tew

In case you REALLY don't see why it would take off, I'm going to be nice and explain it.

The conveyor belt in no way prevents the plane from moving forward. Thus, air moves over the wings and it takes off.

Imagine a treadmill. You put an RC car on it. The treadmill goes 10 mph one way and the car 10mph the other way so it goes nowhere. That's right enough.

But imagine you're on that treadmill on a skateboard with a rocket strapped to your back. You certainly move forward, even if the treadmill is going 100 mph you're still going forward.

What's the difference. The car propels itself by spinning its wheels and exerting a force on the GROUND to move. The rocket (like planes and jets) exerts the force on the air.

Now if you blew air at 300 mph into the inlet of the jet engines (but not over the wings), then the plane would not take off because it is the acceleration of the air that gives it thrust.

10/27/2007 1:03:34 AM

Quote :

"Why would it possibly take off? It's not moving. There's no air to lift the wings. Or is everyone being sarcastic?"

it IS moving.

Quote :

"this "myth" depends on how the myth is "read"... the propulsion is provided by the jet engine, sure..i'll give you that. but when the jet is on ground, the wheels move as the jet is propelled along... if the treadmill matches the speed of said wheels, then the jet is still just sitting in one area...i dont' care how much the jet engine is putting out....the total displacement of the jet is still 0... unless it is a harrier, it is just sitting in one spot with jets going full force."

no, there is no room for interpretation...there is only room for misunderstanding.

Quote :

"i understand that...but the thing says the belt is matching the speed. . that's the part that you have to watch. if it is matching the speed of the wheels, then there is no forward movement...skidding isn't allowed, sliding isn't allowed."

it isn't skidding or sliding. the wheels are simply turning twice as fast as they normally would for the aircraft moving through the air at any given speed.

Quote :

"wasn't there a 20 page thread already sparking this debate?"

yeah, at least twice (i don't know if it was 20 pgs, but it was long). I (and some others) explained it really well, making lots of analogies as well as looking at it from a more engineering oriented perspective. I don't know how anyone could not understand by the end of the thread, but there still were a few.

Quote :

"it wouldn't move forward if the treadmill matched the wheel speed which is driven by the propulsion of the jets"

the wheel speed has nothing directly to do with the thrust.

Quote :

"Now if you blew air at 300 mph into the inlet of the jet engines (but not over the wings), then the plane would not take off because it is the acceleration of the air that gives it thrust."

Yeah you would! your engines would just make even more thrust with the additional air.

10/27/2007 1:24:35 AM

Quote :

"Now if you blew air at 300 mph into the inlet of the jet engines (but not over the wings), then the plane would not take off because it is the acceleration of the air that gives it thrust."

Quote :

"Yeah you would! your engines would just make even more thrust with the additional air."

i was gonna say...maybe a prop engine wouldn't be able to do anything with a 300mph headwind, but a turbofan (jet) is exactly that, a fucking TURBO fan. the faster you blow air into it, the faster the downwind fans spin, the more compression you get, the more thrust you get.

[Edited on October 27, 2007 at 2:13 AM. Reason : adsfasdf]

10/27/2007 2:11:34 AM

^^ Haha i'm a tard.

Yes it would thrust. Depending on how you blew the air into the jet, you might get increased or decreased thrust.

What I meant was if you blew air through it at a velocity much greater than the engine is capable of moving air (which 300 mph definitely is not... maybe i meant to sat 3000 mph), the friction in the engine would be so great the the combusting fuel wouldn't accelerate the air any further.

[Edited on October 27, 2007 at 2:15 AM. Reason : ]

10/27/2007 2:13:12 AM

wat?

10/27/2007 6:32:54 AM

totally deleting my post preventing r-tardation.

[Edited on October 27, 2007 at 7:25 AM. Reason : .]

10/27/2007 7:14:55 AM

totally deleting my post preventing r-tardation.

[Edited on October 27, 2007 at 7:25 AM. Reason : .]

I'll begin by saying I'm pretty certain I have no reason at all for being in this thread/debate.

