off.
        Lee

        Comment

        • LarryG
          The Full Monte
          • May 2004
          • 6693
          • Off The Back
          • Powermatic PM2000, BT3100-1

          #79
          The scan Niki posted suggests that for the plane to move forward, it only has to overcome the frictional resistance of its wheels. But this assumes it is sitting on a non-moving piece of ground. When it's sitting on a conveyor belt that is moving backwards, it must counteract not only the frictional resistance but also the distance it is moved backwards by the conveyor belt.

          Surely we can all agree that if the plane was sitting on the conveyor belt with its engines shut off, it would move backwards with the conveyor belt, at the same speed as the conveyor belt. What would make the component of gravity magically disappear just because the engines start and begin producing thrust?
          Larry

          Comment

          • Stytooner
            Roll Tide RIP Lee
            • Dec 2002
            • 4301
            • Robertsdale, AL, USA.
            • BT3100

            #80
            Okay, lets let gravity do the work then.
            Say the conveyor is on the side of a mountain like a ski lift. Turn it on going 100 MPH going uphill. Sit a plane on this conveyor pointing downhill. Let it go.
            It's going to only go uphill a little until gravity takes over. The wheels will be spinning, but the plane will be moving down even though the belt is turning trying to get the plane to the top. Gravity will pull it downhill. It will be moving pretty fast when it gets to the bottom of the mountain all the while the belt is moving rapidly uphill.
            Lee

            Comment

            • LCHIEN
              Internet Fact Checker
              • Dec 2002
              • 20914
              • Katy, TX, USA.
              • BT3000 vintage 1999

              #81
              Originally posted by LarryG
              The scan Niki posted suggests that for the plane to move forward, it only has to overcome the frictional resistance of its wheels. But this assumes it is sitting on a non-moving piece of ground. When it's sitting on a conveyor belt that is moving backwards, it must counteract not only the frictional resistance but also the distance it is moved backwards by the conveyor belt.

              Surely we can all agree that if the plane was sitting on the conveyor belt with its engines shut off, it would move backwards with the conveyor belt, at the same speed as the conveyor belt. What would make the component of gravity magically disappear just because the engines start and begin producing thrust?
              Sigh, Larry,
              if there were no initial friction of the wheels, moving the conveyor belt would actually leave the plane behind because of inertia, one of newtons laws, things in motion tend to stay in motion (and things that are still tend to stay still). Because large objects like conveyor belts big enough for a plane) tend to have large masses, they tend to start slowly. Friction has a component called stiction that is large when the object is at rest, that must be overcome to start it moving.
              Try pulling a tablecloth quickly from underneath a table setting.
              Loring in Katy, TX USA
              If your only tool is a hammer, you tend to treat all problems as if they were nails.
              BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions

              Comment

              • LarryG
                The Full Monte
                • May 2004
                • 6693
                • Off The Back
                • Powermatic PM2000, BT3100-1

                #82
                Originally posted by LCHIEN
                Because large objects like conveyor belts big enough for a plane) tend to have large masses, they tend to start slowly.
                That's exactly my point, Loring -- also Lee's inclined conveyor, which illustrates how gravity continues to act on the plane regardless of what the conveyor belt is doing. The conveyor will start slowly. The plane will start rolling slowly, even if the throttles are firewalled abruptly. Assuming the conveyor's speed tracking mechanism is able to exactly and precisely match the plane's movement in the opposite direction, as the OP stated it can, the plane won't move relative to the fixed ground.
                Larry

                Comment

                • Stytooner
                  Roll Tide RIP Lee
                  • Dec 2002
                  • 4301
                  • Robertsdale, AL, USA.
                  • BT3100

                  #83
                  Okay, the conveyor is on a mountain for a new test.
                  Start the coveyor running toward the bottom of the mountain. Now set the plane on the conveyor with the nose pointing toward the top of the mountain. The plane will be moving down hill with the conveyor and may actually start to outpace the conveyor by rolling down some depending on it's weight and the conveyor speed.
                  Now fire up the engines and get to full throttle. At some point, the plane will stop moving downward even though both the conveyor and gravity are working on it. Thrust will eventually hold it momentarily still and then the plane will move forward toward the peak of the mountain against both gravity and any speed on the conveyor. The plane's tires are still rolling with the conveyor, but the plane moves separately from the roll of its wheels.

                  It's quite simple really once you overcome the plane to wheel relationship. The tires only bear the weight of the plane and have no relation to any of the planes movements. The plane moves forward because of thrust and not because of anything the wheels do.
                  Lee

                  Comment

                  • scorrpio
                    Veteran Member
                    • Dec 2005
                    • 1566
                    • Wayne, NJ, USA.

                    #84
                    In my original statement I had same error as many others had - I was forgetting that plane rolls not due to torque applied to the wheels - but due to engine thrust. I about changed my mind, but started thinking more about the problem, and now the answer will be: depends on how much plane's landing gear can take.

                    Now, in order for the plane to keep moving forward, it has to overcome two forces: air resistance and friction in its wheel bearings. Gravity is not in the equation yet. Once plane gains some speed and oncoming air pressurises under the wings, main purpose of thrust becomes 'cramming' enough air under the wings to keep the plane aloft, but in the beginning, rolling friction is the main enemy.

                    Now, the only thing we know about the conveyor, is that 'it is tracking the plane speed trying to match it backwards'. It seems that system is essentially trying to keep the object motionless. If this system detects that object is moving forward, it accelerates the belt further. If the propulsion principle of the object is based on pushing off the surface it travels on (i.e. torque to the wheels, rowing a boat etc), then yes, the object will stay motionless.

