| T O P I C R E V I E W |
| PixieSteve |
Posted - 12/29/2005 : 08:47:21
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 opposite direction).
Does the plane take off?
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| 32 L A T E S T R E P L I E S (Newest First) |
| misleadtheworld |
Posted - 12/30/2005 : 18:23:28
Heh, you know, I understood this a few replies back --the engine uses the air for resistance, as opposed to the wheels using the ground-- but the more I read and thought about it (and got caught up with the idea of the plane taking off static), the more I began to doubt it and get mixed up. Then I thought it was a mind trick, like 'what does a cow drink?' or 'What do you put in a toaster?' Quite terrible for someone who once considered a career in aviation. Not me, that is, but can you imagine?
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| hammerhands |
Posted - 12/30/2005 : 12:35:41
Now imagine the same toy car on a belt sander. |
| Steak n Sabre |
Posted - 12/30/2005 : 09:36:03
If the plane is a Harrier Jet the condition of the runway would not matter.....
 The Cult of Frank : 10,000,000 Points Of View... |
| Cult_Of_Frank |
Posted - 12/30/2005 : 07:33:13
Mislead, try this idea. Take a toy car, hold it in your hand, and spin the wheels at the bottom. They move but the car doesn't. Now move your hand. The wheel speed is independant of the plane's speed because they (ideally) are frictionless against the ground - just like the car's wheels are frictionless in the air. Your hand represents the thrust from the engines, acting on the air and pushing the plane forward through the air. You could put this same toy car on a conveyor belt and as long as you are pushing it with your hand, the car will move forward at a speed independent of how fast the wheels spin and only as fast as your hand pushes it forward.
Now another interesting thing is that if the plane WAS staying still relative to the air, it probably would still lift off if the conveyor was fast enough due to the conveyor moving air quickly under the wings whereas above the wings there'd be little to no movement as well as something called ground effect: because air is forced between the ground and the wing, there is much higher pressure below the wing than when the plane is flying at 30,000 ft. This enables planes to actually take off below their normal stall speeds. But that's an aside.
"If we hit that bullseye, the rest of the dominos will fall like a house of cards. Checkmate." |
| misleadtheworld |
Posted - 12/30/2005 : 03:12:58
So it's a trick? You're engineering it so the idea is it has zero speed?
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| PixieSteve |
Posted - 12/29/2005 : 14:41:29
quote:
Originally posted by misleadtheworld
quote:
Originally posted by PixieSteve
if you then gave the bottle forward thrust with your hand, representing the thrust given by the engines on the air, the bottle moves forward. the only difference is the bottle spins at twice the speed.
once we've established the plane can move forward despite the moving runway, we can accept the plane will pick up enough speed to lift.
OH. I thought the conveyer also matched the thrust of the engines. Hammerhands is right, it was poorly worded- Mr. PixieSteve!
you still dont get it if you're saying things like this.
the thing matches the speed of the plane.
but it doesn't even matter how fast the runway is moving, the plane will always taske off the same. well, if it goes too fast then the wheels will be forced to spin at a rate they're not built to, but forget this.
the only reason the thing says the belt moves at the same speed of the plane, is to trick those into thinking the plane will have 0 overall speed. but whether the belt moves at a slower or faster speed doesn't matter. you see, planes move by gripping AIR not ground.
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| TRANSMARINE |
Posted - 12/29/2005 : 14:23:23
quote:
i dmeonstrated this by putting a bottle on its side on a piece of paper that was lying on the floor and pulling the paper. the bottle remained roughly in the same place (relative to the floor). if you then gave the bottle forward thrust with your hand, representing the thrust given by the engines on the air, the bottle moves forward. the only difference is the bottle spins at twice the speed.
Not since Anthony Hopkins made a compass out of wool, needle, and a leaf in THE EDGE (1997) have I been as impressed!
Hank the 8th was a duplicated man
-bRIAN |
| misleadtheworld |
Posted - 12/29/2005 : 14:22:06
quote:
Originally posted by PixieSteve
if you then gave the bottle forward thrust with your hand, representing the thrust given by the engines on the air, the bottle moves forward. the only difference is the bottle spins at twice the speed.
once we've established the plane can move forward despite the moving runway, we can accept the plane will pick up enough speed to lift.
