Wednesday, December 06, 2006


Forum question posed: Would an airplane on a gigantic treadmill be able to takeoff?

(17:32:04) BobCortex: i get the whole gyst of the dumb argument
(17:32:17) BobCortex: figured i'd ask someone who flys the things
(17:32:37) Eclipse: if the engines are going, the plane will leave the treadmill
(17:33:08) BobCortex: even if the treadmill can keep it from actually moving foward?
(17:33:12) Eclipse: its like pulling a wagon, the wheels dont do the pulling
(17:33:21) BobCortex: right
(17:33:25) Eclipse: the wheels just let you move
(17:33:34) BobCortex: plane with breaks on and full power wont move or take off
(17:33:41) Eclipse: a treadmill would change the speed of the wheels, but not the body of the plane
(17:33:50) Eclipse: right
(17:34:29) BobCortex: its some weird physics question
(17:34:48) BobCortex: plane + treadmill that can keep up with the plane. prefect world physics
(17:35:09) BobCortex: people come up with all sorts of junk
(17:35:15) BobCortex: like wheels dont do jack
(17:35:22) BobCortex: planes can take off on water
(17:36:08) BobCortex: but i always kinds thought the engines kinda moved air over the wings
(17:36:15) BobCortex: at least with props
(17:36:42) Eclipse: the engines pull the airplane forward, wheter its a prop or a jet (a jet is a prop with a cone around it)
(17:37:03) Eclipse: the moving airplane causes lift under the wings
(17:37:03) BobCortex: but i guess if the plane is not moving. the wings arn't moving. thusly lift is not being generated
(17:37:13) BobCortex: therefore sits still?
(17:37:21) Eclipse: correct
(17:37:31) BobCortex: if eclipse says its so i can sleep easier!
(17:37:45) BobCortex: it was fun reading people bicker about it
(17:38:09) Eclipse: heh
(17:38:26) BobCortex: fun stuff like "piolts" saying they wouldn't even really notice a diffrence on takeoff
(17:38:52) Eclipse: they wouldnt, the airplane would still accelerate like normal
(17:38:59) Eclipse: the wheels would be spinning faster or slower only
(17:39:10) BobCortex: but take off would never happen
(17:39:13) BobCortex: no climax!
(17:39:26) Eclipse: the wheels do not pull the airplane!
(17:39:37) Eclipse: the engines pull it forward and therefore judge acceleration
(17:39:59) Eclipse: a treadmill would only change how fast the wheels spin
(17:40:30) Eclipse: like a car spinning out on ice, the car still moves forward at the same velocity (due to inertia) and the wheels spin really fast
(17:40:53) Eclipse: the airplane is similar except the engine cause the motion instead of inertia
(17:41:05) Eclipse: and the treadmill would only change how fast the wheels spin
(17:41:26) Eclipse: once the airplane accelerates enough, lift will form under the wings and takeoff would happen
(17:41:42) BobCortex: but its not really moving
(17:42:01) Eclipse: yes it IS
(17:42:08) Eclipse: you're thinking of a car
(17:42:13) Eclipse: thats not how airplanes taxi
(17:42:19) BobCortex: ok
(17:42:23) BobCortex: you keep changing your mind
(17:42:29) Eclipse: no im not
(17:42:34) BobCortex: i know the wheels on a plane dont actually move it
(17:42:46) BobCortex: they roll freely and have brakes
(17:43:34) BobCortex: sure the plane would accelerate on the treadmill. but it wouldn't really be moving in physical space
(17:43:46) BobCortex: is the conculsion from earlier
(17:44:28) BobCortex: heh
(17:44:32) Eclipse: wrong
(17:44:49) Eclipse: the airplane only needs air to move
(17:44:58) Eclipse: otherwise it would be a real shitty airplane
(17:45:18) BobCortex: ugh hurting my head
(17:45:18) Eclipse: so the treadmill will not affect the acceleration
(17:45:39) Eclipse: unless the treadmill is in a perfect vacuum, the airplane will move in space
(17:45:53) Eclipse: because it uses the air to accel and not the motion of the wheels
(17:45:55) BobCortex: but the treadmill is preventing it from actually moving foward to get lift
(17:46:08) Eclipse: no, the treadmill only moves the wheels
(17:46:40) Eclipse: if there were no wheels, and the plane's belly was sitting on the treadmill then maybe
