5150 Lego Posted January 5, 2009 Posted January 5, 2009 A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor 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? Quote
CP5670 Posted January 5, 2009 Posted January 5, 2009 http://blag.xkcd.com/2008/09/09/the-goddam...damn-treadmill/ Basically, the answer depends on how you interpret the question. Quote
AgentRick Posted January 5, 2009 Posted January 5, 2009 How bout a more interesting question? A helicopter spins its propellers incoherence with a turntable that spins in the opposite direction at the same speed. So in question... Quote
5150 Lego Posted January 5, 2009 Author Posted January 5, 2009 http://blag.xkcd.com/2008/09/09/the-goddam...damn-treadmill/Basically, the answer depends on how you interpret the question. Now that takes all the fun out of it now doesn't it?? Also, i heard they were going to do this on mythbusters, but didn't think they acttualy got to it. The've been saying they were going to do it for years and hadn't. ....Intill last year i guess. How bout a more interesting question?A helicopter spins its propellers incoherence with a turntable that spins in the opposite direction at the same speed. So in question... Well one things for sure, The pilot is going to have one fell of a dissy spell! Quote
frogstudio Posted January 5, 2009 Posted January 5, 2009 (edited) A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor 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? No, it wouldn't. Why? Simply because a plane takes off because of the sustentation generated by air flowing through its wings, and for this to happen, the plane itself must be moving. In this case, the plane is completely static, the only thing moving are its wheels. :) Edited January 5, 2009 by frogstudio Quote
5150 Lego Posted January 5, 2009 Author Posted January 5, 2009 Yes, but remember that a plane relies on thrust from the engine to move, not the wheels. Quote
Legoman Posted January 5, 2009 Posted January 5, 2009 (edited) As far as i'm concerned, there are two main angles of this. 1 - as 5150 lego just said 'Yes, remember that a plane relies on thrust from the engine to move, not the wheels.' that should explain the first thought. It's what i thought at first. 2 - it would stay still. because... well, here's how i explain it. Take 5 CD's. Glue them together. Get two rocket boosters, 1 on each side, connected through the middle of the CD via a pole. (For the sake of explaining what i'm saying, imagine the boosters are always 180 degrees - parellel to the ground) If the boosters turn on, what happens? the thrust pushes the CD WHEEL forward. If THAT was on the conveyor... What would happen? The wheel would be turning but the engines would be static, because THE THRUST OF THE ENGINE PUSHES THE WHEELS FORWARD. Therefore, the plane would not take of, because as frogstudio said "Because a plane takes off because of the sustentation generated by air flowing through its wings, and for this to happen, the plane itself must be moving. In this case, the plane is completely static, the only thing moving are its wheels." He explained why it wouldn't take of, I'm explaining why it wouldn't move... But the more i think about it, the more i get mixed up Edited January 5, 2009 by legoman Quote
5150 Lego Posted January 6, 2009 Author Posted January 6, 2009 (edited) The wheel would be turning but the engines would be static, because THE THRUST OF THE ENGINE PUSHES THE WHEELS FORWARD. Therefore, the plane would not take of, because as frogstudio said "Because a plane takes off because of the sustentation generated by air flowing through its wings, and for this to happen, the plane itself must be moving. In this case, the plane is completely static, the only thing moving are its wheels." While i see what your getting at with the CD example, a key factor is missing. Your assuming that the engines only have enough thrust to compensate for the reverse motion of the wheels. Think about it. The wheels have nothing to do with the plane moving but simply act as a measure to reduce friction between the plane and the runway. The propeller or jet engine is what makes the plane move. Not its wheels. Even if the plane is being moved backwards at 50 miles per hour the prop will still pull it through the air. All the conveyer belt will do is make the wheels spin backwards (which arent even connected to anything). Weather the conveyer is making the wheels spin backwards at 50 mph or 500 mph the prop will still pull it through the air. Think of a water plane. It has no problem taking off against the current. Edited January 6, 2009 by 5150 Lego Quote
