Can the Plane Take-Off?

[greencactus3]

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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 the opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?"

there's a 988 post arguement going on in this other forum... im pretty positive about the answer... (there is technically no true answer till someone experiments... and many people will argue that they need proof. only proof possible would be by experiment... and so far i know of noone who has done an experiment)
and please dont turn this into a flame war.... as has happened in the other forum. i mean for this to be amusing. ill post what i think a bit later after a couple posts have been made so i wont give any ideas from the 988 post wisdom i have lol. or you can pm me and ill answer what i think.

have fun!

[KenWittlief]

The answer most certainly can be determined by the laws of physics.

All planes move by reacting with the air, either through propellers or jet engines. The speed of the ground under the wheels has almost nothing to do with the speed of the plane down the runway.

In this case, the plane would accelerate down the runway, and the 'runway' would accelerate in the other direction. The plane would still take off more or less normally, the only effect being the wheels will be spinning twice as fast as they normally would when it leaves the ground.

Hey that was easy, we settled the argument on CD in one post! :^)

[Pat Fairbank]

As Ken said, whether a plane can take off or not depends only on how fast the air is moving in relation to the plane's wings.

It's for this reason that airplanes take off facing into the wind whenever possible: the plane's ground speed at takeoff is not as high as it would be if there were no wind, so a smaller length of runway is needed.

[greencactus3]

Originally Posted by KenWittlief

The answer most certainly can be determined by the laws of physics.

All planes move by reacting with the air, either trough propellers or jet engines. The speed of the ground under the wheels has almost nothing to do with the speed of the plane down the runway.

In this case, the plane would accelerate down the runway, and the 'runway' would accelerate in the other direction. The plane would still take off more or less normally, the only effect being the wheels will be spinning twice as fast as they normally would when it leaves the ground.

Hey that was easy, we settled the argument on CD in one post! :^)

yes.... except one thing.
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 the opposite direction).

so. laws of wear and tear and motor output of either engine(plane or conveyor) ignored, but friction not ignored, the conveyor belt spins fast enough that the friction of the wheel bearings keep the plane from accelerating. so if the conveyor really can turn exactly the same speed(but in the opposite direction) of the plane, the plane would not be accelerating. relative to the conveyor belt it may as well be going almost the speed of light but since the conveyor belt is going the opposite way at almst the speed of light, the plane doesnt move.... im redundant arent i?
well anyways. so the plane wont fly then? i unno
but my thoughts are that a conveyor belt would fail before an airplane engine or landing gear so the plane would fly... eventually... but only after the conveyor belt fails.


-edit- addition-..... oh. and i should just say this.. you both completely ignored the way the conveyor belt works. its not a normal treadmill you see...

[sanddrag]

Originally Posted by greencactus3

there's a 988 post arguement going on in this other forum

Wow, that other forum must not be full of really bright people. Easy question. As mentioned previously, a plane's flight is based on it's relation to the air, not to the ground. Thrust comes from engines pushing on the air, not from wheels pushing on the runway.

[KenWittlief]

you are adding conditions to the question that are not in the question.

The runway moves in the opposite direction of the plane, at the same speed. Nowhere does it say the runway moves to keep the plane in the exact same spot. That is your mistaken conclusion of what will happen.

A jet plane is able to produce thousands of pounds of thrust. Lets say its an F15, able to generate 30,000 lbs of thrust.

The wheels, with the brakes off, will have a very low coeffecient of friction. So lets start at zero mph.

The plane speeds up to 10mph, putting out 30,000 lbs of thrust. The runway matches this speed in the opposite direction. The wheels are putting maybe 5 or 10 POUNDS of force in the reverse direction due to the runways backward movement.

The resulting net force on the plane is 30,000 pounds forward minus 10 pounds backwards = 29,990 pounds of thrust (force) in the forward direction

and the plane is moving forward with a 10mph airspeed and its wheels are spinning at 20mph. The runway is moving backwards at 10mph (ground speed)

as the plane continues to accelerate (due to the thrust from its engines) the runway speeds up, but the force the spinning wheels apply to the airframe will never match the force of the engines, unless the pilot does something stupid like stand on the brakes.

[Madison]

Originally Posted by sanddrag

Wow, that other forum must not be full of really bright people. Easy question. As mentioned previously, a plane's flight is based on it's relation to the air, not to the ground. Lift comes from engines pushing on the air, not from wheels pushing on the runway.

Lift actually acts on the wings and is completely independent of a plane's engines. The engines provide thrust to move the plane through the air fast enough so as to create sufficient lift against the wings.

If the aircraft is not moving in relation to the air, as seems to be the case in this example, it would remain on the ground -- no matter how powerful its engines.

The debate rages on...

[sciguy125]

Think of it this way:
If I have a balsa glider in my hand and I run down the sidewalk, there will be lift on the glider due to the moving air. If I run on a tredmill, the glider doesn't have any lift.

The situation at hand is the same. The airplane is on a tredmill, so it can't go anywhere.

