will it take off?

[SPYDERLK]

First a jet doesn't move by "making wind", it moves by reaction to moving mass amounts of air backwards.

Your arguments are getting weaker. First you have to show there is enough rolling resistance to keep the plane from moving with the engine at take-off power. Set it on the ground it moves, set it on the conveyor = it will move as the axle friction is all that provides resistance. That is the same resistance that was there when it was on the ground and it just ain't very much.

For the plane to remain still with respect to the air, you have to provide a major amount of resistance somewhere. You haven't done it yet.

Re the jet and chocks - that goes all the way back to the late 40s & 50s. Lots of people claiming you could hold a jet back as long as you never allowed it to move at all. Of course that was a whole bunch of hooey.

I spent 21 years in the AF, granted not directly dealing with planes, but all on flyiing bases. Yes, a jet will jump a chock.

Harry K

Harry, you are absolutely right about thrust coming from moving air mass backward - not so much just moving it but actually accelerating it backward very quickly. The engine imparts energy to the air and the airs resistance to being accelerated[inertia] results in the engine and plane being thrust forward. The method that has been posed to counter the thrust and stall motion is by using a conveyor that could push backward continuously on the tire contact patch with an amount of force equal to the engine thrust. Regardless that the wheels have small resistance to turning at a contant speed, they do resist being accelerated to rotatefaster and faster. The hypothetical conveyor would have to accelerate the wheels tremendously to offset the engine thrust, but this acceleration would follow directly from applying reverse magnitude engine thrust to the contact patch. This would be only possible for a short time because energy equal to that produced by the engine accelerating the air is being also forced into the wheel by the conveyor -- the wheels would soon explode. However, until they do, an equal and opposite reacton force - a counterthrust - is being applied to the airframe via axle. Did you see the links in post #233?
Larry

 

[MossRoad]

First a jet doesn't move by "making wind"

What is thrust? What is wind? Close enough to be the same thing when its ripping the grass out of the ground. Doesn't matter if its behind an airplane or from a thunderstorm. I've seen many jets held back by chocks. They had to stop the engines so we could pull the chocks. I probably saw it happen several times a month for six years. 3=several X 12 months a year X 6 years at the airport = enough to be fact in my book.

If you don't allow something to move, that means you don't allow it to move. That ain't hooey.

 

[joerocker]

A "magic" chock made of say teflon WILL hold a car. You see AGAIN you ADDED something to the senario...you ADDED the car making the wheel TURN to CLIMB over the chock. If the car wheel would simply slip on the chock and not be able to climb over it, the car wouldn't move. The magic chock would become the magic conveyor belt but the difference now is that the jet CAN overcome its hurdle because it moves by pushing the air and NOT the ground.

 

[MossRoad]

Go back and read the second link that I posted almost two weeks ago.

 

[turnkey4099]

What is thrust? What is wind? Close enough to be the same thing when its ripping the grass out of the ground. Doesn't matter if its behind an airplane or from a thunderstorm. I've seen many jets held back by chocks. They had to stop the engines so we could pull the chocks. I probably saw it happen several times a month for six years. 3=several X 12 months a year X 6 years at the airport = enough to be fact in my book.

If you don't allow something to move, that means you don't allow it to move. That ain't hooey.

I wondered if you would pick up on that.

Yes, as stated the object will not move. Same thing can be said about holding back a car, a locomotive, a big tractor, ad infinitum. The fallacy in it is:

It won't move if you don't let it but, you can't _keep it from moving_.

