So, you have a plane on a conveyor belt...

[Camacha]

Thanks for explaining that same thing for the n^th time!

[Leszek]

Thanks for explaining that same thing for the n^th time!

http://www.ferneanimalsanctuary.org/uploads/images/website/Seal%20photo%204%20520.jpg

Some people will need N+1 times. Really, this isn't the first time I have seen this one on a forum. This one just doesn't die.

[Lukaszenko]

There is a plane parked on a conveyor belt. The plane fires up it's engine and attempts to take off. The belt runs in the opposite direction. Does the plane take off?

Go!

(Sorry mods.)

How fast does the belt run in the opposite direction? 1 mph? whatever it takes to try to stop the plane? Or, as I originally saw this riddle worded, the same speed as the plane? Depending on how the conveyor functions can make the answer to the riddle easier or harder to pin down.

[lajoswinkler]

This is one of those threads which inevitably end in casualties.

[Mad Rocket Scientist]

If the plane is in a wind tunnel it will takeoff easily; however the question is whether the plane has airspeed, which can become confusing because, as that xkcd blog post that was linked awhile ago explains, a treadmill moving backwards at the same speed as the plane moves forward is somewhat ambiguous statement.

- - - Updated - - -

I wasn't expecting to see people still arguing discussing.

Anyway, I found what I think is a PERFECT analysis for Kerbal. Includes KSP-worthy diagram.

http://blog.xkcd.com/2008/09/09/the-...amn-treadmill/

Here's the link.

[Laie]

I'm sorry... I can't resist. Probably all of this has been mentioned already, but quickly browsing the thread I think I can put it better than anyone else so far.

Variant 1, pure thought experiment:

Wheels and conveyor belt will reach infinite speed almost instantly. The plane takes off anyways (which means that relative to the belt, it's moving at a speed of "infinity and then some" -- but never mind, infinity is weird like that).

Variant 2, with a modicum of physics:

Friction in the wheel bearings creates a backward force that matches the thrust of the engines. Plane won't move relative to the ground or air and not take off. The conveyor belts' speed won't be inifinite, but ridiculously fast: 50 times the speed of sound or 10% the speed of light or something.

Variant 3, why stop with just a little physics:

Whatever speed the equation in #2 resolves to, any real-life-like conveyor belt will disintegrate at much lower speeds. The plane will then get stuck in the rollers.

[Sean Mirrsen]

Variant 3, why stop with just a little physics:

Whatever speed the equation in #2 resolves to, any real-life-like conveyor belt will disintegrate at much lower speeds. The plane will then get stuck in the rollers.

Or alternatively, as I said, a plane with a high-friction undercarriage.

Heck, you could twist the definition of "conveyor belt" and have a seaplane try to take off from an artificial "river" with controlled current speed. Crank it up far enough and it won't lift out of the water while going full thrust and staying in place relative to the channel.

[peadar1987]

Or alternatively, as I said, a plane with a high-friction undercarriage.

Heck, you could twist the definition of "conveyor belt" and have a seaplane try to take off from an artificial "river" with controlled current speed. Crank it up far enough and it won't lift out of the water while going full thrust and staying in place relative to the channel.

Actually, kinetic friction is more or less independent of rolling speed. You are going to get some tiny increase due to hysteresis in the rubber of the tyres and viscous friction from the lubricant in the wheel bearings, but mechanical stresses will tear the entire system apart before you see an appreciable increase in resistive force with increased velocity. The same applies for a plane on skids, only the "tearing apart" stage will happen a lot earlier.

[Barrett]

Yes, it does.

I think the idea behind this myth is that a plane on a conveyor belt can somehow build up enough speed despite not actually changing position to the point where it will just suddenly take off vertically off the conveyor belt, maintain that speed, and fly off on it's merry way. Obviously that's not true. We know that air must flow over and under the wings and impact the wings at certain angles to generate lift. If the plane is not changing position, there can be no lift. All this video does is demonstrate that a conveyor belt can't actually keep a plane from moving. This would be the equivalent to a person running faster than a treadmill, and running right off the front of it. Or more accurately, a person attaching a rocket to their back and pushing themselves forward (since in the first analogy the legs are actually generating the movement while in an airplane it's the propeller).

[KerikBalm]

"can a helicopter take off from a conveyor moving at any speed" the answer is also "yes".

No... Helicopters have a maximum forward velocity dictated by retreating blade stall... if its sitting on a conveyor belt moving forward at the same speed as the tip of a rotor blade... then the advancing blade is moving at 2x speed, and the retreating blade is stationary to the air, and there is a massive lift imbalance... and it won't take off.

