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)Airplane on a conveyor belt
Heard this problem?
“Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off? “I say no, because the plane will not move relative the ground and air, and thus, very little air will flow over the wings. However, other people are convinced that since the wheels of a plane are free spinning, and not powered by the engines, and the engines provide thrust against the air, that somehow that makes a difference and air will flow over the wing.”
Jaime and Adam on Mythbusters are tackling this one on Wednesday. I don’t know the results because I’m not that much on the inside track, but here is my take on what will happen:
The question is a little misleading. Why? You can’t stop a planes forward motion with a conveyor belt. Why? Free spinning wheels and power by thrust. You can stop a car with a conveyor belt (or a person walking… who hasn’t done that at least once at the airport), because power by a car comes from friction with the ground.
Two real life scenarios:
You are walking on the ‘people-mover’ at the airport. You are walking normal speed, dragging your travel bag (which is on two free spinning wheels) but you are moving across the floor twice as fast. The moment you step off, your bag is still on the conveyor belt. Does it all of a sudden hit you in the butt? No. The wheels slow down the moment you do. You are the bags power, not the wheels. If you just stand there, with you bag on the belt and you are not, do you constantly have to push the bag back? Nope. It’ll just sit there, with wheels turning.
If you still don’t believe me, hold two matchbox cars at the top of a declining platform (Like a propped up book.) Put a piece of paper underneath one of the cars. Let go of the cars and pull out the piece of paper from under the one car in the opposite direction the cars are traveling. Both cars will reach the bottom of the platform at the same time. Why? The power is gravity. The moving paper will affect the wheels, so the one cars wheels will be moving faster, but the free spinning wheels have nothing to do with the power exerted on the car (in this case, gravity).
With that aside, there still is a question here. Will a plane rise if forward motion is suspended? We can change the scenario from a conveyor belt to simply adding a towline to the back of a plane, and staking it. The plane can’t move forward because it’s hitched up!!! Will it rise with engines blazing? Here is what might shock you:
It depends.
It depends on the kind of plane. A propeller sitting in front of the plane will force air over the wings, and create lift. The force of the propeller is two-fold. One reacts on the wings, forcing air over then to create lift, and another acts in pushing air in front of the plane behind it, forcing the plane forward. If you stop one force (the forward motion), the lift is still being exerted. The plane will rise on the towline, but not move forward. Jet aircraft will merely sit and struggle against the towline. The force of jet engines is only to push the aircraft forward. It is the forward motion of the plane that causes air to move across the wings, causing lift. Lift is a secondary reaction to the force of the jet. It is directly related to the movement of the aircraft through the air. So… a propeller airplane will lift, a jet airplane will not.
I can't wait to see the show on Wednesday now.
By all means... comment.
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Micah Rousey
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Reader Comments (24)
I don't see how this is a "problem" for anyone to solve!
They provide the forward movement of the plane through the air which travels over and under the wings -- which provides the lift.
The only way that propellers in front of the wing could provide the lift is if they were equal to the entire length of the wing.
The whole "could this happen" is easily demonstrated by think of a car on the same conveyor belt. If the car is relatively "motionless" to the room containing the conveyor belt, even though the wheels are spinning at 60 MPH, do you think that if you were in the car and stuck your hand out the window that it would suddenly lift up because the wheels were spinning? Of course not.
2) To get enough air flowing over the wings, you need to have the plane moving forward fast enough (I don't buy the propeller giving the plane enough lift, otherwise the wings on a plane would be much, much smaller)
3) To get the plane moving forward fast enough, you must have the force generated by the propeller overcoming the frictional forces of the wheels, bearings, etc. on the ground
4) I believe the propeller can overcome the frictional forces when the treadmill is moving at the speed of the plane. Ex. the plane is moving 10mph forward relative to a stationary observer, and the treadmill is moving 10mph backwards relative to a stationary observer. Etc, etc. as the plane accelerates and takes off. I.e. the wheels end up spinning twice as fast against the treadmill as the speed of the plane relative to a stationary observer (20mph in the example I gave).
5) If the treadmill can accelerate fast enough to move as fast as the speed of the wheels, then the plane should not be moving forward. For every inch the wheels roll forward, the treadmill should roll one inch backwards, leaving the plane in the same position. Therefore, if the question is stated that the treadmill moves at the same speed as the wheels, then the plane will not take off. The propeller, by itself, will not provide enough air flow to generate enough lift for the plane to take off.
BTW, I think the luggage will still hit you in the butt. It has significant momentum from you moving forward. Also, think of this - put a ball on the conveyor belt. It will move forward, not sit in place and roll. If the conveyor belt is moving very, very fast, it may bounce and spin, but it will be carried forward. Again, there is significant friction on the ball.
Near the end of your article, you change the question to "Will a plane rise if forward motion is suspended?" which is completely different. The answer, of course, is yes. Air needs to flow over a plane's wings from front to back in order to generate lift. If the plane doesn't move, the plane doesn't fly.
But planes use thrust for power (via jet engine or propeller, most commonly), and that thrust has absolutely nothing to do with the wheels. Pontoon planes don't even have wheels (for the most part)!
