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

all i can say about this is, if it is even possible, it wont be feasible, else aircraft carriers would adopt this. instead, aircraft carriers use the catapult system for aircraft takeoff.

no one offessing anyone, no worries

???????????
to win you all over??
is this a competition?

TS, got prize ey??




would u mind transfer it to a diagram like what i did please? or probably just use mine.

page no 3

anyway. just finished exam. so, nbtd



This post has been edited by fantagero: Jun 30 2009, 09:53 AM

It's not a competition. The point of a [structured] argument is to arrive at some final conclusion from some set of initial physical principles. I've attempted that by using a mixture of analogies and arguments from empirical equations to show that the conveyor belt makes absolutely no difference at all, whether it's moving or not.

What I want to see is an argument on why this even occurs:

because I see no reasons on why this should occur, from the arguments and approximations I've made. I might have missed something, so try showing me if I've missed anything.

This post has been edited by bgeh: Jun 30 2009, 10:32 AM

I'll begin with 1 sentence at a time:
Despite whatever the ground speed maybe, the thrust generate force onto the stationary air.
This is self-explanatory. Airplane engine moves air, air exit from the engine and act onto the stationary air, then it become thrust.
If the ground travel at light speed yet the air above it remain stationary, the thrust will generate force onto the air
Ground speed doesn't matter here, engine thrust only acts on air.
unlike car which generate thrust onto the ground. So, the airplane will just keep moving forward.
Explaination on why we should not confuse with car.

WE must remember, mythbuster did the experiment at relatively low speed. Therefore most of the high speed assumption are negligible (like friction, crosswind). The point of argument here is about plane and conveyor, and not about discussion of secondary factor. A lot of things come into play when high speed is consider.

In reality, the airplane will be bound to higher resistance when it try to takeoff from a moving conveyor. It's not part of this discussion to argue about these resistance, but simply the logic of can an airplane takeoff from a moving conveyor.

bgeh: I think you try too hard. Forget about the assumption. Anyone who came in trying to argue about secondary factor and resistance obviously don't understand the question.

ok i know i'm posting in old thread but i cant resist.
airplane on a conveyor belt wont fly. simply because we cant create the air pressure diff between the upper and lower part of wings. its diff wif rockets where rockets use thrust to go upward. airplane uses thrust to move it forward so that there will be a pressure diff between upper and lower wings.

being said so, i just wondering, if we shoot enough air over the airplane wings, will it fly? (without engine on) i believe it will providing we can create enough air pressure difference. thats aerodynamics

There is a plane out there its called a VTOL, vertical take off and landing.

the answer is solid NO..STATIONARY plane cannot generate lift force on both its wings.

Hmm... read a couple of explanations here, doesn't explain the fundamentals.

I’ll try to explain here why an aeroplane can take off from a coveyor belt and do a comparison with cars on a coveyor belt. Here are the steps we'll take towards understanding the concept:

Step 1 = How a car moves forward ("gains forward momentum" in TS's words) on a normal road
Step 2 = How an aeroplane moves forward ("gains forward momentum" in TS's words) on a normal runway
Step 3 = How a car reacts on a conveyor belt
Step 4 = How an aeroplane reacts on a conveyor belt

But before we get into all that, let me first describe how a car works and how an aeroplane works in a normal scenario... meaning like normal lah, without coveyor belt and all that.

The Car
First let’s take a look at the car. A car is able to move forward because it has an engine which is connected to a gear box which in turn is connected to the wheels. When you pay money to Petronas, they’ll give you petrol which when poured into your engine will cause the engine to rotate and spin the gears in the gearbox which will then cause your wheels to spin.



The rotation from the wheels then cause the tires to push backwards against the road surface and move the car forwards, thus making you spend more money and making Petronas richer. But also making your car go forwards at the same time.

So at this point, I'd like to introduce a new term just to make the explanations easier. The term is "Medium-of-motion". For a car, the petrol gives power to the engine which gives power to the gearbox which then gives power to the wheels and then finally the wheels push against the "medium-of-motion" which is the road surface.

One important thing to note here in this scenario, is that the "medium-of-motion" (ie. the road surface) is not moving and is fixed.



The Aeroplane
Okay now let's look at how the aeroplane moves forward. Forget about the whole thing about "aerofoil" and "lift" and all that stuff, we don't really care about that, let's just look at how an aeroplane moves forward on the ground at an airport runway.

Click to see how aeroplane moves on ground:


Lol... but seriously though, an aeroplane moves forward both on ground and on air by using massive engines that suck in air from the front and blows it out the back. If you're wondering how something can move forwards by blowing air out the back, try inflating a balloon and then letting it go with the end untied. It flies forwards doesn't it? Same thing with the aeroplane, just that instead of keeping a tank full of air to blow backwards, it simply sucks in the air from the front and blows it out backwards.



At this point, let's stop for a while and look at the "medium-of-motion" for aeroplanes. Again, Petronas's petrol gives power to the engines which give power to the propeller which then sucks in the air from the front and blows it out the back really fast causing the high speed air to push against the stationary air behind the propeller causing the plane to move forward. So the "medium-of-motion" here in this case is the air.

So now we begin to see that the wheels of the airplane serves no purpose at all, except to hold the aeroplane up. That's why aeroplanes can also use skis or floaters instead of wheels.




Introducing the conveyor belt!
Okay, now that we understand the fundamental difference between how a car moves forward and an air-plane moves forward, let's bring in the problem... the conveyor belt!

