i guess you didn't see the QSRA video posted by me couple of pages ago. but hey, it's all about the 747. in fact a specific 747 on a conveyor belt, right? do not attempt to generalize the problem :)Quote:
Originally Posted by OverTurn
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i guess you didn't see the QSRA video posted by me couple of pages ago. but hey, it's all about the 747. in fact a specific 747 on a conveyor belt, right? do not attempt to generalize the problem :)Quote:
Originally Posted by OverTurn
If the treadmill matches the speed of the plane, the plane takes off. If the treadmill matches the speed of the wheels, the wheel bearings seize before the plane takes off.
It's really a matter of interpretation of the question. The plane has to move forward to generate lift. If the treadmill matches the wheel speed then no lift is generated. If (as in the mythbusters scenario), the treadmill matches the speed of the plane then the plane still moves forward and lift is generated.
Everyone is right, and everyone is wrong.
I'm not debating the 747 on a conveyor belt thing here, but merely your comparison of vertical takeoff aircraft to this question. Not the same thing. Two entirely different mechanisms.
you can generate enough "lift from the air rushing over [an aircraft's] wings" even when you have a stationary aircraft. it's 50s technology, nothing "special".Quote:
Originally Posted by OverTurn
True, but that has nothing to do with helicopters or vtol aircraft.
oh, fuck off.
Ok, I'm done. But just so long as nobody continues to use helicopters and vtols to explain why can take off with no air moving over the wings.
I guess I just assume in this ridiculous hypothetical situation that no matter how fast the wheels spin, the conveyor belt goes exactly in reverse. This would create a situation in which the plane does not move through the air.
Oh. My. God.
I cannot believe there are people who still don't get this.
Forget the fucking wheels. They are meaningless. They're just along for the ride on an airplane, they are the lowest friction option so that an airplane can move across a solid surface until it gets enough speed to take off. They have nothing to do with moving the plane.
If you think an airplane can't take off because of a conveyer belt, you must really be confused by airplanes that don't even have wheels. How the fuck does a float plane take off? Or an airplane with skids that can land on ice? How the hell does that work?
But what about this?
http://t2.gstatic.com/images?q=tbn:A...T2FpOxn2DKz5XQ
It depends on how you determine the speed that the treadmill is matching.Quote:
Originally Posted by stuckathuntermtn
Read this, it'll make things clearer: http://blog.xkcd.com/2008/09/09/the-...amn-treadmill/
No, it's a toy helicopter.Quote:
Originally Posted by Chainsaw_Willie
NOW WE'RE TALKIN! Except the answer is obvious. It will fly, turntables turn clockwise, heli-rotors turn counter.Quote:
Originally Posted by Chainsaw_Willie
What if we put a osprey on it?
um.....................................
um.....................................
I still can't picture how the plane moves forward relative to the air if the wheels and the treadmill negate each other.
Yeah, I know it's being powered by the jets. So what?
If it were on floats and pointed up river and the river speed matched the speed the floatplane would be going at full power, it wouldn't move relative to the river bank (or the air).
Picture this: the conveyor belt causes the wheels to spin really fucking fast but the 747's jets continue to move the plane forward. That is, the wheels and conveyor do not "negate each other." You're stuck on the driven wheel vehicle thing.
Sound analysis, but the hypothetical posed in that discussion (treadmill matches wheels' speed) is different than OP's. In OP's hypothetical, a given is that the conveyor belt matches the airplane's speed. The only reasonable definition of "plane" as used in OP's hypothetical is the entire body of the plane, not the outer diameter of the wheel.Quote:
Originally Posted by OverTurn
I still don't get it. It's wheels are the only part touching the ground. You're basically saying, yet it would take off because it would overpower the treadmill.
I thought we were saying the treadmill matches the wheels speed and negates it. I don't see why it matters where the thrust comes from, as long as it's forward(backward?) and not up(down?).
No, OP's hypothetical assumes the conveyor matches the airplane's speed, not the wheels' speed. The conveyor does not negate anything -- they are not driven wheels -- but it does make the wheels spin faster while the jet moves forwardQuote:
Originally Posted by stuckathuntermtn
Oh good lord.
Go get a toy car and a treadmill. Turn the treadmill on and set the toy car on it and hold the car in place by hand.
Now, push the car forward against the motion of the treadmill. You are providing thrust to the car the same way the plane's engines provide thrust to it. Can you do it? Do you think it will matter how fast the treadmill is going?