                    However, the propulsion is independent from the belt. What happens? The object(plane) starts moving forward. The conveyor, detecting this, will increase its speed. Conveyor goes to 100mph, causing wheels to spin madly, yet plane keeps moving forward. Conveyor goes to 200, 300, 1000, 5000mph, trying to keep the plane motionless. How quickly does the conveyor accelerate? Does it have a limit?

                    Potential scenarios:
                    1. As RPMs increase, friction forces go beyond bearing ability to handle. Either plane thrust is unable to fight friction or ensuing heat melts the bearings.
                    2. Wheels, subjected to ultra high RPMs, burst.

                    However, this can go in a different direction. Consider phenomenon known as hydroplaning. In heavy rain, a film of water forms on the road. Tires of cars traveling over the road, have treads to channel that water away from between the tire and the pavement in order to keep traction. When treads cannot channel the water away due to either high speed or insufficient tread capacity, tire floats on a cushion of water, losing traction with road (bad). But in this case, a fast moving conveyor will be dragging along a film of air, nad if speed is high enough, it might cause 'aero-planing', so plane essentially rolls over a cushion of air, and takes off.

                    Question is, which happens first.

                    Comment

                    • Russianwolf
                      Veteran Member
                      • Jan 2004
                      • 3152
                      • Martinsburg, WV, USA.
                      • One of them there Toy saws

                      #85
                      Originally posted by Stytooner
                      I have a question for Russian Wolf. Did you have an answer for this when you popped it to us?
                      My answer is on page 5.

                      She will fly.

                      It's a "trick" question in that the "trick" is the junction between the conveyor and the plane.

                      The wheels.

                      If the plane were dependent on the wheels for propulsion, it would not be able to fly as it would remain in one place.

                      But as the wheel are free spinning and not needed for propulsion, the plane can take off almost as normal (wheels will spin at twice rpm as normal but not impede the takeoff).
                      Mike
                      Lakota's Dad

                      If at first you don't succeed, deny you were trying in the first place.

                      Comment

                      • JR
                        The Full Monte
                        • Feb 2004
                        • 5633
                        • Eugene, OR
                        • BT3000

                        #86
                        Originally posted by Russianwolf
                        She will fly.

                        It's a "trick" question in that the "trick" is the junction between the conveyor and the plane.
                        It's not much of a trick, IMO. Your question was "Will the plane take off or not?"

                        No, it will not take off unless it fires up the engines!

                        Humbug!

                        JR
                        JR

                        Comment

                        • LarryG
                          The Full Monte
                          • May 2004
                          • 6693
                          • Off The Back
                          • Powermatic PM2000, BT3100-1

                          #87
                          Originally posted by Stytooner
                          The tires only bear the weight of the plane and have no relation to any of the planes movements.
                          Aw, geez. Why didn't you say that seventy-odd responses ago?!?

                          Seriously, it's amazing how a simple statement -- providing it's presented the right way -- can provide so much more clarity than can pages and pages of highly-detailed explanations.

                          Also, prior to seeing your latest reply, I had a troubling thought (troubling WRT my argument, I mean) that helped open my mental blinders. The thought, which should have occurred to me sooner, was what happens when a vehicle on glare ice loses traction and begins to slide. It's exactly as you said: the wheels are merely supporting the vehicle's weight and have no relation to any of the vehicle's movements. Okay, in this scenario, the ice under the wheels is not moving. What I suddenly realized was that it could be moving and it wouldn't matter at all.

                          I'm still going to maintain, however, that this hypothetical airplane would not experience an entirely normal take-off roll, but one that was longer than normal. The plane would still have to overcome the conveyor's tendency to try to carry it along with it. Exactly how long this would take would be a function of how abruptly the plane accelerates and how abruptly the conveyor starts running.

                          BTW Scorpio your scenario doesn't apply because the OP stipulates that the conveyor only matches the plane's speed, not exceeds it.
                          Larry

                          Comment

                          • scorrpio
                            Veteran Member
                            • Dec 2005
                            • 1566
                            • Wayne, NJ, USA.

                            #88
                            Originally posted by Russianwolf
                            But as the wheel are free spinning and not needed for propulsion, the plane can take off almost as normal (wheels will spin at twice rpm as normal but not impede the takeoff).
                            See my above post. In its effort to match the speed of plane, the conveyor will accelerate to the limit - weather it's the limit of:
                            a. conveyor speed.
                            b. wheel strength.
                            c. bearing capacity.
                            d. tire traction.

                            My vote goes to blown tires and a nasty accident.

                            Comment

                            • LarryG
                              The Full Monte
                              • May 2004
                              • 6693
                              • Off The Back
                              • Powermatic PM2000, BT3100-1

                              #89
                              Originally posted by scorrpio
                              the conveyor will accelerate to the limit
                              As I say in my Okay-Now-I-Get-It post above, the limit is imposed in the OP: Mike clearly stated that the conveyor matches the plane's speed, not exceeds it. So "the limit" is exactly the same speed at which the plane is moving:

                              "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)."
                              Larry

                              Comment

                              • Russianwolf
                                Veteran Member
                                • Jan 2004
                                • 3152
                                • Martinsburg, WV, USA.
                                • One of them there Toy saws

                                #90
                                Originally posted by scorrpio
                                See my above post. In its effort to match the speed of plane, the conveyor will accelerate to the limit - weather it's the limit of:
                                a. conveyor speed.
                                b. wheel strength.
                                c. bearing capacity.
                                d. tire traction.

                                My vote goes to blown tires and a nasty accident.
                                now you are rationalizing. what if I stated:
                                a. conveyor has no speed limitation
                                b. wheels are indestructible
                                c. bearings are indestructible and frictionless
                                d. tires have 100%traction at all times (until/if they leave contact with the conveyor in takeoff)
                                Mike
                                Lakota's Dad

                                If at first you don't succeed, deny you were trying in the first place.

                                Comment

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