This question was answered by a guy who rites for the Chicago Tribune: http://www.straightdope.com/columns/060203.html Dear Cecil: Please, please, please settle this question. The discussion has been going on for ages, and any time someone mentions the words "airplane" or "conveyor belt" everyone starts right back up. Here's the original problem essentially as it was posed to us: "A plane is standing on a 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 the opposite direction). Can the plane take off?" There are some difficulties with the wording of the problem, specifically regarding how we define speed, but the spirit of the situation is clear. The solution is also clear to me (and many others), but a staunch group of unbelievers won't accept it. My conclusion is that the plane does take off. Planes, whether jet or propeller, work by pulling themselves through the air. The rotation of their tires results from this forward movement, and has no bearing on the behavior of a plane during takeoff. I claim the only difference between a regular plane and one on a conveyor belt is that the conveyor belt plane's wheels will spin twice as fast during takeoff. Please, Cecil, show us that it's not only theoretically possible (with frictionless wheels) but it's actually possible too. --Berj A. Doudian, via e-mail Cecil replies: Excuse me--did I hear somebody say Monty Hall? On first encounter this question, which has been showing up all over the Net, seems inane because the answer seems so obvious. However, as with the infamous Monty-Hall-three-doors-and-one-prize-problem (see The Straight Dope: "On Let's Make a Deal" you pick Door #1, 02-Nov-1990), the obvious answer is wrong, and you, Berj, are right--the plane takes off normally, with no need to specify frictionless wheels or any other such foolishness. You're also right that the question is often worded badly, leading to confusion, arguments, etc. In short, we've got a topic screaming for the Straight Dope. First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly. A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily. As you point out, one problem here is the wording of the question. Your version straightforwardly states that the conveyor moves backward at the same rate that the plane moves forward. If the plane's forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH. Assuming you've got Indy-car-quality tires and wheel bearings, no problem. However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off. --CECIL ADAMS
But if the plane is moving down the runway, the runway is just going to speed up. At all times, the runway is moving at the exact same speed as the plane.
exactly what i said This is a problem if the plane uses tires to move itself forward. You and I would both not be moving because we use the friction between our shoes and the ground to move forward so we will always be subject to a ground that is moving in the opposite direction. A plane uses force independent of the ground to move though. If force is being enacted on the plane forward then you need a force equal and opposite that is acting on the plane from the opposite direction. The only way to do this is to add jet engines to the front of the plane and face them the other way. the speed of the tires has no effect on the plane itself.
For this to work, does it follow that if you put a stationary plane on a conveyor belt and turn it on, the plane's wheels will spin but it will not go anywhere?
It seems like everyone who talks about lift and understands how a wing works, where the air flowing above the wing has to move further to match the speed that the air moving under the wing is flowing, says the plane won't be able to takeoff. But I don't think people understand that the conveyor isn't stopping the plane from moving forward. The thrust generated by the engines pushes the aircraft through the air, which generates airflow. And enough to get lift and take off.
I think even the straight dope guy exaggerates the effect of the conveyor belt. The only force the conveyor belt exerts on the plane is the friction within the ball bearings in the wheels. If the plane were sitting on the conveyor belt with it's engine off, yes, the belt would move the plane backward. But add the efficiency of a well greased ball bearing, friction and gravity aren't that hard to overcome in the horizontal plane. An engine that can accelerate a plane to 500 miles an hour would not be overly taxed to reach a take off speed of around 190 mph though it would require a bit more power than from a stationary runway. I did learn something new during this exercise though. Wiki says Bernoulli's principle is not the principle force in lift on an airplanes wings. I've had the wrong idea for arond 40 years.
That's almost correct... it's conceptually correct (assuming that the pilot had not engaged the plane's brakes). The weight of the plane pushing down on the wheels would certainly cause the plane to move backwards. But, if the plane were very very light (perhaps weightless) the wheels would just spin. If a car was in neutral (where the wheels can spin freely) and was standing still on a moving conveyor, it, too, would not move if the car was nearly weightless.
The plane will move in the beginning with the belt. With the jet engine off, there is nothing to give it motion independent of the ground. Once you turn the engines on however and put a force on it, the plane will start to slow down and the wheels will start spin.
If the belt is outputting the same amount of force as the engines only in the opposite direction the plane will not move forward.
I think this is a poorly worded riddle. We all can agree .... if there is no forward motion or wind being supplied (which there isn't), then there will be no lift off. Approach it from this angle; If the plane is on a movable runway that can match the planes speed (assuming the wheel speed), will that prevent the plane from making forward momentum? Theoretical answer is no, as the plane will never create any momentum. The realistic answer is yes, because the movable runway will not be able to keep up with the speed of the wheels as the throttle increases. Once the plane gains any momentum, there will be a "bouncing" effect that will further negate the conveyors job.
Think of a sea plane Its engines generate thrust and move the plane forward bringing wind against the wings, and the plane starts flying... and none of that depends on the wheels ('cause there aren't any!). The same goes for a plane on a conveyor. The wheels aren't what's thrusting the plane - that's being done by the engines. The wheels just spin. So, as with a seaplane, regardless of what the wheels are doing (or aren't doing) the plane will be thrust by the engines, will bring wind against the wings, and will take off.