I'm now batshit crazy confused from this whole mess. I orginally had Duke obnoxiously supported as well as numerous clauses and acknowledgments that his balls indeed grazed my face yet by the law of physics, completely missed my mouth.

Now i'm trying to visualize running on a treadmill with a paper plane in my hand and dropping it. My first inclination is that it would fall straight to the ground. How is this not what would happen on mythbusters? Is there some law that applies where the only way the plane would take off is if the jet engines could overcome the speed of the treadmill? If the belt keeps moving at the precise speed (whatever those fuckers say it has to be) would there never be wind resistance? So now are we dealing with a treadmill that has a speed limitation or are we working with infinites?

This post is probably 80% wrong and 20% crazy stupid, but yeah I really want to grasp this concept and failing out of engineering school doesn't help at all.


maybe it will help if someone explains to me how the plane is moving with respect to the rest of the world, or w/e?

[Edited on October 27, 2007 at 8:24 AM. Reason : .]

10/27/2007 7:57:26 AM

Quote :

"shit i had the myth all confused, yeah it would be able to move forward on a treadmill, groundspeed is irrelevant to airspeed"

bingo... the thrust is being applied to the air, not the ground...

10/27/2007 8:35:58 AM

haha k00 yew crazy j00

WAT

10/27/2007 9:15:51 AM

I hate this question... and WHO THE FUCK would put a tredmill on a runway!?!?!?

10/27/2007 9:55:01 AM

for page 2



every time i read it i lol

10/27/2007 10:11:43 AM

i just spent 3 hours listening and arguing with people about this.

10/27/2007 10:34:15 AM

ok ok

lets say there are no jets engines on the plane.

its one of those rubber band propelled planes.

wat?

10/27/2007 10:37:34 AM

lol.

treadmill + plane - airflow = no launch

wat?

10/27/2007 10:39:30 AM

Grandmaster: ask yourself this

-What devices on an airplane provide thrust for its movement?

-What medium do these devices operate on?

-What does the treadmill, moving at any given speed, have to do with this operation?





Answers:

- Engines

- Air

- Nothing. The treadmill could be spinning at 1200mph in the opposite direction, but as long as there is air moving through the engines, there will be positive thrust. This is, of course, ignoring any friction created by wheels rotating against the 1200mph treadmill. You're thinking about a car where forward movement is afforded by wheels pushing on asphalt; on an airplane, this movement is provided by engines pushing on air. The wheels on a plane are free to rotate in either direction, so while the treadmill may be moving faster than the plane in the opposite direction, this only means the wheels are spinning backwards very fast while the plane itself is still moving forward and creating lift.

10/27/2007 12:04:34 PM

the airplane tires will blow out once they start spinning at twice their rated speed, and the plane will crash and burn.

10/27/2007 12:12:53 PM

yeah i know that the plane uses the engines to generate thrust and wheels play no part in that.

I think it's a very very poorly worded question designed to do just what has happened in this thread. Fuck with everyone and get them all heated. I can't take a side for the very reason that I guess I don't understand aerodynamics and physics enough to completely justify one answer over the other. I'm not a complete retard and I do understand the basic concepts, but so many factors are getting thrown into or out of this problem with each "ur rong im rite, diaf" response.

People take the original question and they spin it 11 different ways and either add variables based on their mood that day or take them away and ignore other factors.

So are we talking about implementing this in the practical sense(ie mythbusters), where despite what the "flyers" imply, there WILL be friction on the wheels, they WILL be susceptible to heat or bearing failure. What is the exact length the treadmill has to be for the thrust of the engine to overcome the backwards force of the conveyor belt and not just do this, but do it in a manner where the plane achieves it's required speed to take off before the end of the treadmill and avoid a Napoleon Dynamite-esque bike ramp situation and just trickle off the front end?

Or is this Magic Land where treadmills are of infinite length and there are no such words as friction, bearing failure, tires melting, or engine failure. And since we're in magic land, do we have magic jet engines capable of the speed of light to compliment the aforementioned infinites and indestructibles? And then leik, once the flux capacitor is generating the precisely calculated gigawatts (much more than 1.21) and the plane ends going the speed of light what's gonna happen when the pilot turns on his headlights?