OH. I thought the conveyer also matched the thrust of the engines. Hammerhands is right, it was poorly worded- Mr. PixieSteve!
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| 50 Pence |
Posted - 12/29/2005 : 14:17:03
I was suprised i got this right, usually these logic things get me.
Blats |
| PixieSteve |
Posted - 12/29/2005 : 12:54:47
quote:
Originally posted by hammerhands
If you begin to turn the conveyor belt under the aircraft, the aircraft will not move because it is tied to the pole.
the point is that the plane would not move even if it wasn't tied to a pole. well, it probably would slightly because there will always be SOME friction. but if we see that as negligable, the plane will stay still. i dmeonstrated this by putting a bottle on its side on a piece of paper that was lying on the floor and pulling the paper. the bottle remained roughly in the same place (relative to the floor). if you then gave the bottle forward thrust with your hand, representing the thrust given by the engines on the air, the bottle moves forward. the only difference is the bottle spins at twice the speed.
once we've established the plane can move forward despite the moving runway, we can accept the plane will pick up enough speed to lift.
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| hammerhands |
Posted - 12/29/2005 : 12:39:27
If the sensor were located in the belt of the conveyor itself then it would spin fast enough to counteract the speed of the plane (the speed relative to the conveyer). If the sensor were located in an geo-stationary position the belt would only turn fast enough to marginally increase the friction on the plane.
It's a poorly worded question. |
| hammerhands |
Posted - 12/29/2005 : 12:21:29
And I guess that's PixieSteve's point, to be stationary you would need to spin the conveyor fast enough to create enough friction to counter thrust, not just fast enough to match the speed of the plane over solid ground.
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| hammerhands |
Posted - 12/29/2005 : 11:53:40
The problem with this question is the mix of real and hypothetical objects.
Let's use an analogy that's easy to grasp.
Imagine an aircraft on a conveyor belt tied by a rope to a pole.
==>=--o
==>= is the aircraft
-- is the rope
O is the pole
If you begin to turn the conveyor belt under the aircraft, the aircraft will not move because it is tied to the pole.
We are seeing a few forces, the force of the conveyor belt on the aircraft (which is really the force of friction between the conveyor belt and the tires), the force of the rope pulling on the aircraft to keep it in place, "never forget the force of gravity", and the force of the earth pushing against the plane that counteracts gravity. These forces are in balance and the aircraft does not move.
You can draw a diagram of these forces:
Draw a line to represent the ground
On the line draw a block to represent the aircraft.
Draw an arrow down from the block marked G to represent gravity.
Draw an arrow up from the block marked g to represent the force of the earth on the block that counteracts gravity.
Draw an arrow forward (to the right if you like) from the block marked t to represent the "tug" of the rope.
Draw an arrow backwards marked F to represent the force caused by friction.
You can draw a curvy arrow marked with the Greek mu to show the coefficient of friction between the aircraft and the ground.
So what would the force of the thrust of the engines look like? That's the same t as the tug of the rope. Now you have a vector diagram of the proposed situation.
The thrust itself is a force acting against the air, but we are not given very much information about that.
So what are we asking the conveyor belt to do? We are asking it to spin fast enough to produce enough friction to counteract the thrust from the engines (not just to match the speed of the plane).
I can't imagine you can go any further without speculation, adding your own general conceptions.
In order to take-off an airplane needs to move fast enough through the air to create lift. The basic diagram of forces they would show you in ground school is:
a block to represent the aircraft
an arrow pointing down marked G for gravity
an arrow pointing up marked L for lift
an arrow pointing forwards marked T for thrust
an arrow pointing backwards marked D for drag (the force of friction between the air and the body of the aircraft)
I like what misleadtheworld brings up about the air being moved by the engine. The engines are going to create a miniature weather system, with air being siphoned along the aircraft. Unfortunately we don't know what kind of engine we are talking about, or where the engines are mounted or what type of air currents would be made (or even the qualities of the air itself). Speculation.
We don't know much about the tires either, but if you think about the force that the engines can produce mostly being absorbed as heat in the tires, I don't think they would last very long. It just takes a plane landing a little too fast or a little too heavy to pop a tire. (This is what I think would happen, catastrophic failure of the tires, and then a big mess.) Maybe the tires are some special material? It's all speculation.