I have watched MythBusters a handful of times, and have generally enjoyed the show, so it will probably be interesting either way. However, I am scratching my head, simply because I don't think I understand the set-up clearly. So let me see if I have a grasp of the 'experiment': The idea is that a standing-still airplane is on a ''runway' which will move under the wheels in the opposite direction that the plane ACTUALLY moves in. The point appears to be to see what happens if you rob an airplane of its forward momentum and leave all of its 'other' aspects intact. To me, it doesn't seem like much of an idea. If you TRULY were able to have a large 'precision-controlled moving runway' which would truly move in exactly the same speed in the opposite direction as any movements the wheels *touching the runway* were to make (which is a silly idea in itself), then what you are doing in reality is the same thing, in concept, that noise-canceling headphones do - they generate and equal and opposite force against a specific energy. In other words, if I understand the set-up correctly, the plane will have NO forward momentum, no matter what. Now keep in mind the basic physics required to put an airplane in the air - 1) air must flow over wings, and the distance the air travels over the top must be greater than the distance through the same air to travel across the bottom, 2) the area of the wings must be of sufficient size to generate enough lift to support the mass of the airplane, and 3) the air must be flowing over those wings at a fast enough speed to generate that lift. If those criteria are met, the plane will rise into the air. What is not explained in the set-up in the original post is exactly what kind of airplane this is: is it a prop plane, a jet plane, a single-prop plane, a multi-prop plane with propellers mounted on each wing, etc. In other words, without knowing exactly how this particular airplane is intended to generate its lift, we cannot answer the question. Typically, an airplane generates its lift by using 'thrust' - using the engine on the vehicle to push it into the air at high enough speeds to create the necessary 'lift'. Usually, an airplane (whether a prop plane or a jet plane) only uses its engine to create forward momentum, and the airflow over the wings is a by-product of this momentum. Under most circumstances, the engines do NOT 'create' the airflow themselves. This is because you would need several propellers, spread out all along each wing, in order to create enough high-speed airflow over the entirety of the wings' top and bottom surfaces to generate 'lift'. Therefore, the 'experiment' seems to be set up in such a way as to negate an airplane's typical method for generating its lift: its actual physical rapid motion through the air. So can the airplane still lift into the air? Yes, if the above criteria are met. This is regardless of 'speed', simply because speed is only one method for generating airflow over the wings. IF the engine(s) of the airplane in question is able to create actual airflow over the wings in a large enough quantity, then yes, *lift* could still be possible. Would it *fly*? No. If lift occurred at all, if would only lift maybe a couple of feet into the air, wobble a bit, lurch forward and smack back down again. Generating lift is one thing, controlling it is another. An airplane controls itself in the air using ailerons and rudders, and this control is dependent NOT upon the lift, but upon the combination of lift AND momentum. Specifically the rudders, but also the ailerons, and more specifically on the tail - you MAY be able to artificially generate enough airflow over the main wings to generate lift, but you certainly and not going to have that same airflow over the tail, and definitely not in the same way that you have when the entire airplane is moving through the air at a high rate of speed. It MAY rise, buck around, and flop. However, the whole idea is silly, as mentioned before. There is no need to test out a theory of having a 'magic runway'. All you would need to do is chain the airplane to a wall behind it. Same effect. No actual forward momentum allowed. Now, the other possibility is this: actually canceling out all momentum is not what happens in the experiment, and the plane takes off normally. As someone else mentioned, the engine and the wheels are not related. The wheels only serve to remove a large amount of friction between the airplane and the ground. All of the driving force of the airplane is between the vehicle and the AIR, unlike, say, a car, where the relationship is between the vehicle and the GROUND. Anyway, if they simply tried to put an airplane on some kind of conveyor belt that would move backwards while the plane tried to move forward, but did not exactly cancel out all forward momentum, then once the plane managed some forward motion, what the wheels are doing would not matter. But in that case, the experiment would be totally different, and pretty useless to boot. It's just a reflection of how airspeed affects flight. It's why Aircraft Carriers face into the wind when planes need to take off or land. I guess what I am saying is, the parameters and goals of the 'experiment' are not explained well enough to make an informed prediction about what will happen.
How can the belt output force? That's impossible. The only force that is being generated is from the engines. Force can be extracted from the conveyor belt IF THE wheels are depending upon their interaction with the ground for force but wheels on planes are designed to ignore Force that can be extracted. They don't want some random thing on the runway to slow down one side back tire and crash the plane. Guys, just think of it as if you were on roller skates on a treadmill. The treadmill can even go MUCH FASTER than you in the opposite direction but if you can grab the handrail and give yourself forward motion independent of the moving ground then it's only the wheels on your skates that are affected.
Originally Posted by Major For this to work, does it follow that if you put a stationary plane on a conveyor belt and turn it on, the plane's wheels will spin but it will not go anywhere? Don't forget inertia. No machinery is 100% free of friction. Even in a vacum, moving masses exert gravitational pulls.
The conveyor will cancel out any forward force generated by the plane's wheels.... .... but on a plane, the wheels aren't generating any forward force! The ENGINES generate the forward force.
The wheels just turn faster. Till the centrifugal force throws off all the rubber and the struts dig into the conveyor belt and the plane crashes and burns.
