This is highly controversial debate that's still raging I think. And I've edited some details to make it clear of the scenario. It is reflected in bold text

The question is that can a plane take off on a conveyor belt moving at the same speed in opposite direction?

In simple form - Can a plane take off while being stationary? Having it's forward rolling momentum elliminated?

Both camp of yes and no are split quite evenly.

The 'Yes' camp argued that :

- Plane do not use wheels to fly, so it can lift off even if the ground is moving away at the opposite direction at the same speed.

The 'No' camp argued that :

- If the plane remained stationary, it wouldn't have forward momentum for air to slice through to the wing to generate lift, hence it won't have lift off.

Sure, Mythbuster tested the myth, but through proper observation you will notice that both model and full scale plane in the myth has some forward momentum.

In my opinion, not definite fact, there's flaws in this debate.

- Propeller propelled air into the wing, thus able to generate lift despite being stationary.
- If plane are able to lift up without moving, why does it still need a runway? VTOL would be laughing stock since it's an invention that fixes nothing!

So IMO, a plane on a conveyor belt wouldn't fly away. I am on the 'No' camp

This post has been edited by wKkaY: Jun 20 2009, 03:33 PM

basically my idea is that the plane to obtain some minimum speed with respect to the AIR (v). So a conveyer (u) will certainly slow down the speed of the plan with respect to the air.

new take off speed = x = v+u

Hence, you will require a higher speed from the plane itself to take off.

Do tell me if i do any stupid mistake here.

Uhh, I don't think you have the setup right

The conveyor belt is moving away in a way that it's making the plane stationary.

what i mean the plane would require a higher speed than normal condition. The extra speed is to overcome the conveyor belt speed and to reach the take off speed.
If it remain stationary, this means the speed of the conveyor belt is the same as the plane. Certainly it wont take off.

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Added on June 16, 2009, 12:58 amactually another important factor would be the type of the plane. Whether it is jet engine, propellor engine also matters.

This post has been edited by ModularHelmet: Jun 16 2009, 12:58 AM

Yeap, I meant the latter, the conveyor belt moves away at the same speed at every condition. Meaning the plane is always stationary.

Consider another body that requires airspeed to fly - a kite. If you were to run on a conveyor belt holding a kite, will the kite take off?

Kites are very light and I think the wind itself can make the kite 'take off' if you just raise it with your arms while tilting it. But for planes I don't think this would be possible.

@TS
I agree with your opinion. If the plane can take off without a runway, why do we still need the runway? Hehe..

Correct me if I'm wrong,

The lift is created by the difference in pressure on the top of the wing and the bottom. This difference in pressure is due to the air velocity which is caused by the jets and also when the plane moves.

But, we're speaking ideally, since the conveyor's velocity is always changing due to the change in the velocity of the plane to keep it stationary right?
So, ideally if there exists a jet that could make the air velocity fast enough to create sufficient lift without moving the plane it should be possible.

What i think is:

A plane fly or take off because of the lift force generated by the pressure difference between the moving fluids around the aerofoil,which is also the reason why most planes need a runway to take off.Ignoring VTOL and vector trusting.Just a typical 747.

Since the conveyor belt always move in the opposite direction of the plane,the relative velocity of the plane and the conveyor belt is zero.This also mean the plane and the more importantly,the air around the plane is stationary.Or,in another words,the velocity of the air is zero.

By the drag force formule,

F = 0.5*C*A*p*v^2

where,
C is coefficient of drag
A is projected area of the plane to the flow
p is density of air
v is velocity of the fluid,which is air here.

Since velocity of the air is zero(boundary condition cant be formed here),there isnt any lift forceprovided by the aerofoil.The plane wouldnt take off.

This is just my 2 cents,as this is what i know.

Do correct me if im wrong.

for me, plane on a conveyor belt, wont take off on opposite direction. so, i think im in no camp.

The answer provided by "Cheesenium" is the most acceptable for your "curiousity", Thread Starter.

It takes an understanding in the concept of Lift Induced Drag.

Regards, Joey

Kite would fly IF you yanked on the string, providing movement for the kite through the air. It's the same why kite fly when it's high up even if you're standing still because there's enough gale in the atmosphere to keep it flying.

So far everybody saying no, I don't believe it's possible to fly either but anyone who's a fan of Mythbuster will notice that their plane took off on a conveyor belt so... explanation?

The video does not seem convincing that the speeds match exactly at every moment. How do they coordinate? Comparing a car and a plane, shouldn't the plane have a higher acceleration?

They did a control test to determine the minimum take off speed of the plane and it's shown that the car can match that speed.

I am not very convinced because I do noticed some forward momentum in the plane. I really hate to prove them wrong because I am a big fan of them but in truth I still don't see how a plane can fly while remaining stationary.

Another possible explanation is the plane design itself. The air pushed by the propeller actually passing through the wing, and there's enough lift to lift the wheel away from the conveyor belt, allowing it to accelerate forward and fly off.

This post has been edited by SeaGates: Jun 16 2009, 06:54 PM

Ah..a control test. But the accelerations would still be different (since they didn't mention the synchronisation method), right? This would cause a net force.

