You have a plane and a conveyor belt.

[thirdlegstump]

Originally posted by: PurdueRy

Originally posted by: TankGuys
The bottom line here is that it depends on the wording of the question, and how people interpret it. There are some "wordings" where the plane won't take off, and some that the plane will take off.

If people make the strict assuption that the treadmill moves backwards at the same speed as the wheels rotate forward, you run into issues of infinite acceleration and other fun stuff, and the plane won't take off.

If people assume the belt moves backward at a velocity equal to and opposite the plane's speed, then the plane takes off just fine, wheels spinning 2x the normal speed.



Consider this thought experiment:

You know those toy cars that you roll backwards, and when you let go, they move forward? Let's say you put it on a STILL treadmill. The car moves forward fine right? Now, put said car on treadmill that's moving backwards, and we all know the car moves at it's speed minus the speed of the treadmill, right? This is because the propulsion system of the car depends on the spinning of its wheels, which are affected by the treadmill. This all makes sense so far, yes?

Now, consider a standard matchbox car. Put that car on a non-moving treadmill. What happens? It stays still. Now, turn the treadmill on, and what happens? Car rolls backwards, right? Now, reach out your hand and hold the car still, while the readmill is moving. What happens now? The car stays put right? Do you think it takes much force to actually hold it still? (try it!) NOW... try pushing the car forward, while the treadmill is moving. You can do it, right? Is it hard? Is it noticably harder than pushing the car forward when the treadmill is not moving? (again, try it!)

This is exactly what happens with a plane. The force of the engines is pushing on the plane - NOT the treadmill. Aside from the bit of friction at the axle that the engine thrust has to overcome, the engines, air, and plane don't really care what happens to the treadmill.

Seriously, anyone who has a treadmill and a matchbox car at home can easily prove this to themselves

excellent example.

Think of a treadmill going downhill. The force cause by gravity simulates the engines force. The treadmill can rotate all it wan't backward but that won't do anything to stop the acceleration of a toy car.(wheels are not tied to power on toy car)

Actually it's a bit more complex than this. There is no 'hand' in the case of an aircraft so there's nothing that's holding the plane in place. The engine thrust has to do this in order for the plane to appear still. Therefore, in order to create the necessary headwind, the engine has to work harder than the usual take off power to compensate for this reverse force. Therefore it's not a fact that the engines don't have to work very hard to produce necessary thrust. It actually has to work much harder than usual (depending on the velocity of the treadmill)

 

[TankGuys]

Originally posted by: mugs
The wording is fine. People always blame the wording and never back it up.

I know there isn't much explaination into this - the problem is that the explaination is quite complicted, and people who are already confused would get even more so

Even if the speed of the wheels is infinite, the plane could take off.

Nope... consider this: The major reason the treadmill affects the plane at all has to do with the friction at the axle, and to some extent, some angular momemtum issues. There is a point where if the treadmill can speed the wheels up enough, those otherwise minor factors would in fact overcome the force the engines were capable of providing. This is why the wording is actually important.

Going back to the thought experiment, if you have your matchbox car on a treadmill rolling back at only 5mph, it's trivial to hold the car in place. If the treadmill is ripping backwards at 150,000 miles per second, it's far less simple. Again, this can be easily shown with a spring scale, car, and a treadmill.

 

[PurdueRy]

Originally posted by: deathkoba

Originally posted by: PurdueRy

Originally posted by: TankGuys
The bottom line here is that it depends on the wording of the question, and how people interpret it. There are some "wordings" where the plane won't take off, and some that the plane will take off.

If people make the strict assuption that the treadmill moves backwards at the same speed as the wheels rotate forward, you run into issues of infinite acceleration and other fun stuff, and the plane won't take off.

If people assume the belt moves backward at a velocity equal to and opposite the plane's speed, then the plane takes off just fine, wheels spinning 2x the normal speed.