It comes down to small details... in order for the plane to not be able to takeoff, the wheel friction must produce a significant force on the plane. Pretty much all modern, powered planes, are capable of much more thrust than needed for level flight. The force from wheel friction must equal or exceed the planes excess thrust.

In pretty much every case, this would result in so much wheel friction, that the wheel would fly apart due to centrifugal force/the bearings would fail/the whole thing would melt.

If on the other hand, you take a human powered plane... and put someone out of shape on there... the plane might not even take off if you use bad wheels instead of very low friction wheels.

Sure there are fringe cases, but for practical purposes, the plane takes off, or it was never much of a plane. I'm not even sure what principle this question is supposed to test...

Is it people's understanding of airspeed? Sure the plane won't take off if it has no forward airspeed... but a conveyor belt won't be sufficient to stop it from building up forward airspeed.

Its essentially equivalent to asking if a plane can takeoff going downwind... and the answer is yes, a plane can, it just has a higher groundspeed and uses more runway.

Planes in general can. If you want to take a specific plane with poorly suited undercarriage, or not much excess power, that can barely takeoff in the first place... then sure, this change will increase the takeoff requirements a bit, and may "tip the scales".

[Deutherius]

I'm not even sure what principle this question is supposed to test...

The principle of trollbait.

[magnemoe]

No... Helicopters have a maximum forward velocity dictated by retreating blade stall... if its sitting on a conveyor belt moving forward at the same speed as the tip of a rotor blade... then the advancing blade is moving at 2x speed, and the retreating blade is stationary to the air, and there is a massive lift imbalance... and it won't take off.

It comes down to small details... in order for the plane to not be able to takeoff, the wheel friction must produce a significant force on the plane. Pretty much all modern, powered planes, are capable of much more thrust than needed for level flight. The force from wheel friction must equal or exceed the planes excess thrust.

In pretty much every case, this would result in so much wheel friction, that the wheel would fly apart due to centrifugal force/the bearings would fail/the whole thing would melt.

If on the other hand, you take a human powered plane... and put someone out of shape on there... the plane might not even take off if you use bad wheels instead of very low friction wheels.

Sure there are fringe cases, but for practical purposes, the plane takes off, or it was never much of a plane. I'm not even sure what principle this question is supposed to test...

Is it people's understanding of airspeed? Sure the plane won't take off if it has no forward airspeed... but a conveyor belt won't be sufficient to stop it from building up forward airspeed.

Its essentially equivalent to asking if a plane can takeoff going downwind... and the answer is yes, a plane can, it just has a higher groundspeed and uses more runway.

Planes in general can. If you want to take a specific plane with poorly suited undercarriage, or not much excess power, that can barely takeoff in the first place... then sure, this change will increase the takeoff requirements a bit, and may "tip the scales".

Airspeed is relevant not ground speed, we can ignore wheel rotation as its an minimal effect even at very high speeds, plane must anyway be able to take off on an reasonable runway. Now if you have an seaplane or skis you have far higher friction and might not work.

Think people either think planes use power to wheels or mess up this with carrier operations and launching into the wind while moving.

[FlyingPete]

Simple: can a plane take off with a strong tailwind (assuming a long runway). The situation is identical apart from the observer's frame of reference.

[andrewas]

Think people either think planes use power to wheels or mess up this with carrier operations and launching into the wind while moving.

Theres an XKCD article linked a few pages back that goes into why some people don't get that the plane (will/will not) take off. The question is slightly ambiguous as to how the conveyor moves in response to the plane.

Some assume that the conveyor accelerates backwards as the plane accelerates forwards, and since the wheels mostly decouple the conveyor from the aircraft, the plane takes off more or less as normal - unless the wheels spin quickly enough to fail, in which case bad things are going to happen.

Others assume that the conveyor accelerates backwards until the force it exerts through the wheels is enough to cancel the thrust from the engines. In theory, that would keep the plane from taking off. In practice, the speed needed to exert that much force through the wheels means that that *something* is going to take off.

[Fel]

The principle of trollbait.

Pretty much, yeah. If we just tested the components rather than focusing on needing a whole airplane, we'd get faster results... instead we get "this." It's just a matter of force, so get some wheels and start building! "Wheel on a conveyor belt" has a better ring to it.

[Camacha]

Pretty much, yeah. If we just tested the components rather than focusing on needing a whole airplane, we'd get faster results... instead we get "this." It's just a matter of force, so get some wheels and start building! "Wheel on a conveyor belt" has a better ring to it.

What is there to test? The answers are pretty clear, only a fairly intentional misinterpretation of what is going on will yield other results.

[WinkAllKerb'']

The principle of trollbait.

