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Author Topic: Can an aeroplane on a conveyor belt get airborne?  (Read 13886 times)

Offline Bored chemist

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Can an aeroplane on a conveyor belt get airborne?
« Reply #25 on: 02/04/2011 21:23:32 »
Do you realise there's a difference between cars which have powered wheels and aircraft which use props or jets?

(and I'd still like an answer to the questions I asked earlier too)
 

Offline Phractality

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Can an aeroplane on a conveyor belt get airborne?
« Reply #26 on: 02/04/2011 21:48:13 »
Airspeed is what produces lift. If the air moves over the wing fast enough, it will lift the plane. If the plane is stationary relative to the ground, it may get some lift due to the fact that the conveyor belt's friction generates wind. If lift-off speed is 80 knots, the belt will have to go considerably faster than that, and the wind it produces will only go up a short distance. An airplane hovering above the belt would be no different from hovering above the ground in an 80 knot wind, except it would have to hover very low where the wind is strong.
 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #27 on: 02/04/2011 23:47:55 »
Are you saying that a aircraft don't rest on its wheels on ground BC?
Or do you mean the fact that as they get up to speed (take off) the weight resting on the wheels get progressively smaller? If it is the later I agree, my car idea is in no way a perfect fit but it will give a general idea of the condition for takeoff. As for the question if I see the difference between cars and props or jets. Yes, I believe I do? I'm not saying that a car is as a aircraft?

I'm interested in putting some few figures to it, seeing the time needed for that possible, as judged by some, inevitable takeoff. To do that we can use a 'takeoff'-speed by a prop or jet, and just look at the time it takes to reach that speed. Or a car, doesn't really matter for this question. when it comes to how much a lift will matter before takeoff I don't really know? But that's not the point either, I'm just interested in how long it will take to reach those 250 km/h on that treadmill.
as for the discussion how the atmosphere might move at different levels? What has that to do with the aircraft standing on the treadmill? If you don't want to assume a hurricane coming?
 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #28 on: 02/04/2011 23:53:43 »
Maybe it could get a 'lift' on air pushing it up, from the treadmills motion, but my guess is that it would be rather unstable. I think Rosy wrote something about that? (The thread BC linked to.)
==

Thinking again, nah.
 

Offline Geezer

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Can an aeroplane on a conveyor belt get airborne?
« Reply #29 on: 03/04/2011 05:25:03 »
OK Yoron, try this.

The conveyor belt has a layer of ice on it (yes, I know that's a bit difficult to achieve) and the aeroplane has ice skates instead of wheels.

So, what happens when the conveyor belt starts moving and the plane's engines are off? I'll tell you what will happen. Not a dang thing! Because of its inertia, the plane will stay in the same position while the belt moves under it.

The only difference between wheels and skates is that wheels produce a little more friction, so the engines have to generate a little bit of thrust to compensate for that. Other than that, whether there is a moving belt or not makes almost no difference.

 

 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #30 on: 03/04/2011 05:33:23 »
Well, if it was ice and skates that plane would find it a lot easier to get into a 'real motion' relative the earths 'starting-mark' I think. But when it comes to the tread belt, and the invariant mass of that aircraft resting on its wheels I'm not as sure of that motion.

« Last Edit: 03/04/2011 05:35:28 by yor_on »
 

Offline Geezer

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Can an aeroplane on a conveyor belt get airborne?
« Reply #31 on: 03/04/2011 05:42:23 »

Well, if it was ice and skates that plane would find it a lot easier to get into a 'real motion' relative the earths 'starting-mark'


No, it would not!! You seem to be making a very bad assumption about how much friction wheels produce. They are hardly any different from ice skates.

The amount of thrust that the engines have to produce to overcome the rolling resistance of the wheels is negligible compared with the thrust required to accelerate the mass of the plane and the thrust required to overcome wind resistance.
 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #32 on: 03/04/2011 05:52:21 »
I'm not speaking about friction?
You are Geezer.

I'm speaking of the mass, the weight resting on the wheels before it gets its lift. Unless you mean friction to be the sole cause? It can mean many things friction? If the texture of the treadmills band is sandpapered you will find a greater friction too.