Before looking further into this problem, let's make a few assumptions:
1. The conveyor belt is very long, longer than the distance required for an air-plane to take off.
2. The conveyor belt is going 'backward' (i.e. opposing the direction we want to go).
3. The conveyor belt, aeroplane and car speeds can be controlled *very* accurately.
4. The car and the aeroplane has a GPS receiver that reads out the coordinates of it's current location.
5. The aeroplane has an additional speedometer that is connected to the aeroplane's wheels

So now, we put the car on the conveyor belt. Let's see what happens.

So we put the car on the belt, turn on the conveyor belt at 35KM/h. At the same time, we on the engine and start driving forwards at car-speedometer's 35KM/h in the opposite direction. Guess what? The car doesn't move forward or backward! It stays in the same spot although the wheels are spinning furiously. The GPS reading too doesn't change and it shows the same coordinates and a speed of 0KM/h!

But why?

Well, when you burn your petrol to make the wheels spin, normally the wheels will push against the road surface to go forwards. But this time, since the conveyor belt too is going backwards, the pushing action of the wheels is negated. It's like driving on a normal road, but instead of you going forwards, the whole world is going backwards!



Because of this, all the force that the wheels is trying to apply to the ground to make the car go forwards is instead being used to stop the car from going backwards. In other words the forward momentum that the car is generating is being cancelled by the backward momentum of the conveyor belt. What does this mean? Well let's jam on the breaks!!

Suddenly when you jam on the breaks, you realise that you're hurtling backwards at 35KM/h. Your GPS reader shows your previously stationary coordinates now moving backwards and your speed as 35KM/h!! So you press the fuel again all the way until your speedometer shows 35KM/h. Look now at your GPS reader and you see that your coordinates are stationary again and you're going at 0KM/h.

Therefore, when your car says you're moving at 35KM/h, this is the speed of your car relative to the "medium-of-motion (ie. conveyor belt), it's not the true speed of the car. The true speed - relative to the Earth - of the car is given by the GPS receiver, which is 0KM/h.

This happens to the car, because the "medium-of-motion" for the car is the road surface and this scenario, the conveyor belt simulates the road surface moving in the opposite direction.

Now let's do it again, this time with an air-plane! Since air-planes move faster, we're going to set the conveyor to 100km/h! Also remember that we have connected an extra speedometer to the aeroplane's wheels, we'll be monitoring this as we do the experiment.

Okay, first lets turn on the conveyor belt at 100KM/h. Immediately we see that the aeroplane is hurtling back at 100KM/h. Let's check the meters:

GPS: 100KM/h -- going backwards
Aeroplane's speed indicator: 0KM/h
Speedometer connected to the wheels: 0KM/h

Okay now we turn on the engines, and start pressing the fuel just a little. Look at the meters!

GPS: 1KM/h -- going forwards
Aeroplane's speed indicator: 1KM/h
Speedometer connected to the wheels: 101KM/h

Notice how fast the speedometer readings jumped? Why?

To understand why we notice such a huge difference in the speedometer reading, we need to look again at the "medium-of-motion" idea. Remember that the speedometer is connected to the tires of the aeroplane, so it measures the velocity of the aeroplane relative to the conveyor belt.

When the propellers of the aeroplane starts spinning, they begin to suck the air from the front and blow it backwards, this creates a forward momentum for the air-plane.

Now this is where the magic happens:

Because the "medium-of-motion" that the aeroplane uses is air and not the road surface, the conveyor belt cannot cancel off the forward momentum of the aeroplane! So almost immediately when you press the fuel, the air-plane will start moving forwards relative to the Earth! That's why your GPS will start showing you that you're moving forwards at a certain velocity.

Now let's go full throttle!!

GPS: 100KM/h
Aeroplane's speed indicator: 100KM/h
Speedometer connected to the wheels: 200KM/h

So now we see that as we really start burning the petrol and making Petronas richer, we continue moving forwards relative to the Earth really fast!

And since we are now moving forwards relative to the Earth really fast, it's exactly like taking off from a normal runway!

But wait! Let's try something extra here. Let's maintain the aeroplane throttle but now pump up the conveyor belt speed to 300KM/h!!! Look at the meters!!

GPS: 100KM/h
Aeroplane's speed indicator: 100KM/h
Speedometer connected to the wheels: 400KM/h

Now the speed of the aeroplane - relative to the conveyor belt - increases to 400KM/h but the aeroplane's true speed relative to the Earth is unaffected!

This is because the conveyor belt - which simulates the road as "medium-of-motion" - does not affect the aeroplane which uses the air as "medium-of-motion".

Conclusions
1. Yes, an aeroplane can gain forward momentum while on a conveyor belt regardless of the speed of the conveyor belt.
2. Yes, an aeroplane can take off from a conveyor belt, because of #1.
3. No, taking off on a conveyor belt will not result in the aeroplane needing a shorter runway, it will still need the same length of runway to take off.
4. It's almost impossible to make an aeroplane stay stationary with it's engines on on a conveyor belt, unless you put the whole experiment inside a wind-tunnel. A wind-tunnel is to an air-plane what a conveyor belt is to a car.

Alternative way to look at it
Another way to look at this is through seeing humans running on a treadmill.

In the first scenario depicted below, the man has to run faster than the treadmill if he wants to reach the wall. His legs are representative of the engines, gearbox and wheels of the car.



In the second scenario depicted below, the man needs just to pull himself with the string to reach the wall. The string is representative of the air as the medium-of-motion while the hands are representative of the engines of an air-plane and it's propellers.



Hope this helps you understand the concepts.



This post has been edited by DeniseLau: Jul 27 2009, 05:15 PM

Hey bro... Very detail explaination with all the diagrams.