I havent watch the video yet,but what im sure is,the forces around the plane arent scaled properly which might give different result compare to real life.

my thought is as what you mentioned.

if plane going at X speed and belt going at -X speed, then it should be stationary.

in the video, the plane was moving at quite a substantial speed.

when stationary, there's no way a lift can be achieved with little to no air-movement.

im in 'yes' camp..

y i said so? it because the plane use engine thrust (air) to create a lift. it doesnt depends on the rotation of the wheel. this means the wheel just only a tool to supporting the plane to move..so when the conveyor belt moving in opposite direction of the plane at the same speed (theoretically) while the plane is in motion, the only thing different is just the wheel is rotate twice as much compared when the plane take off on normal ground.

or simply, the plane use thrust created by the engine to move forward, not by the rotation of the wheel.

That's my answer for now

@blue
Still...if you only use the machine thrust to move forward, it still need the air-movement to push it up. It's kinda different with Helicopter.

Thanks
Rcode

Once again , you guys do not understand that the wheels are FREE to move in forward or backward direction . The thrust will definitely be able to move the aircraft , regardless of what the conveyor belt is doing . Definitely definitely .

Absolutely no doubt about it !

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Added on June 17, 2009, 12:40 amIf you give this question to the guys at Airbus / Boeing / Bombardier / Embraer . They'd tell you its a NO BRAINER .

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Added on June 17, 2009, 12:42 amIf you are talking about WIND SPEED in the SAME DIRECTION as the aircraft is moving . Then that aircraft would definitely not fly . But this one , tyre's doesn't really affect much . Think about the thrust these engine's produce

This post has been edited by gavind87: Jun 17 2009, 12:42 AM

This is first time I saw this kind of topic. My answer is "YES". The plane can fly on a moving conveyor which travel at the same speed in the opposite direction.

I think we need to read the statement again. Can a plane take off on a conveyor belt moving at the speed in opposite direction?. It didn't mention that the plane will be stationary. Reason why there is a large number of people in the "no camp" because they misinterpret that the airplane will be stationary when the conveyor is moving in the opposite direction. Allow me explain a bit.

Cheesenium has post an extensive lift principle, so I'll skip that. For my explaination, you need to know at least the definition of newton's third law.

In newton's third law, every action has a reaction in the opposite direction. On a moving conveyor, an object will remain stationary (to a third eye) if there is a force in the opposite direction reacting on the object. For example, a car. The reaction force will be driving the car at the same speed of the conveyor, in the opposite direction. Because the conveyor is moving, the car appear stationary, like a dyno machine. Agree?

Now, in airplane, the reaction force comes from the thrust of the engine. When we run the engine, the engine move air from the suction to the exit. Basically it doesn't move air on top and below the wing, but across and within the engine. So there is plenty of thrust generated, but without air moving across the wings, it will never fly.

Next, imagine we mount the engine on a trolley, or cart (just the engine, without the wing). The thrust generated will transfer the force to the trolley, and hence pushing the trolley forward. The same effect can also be achieved with fan on a trolley. As the trolley travel forward, air travel across it. Although trolley cuts through the air, it don't fly because there is no wing. Now, we exchange the trolley with a hull and wings, so we get an airplane.

WHEN a plane travel forward (on the runway), only will, the air move across the wing, and create lift. (Hope everyone can keep up with this.)

Now, we upgrade the experiment a bit. We put a conveyor at the bottom. By running the airplane on a conveyor, the engine actually create thrust onto the hull. Despite whatever the ground speed maybe, the thrust generate force onto the stationary air. If the ground travel at light speed yet the air above it remain stationary, the thrust will generate force onto the air, unlike car which generate thrust onto the ground. So, the airplane will just keep moving forward.

When it gain enough speed, sufficient air movement across the wing, the airplane will take off. In fact, it will still require the same runway length disregard of whatever speed the conveyor maybe. Mythbuster is right.

This post has been edited by Aurora: Jun 17 2009, 01:01 AM

Ok ok, when you mentioned regardless of what the conveyor belt is doing. I think you still mistunderstand the concept of the conveyor belt here.

The conveyor belt is moving at a constant speed to match the forward speed of the aircraft so the aircraft is effectively stationary. The conveyor belt accelerate at the same rate as the engine would accelerate the aircraft. Effectively canceling it's forward rolling speed.

Elliminating the conveyor belt, and let's say use an alternative method to keep the aircraft stationary at the same spot- wheel brakes.

Will the aircraft lift off?

There's another clip that support the 'no' camp argument. By, guess who? The one that busted the myth themselves.



Problem with this argument is, not all the aeronautics experts are in the same camp. Asking several engineer from Airbus/ Boeing / Bombardier/ Embraer may yield different answer. So far the debate carried on because experts in this field are yet to come together to give a definite answer.

This is basic answer.

The conveyor is just a distraction, and having nothing or contributor to the airspeed on the wing.

If it can, carrier already build a conveyor on its on board runaway so that fighter jet can fly without the aid the steam pressure jack (I don't what they called it) under the jet wheel to assist the take-off on a short run way.