Consider this thought experiment:

You know those toy cars that you roll backwards, and when you let go, they move forward? Let's say you put it on a STILL treadmill. The car moves forward fine right? Now, put said car on treadmill that's moving backwards, and we all know the car moves at it's speed minus the speed of the treadmill, right? This is because the propulsion system of the car depends on the spinning of its wheels, which are affected by the treadmill. This all makes sense so far, yes?

Now, consider a standard matchbox car. Put that car on a non-moving treadmill. What happens? It stays still. Now, turn the treadmill on, and what happens? Car rolls backwards, right? Now, reach out your hand and hold the car still, while the readmill is moving. What happens now? The car stays put right? Do you think it takes much force to actually hold it still? (try it!) NOW... try pushing the car forward, while the treadmill is moving. You can do it, right? Is it hard? Is it noticably harder than pushing the car forward when the treadmill is not moving? (again, try it!)

This is exactly what happens with a plane. The force of the engines is pushing on the plane - NOT the treadmill. Aside from the bit of friction at the axle that the engine thrust has to overcome, the engines, air, and plane don't really care what happens to the treadmill.

Seriously, anyone who has a treadmill and a matchbox car at home can easily prove this to themselves

excellent example.

Think of a treadmill going downhill. The force cause by gravity simulates the engines force. The treadmill can rotate all it wan't backward but that won't do anything to stop the acceleration of a toy car.(wheels are not tied to power on toy car)

Actually it's a bit more complex than this. There is no 'hand' in the case of an aircraft so there's nothing that's holding the plane in place. The engine thrust has to do this in order for the plane to appear still. Therefore, in order to create the necessary headwind, the engine has to work harder than the usual take off power to compensate for this reverse force. Therefore it's not a fact that the engines don't have to work very hard to produce necessary thrust. It actually has to work much harder than usual (depending on the velocity of the treadmill)

I mentioned no hand in my example. Just drop the car on the track(stationary initially) and speed it up as fast as you want.

 

[InlineFour]

think about how an airplane moves. it requires thrust to propel itself forward. since there is no force pushing the airplane backwards, the thrust will always be greater. the wheels and treadmill are irrelevant.

 

[TankGuys]

Originally posted by: InlineFour
tankguys i have an treadmill at home and will experiement with this in a minute. but i think you're forgetting that the conveyor belt can change speeds. it will match the speed of the wheels.

Yep, and as I said, that's why wording is important.

You're assuming the treadmill matches wheel speed which causes lot of problems.

The question is GENERALLY worded that the conveyor belt/treadmill moves backwards at the same rate that the plane moves forwards, which means if the plane is going forward at 10mph, the treadmill moves backward at 10mph, and the wheels "Spin at 20mph"



You can't test the first option at home - but you can test the second. If you had a spring scale, it'd be really easy, and you'd find that the force required to pull a car @2mph on a treadmill moving backward @2mph is remarkably similar to the force required to pull it on a still treadmill, which tells you the plane can take off quite easily.

Again, the major caveat out there is how people interpret the treadmill in the first place. Moving at the same speed as the wheels causes a host of problems and is too complicated to really easily discuss (within the confines of ATOT ). However, if we stick to the belt moving at equal/opposite speed of the plane, it's quite straightforward, and easy to test for yourself.

 

[mugs]

Originally posted by: TankGuys

Originally posted by: mugs
The wording is fine. People always blame the wording and never back it up.

I know there isn't much explaination into this - the problem is that the explaination is quite complicted, and people who are already confused would get even more so

Even if the speed of the wheels is infinite, the plane could take off.

Nope... consider this: The major reason the treadmill affects the plane at all has to do with the friction at the axle, and to some extent, some angular momemtum issues. There is a point where if the treadmill can speed the wheels up enough, those otherwise minor factors would in fact overcome the force the engines were capable of providing. This is why the wording is actually important.

