Jet Fighter Madness! Physics people come here!

[garublador]

So in this instance, how is lift generated? I see thrust but not lift. I'm not contradicting you mind, just curious.

It generates lift by moving forward through the air.

[Pierre]

I think I see. Without physics knowledge how are we to assume that the plane WILL at some point move forward though? Is it because there is no proper force linking the belt and the plane, since the wheels are assumed to be free spining?

[Markk]

I think we've discussed this. The answer is YES, the plane will take off. +1 to D-EJ915.

cg

We did discuss this! Please, keep up with the rest of the board lol :P

[garublador]

I think I see. Without physics knowledge how are we to assume that the plane WILL at some point move forward though? Is it because there is no proper force linking the belt and the plane, since the wheels are assumed to be free spining?

Exactly!

[Pierre]

So what exactly will happen? The plane will move from the start, or remain stationary THEN pickup speed after... well I don't know after what... what would determine the point when the plane starts taking up airspeed?

[garublador]

It would move just like normal, assuming the wheels can take the extra speed. They'll be spinning twice as fast as normal when the plane takes off.

[Pierre]

Makes sense. From what people said I gathered that the plane would first be stationary THEN pickup speed... hence the confusion. I thought it'd either not move at all but if the wheels are free spinning it'd move at normal speed... just using up them tires way faster.

[Davey]

Makes sense. From what people said I gathered that the plane would first be stationary THEN pickup speed... hence the confusion. I thought it'd either not move at all but if the wheels are free spinning it'd move at normal speed... just using up them tires way faster.

exactly

[Pierre]

I think a simpler way of putting it is simply:

Wheels counter belt
Thrust counters drag and pushes the plane forward

So the forces which make the plane stationary are countered (thrust) and the belt too by the free spinning wheel.

[Zerberus]

And now that's just my ignorance coming to play but unless the plane is a VTOL (specifically designed for it), how can it take off without any lift?

NO aircraft can take off without lift, including VTOLs, VSTOLs and Jetpacks.

And Yes, I know the spaceshuttle will be named now, but at takeoff the space shuttle is no more an aircraft than a Fourth of July Rocket is. During landing it actually proves just how crappy it´s glide performance is. Barely an aircraft by any definition, but most definitely a trans-atmospheric vehicle.

But the plane isn´t taking off w/o lift. As soon as an airfoil moves, AT ALL, it generates lift. And a plane on a conveyor belt will be almost undiscernible from one on a standard runway in this situation, meanimg that airfoil is gonna be moving pretty fast pretty soon

[Ken]

***Subject to change but this is a preliminary response to Zerb, I'm still thinking through what he said.***

I'm still not seeing it. I agree completely that the jet turbines are creating a horizontal force (like you feel if you stand in front of a fan). However, without a motion relative to the air, I do not see air flowing OVER and UNDER the wings to create lift. I only see air flow at the points of the engines, not along the entire length of the wings.

As I see it, without a motion relative to the air to create a vertical lift, the turbines are creating ONLY a horizontal force to counteract the backward motion of the conveyor. Without airflow above and below the wing, there cannot be lift.

[Zerberus]

***Subject to change but this is a preliminary response to Zerb, I'm still thinking through what he said.***

I'm still not seeing it. I agree completely that the jet turbines are creating a horizontal force (like you feel if you stand in front of a fan). However, without a motion relative to the air, I do not see air flowing OVER and UNDER the wings to create lift. I only see air flow at the points of the engines, not along the entire length of the wings.

Correct, and Newton´s Law does the rest. Ther MUST be an equal and opposite reaction to the huge mass of air being forced through /around the engine. Again, forward movement of the craft is the only possibility save the disintegration of the vehicle /tearing out of the enine from it´s bulkhead. The engine isn´t on a test stand, it´s mounted in a functional aircraft.

As I see it, without a motion relative to the air to create a vertical lift, the turbines are creating ONLY a horizontal force to counteract the backward motion of the conveyor. Without airflow above and below the wing, there cannot be lift.

Yes, but the force necessary to hold the position on the belt is almost negligible (Try holding a skateboard in position on a treadmill), then see how easy it is to push it against the tread direction. And the force generated by the engines is supernaturally strong in comparison

[Guitar Toad]

I ignored this thread when it first appeared. I guess I was preoccupied with other things at the time.

Frame of reference. Let me reset the givens from the initial question.

Initially: The jet fighter is stationary. let me assume that there is no wind. The wheels have frictionless bearings, ie, there is no initial rolling force to overcome.

The motive power comes from the moving of air via the jet engine. The jet fighter begins moving at 30 km/h relative to the ground, then 50 km/h relative to the ground, then 200 km/hr relative to the ground. Speed relative to the ground is very important.