Ok I've edited first post to reflect the purpose of the conveyor belt and clarity of the quesiton. There are other better method to keep a plane stationary I guess

eh, who's talking about the wheel la?

btw the engine just moves the plane forward. it only creates the thrust, not the lift.

the lift happens when the air passes above and under the wing, creating the pressure that generates the lift.

the idea behind the conveyor belt going backwards is for it to counter the force generated by the propeller / engine.

That's a difference in magnitude, not in principle. Look at this wind tunnel where model planes are tested (the planes are towed, like kites) -


They would fly up momentarily if you tug on the string (as this creates airflow against the kite, and subsequently lift), but unless you continue running backwards it will fall back down as it loses airflow/lift.

Try flying a small and heavy kite on a windless day. It's frustrating and your legs will get a good workout

If it generates thrust towards the hull, doesn't that create drag and result in force going in the backwards direction, working against the airplane?

And besides, the point of the conveyor is to match the airplane's forward speed but in the opposite direction, so "keep moving forward" won't happen unless the wheels loose frictional contact with the belt.

Seagates: The way you've formulated the question will only lead to the single answer that the plane doesn't take off, because the plane's wings do not move relative to the air [at least the simplified version - which you seem to have incorporated your own assumptions into, thus solving the problem].

Let me ask the following question then:

Assume ideal conditions (perfectly horizontal belt, perfectly round wheel). Take a plane, with the brakes on the wheel off. Put it on a conveyor belt. Rotate that conveyor belt. What do you expect would happen?

The (perhaps unexpected) answer is that the plane would not move at all, even with friction between the belt and the plane's wheels, since this friction only serves to rotate the wheel, and not drag it behind along with it.

So, now the question is: If we turned on the engines, would the plane move forward? Yes it would, because when the engines were switched off and the belt running, it did not move, and also because it's the engine propels the plane forward by pushing the air backwards, and not through the wheels of the plane. The plane would then take off as usual.

But of course, we don't live in ideal conditions, and we then bring in a new concept: rolling resistance. This resistance leads to the wheel slowing down and ending up being dragged along by the belt. But let's now consider the (linear approximation) form of the equation, which we can get off Wikipedia [http://en.wikipedia.org/wiki/Rolling_resistance#Physical_formula]



where F is this rolling resistance that opposes the motion of this force
Crr a coefficient that relates the rolling resistance with the normal force of the aircraft on the wheel and the wheel's weight down onto the ground
Nf is the normal force of the aircraft on the wheel and the wheel's weight down onto the ground

So what do we get here? We have that the resistive force on the wheels (in a sense, 'pulling it backwards') is proportional to the force applied on the ground by the aircraft and the wheels, and not proportional to the velocity at which the airplane moves relative to the belt, i.e. the rolling resistance force is constant, assuming away fuel being burnt out and stuff factoring into that normal force and also assuming away any lift generated by the plane's wings when it moves.

It then becomes a question of: Can the engines then create enough forward thrust, or in essence, a force pushing the plane forwards such that it will overcome this rolling resistance being applied on the wheels which is 'pulling the plane back'? If yes, the plane will move forward, which does not comply with your question at all. If not, the plane won't move forward, and either stay still, or move backward if the thrust isn't large enough to overcome the rolling resistance.

And there's the answer: If the forward thrust is large enough as to overcome this added rolling resistance (added to the other dragging forces on a plane), it will still move forward on this conveyor belt, no matter how fast the conveyor belt spins, because the rolling resistance force remains constant, and the speed of the conveyor belt will only lead to the wheel spinning faster and faster, and the wheel will still go forward, because the added thrust of the plane will cause it to spin even faster than the conveyor belt ever can. And if this thrust can overcome the usual drag forces present when the plane's in motion wrt the air, plus this rolling resistance, it'll achieve the velocity needed to take off and it will take off.

Note: We only consider a linear approximation, it might be that the force is indeed dependent on the relative velocities at higher orders, and this argument will fail, but this semi-ideal model should give you an idea.

Note 2: The wings, etc, etc, don't matter at all, because the question that matters is: Will the plane move at all relative to a stationary observer not on either the belt or the plane, but say standing at the airport watching, and we also assume that the air remains still in the inertial frame of that observer (or to simplify further, will the wheels move forward). If it does move forward, it will also move forward wrt the air, and thus you have lift, and takeoff soon after if the thrust is also large enough

Note 3: The key word here is that we're talking about wheels, not cuboids on the belt or something. Friction on a cuboid will drag it backwards, but friction on a wheel will cause it to only rotate and nothing else [assuming again ideal conditions with no rolling resistance].

This post has been edited by bgeh: Jun 17 2009, 09:30 AM

I don't follow the discussion.

but as long as there's sufficient airflow across the wings, the plane will take flight.

It's the airspeed we should be looking at, not the ground speed. ie: in strong headwinds winds, a light aircraft can appear to be "hovering" above the ground.

In my opinion this is not done cause it will be wasting too much energy.

I'm struggling to follow the discussion as well.
But I agree with as long as there is sufficient airflow across the wings it will take off.

