Taken from another forum I use, this poser was put forward and argued with incredible conviction from 5 angles or more, to reach either of two possible answers.
It's a pretty simple question but had some brilliant rows about all sorts of things going on to justify the answers.
Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
Your answers please, and please expand on them if possible
It's a pretty simple question but had some brilliant rows about all sorts of things going on to justify the answers.
Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
Your answers please, and please expand on them if possible
The plane can never take off. To take off, air is required to flow over the wings to generate lift, but since the plane remains stationary with respect to the ground (if I interpret the question correctly) then, given that there is no wind, it is also stationary relative to the air = no lift.
What arguments were used in favour of it being able to take off? I cannot conceive of any sensible ones, unless I have misunderstood.
What arguments were used in favour of it being able to take off? I cannot conceive of any sensible ones, unless I have misunderstood.
Hmm
Are you saying if the plane switched off power then it would travel backwards..?
With several exceptions (harrier helicopter) an normal aircraft wing needs forward motion to generate lift.
So in short your plane would not take off.
walk against an escalator at the same speed you will appear to be stationary.
Are you saying if the plane switched off power then it would travel backwards..?
With several exceptions (harrier helicopter) an normal aircraft wing needs forward motion to generate lift.
So in short your plane would not take off.
walk against an escalator at the same speed you will appear to be stationary.
If the belt rotates in opposite direction of the wheels and the speed matches, the plain stands still.
Even with no wind a plane take-off is established by the speed of the plane, generating wind or better said air pressure under the wings.
Planes should preferably take off with opposed wind or head wind to increase that effect.
So even if there was wind, it could never be enough to take off.
/Hugo
Even with no wind a plane take-off is established by the speed of the plane, generating wind or better said air pressure under the wings.
Planes should preferably take off with opposed wind or head wind to increase that effect.
So even if there was wind, it could never be enough to take off.
/Hugo
quickshift said:The conveyer simply matches the speed of the wheels but moves in the opposite direction - there isn't any more information than that.
Whether the plane takes off or not.... That's what you have to work out
I won't add anything anyone from the other forum has posted yet
But the "speed of the wheel" could be any of several different things! Be it angular velocity, the free tangential velocity of the wheel on its circumfrence, the velocity of the wheel surface in refrence to the fixed friction point in the negative theta direction, or the net horizontal translation speed of the entire wheel. Each one of them leads to different answers, althought I think the formulator of this question haven't quite thought this far. Similar ambiguity arises with "opposite direction". Each of those different definitions of "speed" listed above is a vector pointing in various different directions.
I think the main difficulty comes from how poorly this question is worded, not in how cleaver or decieving it is.
quickshift said:
Any more "intrepretations" without further specification is like arguing about religion. Insofar, as such, the quesiton has no meaning.
Maybe it'll be much more fruitful (hard to do worse than completely useless) to post arguments from the other thread this was posted from, so we can all get a good laugh
Do you all really think that an airplane takes off by powering the wheels and uses wheel-ground friction to propel itself? 
The engines, be it jet or propeller ones, excert a force on the air which makes the airplane move relative to the air above the ground. As long as the rolling friction of the wheels is not extreme, it doesn't matter what the speed of the plane relative to the ground is, or even what direction the wheels are rotating. That is, the ground is simply not interesting. The aircraft moves relative to the air mass of the atmosphere, not primarily relative to the ground, although it indirectly moves also realitve to the ground.
Edit: Seems somebody else posted the same answer while I was writing this post. There is hope for the forum after all.
The engines, be it jet or propeller ones, excert a force on the air which makes the airplane move relative to the air above the ground. As long as the rolling friction of the wheels is not extreme, it doesn't matter what the speed of the plane relative to the ground is, or even what direction the wheels are rotating. That is, the ground is simply not interesting. The aircraft moves relative to the air mass of the atmosphere, not primarily relative to the ground, although it indirectly moves also realitve to the ground.
Edit: Seems somebody else posted the same answer while I was writing this post. There is hope for the forum after all.
eVITAERC said:
Any more "intrepretations" without further specification is like arguing about religion. Insofar, as such, the quesiton has no meaning.
Maybe it'll be much more fruitful (hard to do worse than completely useless) to post arguments from the other thread this was posted from, so we can all get a good laugh
Here are some from the original forum the question is taken from - not the one I use.