So, assuming the plane's wheels can stand up to the friction of normal take-off speed PLUS being on a conveyor belt moving an equal speed in the opposite direction, then the propeller will be completely unaffected by the conveyor belt. Thus, the plane will move forward, providing the necessary air flow over the wings to create lift.
The plane will, in my opinion, be able to take off.
Propellers and jet engines perform fundamentally the same job in different ways - they both move the plane forward through the air (propellers by pulling the plan with them, jet engines by pushing the plane in front of them). Once they are moving fast enough, a low pressure area is created above the wings, which causes the plane to lift off from the ground (this is a simplified explanation).
So no, a plane cannot lift off just because it has propellers on it's wings. The plane still needs to be moving forward with enough speed to create that low-pressure area above the wings.
But think about it Rust... if the props are pulling the plane through the air, but we tether the plane to the ground, what are they now doing? Nothing but moving air. Put those props in front of the wings, and won't you achieve lift?
Someone made a good point about wing size... that if that would work, wings could be much shorter. I'm willing to admit I may have been a bit optimistic about the amount of air a prop can move over wings.
But I think we agree that a conveyer belt won't suspend a planes forward motion. Am I right? I hope at least that one the show will answer.
Yes we agree that a conveyor belt will not suspend a plane's forward motion, so by extension we agree that a plane on a conveyor belt will be able to take off (since it will still be able to generate forward motion as normal).
I think a plane could acieve lift tethered to the ground. I never said it would be easy or even possible to control, but it could.
You know... using definate phrases like 'will not'; 'no planes that can', etc... usually isn't a good idea. I mean... lets face it... you don't really know for sure do you?
This is the part that cleared up my confusion:
"If you still don’t believe me, hold two matchbox cars at the top of a declining platform (Like a propped up book.) Put a piece of paper underneath one of the cars. Let go of the cars and pull out the piece of paper from under the one car in the opposite direction the cars are traveling. Both cars will reach the bottom of the platform at the same time."
An better example would be to put a glider on a skijump with the conveyer belt under it. Gravity starts to pull the glider down the ramp, but the conveyer belt starts to spin backwards at the rate the wheels are turning. At first, I thought, this would hold the glider in place, but of course it doesn't. Why not? There is insufficient friction between the turning wheels and the conveyer belt. In essence, the glider starts sliding down the ramp. Same thing with the powered plane on a level surface with the same conveyer. As the physics people have said, there is no countervailing force to offset the thrust of the engines. To us non-physics people, it seems intuitive that the wheels have to turn to allow the plane to move. But that's not true. The wheels can slide over the conveyer belt without turning faster, so the plane will move forward until it reaches takeoff speed.
Plane engines are matched with the aircraft so they can achieve the necessary forward motion on the ground to lift off, and keep that air speed once in flight. There's a whole lot of math going on there to make sure the engine(s) generate enough power to counter the weight of the plane (and engines and passengers and cargo, etc.), to make sure the wings can provide enough lift and stability at various speeds, and of course keeping the thing from both falling out of the sky or flailing madly about like a popped balloon.
The one arguable exception I can think of is certain military jets. There are a couple of VTOL jets (Harriers, for example) that redirect the jet exhaust downwards to provide altitude without needing to use a runway, but even those jets still gain forward momentum (and thus, lift) before being able to fly, so I wouldn't consider them to be exceptions to the rule. They can leave the ground while tethered, though, so maybe that counts if that qualifies as "lifting off", even though they still require forward air speed to actually "fly".
Anyways, I stand by it all. A plane (especially a prop plane) cannot lift off without forward momentum. A conveyor belt that matches the wheel speed will have little to no effect, since wheels do not provide any power to a plane's forward momentum, only braking. VTOL jets are exceptions in practice, though their takeoff method is analogous to rockets, not other planes.
Blah, blah blah, etc. etc. I guess we'll find out tomorrow :)
Everyone agrees that lift is created by wind flowing over the wings, that a stationary plane will not lift off.
The real question to this problem is "is there forward motion?"
You used the perfect example with the people mover but came to the wrong conclusion. Take for example, you exit the people mover and are standing on stationary ground while the bag remains on the people mover with the wheels spinning. At this point the bag represents the airplane, the wheels represent the wheels of course, the people mover represents the treadmill, and you or your feet represent the engine and propeller.
The treadmill is moving backwards, but you stand still. As soon you walk forward, the bag moves forward regardless of the people mover's speed. You are propelling yourself forward against the stationary ground and the bag moves forward, the people mover is inconsequential. The engine and propeller propels itself forward against the stationary AIR regardless of the treadmill's speed and it moves forward. Forward motion creates airflow, creates lift.
This works the same for any airplane regardless of whether the propeller is in front of the wings, behind the wings, or is a jet. It's the propulsion against the, stationary air the moves the airplane forward, the treadmill actually has nothing to do with the problem.
1. A car or person on a treadmill is not the same thing as an airplane on a treadmill. The car/person propels itself against the backwards moving treadmill, while the airplane propels itself against the stationary air. Wheels/feet vs propeller/jet.
2. A conveyor belt, people mover, or treadmill CANNOT be substituted for a rope holding the plane stationary. The actual question is: "Will the plane move forward?"
3. It doesn't matter what type or size of propeller or jet engine or it's location relative to the wings. Prop blast over the wings has nothing to do with the problem. The actual question is: "Will the plane move forward?"