The plane's relative velocity with respect to the conveyor belt will be twice the speed then if it is actually moving on the ground.

google mythbuster, they busted the myth long time ago....

and yes, the plane can take off on a conveyer belt

This post has been edited by styrwr91: Jun 18 2009, 01:10 AM

Assume that instead of a conveyor belt, that we have 1000000 idiots pushing the plane because they have nothing better to do. Also assume that the surface area of the aircraft is so high that and it's weight so low that take off speed becomes largely irrelevant. Since there is motion and thus airflow, the plane will take off. As long as the brakes on the wheels are on and the coupling of energy complete, it should fly.

The problem with the conveyor belt ideal is actually a transfer of power at the wheels, because it's well known that you can throw aircraft off other devices that deliver power, like the steam catapult on US Navy carriers. When you turn the conveyor belt, the wheels would turn in the opposite direction, but this assume the wheels are perfect and have no loss, meaning they turn the forward energy of the conveyor belt perfectly into rearward energy in the wheel, thus you have an equalibrium.

The great big question is, due to losses, it would move, but in what coupled direction?

Yet the video clearly shows the ultralight moving with respect background behind it, thus generating air speed and lift.
While the thrust is indeed relative to the air (and not the ground), it is the air speed that generates the wings' lift.
So I'm not clear what the Mythbusters guys supposedly busted.

If the aircraft is on the conveyor belt . Parking brake on , the aircraft will not be going anywhere then

i think every physics question need diagram so that everybody wont misinterpret. because some people understand sentence differently

so yeah, a diagram would help

it very simple actually.if u have a miniature plane made of foam,tied one end of a string to the front of it and another to a rigid stationary object.switch on a table fan in front of it.u will see the plane flying rite?more or less.what happens is the and pushes the air through the wings of the plane to create lift.imagine the fan is swapped with propellers attached to the plane.it will do the same thing.so yes plane could take off on conveyor belt but only for very light propeller plane.air pushed by the propeller must be strong enough to create lift on the wings.i think u can visualize this, while u're at it try jet planes being put in similar condition,it wont take off

sometimes mythbusters do overlook some variables in an experiment.maybe we should call them missbusters?

now im confuse, mind explaining more on the jet aircraft?

Jet aircraft would be the worst type of aircraft to demonstrate this problem.

Somehow I realize this debate would not exist if somebody say 'Hey! Conveyor belt is the worse method of keeping a plane stationary'

waaa.. where's the diagram..
coz my lecturer told me, that he think in picture. so,
diagram please.
coz i could imagine 2 possibilities of the situation here..

Cherroy is right. The belt is distraction.

Imagine this:
A person running on a threadmill, then he gets a push from the back (which is the forward momentum from propeller/jet engine).
The guy either falls down face first, or he moves forward and run in front of the threadmill.

Same thing to the plane:
The conveyor belt will not be effective in keeping the plane stationary.
The plane will move forward DESPITE the conveyer belt.

OR,
there might be a possibility that the plane will just nose dive into the ground before it even moves anywhere (if the force applied is off centre)

The only way to get a plane to fly stationary is in the wind tunnel.

This is on mythbuster. It is busted if my memory serves me correctly.

Hate to quote myself. The correct answer is in my post. I take it some of the reader is from various background and age group, so I ignore all the physic equations, assuming an ideal case.

Yup, the hull is actually holding the engine from moving forward. However given enough thrust, it will overcome this resistance and start moving forward. If you take a close look at Boeing, you will notice a "lump" at the bottom of the hull, which connect to the wing.

The point of the discussion is conveyor belt and take-off. Friction is secondary. In fact, if there is zero friction and resistance, the plane will move with a household fan!!! Of course, if we were to discuss about extreme case, secondary factor will become big enough and will have significant effect. But that is not the point here.

Mythbuster target audience is general public, hence they try to make it less technical. bgeh pointed the friction, which would just bored the viewer if mythbuster broadcast this much.

im in the no camp...

When it starts moving forward, the conveyer belt spins backwards faster (e.g. hook it up in a closed feedback loop), keeping the plane where it is.

How does the plane overcome this and move forward?

the plane will not take off,
but if we stop the conveyor immediately the plane will take off....

[Assumption: In airplanes, the wheels are generally free spinning ones (assuming brakes not on - which is something we would probably assume for a plane that's taking off) so it shouldn't affect the motion of the airplane as a whole on the belt.

Perhaps an illustration of why a free spinning wheel is so important can be shown in the following example:

I'm assuming that most of us think of a car on a conveyor belt when we think of the question, and then think, hm, this would also apply to to an airplane. Let me try to make the connection of this 'free spinning wheel concept' with the car then. Take a normal car, but put ball bearings between the tyres and the driveshaft. Now press your pedal. The driveshaft will rotate like mad, but the rotation does not transfer to the tyre because of these ball bearings - in reality there probably will be a little rotation, though it'll probably require high RPM for this to be seen. In effect you have a *(sort of) reverse version of the conveyor belt, in which the shaft stays still, but the belt and wheel move instead. The point is the rotation of the driveshaft leads to no linear motion of the car, and this also happens in reverse for free spinning wheels]

My answer would be that the conveyor belt will end up spinning the wheels faster, but the wheels will always spin faster than the conveyor belt can ever do because of the forward thrust - which does not come from spinning any wheels (i.e. the source of the motion of the airplane is not due to spinning the wheels below, but by the engines blasting air backwards), so the conveyor belt will never be able to negate that thrust no matter how fast it moves, so the plane will move (or to use an analogy from analysis, for any large natural number M you give me, I can always find a natural number M + 1 such that M + 1 is always larger than M - that + 1 is due to the forward thrust), and the plane will continue taking off. See my above posting, but I guess that I didn't really explain that very clearly.