And who's going to build this conveyor belt anyways? Let's call the Mythbusters. |
| misleadtheworld |
Posted - 12/29/2005 : 11:53:11
quote:
Originally posted by Cult_Of_Frank
I thought of that, but I'm afraid I disagree. The plane can only move forward if there is greater thrust than drag. When the plane sits on a moving platform that opposed all its forward motion, effectively moving the plane backward as fast as it moves forward, it acts as a source of drag equal and opposite the thrust so no, it doesn't move.
Oh, wait, I just realized what you meant... The wheels don't offer resistance because they're free spinning so there is no relevant force exerted. I was thinking of them as locked. Well done.
"If we hit that bullseye, the rest of the dominos will fall like a house of cards. Checkmate."
This is what I thought, and it did seem to make sense, but something still conflicts- I think it's these new trousers.
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| PixieSteve |
Posted - 12/29/2005 : 11:36:37
if the wheels were locked of course it wouldn't take off. just like on a normal runway..
the lesson is basically that the ground doesn't have any affect on the plane. moving runway or not, the plane takes off in exactly the same way. the only difference is the plane's wheels will be spinning twice as fast.
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| Cult_Of_Frank |
Posted - 12/29/2005 : 11:18:42
I thought of that, but I'm afraid I disagree. The plane can only move forward if there is greater thrust than drag. When the plane sits on a moving platform that opposed all its forward motion, effectively moving the plane backward as fast as it moves forward, it acts as a source of drag equal and opposite the thrust so no, it doesn't move.
Oh, wait, I just realized what you meant... The wheels don't offer resistance because they're free spinning so there is no relevant force exerted. I was thinking of them as locked. Well done.
"If we hit that bullseye, the rest of the dominos will fall like a house of cards. Checkmate." |
| Llamadance |
Posted - 12/29/2005 : 11:18:41
Aha!! I understand it now. Yes, yes it does take off. Don't ask me to explain it though.
That which does not kill me postpones the inevitable.
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| TRANSMARINE |
Posted - 12/29/2005 : 11:14:21
Who would build a runway like this?!
Hank the 8th was a duplicated man
-bRIAN |
| PixieSteve |
Posted - 12/29/2005 : 10:29:25
no, the air is still relative to the ground at all times.
just like normal, when theres no moving platform??
what i meant was: at the point the plane turns the engine on the air is still relative to the plane. this is just like a normal plane. it starts to move forward because the engines are sucking in air or watever, nothing to do with the wheels moving it (like in a car). eventually the plane will be moving through the air at enough speed to get lift.
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| misleadtheworld |
Posted - 12/29/2005 : 10:25:22
But the thing is, as you pointed out, the air is still, and since the runway and ground have no effect on the air, where does the aeroplane find this mystery air stream flowing about the wings to give it lift? Apologies if I'm missing something obvious here, I've got new trousers on.
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| PixieSteve |
Posted - 12/29/2005 : 10:18:13
unlike cars, planes don't use the ground to get thrust. plane wheels are not powered by the plane... the plane uses AIR.
if you held a plane in the air with strings and the plane turned its engines on, would the wheels start to spin? no. wheels are simply there to reduce the friction between the ground and the plane to a negligable value.
if the plane was off (engins off, wheels unlocked though) and the runway starting moving backwards, does the plane move backwards? no. the wheels SPIN at whatever speed the runway is moving, but they do not move backwards, and neither does the plane.
the plane turns its engines on. it uses the air, which is STILL. the ground moving below has no effect on the air. the plane moves forward, relative to still land, and over the moving runway it would appear the move at twice the speed. the wheels wouldbe spinning at twice the speed too but that has no effect on the planes movement.
the plane picks up enough speed, apply normal aerodynamic theory, and the plane takes off.