Going back to the thought experiment, if you have your matchbox car on a treadmill rolling back at only 5mph, it's trivial to hold the car in place. If the treadmill is ripping backwards at 150,000 miles per second, it's far less simple. Again, this can be easily shown with a spring scale, car, and a treadmill.

The friction in the wheels is not dependent on the speed of the wheels.

 

[thirdlegstump]

Originally posted by: InlineFour
think about how an airplane moves. it requires thrust to propel itself forward. since there is no force pushing the airplane backwards, the thrust will always be greater. the wheels and treadmill are irrelevant.

Actually they are completely relevent as the plane is sitting on the treadmill. There is direct contact until the aircraft achieves lift. This contact and the motion of the treadmill is what's causing the backwards force. The engine would have to compensate for it via thrust.

 

[mugs]

Originally posted by: Indolent
This is the stupidest, most pointless questions I've ever seen. I think the biggest confusion is that most people get the idea that the conveyor belt is similar to a gym treadmill. If someone would have just explained that the conveyor belt runway would have to be just as long as a regular runway, most people would understand that it would take off. It seems that the people saying it won't take off read the question as the airplane is not moving with respect to the ground next to the conveyor (as I also first thought).

edit* unless someone did explain this and I just missed it reading through this thread.

It's not a stupid question at all. Explaining that the conveyor belt runway would have to be just as long as a regular runway would give away the answer. The question is clever because people just look at the surface and assume the question is trying to see if they know lift is dependent on airspeed, not ground speed.

 

[Yossarian]

Originally posted by: deathkoba

Originally posted by: InlineFour
think about how an airplane moves. it requires thrust to propel itself forward. since there is no force pushing the airplane backwards, the thrust will always be greater. the wheels and treadmill are irrelevant.

Actually they are completely relevent as the plane is sitting on the treadmill. There is direct contact until the aircraft achieves lift. This contact and the motion of the treadmill is what's causing the backwards force. The engine would have to compensate for it via thrust.

just fyi, there are several pilots on this forum (including myself) who have reached the conclusion that the plane does take off. I believe this includes skyking and giantpinkbunnyhead, who are both more qualified pilots than you and I are (instrument rated private).

 

[thirdlegstump]

Originally posted by: mugs

Originally posted by: TankGuys

Originally posted by: mugs
The wording is fine. People always blame the wording and never back it up.

I know there isn't much explaination into this - the problem is that the explaination is quite complicted, and people who are already confused would get even more so

Even if the speed of the wheels is infinite, the plane could take off.

Nope... consider this: The major reason the treadmill affects the plane at all has to do with the friction at the axle, and to some extent, some angular momemtum issues. There is a point where if the treadmill can speed the wheels up enough, those otherwise minor factors would in fact overcome the force the engines were capable of providing. This is why the wording is actually important.

Going back to the thought experiment, if you have your matchbox car on a treadmill rolling back at only 5mph, it's trivial to hold the car in place. If the treadmill is ripping backwards at 150,000 miles per second, it's far less simple. Again, this can be easily shown with a spring scale, car, and a treadmill.

The friction in the wheels is not dependent on the speed of the wheels.

That's like saying my buggers are not dependent of my nostrils. I mean they certainly relate to an extent.

 

[mugs]

Originally posted by: deathkoba

Originally posted by: mugs

Originally posted by: TankGuys

Originally posted by: mugs
The wording is fine. People always blame the wording and never back it up.

I know there isn't much explaination into this - the problem is that the explaination is quite complicted, and people who are already confused would get even more so

Even if the speed of the wheels is infinite, the plane could take off.

Nope... consider this: The major reason the treadmill affects the plane at all has to do with the friction at the axle, and to some extent, some angular momemtum issues. There is a point where if the treadmill can speed the wheels up enough, those otherwise minor factors would in fact overcome the force the engines were capable of providing. This is why the wording is actually important.

Going back to the thought experiment, if you have your matchbox car on a treadmill rolling back at only 5mph, it's trivial to hold the car in place. If the treadmill is ripping backwards at 150,000 miles per second, it's far less simple. Again, this can be easily shown with a spring scale, car, and a treadmill.