Edit: Reworded the whole post

*just realised that it probably doesn't work like that, but the principle still holds, that in a 'free spinning wheel', the shaft and the wheel itself are quite independent assuming the usual wheel assumptions which I'm lazy to type out

This post has been edited by bgeh: Jun 20 2009, 07:15 AM

<removed/> didn;t understand the scenario properly earlier =P

This post has been edited by chezzball: Jun 20 2009, 01:40 PM

Thanks bgeh, I'm an ass for not reading your first post here, because it started with ".. doesn't take off .." and was freakin long. I now understand the importance of Aurora's statement that a car generates thrust into the ground while a plane generates thrust into the air, because when put together with freely-moving wheels it leads to the result that the conveyor belt can never hold a plane back*.

* - taking this at face value, in all honesty i am probably never going to cross-check with the equations coz i'm not a physicist

It's an empirical equation, so yeah at best we take it as some approximation (which will probably fail at some limit). Actually if you used the same linear approximation, given powerful enough engines, you would reach the same conclusion that a plane on wheels with brakes applied will also fly off given the same condition of a conveyor belt.

Also, the above explaination only explains the first part of my post, where I claim that in the ideal case with friction and no rolling resistance, the plane simply doesn't move when the conveyor belt moves.

This post has been edited by bgeh: Jun 20 2009, 06:05 AM

actually la. the videos posted so far, we can't conclude what exactly is the Mythbuster test about since we'll need more info from the starting. they always explain.

so if we go by the thought of the propeller's thrust finally overcoming the speed of the conveyor belt then I wouldn't disagree on anything. surely it'll reach a point where airflow creates sufficient lift to takeoff.

however in the case of what seagates mentioned............

then clearly it's no.

why? because it's no different from tying the plane to a solid object - it's not moving, thus won't takeoff.

I didn't watch the Mythbuster videos, so I don't know what they were talking about to be honest.

As for Seagates question, his question is badly formed, because it doesn't mention at what it's measured relative to. Suppose it's measured relative to that airport observer I was talking about, and suppose that the conveyor belt will track the velocity of that plane and then increase its own velocity. That still makes no difference, because the plane will still be moving forward nonetheless, and if the thrust is larger, it would accelerate too [I'm assuming rolling resistance - without rolling resistance, any nonzero, positive thrust will cause it to accelerate forward no matter what the speed of the belt is]. The tracking of the velocity of the plane makes no difference because the wheels just spin faster to compensate.

Edit: This applies to both propeller and jet planes.

This post has been edited by bgeh: Jun 20 2009, 06:27 AM

yes, from my POV - the wheels don't matter.

i'm imagining that if the thrust generated results in a 75km/h forward motion, the wheel will spin the other direction with the conveyor belt at 75km/h.

in the end it's like running on a treadmill. power is used to go forward but the object remains stationary due to the area that the object is moving on, is moving backwards. and just like running on a treadmill, you can run at 12km/h and still don't feel any air going around your body.

so if we were to go with Seagate's 1st post - clearly there's only 1 answer.

No, because the treadmill analogy fails to compensate for the fact that we're no longer talking about cuboid like objects, which experience massive friction that literally causes us to stop, but wheels instead, where the friction instead speeds up the wheel's velocity. Try this question then. Take a roller skater moving at say 5km/h. Let him skate to a walkalator moving in the opposite direction, at also 5km/h. Guess what will happen when he reaches the walkalator? [You could also substitute the walkalator with a treadmill, as long it's perfectly horizontal and stuff]

Solution in the form of an edit, after you reply (will take a while though)

Edit: 2nd analogy I just thought of: (True story)

When I was a kid, I always wanted to do roller skating/roller blading, and spent quite some time admiring and looking at my neighbour and cousin's skates. Naturally with my itchy hands I liked to roll the wheels using my hands by running my palm across the wheels. I hope you've done this too, but I noticed that the faster you move your hand across the wheels, the faster they rotate. Why? Same principle applies to the walkalator question, and same principle applies to the 'free spinning wheels' on the plane

This post has been edited by bgeh: Jun 20 2009, 07:21 AM

my answer....

wind / move air will blow plane away.....




explanation :
wind : very strong wind can make plane blown away....
move air : control move air can make plane fly... Thats y plane use jet engine / propheller to create moving air at wings area.... the rest principle i think u know better than me

after putting some thought of what you mentioned, i think i finally understood your point.

so assuming i get your point right, we're talking about..........

conveyor belt moving backwards at X speed. Free wheel also will spin at that speed to counter belt's backward movement. that effectively nullifies the speed, while the propeller is still giving the thrust. if that's the case, then it's clear that in the end as long as the thrust provides forward movement, the plane will take off when the air pressure generates sufficient lift.

short version - the free wheel spins to the speed of the conveyor belt, reducing the total effect of the conveyor belt on the plane's forward motion.

so what is the myth is about actually? is it 1. whether a plane can take off on covveyor belt moving on opposite direction or 2. whether a stationary plane can take off using conveyor belt to keep it stationary.

if its about myth no 1, answer is YES. myth 2, NO,conveyor belt cant hold the pane stationary in the first place,rite?

do plane really need a wheel to move forward??
i mean, let say the body of the plane is on ice, the thrust from engine is enough to make the plane go forward.