[greencactus3]

Originally Posted by sanddrag

Wow, that other forum must not be full of really bright people. Easy question. As mentioned previously, a plane's flight is based on it's relation to the air, not to the ground. Lift comes from engines pushing on the air, not from wheels pushing on the runway.

true. but some are quite smart. and well...most are the normal "ur stupid" flaming.... but you gotta have an open mind to understand why both opinions are valid. everyone agrees that the plane must be moving forwards relative to the air to fly... but the plane isnt moving forwards relative to the air.. because the wheels are mounted to bearings which are on the axles... and to my knowledge there are no true frictionless bearings... and because of that friction, the conveyorbelt can keep the airplane from moving forwards relative to the air completely. ok. now shake your brains around and give them a nice stretch... and think slowly... at first the plane is sitting still. now the pilot turns on the enignes. afterburners are fine too. just no vtol technology allowed. so the plane should move forwards... but no, the plane CANT move forwards because the conveyorbelt tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). ... so the plane does not move. because if the plane moves 1mm forwards that would mean the conveyorbelt is not turning the belt at EXACTLY the same speed as the plane.(but in the opposite direction)..
so simply put, the plane DOES NOT MOVE. but in which case where does the air being pushed by the plane's engines go? backwards. but since air molecules have inertia too, the plane should move forwards now. but it doesnt.. so in this problem, because of an improbable conveyorbelt control system, some parts of physics must be ignored... or looked into detail... depending on how you look at it.

[Elgin Clock]

Yeah.. so most people know that I am a student who absloutely hates the book work of physics, but gets what is going on in the experiements.

I'm gonna switch to common sense, or lack their of in this analogy, and say that since Plane (A) is "accelerating" in a positive direction, and Conveyer belt (B) is "accelerating" in a negative direction based on Plane (A) then in all essence the plane is not moving (linearly) at all on this conveyer belt and the plane will not take off.

A = +X speed
B = -X speed

They cancel each other out, and there is no linear displacement.

And my physics training will say that distance (or the X amount of miles the plane is racking up spinning it's wheels on the conveyor belt, does not equal the displacement required to take off.

For example:
What would happen if you put a car on a dyno machine.. It accelerates, and adds up milage, but it's not going anywhere.. (as long as your dyno doesn't fail and freeze up) lol

And yes, I know cars and airplanes are not created equally, but I'm just sayin' - It's the same kind of setup.

[KenWittlief]

oh no, Madison! Its late sunday night so I forgive you! :^)

look at it another way - an F15 comes in at 200mph for a touch and go landing pass. It flies down on the runway, but maintains its airspeed, so the wheels touch the ground, then it throttles up and flys away.

Ok, so add your moving runway to the picture. The plane comes it at 200mph again, the wheels hit the runway, which is going backwards at 200 mph, so what happens? does the plane instantly stop, ignoring the laws of momentum?!

no, the wheels go BRIRP! and spin up to 400mph, the pilot hits the throttle again and does his 'touch and go', just as before.

The engines on a plane do not react off the runway. If they did, then how do planes in alaska with skis instead of wheels get off the ground?!

[greencactus3]

Originally Posted by KenWittlief

you are adding conditions to the question that are not in the question.

nuhhuh.. its been there all along. you say it yourself.

Originally Posted by KenWittlief

The runway moves in the opposite direction of the plane, at the same speed.

same speed as in...... ZERO mph? which means its not moving?

Originally Posted by KenWittlief

...and the plane is moving forward with a 10mph airspeed ...

wait a second.... the plane does not get to 10 mph.... it does not move

Originally Posted by KenWittlief

...as the plane continues to accelerate

again, IT DOESNT!




yes,, arguements are finally starting.. i have succeeded in starting a big arguement! yay!!


ps. please rememebr to be nice to everyone. even i get excited and then agitated by seemingly nonunderstanding members of the opposing group.. but then again the other side thinks the same about me.

[Madison]

Originally Posted by KenWittlief

oh no, Madison! Its late sunday night so I forgive you! :^)

That's why I initially edited my post and added, "That said, Ken's right," but subsequently removed it. I understand your explanation completely and it makes sense, but I don't fully comprehend why -- so I removed my edit. When I can completely describe what's happening mathematically as I know it to be happening anecdotally, I'll be happy.

This is, in a lot of ways, quite similar to confusion about whether its Bernoulli or Newton who've got the proper theories about what causes lift -- when in reality, neither theory fully explains things.

[KenWittlief]

Originally Posted by greencactus3

...so the plane should move forwards... but no, the plane CANT move forwards because the conveyorbelt tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). ...

ok, now you got me laughing :^)

if the plane cannot move then the conveyor belt cannot move either. When the planes 'speed' is zero the conveyor belt speed is also zero !

[sciguy125]

Ah, I now see the error in my thinking..

Let's say we drop a missile from an airplane. Better yet, from a helicopter that is hovering in place. The missile will still go forward even if it has no contact with the ground. So, the ground has nothing to do with the airplane's ability to take off.