The friction in the wheels is not dependent on the speed of the wheels.

That's like saying my buggers are not dependent of my nostrils. I mean they certainly relate to an extent.

They relate to the extent that the wheels are actually moving. If they are not moving, the amount of friction is different. If they are moving, regardless of speed, the friction is the same.

 

[skyking]

Yeah, tire friction is a huge thing, and dependent on velocity and coefficient of friction. They choose to provide you with unlimited performance and ignore real world issues like maximum tire velocity in this question. And conveyor length

 

[thirdlegstump]

Originally posted by: Yossarian

Originally posted by: deathkoba

Originally posted by: InlineFour
think about how an airplane moves. it requires thrust to propel itself forward. since there is no force pushing the airplane backwards, the thrust will always be greater. the wheels and treadmill are irrelevant.

Actually they are completely relevent as the plane is sitting on the treadmill. There is direct contact until the aircraft achieves lift. This contact and the motion of the treadmill is what's causing the backwards force. The engine would have to compensate for it via thrust.

just fyi, there are several pilots on this forum (including myself) who have reached the conclusion that the plane does take off. I believe this includes skyking and giantpinkbunnyhead, who are both more qualified pilots than you and I are (instrument rated private).

I've stated that the plane can take off IF it has enough thrust to gain positive airspeed regardless of given conditions. As a fellow pilot, I trust that you know and have the natural senses to understand the given circumstances and the required circumstantial parameters to make such a question valid and to reach a produce able answer.

 

[thirdlegstump]

Originally posted by: mugs

Originally posted by: deathkoba

Originally posted by: mugs

Originally posted by: TankGuys

Originally posted by: mugs
The wording is fine. People always blame the wording and never back it up.

I know there isn't much explaination into this - the problem is that the explaination is quite complicted, and people who are already confused would get even more so

Even if the speed of the wheels is infinite, the plane could take off.

Nope... consider this: The major reason the treadmill affects the plane at all has to do with the friction at the axle, and to some extent, some angular momemtum issues. There is a point where if the treadmill can speed the wheels up enough, those otherwise minor factors would in fact overcome the force the engines were capable of providing. This is why the wording is actually important.

Going back to the thought experiment, if you have your matchbox car on a treadmill rolling back at only 5mph, it's trivial to hold the car in place. If the treadmill is ripping backwards at 150,000 miles per second, it's far less simple. Again, this can be easily shown with a spring scale, car, and a treadmill.

The friction in the wheels is not dependent on the speed of the wheels.

That's like saying my buggers are not dependent of my nostrils. I mean they certainly relate to an extent.

They relate to the extent that the wheels are actually moving. If they are not moving, the amount of friction is different. If they are moving, regardless of speed, the friction is the same.

Yes but do you understand how the friction affects takeoff performance?

 

[TankGuys]

EDIT: Post below...

 

[TankGuys]

Re: Speed of wheels vs. Friction...


Edit 4:

Okay, this gets complicated worrying about the whole speed of treadmill vs speed of wheels, so I'm not going to bother getting into it, for fear of this whole thread getting even more confusing. I'm also not quite qualified enough to really give any solid answers on that scenerio, so I don't want to post any mis-information.

 

[LazyGenius]

WOW!

I actually joined this forum to add my 2 cents worth to this discussion. I have not heard one explanation as to how the mass of the air moving through the engine is negated.

F=m*A plain and simple.

If you have enough mass flowing through the engine - the engine itself will move forward independent of what the wheels are doing. This is because the engine is grabbing the air. If you could somehow impose the same frictionless, massless properties on the air then this whole thing could work. If not - then the entire plane would move forward relative to the air and not the treadmill. You also have to take into consideration frame of reference also. Because if you were a stationary dot on the treadmill the plane would indeed be moving (but probably not flying).

