[Dakar the Fourth]

The entire MythBusters thing left me unsatisfied. If the "conveyer belt" was moving at the same speed as the plane, how was it the plane moved forward *before* it took off?

[jokell82]

Originally Posted by Dakar the Fourth 

The entire MythBusters thing left me unsatisfied. If the "conveyer belt" was moving at the same speed as the plane, how was it the plane moved forward *before* it took off?

Because the plane will move forward regardless of how the conveyor belt moves. If you notice the whees of the plane are not hooked up to a motor, so how they spin is irrelevant to how the plane moves.

[Dakar the Fourth]

Originally Posted by jokell82 

Because the plane will move forward regardless of how the conveyor belt moves. If you notice the whees of the plane are not hooked up to a motor, so how they spin is irrelevant to how the plane moves.

I understand the wheels aren't providing the thrust,
but if the plane is moving forward then the wheels are either moving faster than the conveyor belt, or they are not in (consistent) contact with it.

If there was a way for them to measure the speed of the wheels when take-off occurred, I bet that'd shut me up. $5 says they were moving faster than Jamie's "conveyor belt" though.

[jokell82]

It's just not possible. The wheels and conveyor belt create a (mostly) frictionless environment, and the plane moves forward. It's the exact same theory as this kind of plane:


Just try to think about how the conveyor actually puts no force on the plane itself, only on the wheels that spin independent of how the plane moves.

[Dakar the Fourth]

That's interesting.

However, I think in a perfect environment, the conveyor and wheels would accelerate to infinity as the plane's engine kept trying move them forward. As I said, I'd like to see a comparison of the wheel speed and conveyor speed in that experiment. I argue they weren't synchronous.

[jokell82]

Except the wheels only accelerate at the rate the conveyor belt does, not the plane.

[Dakar the Fourth]

So you argue they were synchronous in the MythBusters experiment?

[jokell82]

Originally Posted by Dakar the Fourth 

So you argue they were synchronous in the MythBusters experiment?

Dunno. Like most things they do it wasn't very scientific, so it'd be hard to tell from the little clip we saw. Does make for good television, though.

[Dakar the Fourth]

I know. I don't think they'll revisit it (What could they change/improve significantly?), and I doubt people are that rapid about this myth.

[olePigeon]

Originally Posted by Dakar the Fourth 

If there was a way for them to measure the speed of the wheels when take-off occurred, I bet that'd shut me up. $5 says they were moving faster than Jamie's "conveyor belt" though.

The speed of the wheels is completely irrelevant.

[Dakar the Fourth]

Originally Posted by olePigeon 

The speed of the wheels is completely irrelevant.

Not according to the experiment: "The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. "

If I'm right, then the MythBusters failed to replicate the experiment accurately.

[jokell82]

Originally Posted by Dakar the Fourth 

Not according to the experiment: "The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. "

If I'm right, then the MythBusters failed to replicate the experiment accurately.

I believe he meant that it's irrelevant with regards to the plane taking off or not.

[Dakar the Fourth]

Alright then, I think there's a much simpler way to do this then--tether a plane and see if it can take off (gain altitude). That is unless I'm missing the point of the experiment, but my take on it was that matching the tire and conveyor speed was to eliminate any forward movement.
(Aren't their toys that do this?)

[jokell82]

The point is trying to make you realize that matching the tire and conveyor speed does not eliminate forward movement because that movement is created by the propellers/engines of the plane regardless of what the wheels are doing.

[Chuckit]

Originally Posted by jokell82 

The point is trying to make you realize that matching the tire and conveyor speed does not eliminate forward movement because that movement is created by the propellers/engines of the plane regardless of what the wheels are doing.

Wouldn't the friction from the skidding tires mess you up, though?

[Dakar the Fourth]

Originally Posted by jokell82 

The point is trying to make you realize that matching the tire and conveyor speed does not eliminate forward movement because that movement is created by the propellers/engines of the plane regardless of what the wheels are doing.

What I'm not getting past is if there's forward movement, then the wheels must be moving faster than the conveyor belt (so long as they are in contact with it). Is it possible to move something forward on a conveyor belt without it exceeding said conveyor belt's speed?

[smacintush]

Originally Posted by Chuckit 

Wouldn't the friction from the skidding tires mess you up, though?

Why would they be skidding?

[Chuckit]

Originally Posted by smacintush 

Why would they be skidding?