(17:46:52) Eclipse: but the wheels spin freely
(17:47:28) BobCortex: plane acceleration -----> <------ treadmill
(17:47:59) BobCortex: if its moving the plane backwards as its trying to move foward. its not really going anywhere
(17:48:36) Eclipse: thats what i am trying to say, it doesnt move the airplane
(17:48:47) BobCortex: right its spinning the wheels
(17:48:54) BobCortex: and the wheels are connected to the plane
(17:49:05) Eclipse: i think that's where you are not getting it
(17:49:22) Eclipse: are the brakes on?
(17:49:24) BobCortex: no
(17:49:38) Eclipse: then why would moving the wheel move the airplane?
(17:50:06) BobCortex: but the engines are pulling it foward
(17:50:28) Eclipse: if you attach a rope to a wagon and put the wagon on the treadmill, will the treadmill pull you?
(17:51:00) Eclipse: now what happens if you pull the wagon?
(17:51:07) Eclipse: the wagon comes towards you
(17:51:18) Eclipse: the wheels are what prevents the treadmill from having any effect!
(17:51:32) BobCortex: trickery
(17:51:33) Eclipse: now the airplanes engines provide that same force as you pulling the rope
(17:51:59) Eclipse: because the airplane can accelerate independantly of the treadmill
(17:52:11) Eclipse: thats not true for a car
(17:52:25) Eclipse: cars need the friction between the wheel and the ground to move
(17:52:27) BobCortex: i thought the treadmill would act to keep it in place. as in someone running on one. like a propelled treadmill. not one of thoes free running ones
(17:52:27) Eclipse: airplanes do not
(17:52:45) Eclipse: the person would need the friction
(17:52:56) Eclipse: airplanes do not need friction under the wheels to move
(17:54:11) BobCortex: so glad i never took physics
(17:54:22) Eclipse: do you get it though?
(17:55:16) Eclipse: i have to go soon
(17:56:11) BobCortex: so the engines pull the aircraft. regardless of what the wheels are doing. beacuse the wheels do absoluty nothing except keep the plane from falling over.
(17:56:19) BobCortex: someone needs to build a giant ass treadmill
(17:56:32) Eclipse: thats right
(17:56:48) BobCortex: no matter how fast the mill goes it wont affect the plane
(17:56:54) Eclipse: right
(17:56:58) BobCortex: just makes the hwels spin faster
(17:57:01) Eclipse: right
(17:57:13) Eclipse: or slower, if the treadmill is going the other way
(17:57:41) BobCortex: crazy airplanes
(17:58:12) Eclipse: they have to rely on the air or they will be useless after leaving the ground
(17:58:20) Eclipse: like a car in the air can't do anything
(17:58:43) BobCortex: so the plane will justhop off the treadmill and carry on its way
(17:58:48) Eclipse: yup
(17:59:00) BobCortex: sorcery!
(17:59:09) Eclipse: physics!

The answer: The airplane will take off and the pilots will not notice any difference

I’ll admit that the beginning is a little confusing since I didn’t really get what he was talking about.  I thought he was saying that if a treadmill was placed under an airplane and started, that the plane would just take off without using any runway.  That is not true.  All good in the end though!

Rombus: Rebuttal to your rant: If what you say is true, Then why dont airports just use something like the use to test cars (The rollers for the wheels) instead of runways for takeoff?

Eclipse: Because the whole point is that the treadmill does not affect the airplane!  It still takes the same distance to takeoff!

A long and painful version of the arguement

Posted by eclipse on 12/06 at 10:06 PM
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Thank God for physics… It actually hurts me inside that people can’t get the idea… although… a better way to explain is that in a car thrust comes from the wheels hence treadmill absorbs the thrust whereas in a plane the thrust comes from forced air. The analogy to a car on a treadmill would be a plan in a wind tunnel.

Posted on Wednesday, March 12, 2008  at  08:58 PM




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