Bingo. The 'free spinning wheel' will spin faster, and nullify the effect of the conveyor belt. So no matter how fast that belt moves, it doesn't matter, because the wheel will always compensate for it. Next part about why the plane moves forward - the thrust handles that, in effect making sure the wheel will always spin faster than the conveyor belt ever can, and thus the plane will go forward.

This can be generalised further actually for wheel-less cases, even with a constantly compensating conveyor belt

Note: A much simpler analysis using forces also exists.

This post has been edited by bgeh: Jun 20 2009, 08:56 AM

i don't think you get what i was saying. btw never once did i mention using wheel to move forward.

try to understand what me and bgeh talked about in this page, we've come to an agreed understanding already.

i'll try to make it even more simple

the counter movement of the conveyer belt at the same speed of the thrust has no effect on the plane as the contact point between the plane and the belt is the wheel, where the wheel will spin according to the speed of the belt thus having little to no effect on the thrust.

yeah.. the same with what i'm thinking.
just asking for the confirmation before witting it out. lol

because the plane just need to be moving relative to the wind to provide the uplift, not relative to the ground.

lol.. we got some example which plane no need wheel to fly like amphbia plane and ski plane...
The plane need moving air at it wings to produce lift... now thats engine job to provide the force...

i know..
i meant i read through the pages, it seemed that some people got the idea that the wheel that provide forward thrust to the plane.
which is not.

yup, the wheel is free to move, but it can be stopped by the brake of course

dont you all think that plane on a conveyor is same like plane with parking brake engaged or plane with engine running but stay stationary?

when a plane remain stationary, how did it air travel above the wing and below the wing, which means there's no low pressure above the wing and high pressure below the wing, also can say as no lift, so how can a plane fly without lift?

let's take boeing 747 or other passenger aircraft for example, the engine produce thrust and push the plane forward, the forward motion of the plane causes the air to around the wing and create low pressure above the wing and high pressure below the wing, thus create lift so the plane takeoff

unless you are putting the plane's engine in vertical position (similar to VTOL), otherwise you can artificially create airspeed at the wings of a plane to create lift, yes, i mean like those wind tunnel test where the model plane are in stationary position.

No it doesn't. It basically cancels out all the friction that is supposed to pull the plane backwards, well, because it has the ability to roll. That's the gist of it. Spin the conveyor belt and the wheel will simply roll, either forwards or backwards. It doesn't provide any thrust at all.

Or put it this way: The plane isn't advantaged in any way if you got the conveyor belt to spin forwards either.

Heck let me put in the force analysis:
Suppose the conveyor belt's already moving backwards, with the plane also moving backwards. Make some simplifying assumptions:

1) The wings do not generate any 'negative lift' if the plane move backwards
2) Fuel isn't lost

These 2 assumptions will be needed to ensure the weight of the plane on whatever object it rests upon stays constant.

Right, from A level/SAM (I think)/F6 Physics, we model friction by F = mu*N, mu the coefficient of friction, N the normal force on the belt

So if the thrust is less than this frictional coefficient (we assume for simplicity's sake that the coefficient of sliding friction = coefficient of static friction = mu. Analysis still holds because usually coefficient of sliding friction < coefficient of static friction)

Suppose the thrust is larger than the coefficient of friction. The plane will accelerate relative to this belt, and the key word is, it will continue accelerating relative to the belt no matter how fast the belt is moving, it only depends on the initial velocity the belt was travelling at when this thrust was applied (remember, I assumed that plane travels backwards with belt). Plane takes off nonetheless because it'll continue accelerating and at some point its velocity to some observer in the airport will turn positive, and take off occurs.

Of course we never see this, because the landing gear would probably shatter into pieces if we attempted it, and even that's assuming we have those mystical super powerful engines that can provide that much thrust.

This post has been edited by bgeh: Jun 22 2009, 03:28 AM

So, since many of us here agree that the plane can take-off even when it is on the conveyor belt, can we use this to decrease the length of the runway?

*Let's assume that we can manufacture a conveyor belt that is strong enough

i don't think the plane can take off...

plane need wind to lift off. thats why they need runway.
if stationary, no wind to lift their wings? then how to fly?

i suggest u to include big fan infront of the plane (to blow wind) + conveyor belt under it's wheels.

then we can start to discuss.

if i dont get you wrong, what you are trying to say is, the plane is moving forward eventhough the conveyor belt is moving backward, because the thrust generate by the plane is more than the speed of the conveyor belt, so the plane accelerate instead of stationary, if that's the case, when there is sufficient pressure build up on the wing then the plane will take off.

then i ask you how can you generate enough pressure to lift the plane??