Because the wheels are only moving as fast as the belt, while the plane is moving more quickly than the belt in the opposite direction. Having the wheels only go as fast as the belt, the plane cannot move forward without skidding (for the same reason that a plane would skid if its wheels were going the same speed as the stationary ground). That's how it seems to me, anyway.

[smacintush]

Originally Posted by Dakar the Fourth 

What I'm not getting past is if there's forward movement, then the wheels must be moving faster than the conveyor belt (so long as they are in contact with it). Is it possible to move something forward on a conveyor belt without it exceeding said conveyor belt's speed?

You are confusing yourself.

The wheels will be moving faster then the conveyor, yes.

The wheel speed will always be the speed of the conveyor PLUS the speed of the plane.

This will have a near zero effect on the planes forward speed. The only factor would be whether the wheels could take the speed before being destroyed but the plane will have take off long before that would happen.

[smacintush]

Originally Posted by Chuckit 

Because the wheels are only moving as fast as the belt, while the plane is moving more quickly than the belt in the opposite direction. Having the wheels only go as fast as the belt, the plane cannot move forward without skidding (for the same reason that a plane would skid if its wheels were going the same speed as the stationary ground). That's how it seems to me, anyway.

Yes I see what you meant now. Sorry.

[Dakar the Fourth]

Originally Posted by Dakar² 

I just need to see this happen on MythBusters and I'll be happy.

Apparently I'm a big fat liar.

[Dakar the Fourth]

Originally Posted by smacintush 

You are confusing yourself.

The wheels will be moving faster then the conveyor, yes.

Not in the experiment, though. Theoretically, this conveyor is matching the wheel speed always.

[Dakar the Fourth]

Alright, I'll try and boil this down to two questions I believe are the heart of the experiment:

1. Is the point of the conveyer in this theoretical experiment to eliminate forward motion by the plane?
2. Can a plane take off with zero forward motion?

If the answer is yes to both, then I have my answer.

[smacintush]

Originally Posted by Dakar the Fourth 

Alright, I'll try and boil this down to two questions I believe are the heart of the experiment:

1. Is the point of the conveyer in this theoretical experiment to eliminate forward motion by the plane?
2. Can a plane take off with zero forward motion?

If the answer is yes to both, then I have my answer.

1. The point is IF the conveyor would stop the forward motion, which is retarded. The plane will move forward regardless of the conveyor.

2. No, but there will be forward motion, even on a conveyor because the plane moves forward based upon the thrust of its propellor/turbines.

[Dakar the Fourth]

So you're telling me, even though the conveyor belt and wheel speed are exactly the same, the plane will still move forward?

[Sage]

For me, the easiest way to think about it is to just draw a free body diagram (this is just on the x-axis, since that’s the part that confuses people).




That’s all you can draw, since we’re assuming relatively frictionless wheels (thus, no net force from the conveyor belt or wheels, relative to the plane).

[Dakar the Fourth]

Hmmm... coming at this from another angle, if the wheels and conveyor belt synchronized, they would basically emulate something like a hovercraft (free standing with no ground friction)?

[Andhee]

No matter what, air is going to go over the being over the wings. Once it gets to a high enough speed (speed it usually takes off at), and the wings are lifted, it will take off, but only for a very short time as the engines arnt running, so it would soon crash down within a matter of seconds due to the mass amount of air resistance.

Who gives a whether the wheels are spinning or not, all it needs is forward momentum and friction.

[Dakar the Fourth]

Originally Posted by Andhee 

all it needs is forward momentum and friction.

What a coincidence that these are the very topics we're discussing!

[Andhee]

Originally Posted by Dakar the Fourth 

What a coincidence that these are the very topics we're discussing!

Ah right sorry about that, I read the first page and though I'd post my thoughts.

But yeah, it will take off, so there is no point in discussing friction/momentum. The friction will be near enough the same on the conveyer belt as it is normally, apart from there will be more drag due to the thrusters not being on. Momentum will be the same, as p=mv, mass and velocity are the same.

[analogika]

Good grief.

It's the return of the stupid thread.

[TETENAL]

Originally Posted by Dakar the Fourth 

So you're telling me, even though the conveyor belt and wheel speed are exactly the same, the plane will still move forward?

