hmmm.... physics question for you guys....

[RobRR]

Re: hmmm.... physics question for you guys....

The plane will achieve virtually the same GROUND speed as it would on a standard runway. The wheels can not hold the plane in one spot on the conveyor, why is that so hard to understand?

[monk]

Re: hmmm.... physics question for you guys....

Newc, no!

Can we start with agreeing on that the engine is producing a force on the plane, aligned in the x-direction in the coordinate system I gave earlier. Ok? Nothing controversial about that.
For things to be in equlibrium, i.e. no acceleration, this force must be balanced by a force equal in size, but in the opposite direction. Standard Newtonian mechanics.

The tires can not support a force in this direction (except for friction, which is << thrust). That is the reason a tire starts rolling when you give it a push in the x-direction but not in the y- and z-directions.

Try this thought experiment. You have a wheel, attached to an axis. You go down in your basement, and start up your thredmill. You hold onto the axis, and put the wheel so that it makes contact with the threadmill (everything aligned as it would be in the case of the plane). The wheel will start to rotate, at the same speed as the thread mill. Now, I am sure you can imagine that the force you need to supply to the axis to keep the wheel in place IS WAY LESS than the force exerted on the wheel by the thread mill. Try stopping the wheel with your hand and you will see this.

In the case of a car, the force the engine has to overcome is exactly that force (when you try to stop the wheel by gripping it with your hand). That is a large force.

In the case of a plane, the force the engine has to overcome is the force you feel on the axis. This is a small force.

[rjohnstone]

Re: hmmm.... physics question for you guys....

Huh??? [img]/images/graemlins/scratchhead.gif[/img]

[monk]

Re: hmmm.... physics question for you guys....

I am not good at explaining this. The axis/wheel thing assumes that the axis is just a handle, and not actually rotating along with the wheel.

Or, if the plane is not moving, why is the conveyor belt moving in the first place?

Or, instead of a jet engine, tie one end of a rope to the plane, and tie the other end to a tow-car not located on the conveyor belt. If the car starts moving at 10 mph, the plane WILL MOVE at 10 mph too, and the belt will move at 10 mph in the other direction. This because the car is not pushing against the belt. It is pushing againt the surrounding ground. No one can possibly deny that.
The rope is pulling on the plane with a force. Replace the rope with the engine, and you have the same force in the same direction. The engine is not pushing against the belt either. It is "pushing" against the air.

[Newc]

Re: hmmm.... physics question for you guys....

[img]/images/graemlins/laugh.gif[/img] You guys are seriously clouded [img]/images/graemlins/laugh.gif[/img]

It's simple:

The engines produce thrust, yes. The thrust of the engines pushing against air is the primary propulsion system, yes.
The force of the thrust MUST be focused at the wheels. Period. This cannot change. The plane will not move forward because of this fact.

If the plane's engines went from dead stop to 3 million pounds of thrust in the blink of an eye, that thrust force must still be transferred to the wheels before the plane will move. You can yank the landing gear and yes, the plane will sail forward with 3 million pounds of thrust pushing it, but you better nose-up quick of you're going to cause a bit of a mess a few yards down the runway.

While the gages attached to the wheels will show you have sufficient ground speed to attain lift, they are gages that measure tire rotation speed, not vehicle forward motion. They ASSUME the vehicle is moving forwards because the tire is rotating, however, they are simply mechanical devices that measure the rotational speed of the wheel, so they don't know either way.
You could jack the plane of the ground and spin the wheel by hand and the speedometer will say the plane is moving.

Tether two identical 747s tail to tail, fire them up at the same time with the same power, pulling against each other with fully equal force with absolutely no variation possible for all eternity.
Will either plane lift off?
No.

Same situation here. The engine's thrust MUST be transferred to the wheels to initiate forward motion from a dead stop. This is unavoidable.
If the plane is on said conveyor, as soon as the wheels start turning, the belt turns in the opposite direction.
This cause/effect relationship is maintained unflinchingly for eternity, and engine thrust has no effect on the situation other than to cause the initial forward motion and and maintain the tire rotation that started this mess.
The plane will not move forward, and thus will not achieve lift.