But assuming no oil leaks, just something resembling a normal surface where the wheels can get a grip then it will be the props, jets, that makes those wheels start to turn, and in my scenario make the tread mill move the exact same amount in the opposite direction one thousand of a second slower/after. And that's what I'm curious about. Assuming a magical lift and no weight leaves me sort of wondering?
« Last Edit: 03/04/2011 05:53:54 by yor_on »
 

Offline Madidus_Scientia

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Can an aeroplane on a conveyor belt get airborne?
« Reply #33 on: 03/04/2011 05:54:06 »
No, it would not!! You seem to be making a very bad assumption about how much friction wheels produce. They are hardly any different from ice skates.

The amount of thrust that the engines have to produce to overcome the rolling resistance of the wheels is negligible compared with the thrust required to accelerate the mass of the plane and the thrust required to overcome wind resistance.

Indeed. You could stand at the back of the conveyor belt and hold the plane on it

yor_on, if you rolled a toy car down an escalator do you really think it's going to go much slower than if the escalator was turned off?
« Last Edit: 03/04/2011 05:57:41 by Madidus_Scientia »
 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #34 on: 03/04/2011 06:00:24 »
This is not a good comparison :)
The wheels will turn, skates don't.

Sh* nobody but me that sees it?
As for the rest, let's take that as a new question as it's not the same.

Because with skates it would be different, as you say.
==

If we really want to prove a point assume a frictionless material.
Then the bigger the mass the more inertia. and so it never will move.

Probably :)
« Last Edit: 03/04/2011 06:05:24 by yor_on »
 

Offline Geezer

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Can an aeroplane on a conveyor belt get airborne?
« Reply #35 on: 03/04/2011 06:02:52 »
I'm not speaking about friction?
You are Geezer.

I'm speaking of the mass, the weight resting on the wheels before it gets its lift.


Then you ARE speaking of friction. If the wheels have no rolling resistance, the weight resting on the wheels is completely irrelevant, so if you are concerned about that weight, you must be making a false assumption about the significance of friction.
 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #36 on: 03/04/2011 06:06:15 »
No rolling resistance?
 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #37 on: 03/04/2011 06:08:29 »
Am I to understand this to mean that, according to you, there is no difference between 10 kg resting on three wheels, or 10 tons?
 

Offline Geezer

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Can an aeroplane on a conveyor belt get airborne?
« Reply #38 on: 03/04/2011 06:10:13 »
This is not a good comparison :)
The wheels will turn, skates don't.

Sh* nobody but me that sees it?
As for the rest, let's take that as a new question as it's not the same.

Because with skates it would be different, as you say.

No, it wouldn't be different. Wheels and skates do the same thing. They support a mass and minimize friction at the interface between the mass and some other surface.

I suggest you go back and study some basic Newtonian Mechanics.
 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #39 on: 03/04/2011 06:13:47 »
:)

We'll see. Now tell me what you meant by 'no rolling resistance'?
You deem it as if the wheels 'friction' is what counts right? And somehow the mass resting on them has no importance? So I could mount Titanic on those wheels then? And give it a flying start? Well, sort of?
 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #40 on: 03/04/2011 06:14:27 »
Ignoring Icebergs :)
 

Offline yor_on

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« Reply #41 on: 03/04/2011 06:16:09 »
You know, this treadmill is indeed a marvelous invention :)
 

Offline yor_on

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« Reply #42 on: 03/04/2011 06:20:02 »
I'm not disputing this Geezer "They support a mass and minimize friction at the interface between the mass and some other surface." But you wrote it yourself. "They support a mass."
==
And yes, looking at it your way I can see that you might think so. But to me the inertia will constantly work against the force trying to make it move, with the tread mills band one thousand of a second behind. And the greater its mass, relative the power, the longer the stretch it will need for that takeoff. Well, as I see it.

Let us assume that the aircraft 'jumps' like blackbody radiation. from 10 to 20, to 30 km/h

10 - treadmill nul
20 - 10 (one millisecond later)

In reality both accelerations are constant ones, so the divergence will widen between the two, to the aircrafts advantage, it's just that I expect it to take a awful amount of time for that plane to ever get airborn myself, if ever.