Harry K

 

[turnkey4099]

Harry, you are absolutely right about thrust coming from moving air mass backward - not so much just moving it but actually accelerating it backward very quickly. The engine imparts energy to the air and the airs resistance to being accelerated[inertia] results in the engine and plane being thrust forward. The method that has been posed to counter the thrust and stall motion is by using a conveyor that could push backward continuously on the tire contact patch with an amount of force equal to the engine thrust. Regardless that the wheels have small resistance to turning at a contant speed, they do resist being accelerated to rotatefaster and faster. The hypothetical conveyor would have to accelerate the wheels tremendously to offset the engine thrust, but this acceleration would follow directly from applying reverse magnitude engine thrust to the contact patch. This would be only possible for a short time because energy equal to that produced by the engine accelerating the air is being also forced into the wheel by the conveyor -- the wheels would soon explode. However, until they do, an equal and opposite reacton force - a counterthrust - is being applied to the airframe via axle. Did you see the links in post #233?
Larry

No energy from the engine is being "forced back into the conveyor". The only way that could be done is for the wheels to be somehow driven by the engine.

It still comes back to 'rolling friction' and 'bearing friction' then plus resistance to being accellerated. Basically you are requiring a massive increase in those friction/inertia forces. The problem there is it fails as I can specify an engine powerful enough to move the plane even with the brakes locked.

Picture the engine running full out on the belt. Per you it remains stationary in the air. Now picture it somehow suddenly sliding sideways off the belt on the ground. What happens? It is pulled -through the air-.

Harry K

 

[HTWT]

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.

as the plane moves

there have been three camps, Yes, No, or If. Yes, no problem, NO, why argue with the irrational, that leaves If. If the plane moves at all it will fly. Does the plane move, maybe Yes, maybe No. Yes, it moves as originally stated, problem over, plane flies. No, plane does not move therefore the tires do not move and the conveyor does not move, no need for a conveyor , NO PROBLEM.

 

[NorthwestBlue]

Pls see previous post. Hit the wrong button.

76000-1177.2=74822.8 N" Im not sre how to deal with energy here since thrust is just force - not energy or power. Yet I know an engine cannot produce more energy than it consumes nor at a faster rate than it consumes it, and the rate that thetires are absorbing energy is increasing.

So we have 74.8kN of thrust to oppose this magical contraption. Throw in that F100 engine in my example with 128.9kN of thrust for even more fun.
No rockets allowed in this problem. We cant stand that much fun.

Air drag is inconsequential because lift will cancel it out as the airplane accelerates through the air and FLYS.

So can anyone see where I went wrong on my thinking. Well yeah. Significant redefinition of the scenario by converting to skidders and a rocket engine notwithstanding, you introduced real world variables that only make it more difficult for the plane to take off.

Oh, if anyone cares I think RobS answered the original question correctly way back on post #5. Yep. Unless the original was asking us to think a little further on the implication that forward motion was prevented.
Larry

Thanks for the point by point. Basically proves everything I was saying. A couple of things though. The F100 is an air breathing, Jet-A burning, turbine engine - not a rocket. Wheels and tires can be made from many materials and compounds... doesn't change the fact that they are wheels. I did not redefine the scenario, I adjusted the design to prove that it was indeed possible for an airplane to fly regardless of the "magic".

It is simply THRUST/LIFT/FRICTION/ACCELERATION.

Sooo, I can agree with what you are saying. Can you agree that if friction and time (to accelerate and achieve lift) are minimized the airplane will fly regardless of the acceleration potential of the MCB?

 

[Egon]

THRUST/LIFT/FRICTION/ACCELERATION.

]

Lift is a very important factor in getting a plane off the ground barring helios and vertical thrust types. Now the big question is

" How is lift acheived ? "​

 

[NorthwestBlue]

THRUST/LIFT/FRICTION/ACCELERATION.

]

Lift is a very important factor in getting a plane off the ground barring helios and vertical thrust types. Now the big question is

" How is lift acheived ? "​

Egon,

Lift is achieved with forward movement of the wings relative to the surrounding air mass. It is dependent on velocity and angle of attack. In order for an airplane to accelerate, its thrust must be greater than drag. In this scenario the only apreciable drag is from the wheel contact with the conveyor belt. Larry, aka. SPYDERLK has argued that if there is sufficient traction or friction then the drag created by the conveyor belt would be greater then the thrust. I don't dispute that part. But, if you minimize friction then the airplane will accelerate through the air mass and its wings will create lift regardless of the acceleration capability of the MCB.