Freddie Posted January 6, 2009 Posted January 6, 2009 The logic here that most people miss, is that the aircraft's means of propulsion are independent of the ground, unless you count hills for quick acceleration/decelleration. Unlike cars, which move by applying rotating force to the wheels, and thus making the car move, aircraft propel themselves by forcing air to move backwards relative to the aircraft itself. Aircraft with some sort of jet system will also use the expanding forces of the expelled warm air to further gain more forward thrust, but that's beside the point here. When the means of propulsion are independent of the force attempting to hinder propulsion, the obstacle remains a moot point, and won't factor in at all. So, the plane would take off regardless if the conveyor attempted to hinder take-off or not. If it was a wind-turbine however, you'd end up with a plane that would fly at 0 kmh. I wouldn't go anywhere, but it'd still fly. Quote
rriggs Posted January 6, 2009 Posted January 6, 2009 Conventional (key word!) aircraft need moving air flowing over and under their wings in order to get lift. With the wheels moving on a conveyor of the same speed the air speed at the wings will remain (as near as damn it) 0 knots and will therefore not produce any lift so the plane won't fly. No lift = No flight. Cheers Rog Quote
Hobbes Posted January 6, 2009 Posted January 6, 2009 1. A plane is not a car, meaning its wheels aren't driven by the engines. They just "hang" on an axle on the landing gear (simplified). Thus, the wheels of a plane turn because the plane is moving due to air flow created by the engines and the wheels having grip at that point. 2. If the pilot starts the engines they create air flow. Due to the plane's design it starts to move. If it "hung" in the air (i.e. if it flew with the landing gear out) the wheels wouldn't move. Thus, starting the engines doesn't turn the wheels but creates air flow and makes the plane move. If the wheels have grip at that point, they move. If not, they don't. If the surface were perfectly slick they would skid along it due to lack of grip and not being connected to anything (except the axle that holds them on the landing gear). 3. If you start the treadmill first the wheels would move because of it. But since they're not connected to anything the plane wouldn't move. 4. As soon as the pilot starts the engines there is air flow, the plane moves and there is no way anymore for the treadmill to keep the plane from moving. The treadmill only has an effect on the wheels, not the airflow created by the engines. There is no way to start the engines first, make the wheels and treadmill turn/move and keep the plane still. If there is airflow and thus thrust that moves the plane - it moves. And eventually takes off. It's actually a stupid question... Quote
CP5670 Posted January 6, 2009 Posted January 6, 2009 (edited) Those of you who are saying that the plane is "moving" or "not moving" should clarify what that means. Is that relative to the ground, the bottom/top of the treadmill or the air particles around the plane? In any case, the real issue here is that the behavior of the treadmill makes no sense and cannot occur in real life. As the article I posted earlier says, the problem is essentially asking what happens if you take an immovable plane and move it. Edited January 6, 2009 by CP5670 Quote
General Armendariz Posted January 6, 2009 Posted January 6, 2009 Yes it can because its like a treadmill, it gives the plane speed than it just lifts off. Quote
AgentRick Posted January 7, 2009 Posted January 7, 2009 But do you realize the treadmill is going in the OPPOSITE Direction? Quote
Wouwie Posted January 7, 2009 Posted January 7, 2009 I have no idea...But in 30 minutes I know the awnser! Mythbusters - Airplane on an conveyerbeld is on the TV now Quote
CP5670 Posted January 7, 2009 Posted January 7, 2009 The question is more of a hypothetical one. The airplane will always take off in real life because it's impossible to make a treadmill that does anything like what the problem says. Quote
Wouwie Posted January 7, 2009 Posted January 7, 2009 The awnser: It can take off. They've made a long piece of fabric, which is pulled by an SUV. The plane is on the fabric with the same speed as the car pulls the fabric under the plane. It takes off. Question awnserd, case solved; Quote
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