Science on TV

14 opinions for MythBusters - Plane on a Conveyor Belt myth

• Raging Bull
  Feb 4, 2008 at 1:47 am

  Well the main thing people need to understand is that a plane on a conveyer belt is not the same as a car on a conveyer belt.

  The easiest way to put it is : “The plane does not care what the wheels are doing”. This is because the wheels of the plane are free-spinning (negligible friction). The plane will move forward from the thrust of its engines and the only thing affecting this is the air infront of the plane. The speed of the wheels may be matched instantaneously or with a slight delay by the conveyer belt but it matters not. For all its worth, the wheels would spin faster than the plane but the aircraft along with its engine wouldn’t feel the difference, therefore taking off as usual.

  The Mythbusters are great and I do not know how they would be able to create a conveyer belt with the ability to match the speed of the planes wheels, but they did a great job with this myth.

  Just to restate my view : The aircraft WILL take off since its movement is independent from the movement of its wheels.

• Healthy Shana
  Feb 4, 2008 at 9:35 am

  Thanks so much for your comment Raging Bull. Prior to watching the MythBusters episode i’d never ever heard of the “Plane on the Conveyor Belt” story…so kudos to you for known all about it!

• Raging Bull
  Feb 5, 2008 at 6:28 am

  Lol no problem. Its not a very uncommon debate. I too learned about it a while back from a friend. Through many hours of reading, even more hours of thinking, and days of imagining different scenarios, I came up with a solid explanation and a brief summary of what exactly is happening. :)

• Healthy Shana
  Feb 5, 2008 at 9:53 am

  I have another mythbusters code for you to crack. Let me know your thoughts!

• Tom
  Feb 15, 2008 at 8:12 pm

  An airplane with mass M sits on a conveyor belt. The airplane’s jet engine is turned OFF. The conveyor belt starts moving backward at a constant velocity V_conveyor. (note that all velocities will be quoted relative to the ground referential)

  Which direction does the plane go? Backward, with velocity V_conveyor.

  Intuitively it is reasonable to expect that the plane will have to overcome this backward velocity before taking off.

  At time T=0, the jet engine is turned on, generating a thrust, F by pressing against (we all agree) the atmosphere. As a result, the plane experiences a forward acceleration A = F/M. Note that the conveyor belt provides no acceleration, just constant velocity. [As an aside, also note that acceleration is independent of any fixed frame of reference].

  We are allowed to decompose the velocity of the plane relative to ground (V_total) into the velocity component due to the conveyor belt (V_conveyor) and the velocity resulting from thrust acceleration (V_plane):

  V_total = V_plane - V_conveyor

  As velocity is the integral of acceleration, at the instant in time T=0 its (forward) velocity component due to the thrust force is still V_plane = 0: the plane is still moving backward relative to earth with velocity V_conveyor - V_plane = V_conveyor.

  Time, T, continues to pass. Thrust F guarantees constant acceleration A=F/M, and the equation for the velocity of the plane due to thrust is V_plane=TA.
  Thus V_total = TA - V_conveyor. The plane’s ground speed V_total will be zero only when

  T_null = V_conveyor/A

  Until this time the plane will continue to move backward. As more time passes, the plane’s forward velocity component due to thrust will continue to increase linearly with time, whereas the backward velocity component due to the conveyor belt motion will remain constant, and so the plane will progressively pick up forward velocity (V_total = TA - V_conveyor > 0 ), develop lift and take off.

  Thus if the conveyor belt has constant velocity the plane will, of course, teventually take off. If the conveyor belt’s velocity increases linearly with time at a rate A, then the plane will not develop forward velocity and will not take off.

  [Formally speaking, the thrust does not arise from "pushing against the atmosphere", but from conservation of momentum P of air with mass m_air being pushed out by the engine with velocity v_air:

  P = m_air X v_air

  and F = dP/dt is simply the mass of air pushed out with velocity v_air per unit time. This changes the result not an iota. Cheers]

• Healthy Shana
  Feb 16, 2008 at 6:15 pm

  TOM??? What in the world? Are you a physicist or something? That kinda went over my head, but at the same time I was oh so impressed. Can I marry you :) JOKE!!

• Gatormike
  Apr 25, 2008 at 12:34 pm

  TOM, very complicated analysis but I don’t think your ultimate conclusion is correct. The plane with the motor off would indeed move backwards when the conveyor belt is activated, especially if the speed of the conveyor was very solw. Why? Because the friction of the wheels on the conveyor would be sufficient to keep the plane in position so the the force of the conveyor would be transferred to the plane. The faster the conveyor moves, however, the more “slippage” would occur because this frictional force is relatively weak.

  Once the motor is turned on, however, the friction provided by the free-spinning wheels would become virtually irrelevant to the equation. Your bottom line point, that the final forward velocity of the plane equals the forward velocity minus the conveyor velocity, is simply incorrect. The aircraft with the wheels on the conveyor would behave very much like (almost identical to) an aircraft suspended 6 inches above the conveyor. The impact of the conveyor is nearly zero…

• Healthy Shana
  Apr 25, 2008 at 12:42 pm

  Hi Gatromike, thanks for coming through and commenting.

