will it take off?

[MossRoad]

...Take the little childs toy with 5 or 6 ball bearings suspended with the strings. Pull one ball back and let her go, that energy causes a reaction making the other ball at the opposite end move. ...

The balls will eventually stop, not on their own, but because of air resistance, gravity, etc... they won't just keep going forever.

As far as the rocket in a dark place, well, that was not very nice! ... but there is a video of someone doing that on you tube (and a link to it will not be posted here on TBN).

 

[shaley]

Tom, Let me ask something about the conveyor here. Are you saying that if a plane is sitting on a conveyor and the conveyor starts that the plane, in this case under no power yet or thrust will stand still in relation to the ground while the wheels turn under it because there is no friction? Where on earth is there no friction. If you placed a model car on a tread mill and started it moving won't the car follow the belt rather than standing still and fall off.

 

[MossRoad]

I'm saying these fish have wings and the beat them pretty hard and fast. You can hear the flutter when the come out of the water. I'm just as amazed at the Tuna for learning how to nail them. Look at the size of the peck wings.

Man, that is some detail. Nice picture.

Do you think he wished the tuna was on a treadmill?

 

[RayH]

Man, that is some detail. Nice picture.

Do you think he wished the tuna was on a treadmill?

The Tuna WAS on a treadmill, thats the only way he could get airborn.

 

[NewToy]

No such treamill exists. If the treadmill could match the wheel spin exactly, it would turn at the exact same moment the wheel turned, keeping it in the exact same place in relation to a point on the ground , no matter how much thrust you applied.

If the plane is standing still it has to overcome the friction with the ground to first get moving. Once it is moving, it can then pick up speed. It will never break this friction if the wheel makes no forward progress due to the magic treadmill. No initial forward motion means no forward motion ever due to the magic treadmill. Now, if the treadmil had a short delay in its initial reaction time, I would say, sure, it will constantly be trying to catch up and the plane will always be ahead and eventually take off. But we are assuming a magic treadmill that would never let the plane leave its initial spot to start with. Nope. No fly today.

Let's take the propeller off the plane and replace it with a winch. Take the winch cable and attach it to a fixed object in front of the treadmill. Activate the winch and treadmill at the exact same time same speed etc, the plane will be pulled forward with the wheels spinning twice as fast as they would be sans treadmill. Same concept with the propeller reinstalled. The plane will move forward when thrust is applied you just can't see a cable pulling the plane the thrust is moving the plane forward in relation to the ground. The plane and treadmill just double the wheel speed over a plain old runway. The surrounding air is NOT attached to the treadmill and the thrust applied to the plane has to move it, there is no other way for it to work. The energy applied to the plane as thrust has to create that equal and opposite reaction thing. The forward motion is not being applied to the landing gear but to the air as thrust. therefore, up, up and away.
John

 

[John_Mc]

Assuming the plane is on wheels and the wheels can spin freely at the speed of the movable runway, the plane will take off when it reaches the airspeed required for take-off. The airflow from the prop or jet or the breeze from the moving runway will never provide enough lift for take-off.

Look at it this way. There is a 1 mile long conveyor 2 feet wide 4 feet off the ground (for convenience). On the conveyor is a box. The box has wheels at each corner. A pole runs through the box and extends 3 feet to either side of the conveyor. You are on one side of the conveyor and schmism is on the other side. You both start walking at the same speed each pushing on the pole running through the box. The box will move along the conveyor. The conveyor is controlled to move in the opposite direction of the box at the same speed as the box is being pushed by you and schmism. The faster you and schmism go the faster the conveyor goes. As you pick up the speed and run pushing the box at 10 mph, the conveyor increases its speed to 10 miles per hour in the opposite direction. Will you get to the end of the 1 mile conveyor (assuming you both can run a mile at 10 mph)? Of course you will. And, in reaching the the end of the conveyor, you would have moved though the air mass surrounding you at a speed of 10 mph.The force moving the box is independent of the wheels contacting the conveyor. The wheels will be spinning at a rate equal to 20 mph during your run.

A similar condition exists with the plane. The plane is being moved by the propeller, or jet engine if you prefer, independent of the wheels contacting the movable runway. The plane will move through the air mass while the movable runway moves at the same speed in the opposite direction. At the proper airspeed (plane moving through the air mass) the plane will take off and the wheels will be very thankful because they can finally stop spinning so **** fast!

Jeff

Nice to see someone get it right, and with a good description to boot! the key here is in the original description of the problem:
[q]a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.[/q]

If the plane is moving forward at 50 MPH, the conveyor belt matches this and spins 50 MPH in the opposite direction. This has no effect on the forward speed of the plane (ok, maybe SLIGHT losses due to friction in wheel bearings). The plane will lift of with it's wheels spinning at 100 MPH (assuming 50 MPH is enough to get airborne)

While I'm not ruling out the possibility, I've never seen a plane where the engine (whether prop or jet) provides enough airflow over the wings for the aircraft to take flight without first rolling forward (excluding VTOL, but then it's actually more of helicopter type mode, and the conveyor belt is irrelevant). Even the military jets don't take off without some forward motion. With enough power, the length of this motion may be rather short, since it can get up to flying airspeed rather quickly.