As I said before, if you ask this question, then you have an impossible system:

The tangential speed of a tyre of an airplane moving forward on a conveyer belt is:
tangential tyre speed = conveyer belt speed + airplane speed

The condition for the conveyer belt as per this thought experiment:
conveyer belt speed = tangential tyre speed

As one can easily see both equations can not be true at the same time. That's why it doesn't make sense to ask the question this way. It defines a logically impossible conveyer belt. So Mythbusters didn't and they picked the more sensible airplane's takeoff speed for the conveyer belt.

[phantomdragonz]

it amazes me that people will still bicker over this subject.... for those of you who believe the plane will take off... you should breed and populate this world with people of intelligence...

for those of you who think it wont should just kill yourself now and spare this world of your stupidity.

I would be willing to bet all of my possessions on this earth that the plane will take off...

this bet will not keep me up at night...

have a good day!

-Zach

[Chuckit]

Originally Posted by phantomdragonz 

it amazes me that people will still bicker over this subject.... for those of you who believe the plane will take off... you should breed and populate this world with people of intelligence...

for those of you who think it wont should just kill yourself now and spare this world of your stupidity.

This question really doesn't have anything to do with intelligence.

[voodoo]

Originally Posted by Chuckit 

This question really doesn't have anything to do with intelligence.

True, it has everything to do with one's interpretation of the question and what physical forces are deemed to apply.

[analogika]

Originally Posted by voodoo 

True, it has everything to do with one's interpretation of the question

That's arguable and doesn't fundamentally affect the answer - either the question makes sense, and there is only one answer, or the question makes no sense and isn't really answerable.

Originally Posted by voodoo 

and what physical forces are deemed to apply.

Not really.

The only force that affects whether it's going to lift off or not is thrust.

Friction only enters into the equation if you *assume* that at some point, the wheel bearings are going to overheat and lock up. That's not going to happen on any aircraft maintained to proper operational standards.

So, since this is an *extremely unlikey* unknown and not mentioned in the question, it can be as safely ignored as the fact that the airplane MIGHT be destroyed by a descending asteroid, or that the kerosine might run out because the plane has not yet been refuelled.

Neither of which have been specified in the question, either.

[analogika]

Originally Posted by Chuckit 

This question really doesn't have anything to do with intelligence.

Does "the ability to reason" sound slightly less insulting?

It's just as true.

[voodoo]

Originally Posted by analogika 

That's arguable and doesn't fundamentally affect the answer - either the question makes sense, and there is only one answer, or the question makes no sense and isn't really answerable.

Of course it's arguable, captain obvious. Check every single public forum for threads on this question.

However, your binary interpretation is completely reasonble, but completely wrong. A question can well be without sense and yet have an answer. It can also make complete sense and have no answer.

You'll learn that one day.

Not really.

Yes really. The world isn't just 'einz, zwei, eins, zwei' you know.

The only force that affects whether it's going to lift off or not is thrust.

No...

Friction only enters into the equation if you *assume* that at some point, the wheel bearings are going to overheat and lock up. That's not going to happen on any aircraft maintained to proper operational standards.

Well you assume quite a bit if you are taking the original question literally. The fact is that friction works on everything at all times.

Of course it applies to perfectly maintained (Prussian even) wheel bearings. So as this plane is taking off it will need far more thrust than a plane not working against the conveyor belt. More thrust means more powerful engines, which means more mass, which means more friction..

Already friction plays a part and it's exponential. A plane takes off at 200 km/h (for instance) which just means it needs to overcome that amount of friction. Do you think a plane can take off at 400 km/h?

Not with normal powerplants.

So, in reality (everything being real except for that mystical conveyor belt) a B777 could never lift off under those circumstances.

Perhaps a special made plane, with super powerful powerplants and virtually no other mass could make it, but then we're back to assuming some special circumstances.

It all depends, my Prussian besserwisser.

[analogika]

Originally Posted by voodoo 

Of course it applies to perfectly maintained (Prussian even) wheel bearings. So as this plane is taking off it will need far more thrust than a plane not working against the conveyor belt. More thrust means more powerful engines, which means more mass, which means more friction..

Already friction plays a part and it's exponential. A plane takes off at 200 km/h (for instance) which just means it needs to overcome that amount of friction. Do you think a plane can take off at 400 km/h?

Not with normal powerplants.

The friction provided by the wheel bearings is negligible compared to the friction provided by the AIR, and the staying power of sheer inertia.

It is NOT a real-world issue. (If it were, there would be no way for an aircraft to take off on skis, whose friction from a stand-still is undoubtedly higher than that of properly maintained bearings.)