No, the thrust of the plane has absolutely nothing to do with the speed of the conveyor belt. The thrust will only determine the acceleration of the plane relative to the conveyor belt supposing the force by the engines is larger than the frictional force. So the question to ask is whether the plane will accelerate relative to the belt. The speed of the conveyor belt becomes irrelevant in that context, and that's the point.


In the case of the 'free spinning wheel', basically even without thrust, and in the presence of a moving conveyor belt, ideally the plane will not move, even though there is friction between the tyre and the belt; this is because the conveyor belt simply causes the wheel to roll, and doesn't move the plane at all. The wheel will continue rolling and rolling matching the speed of that conveyor belt, even with 0 thrust. A simple experiment (I used thumbscrews - which are far from ideal, if possible get as round a cylinder as possible with moderate mass (basically not too light)) is to take a cylinder and put it under say a plastic ruler or a flat surface with enough friction. Pull that ruler and you'll see that the object will just roll and stay at the same place. Why? Conservation of momentum of that object. It'll also seem to roll slowly in the direction that you pulled the ruler. Why? Rolling resistance that I discussed earlier. But in the ideal case, the rolling resistance doesn't exist, and thus the plane will stay in the same place no matter how quickly you move the belt. The semi ideal case can be modelled by adding an extra 'dragging force' term, the rolling resistance. If the thrust can handle that, we're done, and plane will take off. That's it.

so you are in no camp, the plane will not take off unless there is enough pressure generate and lift the wing right?

Airplane can take off because of the wing lifting, which is generate through differentiation of pressure between upper and lower part.

Strip up the wing, no matter how fast the plane travel, it cannot fly.

The thrust on plane is to enable the plane to travel fast enough so that the airflow is fast enough between the wing to have enough pressure of lifting force.

So if the airflow is fast enough, you don't need to have thrust of engine to propel the plane forward to fly (in reality impossible as you won't have those kind of air speed in ordinary situation), just open up the wing will do, just like what they do in air-tunnel test or just like a bird, when you see there is enough wind, they just spread out the wing, then they are hanging up in the air already.

No, I am saying that there is a nuance. It depends on the magnitude of the thrust, that's it. I did not say whether it's a definite yes or no.

I'll state my position again: The thing that matters is to ask whether the plane can move forward with some positive velocity. If it can, then yes it will fly, if it has enough thrust. We've shown (somewhat) that the plane does move forward, so it indeed can fly. The wings and pressure difference etc, etc aren't really important, because all you'll need is some mystical 'more powerful engine' to solve that problem if the engines you already have can move the plane forward, but lack sufficient thrust to cause the plane to fly.

This post has been edited by bgeh: Jun 24 2009, 12:13 AM

discussing physics in words will lead to confusion.


the plane is on infinitely long conveyor belt.

so, if someone could set,
whether
-the force that moves the plane forward solely from the engine. or,
-the force that moves the plane forward solely from the wheel rotation,

whether
-the conveyor belt is freely moving, or
-the conveyor belt in the case of wheel rotation, move freely with the wheel,
-the conveyor belt moves at constant speed

It makes no difference in any above option, what the sole important factor that dictate the airplance can fly or not is the airflow between the wing.

If any method can, carrier already implemented the conveyor method to shortern the run away for fighter jet take off purposesly.

After reading for a while, I think I am switching camp

The explanation given is that, the conveyor only apply force to the wheel, and since the wheel is free rolling, the force doesn't transfer to the plane, so it won't be affecting the plane.

If conveyor belt is moving back at A speed, and plane have a forward trust of B speed. The only difference is that the wheel will be moving at A+B speed while the plane move forward at B speed.

Even if the conveyor belt is moving to match the forward trust of the plane. It simply means the wheel move at 2xB speed while the plane moves at B speed.

So on a infinitely long conveyor belt. The plane will still treat it as if it's a static tarmac and take off after gaining enough forward movement to achieve take off speed.

Reason wheel brakes can keep a plane on the ground is that the brake pads is the transfer medium for the force on the wheel to the aircraft.

Did I get it right?

Exactly, but with one caveat: Thrust is a force, not speed, i.e. it's proportional to acceleration. The only reason we equate the idea of thrust to speed is because the drag force imposed by the air is proportional to the square of the velocity [at higher velocities, it's linear for low velocities], so you need a higher velocity to get a drag force that will cancel out a larger forward thrust.

Yes, the brakes applied suddenly changes the 'free spinning wheel' into a cuboid like object, which requires a lot more thrust for the plane to move, if it manages to overcome the friction (see one of the above posts)

To those who actually support the 'YES' answer, what are you guys thinking?

I'm am not a scientist. In fact, I'm pretty ignorant when it comes to science. But even I know that most airplanes, in this case, a 747, need to have wind pass it's wings in order for it to fly.

If a plane is on a conveyor belt that is moving backwards as fast as the plane is moving forward, then the plane is actually just staying still. If the plane stays still, then there is no wind passing it's wings.

And if you think a plane, with enough propulsion, can take off while staying still, then you are right, but those planes are called VTOLs (Vertical Take Off and Landing), and they use very powerful thrusters to lift them UPWARDS when taking off.