« Last Edit: 03/04/2011 06:30:08 by yor_on »
 

Offline Geezer

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Can an aeroplane on a conveyor belt get airborne?
« Reply #43 on: 03/04/2011 06:20:48 »
Am I to understand this to mean that, according to you, there is no difference between 10 kg resting on three wheels, or 10 tons?

Ah! You are finally getting it.

Assuming the bearings were designed properly, the coefficient of friction would be the same in either case.

Put a mass of 10 kg and a mass of 10,000 kg, both on rolling bearings, on a 1% incline. Which one will have greater acceleration? Answer: Neither. They will have equal acceleration.
 

Offline Geezer

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« Reply #44 on: 03/04/2011 06:25:12 »
If you support a mass of 1000 kg on an air bearing, you can move it quite easily with a force of 1 kg.
 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #45 on: 03/04/2011 06:31:46 »
The inertia will differ, and it won't disappear just because it starts to roll Geezer. It's there along with the mass. Even though it will lessen with speed and lift I guess. Which is a stupid statement as it must have the same inertia always, but it's the lift that confuses me here? As that will change the mass resting on the tread belt.
==

And that's my point I think.
You can assume any acceleration as inertial reactions.
There has to be one to any increase in speed.
 
But I'm not as sure any more :)
« Last Edit: 03/04/2011 06:44:03 by yor_on »
 

Offline Geezer

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Can an aeroplane on a conveyor belt get airborne?
« Reply #46 on: 03/04/2011 06:39:47 »
The inertia will differ, and it won't disappear just because it starts to roll Geezer. It's there along with the mass. Even though it will lessen with speed and lift I guess.

No it won't. The inertia of the aircraft has nothing to do with the speed of the conveyor belt.

Try again  :D

(You're really clutching at straws now.)
 

Offline yor_on

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Can an aeroplane on a conveyor belt get airborne?
« Reply #47 on: 03/04/2011 06:48:17 »
No, I'm not ready to throw in the towel, just yet.
Action and reaction.

The mass rest on those wheels. What mass there is, will be inertial even if the wheels are friction less. So to get it to accelerate will be as constantly making its mass to move from zero, add to that that the treadmill constantly are stealing that motion by accelerating the other way.
==

Think of those objects in space instead. To get 10 g or ten ton to move will take a different force. And an acceleration is like constantly coming back to that ten ton object and push it a little more. And the weight (on earth) is all resting on those three wheels that meet a surface rushing the other way.
==

But I'm not as sure as I was, da* :)
==

the only reason it would matter would then be the magical prop, or jet?
Take a cyclist sitting on his bike, take yourself running on a treadmill.
Exactly how do you see that differing from using a propeller to get that forward moving force?
As long as you're still on that treadmill?
==

Let us assume no prop. What would happen if the treadmill started to move. The aircraft would go with it, do you agree? So the inertia matters, as does its weight relative what it stands on, which are the wheels. To get to your point I will have to assume that the only thing that would matter was the friction the wheels had relative the aircraft and the treadmill, ignoring the aircrafts mass as well as the treadmills opposite motion.
« Last Edit: 03/04/2011 07:10:47 by yor_on »
 

Offline Geezer

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Can an aeroplane on a conveyor belt get airborne?
« Reply #48 on: 03/04/2011 07:06:33 »
You keep going back to the mass resting on the wheels as if it had some magical property. I'm sorry to disappoint you, but it doesn't!

It results in a bit of friction force that has to be overcome, but compared with the other forces involved, it's quite negligible.

If you don't believe me, put your car on a small incline, release the brake, and try to stop it.
 

Offline Madidus_Scientia

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Can an aeroplane on a conveyor belt get airborne?
« Reply #49 on: 03/04/2011 07:07:36 »
The difference between one plane trying to take off on a normal runway and another trying to take off on a conveyor belt will be as insignificant as one ball rolling down a rising escalator and another ball rolling down an escalator that is turned off.

One example gets acceleration from gravity, the other from jet engines.
 

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Can an aeroplane on a conveyor belt get airborne?
« Reply #49 on: 03/04/2011 07:07:36 »

 

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