For you, though, it obviously IS an issue - along with a number of other issues you seem keen on displaying.

[voodoo]

Originally Posted by analogika 

The friction provided by the wheel bearings is negligible compared to the friction provided by the AIR, and the staying power of sheer inertia.

Absoloutly not! The wheel friction becomes unmanageble at that velocity. Try to LAND a plane at 400 km/h and see how the undercarriage farres - just by friction with the ground.

Short answer is that the wheels break off and it melts when it hits the concrete.

It is NOT a real-world issue. (If it were, there would be no way for an aircraft to take off on skis, whose friction from a stand-still is undoubtedly higher than that of properly maintained bearings.)

Heh, I would have mentioned seaplanes as a way better example. Friction against water is much higher than against snow, but well..

The friction against the air at the low airspeed at takeoff is relevant and must be overcome along with the friction of the ground to achieve liftoff.. naturally. Now this friction with the ground is more than double which means more power is needed.

Elementary. Power == more mass => more friction.

==> a normal plane (e.g. B777) isn't going to take off with those extra forces and the undercarriage is compromised, no matter how well it is maintained

For you, though, it obviously IS an issue - along with a number of other issues you seem keen on displaying.

Peh you're far better at being the stereotypical German than making jabs

[analogika]

Originally Posted by voodoo 

Absoloutly not! The wheel friction becomes unmanageble at that velocity. Try to LAND a plane at 400 km/h and see how the undercarriage farres - just by friction with the ground.

Um.

There is no "friction against the ground" in this equation. The only friction that opposes forward motion is that of the axle BEARINGS WITHIN THE WHEELS.

Your example speaks of LANDING, where the problem is getting the wheels rotating at landing speed from zero.

There's a reason you see landing-skid marks, but no take-off skid marks.

Originally Posted by voodoo 

Heh, I would have mentioned seaplanes as a way better example. Friction against water is much higher than against snow, but well..

The friction against the air at the low airspeed at takeoff is relevant and must be overcome along with the friction of the ground to achieve liftoff.. naturally. Now this friction with the ground is more than double which means more power is needed.

Again: It's not friction against the ground, but friction within the wheel bearings. The wheel just rolls along and eliminates ground friction, substituting bearing friction instead - that's why it's there.

How fast can a wheel spin before the bearing locks up?

I'm SURE there is NO WAY a wheel bearing would be approved for commercial aircraft if it wasn't capable of handling AT LEAST twice the take-off velocity...

Originally Posted by voodoo 

==> a normal plane (e.g. B777) isn't going to take off with those extra forces and the undercarriage is compromised, no matter how well it is maintained

What about a normal plane like the Cessna 172?

Originally Posted by voodoo 

Peh you're far better at being the stereotypical German than making jabs

Just knock off the Nazi allusions. They make you look like a total asshole.

[slpdLoad]

See you all at page 15.

[Tiresias]

The plane moves forward and takes off. The conveyor belt only makes the wheels spin twice as fast.

The Straight Dope: An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

[LegendaryPinkOx]

Why in the hell are there 7 pages debating this?

[voodoo]

Originally Posted by analogika 

Just knock off the Nazi allusions. They make you look like a total asshole.

Nazi? That would be way too easy. No no, I would not, have not and will not compare you to a Nazi.

When you get the voodoo-treatment you get high-class quality, not street trash like Nazi comparisons. Silly person.

[smacintush]

Originally Posted by LegendaryPinkOx 

Why in the hell are there 7 pages debating this?

Because people who can't accept that they are wrong in saying that they plane won't take off are arguing semantics in order to make themselves right.

[voodoo]

Originally Posted by smacintush 

Because people who can't accept that they are wrong in saying that they plane will take off are arguing semantics in order to make themselves right.

Same difference.

[Tiresias]

But could a modified high-performance car with wings take off from a runway without a conveyor belt? Eh? Discuss that one.

[design219]

http://forums.macnn.com/89/macnn-lou...-bumping-into/

[ghporter]

Is this thing STILL going on?

An airplane gets lift from air moving over and under its wings. If it gets enough air crosses over and under the wing, then enough lift to counter the weight of the aircraft will be generated. Except for brakes, airplanes generally don't have anything connected to the wheels at all-they just roll. So the given set of circumstances is either contrived to equate to a null situation (the wheels won't be moving by themselves and so neither will the conveyor belt) or it's worded so badly that it's completely meaningless.