Plane on a conveyor belt
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"I watched and listened to the disagreement for a while and was fascinated to see that the argument seemed to split between those who had some engineering or math background, all of whom said the airplane would takeoff and fly without any problem; and those with some other background, who visualized the airplane as having to push against the conveyor in order to gain speed. Because the conveyor equaled the airplane's push against the conveyor, the airplane stayed in one place over the ground and in the calm air could not get any airspeed and fly.
It was an interesting argument, but as things progressed, more rational heads prevailed, pointing out that the airplanes do not apply their thrust via their wheels, so the conveyor belt is irrelevant to whether the airplane will takeoff. One guy even got one of those rubber band powered wood and plastic airplane that sell for about a buck, put it on the treadmill someone foolishly donated to the Lounge years ago, thinking that pilots might actually exercise. He wound up the rubber band, set the treadmill to be level, and at its highest speed. Then he simultaneously set the airplane on the treadmill and let the prop start to turn. It took off without moving the slightest bit backwards."
http://videos.streetfire.net/player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B
Try looking at it another way. Lets pretend a jet plane is flying at 200mph and comes in for a touch-n-go. It does it's touch-n-go on this hypothetical conveyor that is moving 200mph in the opposite direction. When the jet touches down it will not cease to move forward. Instead the wheels of the jet will be moving roughtly 400mph. If the jet touched down on a normal runway then the wheels would be moving at 200mph. According to the "no fly" people, as soon as the jet touched the moving conveyor it would simply stop moving.
What's up for debate is how the wheels affect the plane when they are on the conveyor.
Nope, no debate.
Let me try this another way.
First, image you have a conveyor belt moving at -1 mph.
Next, put a free standing wheel on it spinning at 2 mph. Nothing is wrong with this so far.
The wheel would be moving relative to the air around it at 1 mph opposite the conveyor.
Just attach a plane to the wheel and you're good to go.
The thrust of the plane now accelerates it to 5 mph. The conveyor now is moving at -5 mph.
The wheels are forced to speed up to 10 mph. Once again everything is fine.
Go back to just the conveyor, spinning at -5 mph. You can put a wheel on it spinning at 10 mph no problem right? The wheel would be moving along the conveyor at a relative speed to the air at 5 mph. Just have a plane sitting on top of the wheels, and once again you've got no problem, the plane is moving at 5 mph as well.
As far as the body of the plane is concerned, the conveyor doesn't even exist.
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I already mentioned earlier that speed is inconsequential for a plane to fly. Relatively speaking the plane has a speed of zero because its position has not changed. But there are vector quantities being added here.
What you are basically trying to get at here is that if the plane's wheels are in free motion, then its position is independent of the belt. This is fallacious. When you hop on a moving train, you follow the train. You can't jump up and expect to land on a different traincar. It is NOT independent.
This is what Einstein realized in his theory of relativity, that everything is relative to everything else. We are on a planet that is rotating thousands of miles per hours yet we do not feel a thing. When I hop up into the air, I do not land in New York. If I had wheels attached to me (likewise on the train scenario) I do not stay in one place in space -- I move with the earth.
The plane moves with the conveyor belt regardless if it had wheels or if it "hopped".
The people in your post examples have not taken a class in physics.
This is not a matter of mathematics, or engineering. It's basic physics.
2005 Mercedes-Benz C240 4Matic
1993 Mazda Rx-7 Twin Turbo (sold)
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AcesHigh wrote:
What you are basically trying to get at here is that if the plane's wheels are in free motion, then its position is independent of the belt. This is fallacious. When you hop on a moving train, you follow the train. You can't jump up and expect to land on a different traincar. It is NOT independent.you're talking in 2 totally different directions here..
the planes position IS indepentant of the belt.... (now this isn't TOTALLY true due to the small amount of friction in the bearings, however it will be overcome)..
you're train analogy... you're already have momentum and you continue that momentum. Cause if we want to quote famous dead guys, look at Newton - "An object at rest remains at rest, and an object in motion remains in constant motion, unless acted on by an outside force." If you took a hollowed out train with no ends (to eliminate air resistance)... The train is stopped. You jump and while you're in the air the train moves. When you land you will not be in the same position.. This is because you are not independant of the train and did not already have momentum in either direction. If you're still standing on the ground when the train moves, you are not independant of the train anymore..
with the plane, the wheels make the plane independant of the conveyor (again this is ruling out the small bit of friction that is easily overcome)
How about this.. put the plane on a stopped conveyor and then have the conveyor instantly jump to 50mph. Will the plane stay in the exact same spot on the conveyor belt? NO! Yes it will move backwards some (again because of the easily overcomable friction...hell, it has to be overcome for a plane to fly in the first place!) however it will stay much closer to it's relative position than to it's original spot on the conveyor.