No fucking idea why I keep letting myself get sucked back into this never ending debate. It's simple, the plane is spinning clockwise...no wait counter-clockwise...i mean...bah.

10/27/2007 6:24:13 PM

Some of you guys are fucking idiots.

Yes, the jet applies force to the air. BUT, the plane requires LIFT in order to take off, not just horizontal velocity. The LIFT is generated by the plane's movement with respect to the air around it. The air around the plane is generally in the same frame of reference as the ground. So, if the jet is not moving with respect to the ground, then it is highly improbable that the jet will have enough lift to take off.

Thank you, you have just been owned. Now shut the fuck up with your wise-ass "hahah, durrr, a jet's wheels don't make it move, durr."

10/27/2007 6:44:01 PM

Quote :

"Some of you guys are fucking idiots. Yes, the jet applies force to the air. BUT, the plane requires LIFT in order to take off, not just horizontal velocity. The LIFT is generated by the plane's movement with respect to the air around it. The air around the plane is generally in the same frame of reference as the ground. So, if the jet is not moving with respect to the ground, then it is highly improbable that the jet will have enough lift to take off. Thank you, you have just been owned. Now shut the fuck up with your wise-ass "hahah, durrr, a jet's wheels don't make it move, durr.""

I hope you're being sarcastic. Otherwise you just noobed all over yourself.


There is nothing preventing the forward motion of the plane relative to the air. Jet engines are capable of generating tens of thousands of pounds of thrust (total thrust is in the neighborhood of 100,000 lbs in a large commercial aircraft). The only things resisting the forward motion of the plane are (1)friction between the craft and the air (2)friction in the wheels moving relative to the ground.

During a normal takeoff a jet is traveling between 100 and 200 mph when it leaves the ground. If the friction in the wheels were so great at the takeoff speed as to prevent the forward motion of the plane the plane could not further accelerate and therefore not take off. Furthermore the engines are capable of propelling the plane MUCH faster than the speed required for takeoff, so there is considerable margin for added resistance during takeoff.

If the treadmill is constantly moving 200mph against the plane and the engines maintain exactly enough power to maintain 0 motion, how can I know that the jets are not at full throttle? During normal takeoff the plane is battling not only the 200 mph friction in the wheels, but also the friction forces of the air (which are proportional to the velocity of the air squared). Since at 0 air speed (and 200 mph ground speed), there is 0 friction in the air, the throttle can't possibly be maxed (otherwise it wouldn't be able to take off from stationary ground!). At this point all you have to do is increase the throttle and the plane will begin to accelerate (relative to the air).

The ability for the plane to take off from a moving runway will then be dependent on the ratio of the air friction to the wheel friction. Rather than explaining why (since the retards on this board are not bright enough to comprehend), I will just tell you that the air friction dwarfs the wheel friction.

10/27/2007 7:08:57 PM

the question doesn't ask "can the plane accelerate." It asks "can it take off?" And the answer to that is simple: at the time in question, the plane does NOT have have enough lift to take off. Pull up on the stick all you want, it won't take off.

If the plane accelerates, then it is no longer conforming to the status of the original question, because it is now moving at faster than takeoff speed on the treadmill.

Thanks for playing, though

10/27/2007 7:16:02 PM

Oh look, it's this thread again.

10/27/2007 7:19:14 PM

^^are you serious?

[Edited on October 27, 2007 at 7:19 PM. Reason : dsx]

10/27/2007 7:19:42 PM

wat

10/27/2007 7:39:37 PM

you guys are all fucking dumb

the plane takes off

10/27/2007 7:54:48 PM

please, explain how a plane takes off without lift.

10/27/2007 8:09:52 PM

the treadmill does not prevent the plane from moving forward under the force of the thrust created by its engines. As the wings move forward through the air, lift is created.



