yes if the wind blows hard enough it would pick the plane up off the ground, but thats not in the question.
again the problem possed all over the interweb states that the ground and plane move in opposite directions equally.
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Kaan Wrote:yes if the wind blows hard enough it would pick the plane up off the ground, but thats not in the question.
again the problem possed all over the interweb states that the ground and plane move in opposite directions equally.
You didn't read the whole statement.
But if the interweb says so....
The ground has such an absolutely small fraction of an effect on the movement of the plane through attachment related friction that it's practically irrelevant.
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asteele2 Wrote:Kaan, I think you're missing that the wheels aren't "connected" to the movement of the plane. The frictional losses considered are marginal, at best. You could move the threadmill under the plane at whatever speed you wanted and hold the plane in place with a glorified chain. The moment the thrusters activate the plane begins to move forward and, proportional to foward velocity, lift is generated.
take head out of ass... the plane never gets to move forward according to the problem at hand... if you dont move forward then you have no air under the wings.
 i guess we'll see on mythbusters.
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if this were the case, why dont we have cool small aircraft carriers with big ol' treadmills so the planes can take off instantly?
we have big ass carriers that have to force a plane to a minum speed (aka air under wings) so they can get off the ground.
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.RJ Wrote:the wheels/treadmill are frictionless. frictionless wheels mean the wheels dont roll.
frictionless treadmill means the treadmill doesnt turn.
this riddle has much suck
this is an aerodynamics question NOT a question of thrust (or thrusters for those that watch the sci-fi channel!  )
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The 'myth' is that if you put a plane on a conveyor belt/treadmill, and it is free to move at whatever speed the plane's wheels are turning (and always matching this speed), will the plane take off or remain stationary?
Here's a hint for y'all: The wheels arent driven by driveshafts. They just spin freely (without friction for the sake of argument in this example). Yes, we already knew this. But think about it again in the context of this example.
(09-25-2019, 03:18 PM)V1GiLaNtE Wrote: I think you need to see a mental health professional.
Kaan Wrote:this is an aerodynamics question
Right... so think about it in that sense.
Kaan Wrote:yes if the wind blows hard enough it would pick the plane up off the ground, but thats not in the question.
Getting warmer.
(09-25-2019, 03:18 PM)V1GiLaNtE Wrote: I think you need to see a mental health professional.
Kaan Wrote:this is an aerodynamics question NOT a question of thrust (or thrusters for those that watch the sci-fi channel! )
Get YOUR head out of your ass!
The WHEELS aren't propelling the plane, you dummy. They're just holding the damn thing up. The reason we don't have runway tread mills is that the treadmill must be as long as a traditional runway to generage lift... so what's the point of moving ground in that case?
It doesn't need mythbusters, for christ's sake.
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Evan Wrote:frictionless wheels mean the wheels dont roll.
frictionless treadmill means the treadmill doesnt turn.
Ok, the wheels/treadmill has negligible friction in the context of this exercise. Better?
(09-25-2019, 03:18 PM)V1GiLaNtE Wrote: I think you need to see a mental health professional.
if the plane is not moving relative to its position before the treadmill starts, it will not take off. if the vector is greater than whatever the speed is to take off regardless of how fast the treadmill is moving, then the plane will take off.
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Uh yeah I'm with Kaan. If the wheels just spin and the treadmill keeps the plane in place, no air is being forced underneath to create lift. It's just sitting there making the treadmill go faster and faster. It would be like putting a Corvette ZR1 on a lake of ice. You could mash the go pedal as much as you want, all you're going to get is a ton of exhaust gas and some melted ice. Even then the car would probably eventually move forward a little... in RJs example it would never move if the treadmill just moved under the power of the wheels, which were driven by the jet engines. I think. Ah man I'm confused now ha ha
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It won't fly. If the tradmill can offset the forward thrust it will never have forward motion. No forward motion means no lift. End of story.
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Apoc Wrote:It won't fly. If the tradmill can offset the forward thrust it will never have forward motion. No forward motion means no lift. End of story.
See, this is the problem. RJ didn't present us with a riddle, he presented us with a picture. If the treadmill can offset the forward thrust of the plane, then yes, it won't have forward motion. But the question is usually put that the speed of the treadmill = the speed of the wheels turning, which is independent of thrust and the plane will move forward, easily.
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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.
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WRXtranceformed Wrote:Uh yeah I'm with Kaan. If the wheels just spin and the treadmill keeps the plane in place, no air is being forced underneath to create lift. It's just sitting there making the treadmill go faster and faster. It would be like putting a Corvette ZR1 on a lake of ice. You could mash the go pedal as much as you want, all you're going to get is a ton of exhaust gas and some melted ice. Now think about the same Corvette with a jet engine on the back. The jet engine doesn't need traction to propel the car forward so regardless of whether the wheels have traction it will push the car forward.
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Apoc Wrote:It won't fly. If the tradmill can offset the forward thrust it will never have forward motion. No forward motion means no lift. End of story.
That's the thing, there's no offset occuring. The treadmill doesn't offset actions of the plane.
And Lee, the principal difference b/w a plane and an Corvette.. is that the plane is driven by thrusters attached to the body pushing air. A corvette is driven by wheels pushing against the ground. Think about how that will effect motion.
Haven't you guys ever seen the Bond movie with the jets attached to the ice-racer?
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CaptainHenreh Wrote:See, this is the problem. RJ didn't present us with a riddle, he presented us with a picture.
My fault - I made an assumption there.
WRXtranceformed Wrote:It would be like putting a Corvette ZR1 on a lake of ice. You could mash the go pedal as much as you want, all you're going to get is a ton of exhaust gas and some melted ice.
Planes do not drive the wheels though. No driveshafts.
Apoc Wrote:It won't fly. If the tradmill can offset the forward thrust
How can the treadmill offset forward thrust by spinning the wheels, when the wheels do not drive the forward motion of the plane?
(09-25-2019, 03:18 PM)V1GiLaNtE Wrote: I think you need to see a mental health professional.
put a plane with no thrust on the treadmill, does it move forward? stand still? or move backward?
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i think its the problem as stated that sucks and is unclear....but anyway
what RJ and Ginger is saying is:
the treadmill is as long as a runway. the thrust of the engines push against the air (not the treadmill) to propel the plane forward, so the treadmill really doesnt even move, or if its already moving, it means the wheels are just spinning faster.
remember, a treadmill is turned by the backward (push-off) pressure of your feet or wheels. a plane does not exert this pressure, it uses the air to push off.
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