• brian clubb
  Jun 28, 2008 at 5:17 pm

  i heard that you ould make a gernade out of a light bulb

• Nelson
  Aug 31, 2008 at 9:51 am

  I think one key ingredient is missing from all these conveyor belt ruminations: LIFT. Air must pass over and under the wing surface to generate lift. The only way to do this, outside of a wind tunnel) is to have the plane moving forward. If it is on a conveyor belt, the plane’s forward motion relative to the earth is zero, regardless of the speed attained by the belt or the wheels on the plane. Therefore, there is no air moving across the surfaces of the wing. Simply put, no movement through the air, no possibility of take off.

  Or did I misunderstand the question entirely?

• Frank
  Sep 20, 2008 at 5:50 am

  Nelson, you are correct - the plane does need forward movement to give an airflow over and under the wing to create lift.
  In the episode of the plane on the conveyor belt, the result is “busted” but this is incorrect to what the myth states. The myth identifies that both plane and conveyor are traveling at the same speed - therefore the plane will seem to not move at full throttle due to the conveyor. We can see in this episode in both cases (remote control plane and real plane) that the plane is moving forward (past the cones) therefore giving it lift enabling the plane take off. This myth needs to be tested again and proved correct - that a plane cannot take off when it is not moving forward.

• Steve
  Sep 20, 2008 at 6:10 am

  Where Tom states “the plane will progressively pick up forward velocity…develop lift and take off.” he’s correct in the way that as Nelson said you must have air passing over and under the wing to generate lift.

  I wasn’t sure about the use of an airplane that only requires a forward speed on 25mph to take off.

  Also - and this was and is key for my scepticism about the execution of the myth; from the footage it looks like the airplane is moving forward for a period before take off occurs i.e. forward speed is greater than the reverse speed applied by the conveyor.

  On one level I guess that if your prop is powering you at e.g. 25mph and the conveyor is pulling backwards at 25mph - even though you don’t have forward motion, if you can get the wheels to leave the ground and keep the prop pulling 25mph then in theory you will be moving forward at 25mph ?

  In effect - you could imagine a scenario where the conveyor drops when the optimum speed is reached. Will the plane stay where it is - flying naturally due to lift ? In theory it shouldn’t because it will not be travelling through the air. The prop will seek to pull it forward at a speed of 25mph but the airspeed at that point (of the conveyor dropping) will be 0mph - therefore the plane will lose lift and drop until the prop gets the plane accelerated up to an airspeed of 25mph.

  I think a computer model would show whether this works on a larger and more satisfying scale without the inherant danger.

  Incidentally - the whole prop vs wheels thing is I think a red herring - lift and ‘maintained’ airspeed to achieve lift is all the physics that this myth needs to comply with. Without lift you will not get and stay airborne. “Ye cannae break the laws of physics Jim”… ;) It’s not the speed of the wheels but the speed which air is passing over and under the wings that offsets the weight of the airplane and gives you lift. That’s what the pilot (and all ’successful’ pilots) know.

• Steve
  Sep 20, 2008 at 6:33 am

  OK - From reading another site I’ve got it :)

  The basis of the myth is whether the plane will stay still because the reverse speed of the conveyor matches the speed applied through the prop. The answer was that the plane moved forward and accelerated up to the speed applied (enough thrust for 25mph).

  The wheels don’t really come into it because they just spin forwards as the conveyor is pulled out from under it at 25mph (they spin to match the speed being pulled backwards). The prop pulls with a forward thrust of 25mph and that pulls the plane forward at 25mph regardless of what the wheels are doing, and the plane takes off due to forward motion and lift applying.

  You will not get the plane to stay where it is under the experiment conditions because there is no way of ‘matching your speed to counter the conveyor speed’. The plane will move forward relative to its starting position by the amount of thrust applied (25mph worth for the weight of the airplane). Incidentally - the wheels will be travelling at 50mph at the point of take off but they are both:

  incidental to the myth
  and
  central to the understanding of why the plane moves forward with increasing speed.

  The wheels act in the same way that is displayed when you do the old pulling the tablecloth out from the table setting trick.

• Ian
  Sep 20, 2008 at 7:01 am

  I think the main reason for the differences of opinion on this issue is the ambiguity in the myth statement:
  “A plane is standing on a runway that can move (some sort of brand conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the PLANE SPEED and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?”

  This could have two interpretations depending on whether the plane speed is considered relative to the conveyor belt or relative to the ground.

  Interpretation 1: If plane speed is relative to the conveyor belt the plane’s speed relative to the ground will be zero as long as the control system keeps the speed of the conveyor belt matched to the speed of the plane. In this case, the plane will not take off (unless it is pointed into a really strong wind that matches or exceeds take off speed). Mythbusters did not bust this interpretation of the myth

  Setting up such an experiment would be difficult, as the aircraft thrust would have to be kept just sufficient enough to prevent the conveyor belt pulling it backwards. If too much power was applied, the plane would move forward and the test would be invalid.

  Interpretation 2: If plane speed in the myth statement is considered relative to the ground, it doesn’t matter how fast the conveyor belt is moving, the plane will take off when its ground speed is enough for it to gain sufficient speed to take off. Mythbusters did bust this interpretation of the myth.

Have an opinion? Leave a comment:

• Use this area to discover new blogs and content in the b5media network.
• Beauty & Style
• Business
• Entertainment
• General
• Health & Wellness
• Lifestyles
• Music
• Sports
• Team Blogs
• Technology
• Travel & Culture

• Social Web
• E-mail