Think of it like this:You are holding a rollerskate and letting a moving treadmill roll the wheels on said rollerskate. Even if that treadmill is going 700 mph, you can still move that skate forward with roughly the same amount of force required to move it along the floor. The engines on the plane act to move the plane forward, just like your hand on the skate.

10/27/2007 8:16:03 PM

Quote :

"I think it's a very very poorly worded question designed to do just what has happened in this thread. Fuck with everyone and get them all heated. I can't take a side for the very reason that I guess I don't understand aerodynamics and physics enough to completely justify one answer over the other."

It's not a poorly worded question. It's really pretty straightforward. People are just poor thinkers.

Also, aerodynamics has nothing to do with it. The airplane functions exactly like it would on a stationary runway, from an aerodynamic perspective. Basic physics is what's throwing people for a loop.

Quote :

"So are we talking about implementing this in the practical sense(ie mythbusters), where despite what the "flyers" imply, there WILL be friction on the wheels, they WILL be susceptible to heat or bearing failure. What is the exact length the treadmill has to be for the thrust of the engine to overcome the backwards force of the conveyor belt and not just do this, but do it in a manner where the plane achieves it's required speed to take off before the end of the treadmill and avoid a Napoleon Dynamite-esque bike ramp situation and just trickle off the front end?"

The increased friction in the wheel bearings is the only difference (and maybe a little bit more heat in the tires). I guarantee you that no aircraft's tires and wheel bearings are engineered with such a small margin of safety that they wouldn't be able to withstand one takeoff run at double their normal rotational speed (landings would be much more severe duty, anyway).

As far as requisite length of the conveyor, it would just need to be the same length as a regular runway, plus a small amount extra to account for slightly reduced acceleration on account of the wheels spinning twice as fast (it would have a pretty small impact on takeoff distance).

Quote :

"the question doesn't ask "can the plane accelerate." It asks "can it take off?" And the answer to that is simple: at the time in question, the plane does NOT have have enough lift to take off. Pull up on the stick all you want, it won't take off. If the plane accelerates, then it is no longer conforming to the status of the original question, because it is now moving at faster than takeoff speed on the treadmill. Thanks for playing, though"

The plane's acceleration and whether it will take off are inseperable. Also, the question places no constraints on the airplane's acceleration. The treadmill speed is only linked to the aircraft's wheel speed--not its airspeed.




If anyone really wants to take a bet on this, I'll put up $10,000...or more than that if you really want.

10/27/2007 8:25:37 PM

nice try. If the plane is moving at takeoff speed on the treadmill, it is still moving at 0 mph relative to the ground. thus, no lift.

10/27/2007 8:27:37 PM

Either put your money where your mouth is or stop trolling.

10/27/2007 8:28:46 PM

Quote :

"The plane's acceleration and whether it will take off are inseperable."

False. A plane can technically fly at takeoff speed. Otherwise it wouldn't be "takeoff speed."

Quote :

"Also, the question places no constraints on the airplane's acceleration."

In a sense, yes. The question doesn't specifically state that the plane can't accelerate. However, as I said earlier, if the plane accelerates to a different speed, then it is no longer at takeoff speed, is it? Thus, it's not acting under the parameters of the question. If you allow the plane to accelerate, then I could easily say "yeah, the plane can fly. take it off the treadmill, durrrrr"

10/27/2007 8:41:03 PM

Dude...

I have a degree in mechanical engineering.

I fly airplanes for a living.

I will put any amount of money up to, say, $30,000 on the line as a bet that I'm right on this one.

I have a hard time believing that a NE alumnus could be stupid enough to actually believe what you're saying, so I'm assuming you're trolling.

10/27/2007 8:48:52 PM

how do you keep the plane from accelerating then?

10/27/2007 8:49:10 PM

how do you keep me from taking the plane off the treadmill then? Look, if you are going to change the parameters of the question, then you aren't answering the original question, are you?

It's like saying "hey, there's a car stopped in front of a stop light. can the car move past the stop light?" The answer is "no." The car CAN'T go past the stop light as long as it is stopped.