McDoul
No. The plane is not generating any lift as it is not actually moving.
obvious really.... as whilst the plane may achieve a ground speed of 4-500mph it will have a physical air speed of 0 knots.
No Air speed. No lift. It will not take off.
It will only take off if moving through the air.
Next...
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Ryan
Perhaps if a plane taking off had anything to do with how fast the wheels were spinning around. Which it doesnt.
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Tim2
I would say yes as the speed generated by the plane runnning against the conver belt would be same as if it was running down the runway.
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dafuge
If the conveyer belt is moving in the opposite direction to the wheels, wouldn't it double the plane's speed?
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Doug1985
Thats what I thought, but then managed to confuse myself thinking about it
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Tim2
Is this a trick question because this could never happen in real life?
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Silverx
But wouldn't the conveyor simply pull the plane back?...
It's not like those cogs, it's a flat surface.
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Doog
YES, BECAUSE THE POWER ISN'T TRANSFERRED THROUGH THE WHEELS FOR THE LOVE OF CHRIST
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Gricehead
How can the travelator/conveyor move in the opposite direction to the wheels?
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Alicombol
The answer is no, as I see it.
First of all you do not need air speed to take off, only air flow through the aerofoil (wings), whether it is created by the thrust of an airplane or the force of the wind doesnt matter.
Second of all, if there is no wind, and the conveyor belt is opposing the wheel speed (i.e. net speed of 0), there is a net flow of 0 through the aerofoil, hence no lift, hence no takeoff.
So, cut a long story short, NO. (or 42 as was pointed out.)
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Benka_Williams
i don't believe it til i see it... someone build a scale replica
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Serpico
Lift is created by the air passing over the wing taking longer than air passing underneath it. The plane is not moving forwards, therefore there is no airflow and consequently no lift.
All hail me.
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Sky Blue Tom
There's no airspeed. When you go to the gym and run on a treadmill do you feel the wind in your face?
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Dougie
Yes it will take off because you'll get a head start on the guy operating the conveyor belt, everyone knows airport workers are mongs, like supermarket trolley collectors
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Jono
of course not.
the airplane's lift is generated by air pressure differences above and below the wing.
If the plane is stationary but the wheels moving there's no movement of air round the wings and so no lift.
You could do it in a wind tunnel with an artificial headwind sufficient to generate the required amount of lift.
a similar example would be putting a car on a dyno and getting the needle up to top speed. While it reads 150mph you wouldn't get a speeding ticket since you're not moving.
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Serpico
You can say that as much and as long as you like, but in my head the plane still moves backwards. I believe your analysis is correct, but can't persuade my inner-moppet that it's true.
Like I know the Earth is round/spherical but it sure doesn't look like it from my back garden.
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Doog
if one more person had come in and tried to tell me that I was wrong I probably would have lost the will to live tbh
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Rcjuk
You're wrong doog
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Harry Potter
Is it a magic plane or something?
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The Rafalution
wouldn't it basically be the same if you welded the axles not allowing the wheels to turn? i mean if the conveyer belt is moving at exactly the same speed as the wheels then they can't 'roll' forward while it contact with the belt so must surely skid? if they are moving at twice the speed of the plane as was mentioned earlier then the belt is still moving the same speed as the wheels, not the plane, yes?
A normal airplane will not take off without moving at fairly high speed Its the wings lifting power that gets it off ground! You dont have any lifting power if the airplane stands still Simple enough What is the purpose of a question like this anyway
It ought to be obvious to nearly everyone Are you aware of the things, airoplane designers go through to get the wing profile right?? Nearly as much as we audiofreaks go through to get the right sound Its really as simple as that
Steen
Its the wings lifting power that gets it off ground! You dont have any lifting power if the airplane stands still Simple enough What is the purpose of a question like this anyway It ought to be obvious to nearly everyone
Are you aware of the things, airoplane designers go through to get the wing profile right?? Nearly as much as we audiofreaks go through to get the right sound Its really as simple as that Steen
Oh dear,
sorry no hope for this forum
...18 Answers, 140 times read... and only 2 people who take time to really think about it for a moment...
I don't need to repeat it, as it was already mentioned 2 times, but again.. Has anyone of You EVER seen an airplane that powers the wheels of its landing gear in order to generate thrust for takeoff? Damn, I always wondered what these big, loud hairdryers beneath the wings were for...
I think I'll go to sleep now, this one has definitely grounded me for today
Greetings,
Andreas
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