Tell me exactly how the plane WILL move forward from a dead stop.

[toejam]

Re: hmmm.... physics question for you guys....

[img]/images/graemlins/popcorn.gif[/img] [img]/images/graemlins/sleep.gif[/img] [img]/images/graemlins/popcorn.gif[/img]

[Sandman0170]

Re: hmmm.... physics question for you guys....

Assuming this plane can fly, it will take off. It is a trick question. The only factor the surface a plane may take off from such as pavement, snow, water or even a hypothetical conveyer belt, is the drag created from the friction between the plane and the ground. To minimalize this friction, the plane in this case most likely has wheels which are free rolling. With snow planes, they have skis, sea planes have pontoons or a hull...their purpose is to minimize friction with the surface it is resting on only. Once inertia and this friction are overcome, the plane will move forward. This is because thrust is produced from the engines channeling through the air much like you would channel water when you swim. This air is no different if the surface is moving or not, therefore equal thrust is produced. While this plane moves forward, the wheels continue to spin minimalizing the friction. If the surface the plane is on moves, the wheels will spin at the speed of the plane + the speed of the surface if moving in the opposite direction, or the speed of the plane - the speed of the surface if they are moving in the same direction. Once the plane gains enough ground speed to have enough air to pass over the wings surfaces, it will generate lift.

For those who still don’t get it, next time you take a bath, grab your little rubber ducky and let it float, that is your plane. The water is your wheels. Now take a thin flat sheet of something that isn’t pliable and slide it across the bottom of the tub underneath your ducky moving from in front of its head to behind its tail. That’s your conveyer belt. Your ducky doesn’t move. The water minimizes its drag and is irrelevant to your rubber ducky floating in your tub. Now, if you put wings and an engine in your ducky, it will take off.

Now if you still think the plane wont take off, enjoy knowing the earth is flat and is the center of the universe.

[toejam]

Re: hmmm.... physics question for you guys....

The earth's not flat?!?

[RobRR]

Re: hmmm.... physics question for you guys....

[ QUOTE ]

Same situation here. The engine's thrust MUST be transferred to the wheels to initiate forward motion from a dead stop. This is unavoidable.


[/ QUOTE ]

You have absolutley no idea how a plane really works do you? So taking your theory, you can put a plane on a Dyno and get a reading then? Just quit while your ahead man, every new post you make makes you look worse and worse. [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img]

[zeegler]

Re: hmmm.... physics question for you guys....

The hilarious thing is that NEWC actually thinks he knows what the hell he is talking about, when he's just talking jibberish. He has no clue about the laws of physics, inertia, thrust versus resistance, or anything. his posts are pure garbage.

[GodOfRhythm]

Re: hmmm.... physics question for you guys....

[ QUOTE ]
The things I've read in this thread....

First of all Bernoulli's Principle is not the explanation for flight in se. Try calculating and designing a wing for an airplane supported by Bernoulli, simple hilarity! [img]/images/graemlins/laugh.gif[/img]

The plane would NOT take off, it would remain on the exact same spot (if the theory that is given in the question was indeed practically possible: a plane countermoving at a certain speed on a treadmill going in the opposite direction at the same speed).

Flight of an airplane is achieved by many different factors, but the most important one is lift, as mentioned.

Now let me explain a bit: a plane reaches lift because of air that travels under but particularly over the wing. Because of Bernoulli you'd need a 50% larger top of the wing than the downside to achieve the most minimal of lift, that is unfeasable, though. What really happens is the air on top does move a slight bit faster, a vacuum appears over the top of the wing, this vacuum is filled in IMMEDIATLY by the air even higher than the air already on the top of the wing, so the vacuum pulls in air from above, creating A) an action/reaction equation not dissimilar to Newton and B) lower air pressure (relative to under the wing), pure logic dictates that more speed = more air = more displacement = more lift (hence minimal take-off speeds and the need for engines, namely to create forward motion (or backward motion in some parallel/symmetrically winged stuntplanes)). And as such (albeit simplistically stated) lift is created.