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now I also want to hit another angle...
like I've said, no matter what (once again, once the small bit of friction is overcome)... if the plane and conveyor are moving the same speeds then the plane HAS to be moving forward...
now is it possible for the conveyor to keep the plane in the same relative spot? yes....(most likely). To do this the conveyor would have to be turning at extreme speeds to multiply the drag in the wheel bearings enough to overcome the thrust of the engines...
however in the case, the conveyor would be turning very very fast, the plane wheels would be spinning at the same speed, however the PLANE WOULD HAVE A SPEED OF ZERO! -
In the first example with the toy plane and the treadmill, the plane flew because its vector was greater than the treadmill's. There was a positive net vector and thus it moved.
In the second quote, imagine a plane moving at 400mph. Now put a giant conveyor belt underneath it at 400mph. What happens? Relative to the belt, its speed is ZERO. Another simple vector addition. The earth is still moving at 400mph underneath the belt, but in terms of relativity the plane's movement is zero. Another plane flying parallel to the first plane will note that they aren't moving (provided they cannot see the clouds or the earth) even though they are in space!
I don't even want to read the third one, but I assume it's the same as the last two.
Again, what this all boils down to is relativity and vector addition. We as humans see things relative to other things. We see light as being fast because relative to us, it is fast. If someone were to move at the speed of light, normal humans would look like we were frozen in time.
2005 Mercedes-Benz C240 4Matic
1993 Mazda Rx-7 Twin Turbo (sold) -
I just want to make another quick comment here before I go..
We are not arguing if the the plane has a speed of zero because I already said that it did. Hopping inside a hollowed out train would still do the same thing as the hopping on top of the train.
Here is one last analogy to help everybody understand:
If you were on a train and running as fast you can the opposite way as the train, which is going as fast as you are. Provided you can run forever, you are running towards a overpass bridge which is 10 meters ahead of you. There are infinitely many train cars.
You are at a stationary 10 meters from the bridge. If you jump while running, you remain in the same place. Unless you increase your speed GREATER than the train, you will not move. Given the plane scenario that the belt is continuously variable, if we apply it to this scenario we find that if we run faster the train will go faster the opposite way and --- vector addition, zero displacement. You are still 10 meters away from the bridge no matter how much you run, jump, whatever.
Wheel speed does not matter, it is the overall plane speed that does. If the wheel speed is less than the conveyor belt speed (using vector addition one last time) we have a negative displacement which means the plane will move backwards infinitely many times.
Have a good night you guys.
2005 Mercedes-Benz C240 4Matic
1993 Mazda Rx-7 Twin Turbo (sold) -
Here's my theory...
Think of a Kite; a kite can lift off w/o moving forward; I think the thrust of the engine will pull the air over/under the wings, and therefore it would take off.. I could be wrong, just an analogy...Oh and if it was a propellered plaine, Propellers in front of the wing; wouldn't that create air flow over/under the wing as well?
depends on what type of plane we're talking about too I guess.. -
Aceshigh, you are incorrect that planes using gearing in the wheels for take off, they entirely use their jet engines for this, how else do you think they accelerate so damn fast down the runway? You keep looking at this from the point of view that the plane is not moving which is not true, it may not be moving as fast with x amount of thrust when y amount of conveyor movement is pushing the opposite direction, but it is moving at will once it generates enough forward thrust to start the wheels spinning.
Your last analogy again assumes that the airplane is using its wheels to generate enough speed (approx 150mph I am told is what it takes to get an airliner airborn) to take flight which is fundamentally what is flawing your analogies. The wheels on a jet are free rolling units (at least on take off and landing) whose only purpose is for minor steering correction and to allow the plane to easily roll when thrust is applied. As soon as you realize that fact you will realize that it is possible for the plane to move forward while on a conveyor belt.
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That is an interesting way of putting it, but I hope you can see the fallacy:
The earth is rotating at a speed of 1670km/sec. A Harrier air craft takes off straight up from the earth. Does it magically appear in Seattle as it lands?
2005 Mercedes-Benz C240 4Matic
1993 Mazda Rx-7 Twin Turbo (sold) -
THE PLANE/JET WILL NOT FLY!!!111!!!!!1!!!!
you guys are crazy that think it will fly. put a lil more thought into it.
the plane is standing still, if a plane could just lift off the ground standing still, they wouldnt need engines that creat thrust.
they need to have air movement under the wings. the only thing moving is the wheels, DUH. -
It figures that Chuck started this thread, it's worse than a political thread except people don't get bent out of shape and close it for no reason. LOL
Of course the plane will take off, what the wheels are doing doesn't really affect the plane as it accelerates. The pilot will hit the throttle, the thrust of the jet engine will push the plane forward, irreguardless of whether there is a conveyor under the wheels, or not. The plane will gain airspeed (again, it doesn't matter what the conveyor below the wheels is doing, the wheels would be spinning like a mofo because of the conveyor speed, but it really won't affect the acceleration of the plane relative to the ground). The plane will gain airspeed until the wings provide enough lift for the plane to take off.
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[/size]If it makes it easier to grasp, what would happen if the conveyor was moving in the same direction as the plane? Would the plane take off twice as quickly? No, it would accelerate like normal, but the wheels would be spinning in reverse until the speed of the plane matched the conveyor, once the plane matched and then exceeded the speed of the conveyor, the wheels would come to a stop, and then start moving in the forward direction (again, not affecting the acceleration of the plane).