This is not a question of whether the wheels will be destroyed or if the bearings can handle the added speed or about friction in the wheelbearings or whatever - that's a separate question. The original question simply deals with the effect of the surface the plane is sitting on having the ability to move in the opposite direction of the plane. Assume zero friction wheelbearings and indestructable wheels. If we keep it that simple then the example above makes it perfectly clear that the plane will move forward and take off.
Now, if the premise of your argument demands the inclusion of the physical reality of spinning wheels and bearing friction and whatnot, then you could make a case that the plane won't achieve sufficient airspeed before the wheels seize or fly apart and I'd go along with that depending on what kinds of speeds we're talking about.
When you ski slowly, do your feet feel like they're dragging behind you? What about when you ski fast? Is there an appreciable difference on the drag force on your feet between going slow and fast?Quote:
Originally Posted by stuckathuntermtn
No. There isn't. The skis are a low friction platform that don't really give two fucks about how fast or slow they're going across snow. These are the airplane wheels, a low friction platform. The jets act on the airplane, and the treadmill acts on the low friction platform. Guess what, the low friction platform can't transfer the force needed to stop the jets.
Your example of a float plane is a good troll, but also wrong. If the plane wasn't moving relative to the river bank or air, the water wouldn't be moving either. Float planes just have a larger amount of resistance to overcome, but would still take off. You keep trying to re-word the concept in your head to: if the plane cannot ever move, will the wheels under it spin? The wheels and treadmill do not negate each other.
Stop with the mindfuck shit, WillQuote:
Originally Posted by Chainsaw_Willie
why do you want to ruin my fun, Steve?
Man this is harder than the whole gravity stretches time thing.
We're doomed.Quote:
Originally Posted by stuckathuntermtn
WHO'S TAKING OFF NOW BITCHES???!!
http://farm8.staticflickr.com/7044/6...06e4a32a_c.jpg
If we're talking speed relative to the to the stationary ground and - lets just assume - air next the treadmill and and the plane, then the treadmill will only only match wheel speed if the plane is stationary. The second jet thrust is applied and the plane moves forward relative to the ground and air - as it most certainly will - the wheels move faster than the treadmill and lift will be generated.Quote:
Originally Posted by doughboyshredder
Think of it like walking up the down elevator. if the airplane was driven by its wheels, you would stay in the same place, just like walking up the down elevator. The confusion comes from the fact that the plane is not driven by it's wheels in contact with the conveyor belt, it gets it propulsion from its jet engines pushing air.Quote:
Originally Posted by stuckathuntermtn
Now if you had a rocket pack on at the bottom of the stairs, it would shoot you up the stairs at a fast rate, regardless of how fast the escalator went down, or how fast your legs ran.
Yes, but, if we had a magical treadmill with a motor controlled by a pid that is linked to the wheels of the jet, the treadmill would increase in speed sufficiently to keep the jet from moving forward. The jet wheels and treadmill would reach terminal quickly and bearings would seize, shit would go kaboom. LOL.Quote:
Originally Posted by NorthernSpotted...
If the jet is moving forward relative to the ground it will achieve lift at some point.
In the original hypothetical question (not the op's, but the original argument starter posted online) the real question is can the imaginary treadmill be set up in such a way to keep the jet from moving forward by matching the wheel speed caused by the jet's thrust. In my imaginary world the wheel bearings seize, and then depending on the amount of thrust the plane provides either the nose hits the treadmill and the plane disintegrates, or the plane overcomes the drag of the melting tires and takes off.
In your example, the planes wheels would HAVE to spin at a higher speed than the conveyor belt is moving.Quote:
Originally Posted by Jethro
The confusion comes from the concept that the conveyor belt could be somehow linked to match the speed of the wheels, therefore never allowing the wheels to spin at a higher speed, causing the bearings to seize.
It doesn't matter where the propulsion is coming from. If the wheels can never move faster than the conveyor belt the plane will not be moving forward relative to the ground. In order to move forward relative to the ground the wheels MUST move faster than the conveyor belt.
This is why the answer to the question depends on whether the conveyor belt/treadmill is moving at the same speed as the plane, or at the same speed as the wheels, and why stuck is so confusered.
I think it would be helpful to get two toy cars.
One regular hot wheels type car, or maybe a toy airplane. That would be better, a toy airplane with free rolling wheels.