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| TRANSMARINE |
Posted - 12/29/2005 : 10:18:09
quote:
Originally posted by Cult_Of_Frank
I'm afraid the plane is going nowhere, it's basically a couple really big fans at this point. Planes generate lift, as has been pointed out, by airflow over the wings causing a pressure differential (the top surface is curved so that by Bernoulli's theory air must move faster across it, spreading out the molecules and creating lower pressure than underneath the wing - so really you could say that the plane is lifted from the bottom by high pressure air or sucked from the top by a vaccuum with equal validity imho). A stationary plane can be lifted off the ground if a strong enough headwind comes for the given temperature/pressure. Likewise, a plane with its wheels moving, no matter how fast, can and will generate no lift without air moving across its surface.
The engines do not generate lift, they simply push the plane forward causing air to move across the wing. A pilot can gun the engine at a standstill or with brakes applied and this doesn't translate into the plane jumping into the air. In fact, a conveyor belt that would move the plane backward as fast as it can move forward is basically just a set of brakes, keeping the plane stationary.
So, my guess is that no, the plane doesn't take off.
"If we hit that bullseye, the rest of the dominos will fall like a house of cards. Checkmate."
What if the plane is on strings? Or it is a CGI effect? In those cases, yes...it lifts off.
Hank the 8th was a duplicated man
-bRIAN |
| Cult_Of_Frank |
Posted - 12/29/2005 : 10:10:35
And you're lack of understanding of contractions disappoints me. :P
I'd love to be proven wrong, always more interesting than having someone say, "Yeah, you're right." But that has yet to happen on this particular topic, however often it seems to happen elsewhere.
"If we hit that bullseye, the rest of the dominos will fall like a house of cards. Checkmate." |
| PixieSteve |
Posted - 12/29/2005 : 10:06:18
i love this question, because to be able to answer No you need a basic understanding of aerodynamics. However, to answer Yes, you need even more knowledge.
CoF, you're mean, and your lack of understanding of planes disappoints me...
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| Carl |
Posted - 12/29/2005 : 10:04:10
This forum is turning lame. ;)
 "Yo! Ho Ho! Merry Christmas!" |
| Cult_Of_Frank |
Posted - 12/29/2005 : 10:03:47
I'm afraid the plane is going nowhere, it's basically a couple really big fans at this point. Planes generate lift, as has been pointed out, by airflow over the wings causing a pressure differential (the top surface is curved so that by Bernoulli's theory air must move faster across it, spreading out the molecules and creating lower pressure than underneath the wing - so really you could say that the plane is lifted from the bottom by high pressure air or sucked from the top by a vaccuum with equal validity imho). A stationary plane can be lifted off the ground if a strong enough headwind comes for the given temperature/pressure. Likewise, a plane with its wheels moving, no matter how fast, can and will generate no lift without air moving across its surface.
The engines do not generate lift, they simply push the plane forward causing air to move across the wing. A pilot can gun the engine at a standstill or with brakes applied and this doesn't translate into the plane jumping into the air. In fact, a conveyor belt that would move the plane backward as fast as it can move forward is basically just a set of brakes, keeping the plane stationary.
So, my guess is that no, the plane doesn't take off.
"If we hit that bullseye, the rest of the dominos will fall like a house of cards. Checkmate." |
| PixieSteve |
Posted - 12/29/2005 : 09:20:34
the air is still
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| TRANSMARINE |
Posted - 12/29/2005 : 09:16:37
Is there a wind current? Actually misleadtheworld, air travels over aircraft wing and thru the airfoil, thus the plane is lifted from the top.
Hank the 8th was a duplicated man
-bRIAN |
| misleadtheworld |
Posted - 12/29/2005 : 09:09:35
Isn't it something to do with the sucking of the air by the engines around the wing? Thus creating a difference in air pressure underneath the wing, creating lift. I'm quite skeptical, but I'll certainly eat my hat if someone a) proves it right with facts, figures, and chocolate biscuits or b) someone buys me a tigermoth and demonstrates such a technique.
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| Llamadance |
Posted - 12/29/2005 : 09:04:06
Haha. I saw this on another forum, it caused mayhem. I still don't quite get it, but probably because I don't understand how a plane works.
That which does not kill me postpones the inevitable.
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| PixieSteve |
Posted - 12/29/2005 : 08:57:25
wrong!
next
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| lonely persuader |
Posted - 12/29/2005 : 08:55:12
no, it's travelling at 0 miles in relation to the earth and its air. Thus no lift. |
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