Other, more traditional planes have thrusters (propeller, jet turbine, etc) that move the plane FORWARD so the wind will do the job of lifting the aircraft.

UPDATE: Okay, I've seen the Mythbuster episode, and I am now in doubt about my own answer. But that doesn't mean I fully accept the other camp, though. The plane flown was just a light, propeller based plane, not a huge jet turbined behemoth like a 747.

UPDATE: Okay, I changed my mind. Actually it is possible to take off, even for a 747.

This post has been edited by mylife4nerzhul: Jun 24 2009, 08:48 PM

I think no matter what it will fly.
Cause the wheel is going freewheel.
Is just the matter will it get enough speed.

Nope, it wouldn't help the plane to take off earlier. We will still need a conveyor as long as the runway.


Glad that you change your mind. The thrust is acting on the stationary air, not on the conveyor. If we have wind traveling backward that fast, the airplane will take off vertically. That is how scientist did the simulation in wind tunnel.

i thought myth buster already confirmed this that it can take off?

You mean the conveyor belt does not affect the plane at all? I thought that happens only when everything is assumed to be ideal? Imagine if you set the conveyor belt to turn in the direction at which the plane is about to take-off. Wouldn't that help?

myth buster aint physicist.

anyway.
if ideal, the conveyor would hold the plane stationary relative to the ground.
and the wind moving relative to the wind, the plane wont take off.

This post has been edited by fantagero: Jun 26 2009, 03:18 AM

No fantagero, read the previous explainations. And no, they don't need to be physicists to show something. Physicists do not hold a monopoly on truth in physics.

Edit: The point is that pretty much all the replies here are aware that the critical condition for takeoff is airflow over the wings, but the point of this conveyor belt is to throw this possibility of the airplane moving relative to an observer not on either the belt on the plane, but say on the airport, at which the air is still, relative to this observer, into doubt. The condition for flight is then reformulated in the following question: Does the plane move at all relative to this observer? That then tells you if the condition for flight is achievable and flight occurs.

This post has been edited by bgeh: Jun 28 2009, 02:57 PM

I believe the plane will not take off. Reason? Simply because there is no enough airflow flow through wings that needed to create lift. Some more, the conveyor belt move at the same speed as the plane which mean the plane will stay stationery and that will make wing relative to the plane and it will simply not take off.

what i'm saying is experimental may differ than theory.

no, the plane doesnt move relative to the wind and the earth. so it wont take of.

to take off, the plane need to suck wind using the engine, and move the plane forward so that a different pressure could be created on top of the wing and create uplift.

but conveyor counter the forward movement of the plane and make the plane stationary relatively

aihh can reach page 5 when it's very simple actually - if pressure not created on the wings, no take off. simple as that.

I second your motion. I did some "cacat" sketching to illustrate the said "experiment" but I didn't post it up here. Hmm...maybe I will...

Regards, Joey

This post has been edited by Joey Christensen: Jun 29 2009, 10:53 AM

Try read before posting. For godsake, it's only 5 pages. We don't have that much amount of patience to type it over and over again.

fantagero: try look up mythbuster member profile before making a fool of yourself.

did i wrote something wrong?

i know mythbusterr are professional.

what i meant was, the experiment they did is experiment (where the plane take off). of coz the result gonna be different if u talking in term of simple physics. where assumption could be made.


This post has been edited by fantagero: Jun 29 2009, 06:36 PM

No offense bro

The thrust from the running engine is acting on stationary air. Did the conveyor movement affect or disturb the air?

Hate to repeat the post. Hereby I repost my explaination again.


This post has been edited by Aurora: Jun 29 2009, 07:22 PM

Oh trust me, it can, and in the case of the airplane, it probably will. We don't have anything like those 'free spinning wheels' things I've been harping on so much about, and the weight of the plane is bound to cause some flex in the conveyor belt, which breaks the approximation. There's also the problem of having no perfectly flat surface, having surface imperfections will give you vastly different coefficients of frictions, etc, etc.

But we've always worked things out by making simplifying assumptions (e.g. you assume a brick won't act nuts and deforming like a piece of rubber or plasticine, but for all you know it might just happen. We make approximations by calculating its tensile stress/strain, etc. etc. under certain conditions, while if you really wanted to know it's full behaviour we would need to calculate the Schrodinger equation for all the atoms in the surrounding potential (approximately 10^26 ish atoms, and associated potentials), etc, etc.)

They're models as of now, and will await some experimental confirmation, but aren't you also hypothesizing like all of us here, just trying to use the power of argument to win us over?

[read my previous arguments: You will see that the conveyor belt has absolutely no way of cancelling out the motion of the plane unless the friction generated by this conveyor belt is so large as to cancel out the thrust of the plane completely (again, this is probably a first order approximation), which in reality, tends not to be the case (remember, friction is not proportional to the relative velocities the bodies move past each other, but just the normal force that one body applies on the other)]

Or try a thought experiment: Imagine instead a plane landing on the conveyor belt at a velocity a, and a conveyor belt moving backwards at a velocity -a. Will the plane's motion be stopped the moment it lands? If so, why, or why not?

This post has been edited by bgeh: Jun 30 2009, 02:09 AM