It depend if your considering the question from a "n" dimensional inertial frame of reference point of view or only 3 or 4; ) but usually most specialities in pratice only need to use max 3 or 4 at least 99% of time. ; )

I do believe sometime this is also why the switch from a finite number to non finite and undefined flutctuating value is disturbing for most and could require a lot of time to adapt and selfscale to clearly comprehend.

*shrug*

Edited April 28, 2015 by WinkAllKerb''
0/10 i never seen that movie.

[jara_]

I'm an engineer and I feel sad when people without much knowledge of science ask these kind of questions seriously.

Try explaining them about how to make a plane take off without moving forward and they start to seizure.

[Sean Mirrsen]

I'm an engineer and I feel sad when people without much knowledge of science ask these kind of questions seriously.

Try explaining them about how to make a plane take off without moving forward and they start to seizure.

Er, you can't make a fixed-wing aircraft take off without moving it forward relative to the airstream. The aircraft can theoretically deflect its own thrust downward, via numerous means, but then it earns the label "VTOL" (or some variation thereof), and is no longer in the spirit of the question.

And the scenario is valid for aircraft with high-friction undercarriage, that may generate enough thrust to counteract the friction, but not to accelerate against the airstream sufficiently to reduce that friction via lift.

[VirtualCLD]

Er, you can't make a fixed-wing aircraft take off without moving it forward relative to the airstream. The aircraft can theoretically deflect its own thrust downward, via numerous means, but then it earns the label "VTOL" (or some variation thereof), and is no longer in the spirit of the question.

And the scenario is valid for aircraft with high-friction undercarriage, that may generate enough thrust to counteract the friction, but not to accelerate against the airstream sufficiently to reduce that friction via lift.

You're absolutely right, but I have encountered people similar to the ones that jara_ described who do not seem to comprehend the difference between moving with respect to the air and moving with respect to the ground. Now I realize the original question is left ambiguous on purpose, but I have never considered the scenario of an aircraft with high-friction undercarriage because I don't believe it is a valid scenario.

[LN400]

The question is rather, if you put an F-15 or an SU-35 on that conveyor belt and fire up the engines, would the conveyor belt fly?

[Fel]

What is there to test? The answers are pretty clear, only a fairly intentional misinterpretation of what is going on will yield other results.

There's 72 posts in this thread; getting people to agree about the forces involved removes the whole "I know more than you" b.s. And, quite frankly, we're a community that excels in engineering, given most of these forces aren't strongly interdependent, testing resolves the issue faster and teaches more than just "Team No-Fly" vs "Team Fly"

I expected more from the KSP community, that is all.

[Sean Mirrsen]

You're absolutely right, but I have encountered people similar to the ones that jara_ described who do not seem to comprehend the difference between moving with respect to the air and moving with respect to the ground. Now I realize the original question is left ambiguous on purpose, but I have never considered the scenario of an aircraft with high-friction undercarriage because I don't believe it is a valid scenario.

I've actually provided a very valid scenario earlier in this thread, that only requires you to stretch the definition of "conveyor belt". Expand it to encompass any stretch of moving surface that the aircraft would normally use to take off, and there is exactly one scenario where a person who mistakenly applies the "plane can take off from a conveyor belt easy" paradigm could fail to get his plane to take off.

A seaplane, taking off from a river, upstream.

[VirtualCLD]

I've actually provided a very valid scenario earlier in this thread, that only requires you to stretch the definition of "conveyor belt". Expand it to encompass any stretch of moving surface that the aircraft would normally use to take off, and there is exactly one scenario where a person who mistakenly applies the "plane can take off from a conveyor belt easy" paradigm could fail to get his plane to take off.

A seaplane, taking off from a river, upstream.

I've only flown a seaplane once (and not on landing), so I am NOT an expert, but I believe even that is still possible for reasonable river speeds. Now if we stretch this further and put the gear down while in the water (an often fatal mistake) or say the water is moving over 80 mph, then yes, the plane would not be able to take off. I will add that the reason for not taking off will most likely be due to RUD more than anything else.

[Camacha]

I expected more from the KSP community, that is all.

I feel exactly the same, for different reasons. There is little reason to debate this subject for 8 pages when answers have pretty much been established a long time ago. Insisting on re-examing the same subject ad nauseum serves little purpose.

I've actually provided a very valid scenario earlier in this thread, that only requires you to stretch the definition of "conveyor belt".

There is always a scenario where you can toy with the rules and change the outcome, but like I said before, you would have to be quite intentionally misrepresenting the scenario, or looking at it from the other side, would have to be quite wilfully looking for unusual interpretations of the scenario, even when conceding that different people have different views.