If the thrust is great enough the mass will move forward independent of what the wheels are doing. But if one of the stipulation is that the wheels somehow exert enough force on the plane to keep it stationary then the point is moot. You have exerted an equal and opposite force to keep the object in place - so you are hamstringing the forward movement by the wording of your problem.

 

[skyking]

No problem, TankGuys, you have answered effectively within the scope of the question. It was a limited scope, and a nice little riddle for many. Posts like this one and the vocabulary test keep the wasted time on the internet from being a total waste, both of them got the juices flowing and the blood boiling

 

[cwdegood]

*Forwards thread to the Mythbusters*

 

[Indolent]

Originally posted by: mugs

Originally posted by: Indolent
This is the stupidest, most pointless questions I've ever seen. I think the biggest confusion is that most people get the idea that the conveyor belt is similar to a gym treadmill. If someone would have just explained that the conveyor belt runway would have to be just as long as a regular runway, most people would understand that it would take off. It seems that the people saying it won't take off read the question as the airplane is not moving with respect to the ground next to the conveyor (as I also first thought).

edit* unless someone did explain this and I just missed it reading through this thread.

It's not a stupid question at all. Explaining that the conveyor belt runway would have to be just as long as a regular runway would give away the answer. The question is clever because people just look at the surface and assume the question is trying to see if they know lift is dependent on airspeed, not ground speed.

Ok, it might not be a dumb quesiton, but, a lot of the arguing could have been avoided if this simple fact was explained. It would have cleard up most of the doubters misconceptions.

 

[DrPizza]

*sigh* I was the first one to correctly answer the last thread...

I suppose it's time for me to take care of this thread. At least through about the 125th reply, no one has pointed out that the OP was completely wrong in how he worded the problem.


First of all, let me try to explain the situation for those who think the plane is stationary:
1. Imagine a jet on an ice runway... Let's replace the wheels with some skids for it to slide on. Yet, with enough thrust from the engines, the jet *will* accelerate down the runway, without any wheels spinning.

2. Now, replace that ice runway with a concrete runway. But, this time, put ball bearings under the jet. Will it take off? Again, almost the same scenario. It'll slide down the runway over the ball bearings, without any wheels spinning.

3. Now, hoist that jet into the air and replace the skids with some freely spinning wheels. Make sure the wheels spin fairly freely. It doesn't matter how those wheels are spinning, the jet will accelerate down the runway.

HOWEVER, the OP (nanostuff) thought he was cleverly rephrasing the original post with better wording. I'm sorry, nanostuff, you are wrong. In fact, you've worded the original post such that it is *IMPOSSIBLE* for that to happen... it's not simply an engineering challenge that the original thread was (a very large treadmill.)

You stated, "the belt moves in reverse exactly as fast as the wheels move forward." Now, in the original thread, it stated the treadmill moves backward at the same speed the plane is moving forward. In that case, forward motion with respect to a stationary object on the ground would have made the most sense. However, now that you've rephrased it "as fast as the wheels move forward," I think the most logical place to judge that motion would be with respect to the treadmill. And, this is impossible. Let's keep the plane from accelerating for a moment and simply look at what would happen if the plane was to maintain forward motion at 1 mph with respect to the ground (not the runway.) No matter how fast the conveyor was running, the forward motion of the tires (with respect to the conveyor) would always be 1mph more.

If that isn't quite clear, imagine the jet is rolling forward at a constant 1mph toward this runway. The moment the wheels are on this conveyor runway, the conveyor will start spinning in the opposite direction, trying to match the speed of the wheels. As the conveyor starts to spin, its own motion will simply cause the wheels to spin forward even faster. With the jet moving at 1mph, the conveyor could speed up to 100mph, yet the wheels of the jet would be spinning forward at 101 mph; always 1mph faster.

Just to be sure the OP is wrong, I searched for another post of his in this thread:

The belt just senses rotation of the wheels and compensates for this,

IT CAN'T!!!
It cannot compensate for the rotation in the wheels.