EDIT: oops... I have seen one instance of an airplane taking off without rolling forward. I saw a guy take off into a 30 knot or so headwind. As 30 knots was enough to fly in that aircraft (at least in ground effect), all he needed was for the guys holding his wing struts to keep him on the ground to let go... he just floated right up. I've seen a similar thing when aircraft float up against their tiedown ropes in a strong windstorm.

John Mc

 

[John_Mc]

Put on some roller skates and stand on a treadmill. Hold a CO2 fire extinguiser under your arm and point it backwards. Have someone turn on the treadmill and fire off the fire extinguisher. You will shoot forward no matter what the speed of the treadmill, right? Except that according to the original question, the treadmill will match your wheel speed. In order to go anywhere forward, your wheels would have to turn faster than the surface of the treadmill. Soooo, I am back to the school of thought that this is a tough question.

Except that the original question doesn't say the conveyor will match the wheel speed it says "the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction." Nothing about wheel speed, it's the speed of the plane that matters.

Given this, I'm not sure how the conveyor could ever match the wheel speed, except when every thing is not in motion. Imagine the plane starts moving forward at 1 MPH. The conveyor senses this and matches it with 1 MPH in the opposite direction. But now the wheels are moving at 2 MPH (aircraft moving forward at 1 MPH since it's being pulled by the prop, converyor moving back at 1 MPH = 2 MPH on the wheels). Now, if the converyor is sensing wheel speed, it "senses" 2 MPH and speeds up to that speed. Wheels are going 3 MPH (assuming plane has not accelerated further). The converyor senses this and speeds up again. All it does is keep speeding up if the plane has any forward motion at all. It will never slow or stop the plane unless the wheel bearings tie up.

John Mc

 

[MossRoad]

Let's take the propeller off the plane and replace it with a winch. Take the winch cable and attach it to a fixed object in front of the treadmill. Activate the winch and treadmill at the exact same time same speed etc, the plane will be pulled forward with the wheels spinning twice as fast as they would be sans treadmill. Same concept with the propeller reinstalled. The plane will move forward when thrust is applied you just can't see a cable pulling the plane the thrust is moving the plane forward in relation to the ground. The plane and treadmill just double the wheel speed over a plain old runway. The surrounding air is NOT attached to the treadmill and the thrust applied to the plane has to move it, there is no other way for it to work. The energy applied to the plane as thrust has to create that equal and opposite reaction thing. The forward motion is not being applied to the landing gear but to the air as thrust. therefore, up, up and away.
John

What is the winch in your example attached to? Something that is attached to the same ground that the treadmill is sitting on, that's what. A winch attached to the ground is not the same things as air flowing over that ground.

I believe that the perfect magic treadmill (if it existed) that could exactly counteract the wheel's rotation from the instant it tried to roll would act just like a rope tied to the plane's tail and keep it from moving. The airplane would just be a big fan blowing air behind it, anchored to the treadmill by the friction between the tires and the surface of the treadmill.

To be honest, the entire scenario is not possible, because a treadmill that can exactly counter the wheel's movement does not and cannot exist. It would have to intuitively know when to turn BEFORE the wheel ever turned, otherwise it would be a reaction, not an action.

 

[MossRoad]

Except that the original question doesn't say the conveyor will match the wheel speed it says "the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction." Nothing about wheel speed, it's the speed of the plane that matters.

Given this, I'm not sure how the conveyor could ever match the wheel speed, except when every thing is not in motion. Imagine the plane starts moving forward at 1 MPH. The conveyor senses this and matches it with 1 MPH in the opposite direction. But now the wheels are moving at 2 MPH (aircraft moving forward at 1 MPH since it's being pulled by the prop, converyor moving back at 1 MPH = 2 MPH on the wheels). Now, if the converyor is sensing wheel speed, it "senses" 2 MPH and speeds up to that speed. Wheels are going 3 MPH (assuming plane has not accelerated further). The converyor senses this and speeds up again. All it does is keep speeding up if the plane has any forward motion at all. It will never slow or stop the plane unless the wheel bearings tie up.

John Mc

That explanation I agree with. It is just a matter of how we interpret the scenario. Your exlanation above makes sense to me. My interpretation of the scenario starts with the conveyor countering the wheel rotation as it occurs, not after it occurs.

 

[MossRoad]

Anyone ever read A wrinkle In Time? I had an easier time understanding tessering than this treadmill thing!