Quote :

"I fly airplanes for a living."

Good, then you should be able to understand why a plane cannot take off without lift.

10/27/2007 8:52:50 PM

aaronburro

i don't have anything against you, so don't take this personally, but you're being a moron.

what in the original question says the plane can't have forward motion relative to the ground? it only states that if, for example, the plane is moving at 20 knots, the treadmill is moving at 20 knots in the opposite direction. this still means that the plane is traveling 20 knots and generating the appropriate amount of lift, but the wheels are rotating at 40 knots instead of the 20.

i don't have to explain the rest to you, as i'm pretty sure you can grasp what happens next

10/27/2007 11:24:38 PM

I understand lol.

[Edited on October 27, 2007 at 11:43 PM. Reason : .]

10/27/2007 11:40:31 PM

What is this talk about acceleration and thrust and blah blah blah?

Airspeed is the ONLY factor that is important here. If the airspeed is high enough, the wings will produce enough lifting force for takeoff. If not, they won't. It's as simple as that. Airspeed is independent of groundspeed.

10/28/2007 11:51:51 AM

I think for sake of simplicity the question assumes the air to be static, thus airspeed is the same as ground speed. Doesn't really matter, principles are the same.

10/28/2007 3:34:42 PM

If the plane would take off....why doesnt the military build giant treadmill style runways? then there would be no need for 6000+ ft runways.

[Edited on October 28, 2007 at 3:49 PM. Reason : l]

10/28/2007 3:49:30 PM

I hope you're kidding

10/28/2007 3:51:53 PM

I hope you're kidding

10/28/2007 4:00:47 PM

I hope you're kidding.

10/28/2007 4:04:02 PM

I know he is not kidding.

10/28/2007 4:04:34 PM

Quote :

"If the plane would take off....why doesnt the military build giant treadmill style runways? then there would be no need for 6000+ ft runways."

10/28/2007 4:07:08 PM

Stop. Just stop, before you idiots devalue my degree any further.

10/28/2007 4:17:20 PM

the plane moves forward. no one is arguing it takes off stationary.

10/28/2007 4:54:41 PM

aaronburro is incorrectly arguing the point because he has it stuck in his mind that the measured speed of the aircraft is the speed of the craft compared to the speed of the surface it is on (the treadmill.)

the speed of an aircraft is determined by it's speed with respect to the land, not this silly conveyor belt.


so assuming that there is no wind, etc, if the speed of the airplane (when viewed from the control tower, ground, wherever) reaches takeoff speed, it will lift off, no matter if there is a pesky conveyor belt underneath or not.



the only problem, of course, is that the wheels will generate a little bit of friction (which likely wouldn't cause much of a problem)... also there is the possibility that the wheel assembly wasn't designed with those speeds in mind, and might fail.


but the plane will lift off.

10/28/2007 5:17:44 PM

Quote :

"the speed of an aircraft is determined by it's speed with respect to the land air, not this silly conveyor belt."

10/28/2007 5:18:53 PM

air or ground it doesn't matter. lets talk about the speed with respect to the ground. which in this example (no wind) is the same as the air.


i'm not talking about any silly airspeed measurements to determine lift, i'm talking about the speed of the aircraft to the ground, to make the question simple. (for those people who obviously need it spelled out for them.)

[Edited on October 28, 2007 at 5:24 PM. Reason : ]

10/28/2007 5:23:26 PM

it CLEARLY takes off REPEATEDLY in this video:
http://www.youtube.com/watch?v=AoCm7DaRV4Q

10/28/2007 5:55:49 PM

Quote :

"air or ground it doesn't matter. lets talk about the speed with respect to the ground. which in this example (no wind) is the same as the air. i'm not talking about any silly airspeed measurements to determine lift, i'm talking about the speed of the aircraft to the ground, to make the question simple. (for those people who obviously need it spelled out for them.)"

well last time i checked, aircraft speed was measured in KIAS (knots indicated air speed)

which is the only speed that matters

dont spoon feed people

10/28/2007 6:12:20 PM