If the conveyor belt runs in the opposite direction relative to the jet fighter, and since the conveyor belt imparts no horizontal velocity to the jet plane, then, it doesn’t matter how fast the conveyor is running counter to the jet fighter. Once the jet engines are fired up enough to overcome the jet’s own stationary inertia and moving the plane forward, then the conveyor belt has no effect. If the conveyor belt is moving in the same direction as the jet fighter or in the opposite direction 1x, 2x, or, 10x the ground/ air speed of the jet fighter. The conveyor belt has no real effect on whether or not the plane flies.

Now if the JF is moving forward at 100 mph relative to the ground. Then the JF is moving + 100mph relative to the ground and the ground is moving -100 mph relative to the jet fighter. Then if the conveyor belt is moving in the opposite direction as the jet fighter at a velocity equal and opposite to the JF, Then, the conveyor is really stationary. And is doing exactly what the ground is doing. The ground is not moving nor is the conveyor moving. The conveyor is in fact stationary. That's the way it works out as the question is originally framed. Crazy, eh!

[ArtieToo]

If the conveyor belt is moving in the same direction as the jet fighter or in the opposite direction 1x, 2x, or, 10x the ground/ air speed of the jet fighter. The conveyor belt has no real effect on whether or not the plane flies.

Thats a great way of visualizing it for those who still can't see it. Assume the conveyor is moving the same direction as the plane. The plane is moving down the runway at 200MPH. So is the conveyor, and thus, the tires are not turning. It doesn't change the fact the plane is moving at 200 MPH.

As the question was originaly stated, the tires are turning at 400 MPH. The plane is still moving forward at 200 MPH.

Cute question.

[bert123]

Maybe they could see it if you did a diagram Artie

[Pierre]

***Subject to change but this is a preliminary response to Zerb, I'm still thinking through what he said.***

I'm still not seeing it. I agree completely that the jet turbines are creating a horizontal force (like you feel if you stand in front of a fan). However, without a motion relative to the air, I do not see air flowing OVER and UNDER the wings to create lift. I only see air flow at the points of the engines, not along the entire length of the wings.

As I see it, without a motion relative to the air to create a vertical lift, the turbines are creating ONLY a horizontal force to counteract the backward motion of the conveyor. Without airflow above and below the wing, there cannot be lift.

To each force there is an equal and counteracting force right? Or something like this.

So conveyor belt is CANCELLED by the wheels.

But the thrust of the plane isn't.

LOGICALLY it makes little sense. Physically however, it's correct.
Sorry Zerb didn't express myself correctly there... The lift in a Harrier comes because the air is hitting the ground and then back up under the wings... 'pushing' the aircraft forward. What I meant was 'forward lift' as in...running to it, if you see what I mean. Obviously you're right :P As always

[Ken]

Okay, I think I'm seeing where my error was...

If the wheel bearings are indeed frictionless, the backward force would NOT play a role, and the force from the turbines would propel the aircraft forward. Truly forward relative to an inertial observer. Yes, the plane would take off, because the plane IS moving forward and air is moving about the wings to create lift.


I was reading it before in such a way that the plane would remain stationary -- impossible to generate lift without the forward motion.

So yeah, if the retarding force (the correct technical term! ) of the conveyor belt is such that the plane is able to overcome it and move forward, yes it will takeoff.


I wasn't wrong, I just didn't interpret the question correctly.

[Tor]

Nah, you're all wrong -- this is what'll happen:

The wheels will, due to the twice the speed of what they're capable of, disintegrate quickly. The nose wheel will project large rubber bits towards the air intake of the jet motor, thus stressing the engine to the point where it explodes! Jet fighter in pieces, tell me how that serves for a lift!!



























Uhm, no,

I was reading it before in such a way that the plane would remain stationary -- impossible to generate lift without the forward motion.

Same here; I did not consider another motion possible.. at all! Oh well..

[Willis]

Same here; I did not consider another motion possible.. at all! Oh well..

That's also what I though, since from this

This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

I assumed it meant the speed of the plane relative to the ground (although it doesn't state anything that would suggest otherwise)

[HolyDirt]

The problem with everyone's arguement FOR the plane taking off is the assumption that the wheels rotate freely without friction. No where in the question does it actually say this. If you take into account this friction, the problem becomes much more complex.

This is the Physics rational I can come up with if the wheels are frictionless:
The plane would take off. The force exerted by the conveyer belt does not go into accelerating the jet, but rather into the ROTATIONAL acceleration of the wheels. The force needed to make the wheels turn is equal to and opposite of the conveyer belt force, so those 2 cancel out. Since the Jet Engine creates an unbalanced net force on the jet, it accelerates foreward and takes off. Dooooooooone.