Air speed, relative speed, weight of the plane and sizes of the wings all matter greatly of course. For example, a small cessna needs to create 2,5 ton/s air deplacement for its rougly 3000lbs frame, a jumbojet as is mentioned here is about 800 000lbs, do the math!

[/b]Your point of the weels having minimal impact on the planes forward bearing is interesting to say the least. However there is an equally interesting paradox: the plane is reliant on its weels until it reaches sufficient speed = enough airflow = sufficient lift, however since it's still bound to its weels to keep it on the conveyor belt and counteracting gravity and the conveyor belt is moving in the opposite direction at the exact same speed (at no point does the plane move faster or slower than the belt). Now it's necessary to use some sort of archimedal point, let's say the ground, under the conveyor belt or besides the conveyor belt through this we can also consider another archimedal, solid point: the air surrounding the plane. Asides from the minimal airflow created by the belt itself and the engines (Matt's main theory) the air would remain virtually the same. Of course engines are designed nowadays to not only pull in air into themselves for propulsion but also to offset air to above and below the wing to aid in achieving more lift, this however has to be seen as complementary and not sufficient on its own.[/b]

The plane remains on the exact same spot (for example it will have the exact same GPS coordinates), no sufficient airflow is created by movement (the main instigator of airflow and lift), the plane does not move forward, the plane does not achieve lift, the plane DOES NOT TAKE OFF!

In other words, I'll repeat the same: the plane won't take off, it'll only be drawing a crowd of people going "what in the name of holy fuck are those dumbasses doing with that Boeing and that gigantic conveyor belt".

[/ QUOTE ]

YES! Period

You guys are MISSING the point completely!

You are incapable of fathoming the inherent paradox of the situation? You're saying it will take off, that is fucking IMPOSSIBLE!!!

In simplest possible terms:

Jets= forward thrust = weels go over the conveyor belt (which is going at the same speed but in opposite direction)

The Weels WILL be essential! Since the plane does NOT move forward (relative to the air and the surrounding ground) no sufficient AIR and WIND will be forced over and under the wings to provide THE LIFT!!!

No lift = weels stay on ground = plane stays on conveyor belt!

If you were looking at the plane while it was on the belt but had glasses on so you could not see the weels, you wouldn't even know it was 'on' or was trying to move forward!

No do you understand this? Plane does NOT go forward, NO air is forced up and under the wings = NO displacement of air = NO FUCKING LIFT!

For god's sake people! [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img]

[RobRR]

Re: hmmm.... physics question for you guys....

So why the fuck does the plane NOT go fucking foreward? What the fuck is keeping the fucking plane from fucking moving fucking foreward? Thats what Im not fucking underfucking standing! For god's fucking sake! [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/tongue.gif[/img]

[monk]

Re: hmmm.... physics question for you guys....

[img]/images/graemlins/laugh.gif[/img]

Yes, I would like to know that too. Sure, if the plane DOES NOT MOVE, no lift will be created.

But, what are the arguments against the plane moving? And if the plane isnt moving, then why is the belt spinning?!

[RobRR]

Re: hmmm.... physics question for you guys....

EXACTLY!!! Finally someone gets what Im trying to say... if theres no measuralbe ground OR air speed, the the conveyor is NOT MOVING! Hence your answer right there, if the plane is moving, so is the conveyor! If the plane is not moving, neither is the conveyor!

[wulfe13]

Re: hmmm.... physics question for you guys....

Wow, I can't believe some people are still holding out after that webpage Tim posted!!! How hard is it to understand that page? The plane will move forward hence creating airflow and lift!

Read that whole page!!!