Concluding, the moment the jet turns on its engines and starts to propel itself forward, the wheel/conveyor interface is going to start spinning faster and faster and faster, beyond the limits of what the ball-bearings in the wheels can handle. This will lead rather quickly, I'd think, to a complete failure in the landing gear. Assuming all of the wheels don't simultaneously burst into flames, we'll assume that initially, only some of them do. Because of the OP's incorrect wording, the plane actually *won't* take off, instead some red light in the cockpit is going to start flashing and alarms are going to start going off... "FIRE!" Hopefully before the fire spreads to the jet fuel, and the fire trucks arrive, the passengers can all get out safely. Then again, as they slide down the inflatable ladders to relative safety, they're going to land on the conveyor thingy which will probably still be spinning rather quickly from its own momentum. . The passengers will initially get flipped and bounced around on the runway, as if they jumped out of speeding car onto the road (except it'll be the road moving rather than a car) After a few seconds, they'll be flung off the runway where they'll repeat the process of losing some skin.


Do I get a prize First correct response again

 

[totalcommand]

Originally posted by: LazyGenius
WOW!

I actually joined this forum to add my 2 cents worth to this discussion. I have not heard one explanation as to how the mass of the air moving through the engine is negated.

F=m*A plain and simple.

If you have enough mass flowing through the engine - the engine itself will move forward independent of what the wheels are doing. This is because the engine is grabbing the air. If you could somehow impose the same frictionless, massless properties on the air then this whole thing could work. If not - then the entire plane would move forward relative to the air and not the treadmill. You also have to take into consideration frame of reference also. Because if you were a stationary dot on the treadmill the plane would indeed be moving (but probably not flying).

If the thrust is great enough the mass will move forward independent of what the wheels are doing. But if one of the stipulation is that the wheels somehow exert enough force on the plane to keep it stationary then the point is moot. You have exerted an equal and opposite force to keep the object in place - so you are hamstringing the forward movement by the wording of your problem.

ding ding ding! you are correct sir!

welcome to atot.

 

[DrPizza]

Originally posted by: NanoStuff
The plane increases it's thrust and the wheels begin to rotate. The belt compensates for the rotation of the wheels in reverse, as in the belt moves in reverse exactly as fast as the wheels move forward.

And if it matters, it's a nice sunny day and you have good tires, so you get perfect traction on the belt at all times. Your plane also happens to be very powerful and you can give it as much thrust as you like, but the source of thrust is at the back of the plane so it never provides airflow over the wing.

Does the plane take off?

Oh, while I'm pointing out things that are incorrect in the original post, I suppose I should add that even if the source of thrust was at the back of the plane (either propellers or jet engines), they *would* create some airflow over the wing. Where do you suppose the air comes from that goes into the prop/engine??

 

[imported_Condor]

Repost. Takes airspeed to fly, not tire rotation.

 

[imported_Condor]

Originally posted by: Yossarian

Originally posted by: deathkoba

Originally posted by: InlineFour
think about how an airplane moves. it requires thrust to propel itself forward. since there is no force pushing the airplane backwards, the thrust will always be greater. the wheels and treadmill are irrelevant.

Actually they are completely relevent as the plane is sitting on the treadmill. There is direct contact until the aircraft achieves lift. This contact and the motion of the treadmill is what's causing the backwards force. The engine would have to compensate for it via thrust.

just fyi, there are several pilots on this forum (including myself) who have reached the conclusion that the plane does take off. I believe this includes skyking and giantpinkbunnyhead, who are both more qualified pilots than you and I are (instrument rated private).

They may know the mechanics and the regulations, but they seem to have missed the physics completely! "The belt compensates for the rotation of the wheels in reverse, as in the belt moves in reverse exactly as fast as the wheels move forward." That means that the plane never gains any forward speed to provide lift from the flow around the lift surfaces.