Then get one of those toy cars that you roll backwards and then they go forward on their own...
Then take them to treadmill.
The toy airplane, you will be its power, sort of like a real airplanes jet engines. You can put it on the treadmill, and push it forward, against the direction the treadmill is going and the wheels just spin, right? So the toy airplane's speed is decided by how fast you push it (like a real airplane's speed is dependent on how fast its jet engines push it). The wheels are just along for the ride.
Now take your toy car, and roll it backwards to charge it up. It will be powered by its wheels. If you set it on the tread mill, against the flow, it will probably go backwards, or forward pretty slow because its speed is dictated by the speed of its wheels...
See the difference????
LOL, you're still not getting it.Quote:
Originally Posted by mtngirl79
In your example the wheels on the plane that you are pushing are moving FASTER than the treadmill. In order to create lift, the wheels will ALWAYS have to move faster than the treadmill. If the wheels CANNOT move faster than the treadmill, then the plane CANNOT generate lift.
The wheels aren't "just there". The plane is sitting on them. They are the connection of the plane to the ground. Remove the wheels and replace them with saucers, and the plane takes off. In a scenario where the treadmill is matching the speed of the wheels, the wheels prevent the plane from getting any ground speed which is necessary for lift.
It's pretty basic really.
Actually, you don't get it...Quote:
Originally Posted by doughboyshredder
The original question...
The conveyor is tracking the PLANE'S speed, NOT the wheel speed, and the wheels are free rolling, so their speed is irrelevant, and YES they CAN move faster than the treadmill, and faster than the plane, because they are free rolling! They will go however fast they need to go to keep up with the plane unless the friction becomes too much, and they break...Quote:
Originally Posted by "The Big One"
Uh, you lost me thereQuote:
Originally Posted by doughboyshredder
That's the OP's question, but as I posted not the original question floating around the nets for so long.Quote:
Originally Posted by mtngirl79
Of course if the conveyor is tracking the planes speed the plane takes off.
Stuck and others have posted about the other hypothetical where the conveyor matches the wheels speed. In that scenario the bearings seize and the plane falls over (unless it has so much power it just drags the melted carcass of the tires across the treadmill and takes off.
This is one of those questions designed with an ambiguity for the purpose of creating pointless arguments.
Which can be fun, I suppose.
They're flying saucers.Quote:
Originally Posted by Big Steve
priceless.Quote:
Originally Posted by pisteoff
And a helicopter on a turn table would take off. The thrust is downward. The spin is torque.
Even if the conveyor matches the wheel speed, I'm sure that the wheel bearings could tolerate the 350 kts(twice a 747s takeoff speed) and still take off. Like I said, we're doomed.Quote:
Originally Posted by doughboyshredder
The conveyor cannot match wheel speed if the airplane is moving. Per the question, the conveyor will not move until the plane moves.Quote:
Originally Posted by AaronWright
Here's an example using time as a variable:
Wheelspeed(t+0.001) = airspeed + conveyor speed
Conveyor speed = wheelspeed(t)
Therefore
Wheelspeed(t+0.001) = airspeed + wheelspeed(t)
Once the plane moves, the wheelspeed/conveyor speed would accelerate until the wheelbearings created enough friction to stop the airplane (i.e. seize the bearing). It's a circular reference.
I weep for the future of this country. Jesus fucking christ, is this really so hard? Really?
Plane takes off. If you don't understand why the treadmill has no fucking affect on the jet engine's ability to move the plane forward with respect to the atmosphere, then you are an idiot. The wheels can be on ice. In quicksand. On a treadmill going 9000mph in reverse, or 9000mph forward. It makes no difference. If you really don't get it, then you should burn your GED paperwork and go back to middle school. Fuck I'm glad I don't have kids.
Here is another question, from high school physics 25 years ago. Two identical cars, each traveling 50mph crash into one another in a perfect, straight on head on crash. they come together at a speed of 100 mph. Do the occupants experience a wreck equal to 100mph into a solid concrete wall, or 50 mph into a wall?
Effect, not affect. Who's the idiot?Quote:
Originally Posted by Flyoverland Captive
Really.
If the treadmill is moving at 9000mph backwards then the jet's wheels must rotate at 9200mph. I doubt the wheel bearings and rubber could handle that. So, you are wrong. The speed of the treadmill does have an EFFECT on the jet's ability to move forward.