A plane never moves via gear drive to the wheels. The plane moves forward by the thrust of the engines. The plane has brakes, apply the brake on one side and the plane turns, apply the brake to the other side and it turns the other direction.
Now that I've spoken, there is nothing else to say, you might as well lock the thread now.
DaveH
'94 Supra- 7.77 @ 176mph -
camzaro28 wrote:
THE PLANE/JET WILL NOT FLY!!!111!!!!!1!!!!you guys are crazy that think it will fly. put a lil more thought into it.
the plane is standing still, if a plane could just lift off the ground standing still, they wouldnt need engines that creat thrust.
they need to have air movement under the wings. the only thing moving is the wheels, DUH.You need to put more thought into it...all the wheels do is allow for easy rolling. Nowhere does it say the plane will stay in one spot (it won't) as the force pushing it forward is the thrust of the motors, <u>***NOT THE ROTATION OF THE WHEELS ***</u>if it were dependant on wheel rotation to move forward you would be correct, but since it doesn't (see side car analogy AGAIN) the plane will move forward via thrust from the motors which will cause airflow over the wings which will allow it to take off.
Everyone needs to quit thinking of the plane as a car and realize that forward motion is caused by the prop/jets, the wheels just make it slide along the ground easier.
One more attempt to make people see the light here:
Imagine the most slippery substance on earth was applied to the runway, so slippery that the wheels on a CAR would just sit and spin with the car going nowhere, just doing a burnout upon shifting into gear. Now think what would happen if you attached a jet engine to the roof of the car and turned it on producing thrust. What do you think would happen to the car. Do you think it would stand still? No, of course not, even if you shifted the car into reverse and floored it and somehow managed to match your wheel speed with that of the air speed produced by the jet engine pushing you you would still move forward due to the wheels not getting any traction on this surface and the jet engine being able to push the vehicle at will as it doesn't rely on traction to move it forward..... Same principles apply to the jet on the run way.
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haha, yeah once i read dave's post i understood. i was under the assumption that no matter what the plane was at a standstill and was gonna take off like that! i read over it a lil to quick. ah, oh well. i get it now.
YES the plane will take off. maybe i should have read the 9 or so pages b4 posting
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DaveH wrote:
It figures that Chuck started this thread, it's worse than a political thread except people don't get bent out of shape and close it for no reason. LOLOf course the plane will take off, what the wheels are doing doesn't really affect the plane as it accelerates. The pilot will hit the throttle, the thrust of the jet engine will push the plane forward, irreguardless of whether there is a conveyor under the wheels, or not. The plane will gain airspeed (again, it doesn't matter what the conveyor below the wheels is doing, the wheels would be spinning like a mofo because of the conveyor speed, but it really won't affect the acceleration of the plane relative to the ground). The plane will gain airspeed until the wings provide enough lift for the plane to take off.
If it makes it easier to grasp, what would happen if the conveyor was moving in the same direction as the plane? Would the plane take off twice as quickly? No, it would accelerate like normal, but the wheels would be spinning in reverse until the speed of the plane matched the conveyor, once the plane matched and then exceeded the speed of the conveyor, the wheels would come to a stop, and then start moving in the forward direction (again, not affecting the acceleration of the plane).
A plane never moves via gear drive to the wheels. The plane moves forward by the thrust of the engines. The plane has brakes, apply the brake on one side and the plane turns, apply the brake to the other side and it turns the other direction.
Now that I've spoken, there is nothing else to say, you might as well lock the thread now.
YAY!!! Dave and I agree on something again....maybe there is hope for him afterall.
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I think I finally see where we are butting heads, Tjamz; in your example you assume that the wheels are frictionless; if in fact the wheels were frictionless, then yes the plane would be able to move.
I interpreted the original scenario to be not an ideal world but one in which friction exists. The plane is set such that it's velocity relative to the belt surface is perfectly counteracted by the velocity of the surface. Thus, the plane is stationary with respect to the air, with its thrusters on (after all, isnt that what is moving the plane in the first place?). If this wasn't the case and the plane would be stationary WITH or WITHOUT its thrusters then this really isn't a Physics problem at all, but common sense. A guy on an ice rink holding a big fan would start sliding. It would make more sense to have the question set to where the speed of the plane is matched in an opposite fashion (that is, the belt is actually retarding the advance of the plane).
If the situation was how I interpreted earlier then you would have two outcomes:
Case #1: Wheels are not frictionless Answer = no, the plane doesn't move (as I have been trying to get at). There are static frictional forces that the plane must overcome, and if it is already giving 100% thrust and not going anywhere, it isn't taking off). The postulate here is that the treadmill is imparting enough drag on the plane via friction to counteract the thrust.
Case #2: Wheels are frictionless (belt can do no work) Answer = yes, the plane can take off. If world was frictionless then the surface would not matter. The thrusters might as well be off, for the belt moving wouldn't affect the plane at all.
This question is hard due to its ambiguity, not its physics setup.
2005 Mercedes-Benz C240 4Matic
1993 Mazda Rx-7 Twin Turbo (sold)
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