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Tuesday, December 12, 2006
The stupid hurts my brain.
I realize this is pretty irrelevant but it's amazing what kind of stupid discussions smart people can have. Take this supposed problem of an airplane on a treadmill
the "Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?" Pogue at least gets the answer right, but is upsettingly unsure of it, and worse, other bloggers including Mark at BoingBoing are insisting on the wrong answer. These guys need to go back to physics class. Planes fly due to Bernoulli effect, that is, a wing moves through the air (this is critical) and generates lift, because the air moves faster over the top of the wing compared to the bottom or the wing is tilted creating a downward deflection of air. Slower moving air underneath the wing leads to higher pressure, while faster moving air moving over the wing leads to lower pressure, hence there is an upward lift on the wings. (Wikipedia has a full discussion of lift that I found helpful, including explaining why Bernoulli's equation works, even though the reason it works isn't the one you think is.) The engines and wheels are largely irrelevant to this discussion (assuming we're talking about non-military jets, they have light enough bodies and powerful enough engines that they rely more on thrust than on lift for their flight), engines provide the thrust that moves the plane through the air, that generates lift on the wing, the lift makes the plane fly, not the engines, otherwise cars would be taking off left and right. Cars can take off, at very high speed they'll similarly generate enough lift from air traveling under the flat undercarriage and over the curved body to get airbourne (hence spoilers), but it's like a single wing flying with no control. So, the answer is no, a plane (airliner, cessna, etc.) on a treadmill will not fly, because there is no lift, because there is no airflow over the wings. As anyone who has ridden a stationary bike knows, no matter how fast you pedal, the wind won't blow through your hair. To say a plane on a treadmill will take of is like saying attaching wings to your arms and pedaling on a stationary bike will somehow allow you to take off. It makes no sense. Are they saying it will take off just to piss me off? Am I getting paranoid? Labels: physics |
10 Comments:
It depends upon how much thrust the plane's engines generate. Some fighter planes generate enough thrust to drive the plane even if the plane had no wings at all- they're essentially jet-powered rockets, if that's not an oxymoron. Such a plane would eventually lift off the treadmill of its own accord- probably with disastrous consequences, since without airflow over the control surfaces the plane would not be pilotable.
2:12 PM, December 12, 2006
Exactly, the wings on a fighter pilot mostly exist to give it directional flight, but they are essentially rockets with wings.
I think this example though is more about planes in terms of passenger planes, which absolutely need to gather speed to generate enough lift to take off. The suggestion that their jets are powerful enough to be solely responsible for mobilizing thousands of pounds of plane, passengers and luggage is just ridiculous, the engine would have to be a much larger portion of the plane's total weight, or would otherwise be so powerful it would rip itself off the wings.
2:23 PM, December 12, 2006
I realized just now the real question to ask them. Do they really think a passenger plane would be able to take off without its wings?
If yes, then they're nuts, if no, then what function exactly do they think the wings have?
2:25 PM, December 12, 2006
I liked this guy's two bits: "A thought experiment. Suppose you're on the treadmill at the gym and someone sets the speed to "WHOAH!". You're going to go flying backwards. Now, suppose you have rollerblades on. With just one finger wrapped around the handlebar, you'd be able to resist the force of the treadmill no matter what the speed was set to (up until the bearings on the rollerblades started smoking.) Now, imagine someone gave you a Wiley E. Coyote-style Acme rocketpack and lit it off. What direction do you think you'd go?"
Of course the Rev.Dr. is perfectly correct. The problem with the puzzle that keeps everyone talking is that it's very hard to imagine the speed the treadmill would have to achieve to keep the plane stationary. If the wheels on the plane were frictionless (a loaded term to introduce into a physics problem, but one which everyone keeps saying is an important point in this problem for some reason) it's going to be very hard to keep that plane stationary. I think it might even be impossible due to relativistic constraints. That would certainly be true if we add an unlimited fuel supply and lots of time.
-JE
4:06 PM, December 12, 2006
I think the bigger problem is the use of analogies as the sole argument for the take off side, and the complete misunderstanding of what thrust is.
Analogies are helpful to explain what you mean, but they don't explain where you get the lift.
Looking at the comments there are 2 further miconceptions about the physics of flight.
#1. No one understands that thrust is a forward force. It does not lift up the plain (unless you point the engines up, but that's not the point of the question). The forward thrust pulls the plane forward, as the plane accelerates, the air moving over the wing creates lift, the upward force.
#2. People do not get how jets work. Jets do not blow wind over the wings, nor do propellers. They merely generate forward thrust. Again this does not apply to supersonic jets that have wings that create a bare-minimum of lift, but it is assumed that we're talking about an ordinary plane here, like a cessna, or a passenger jet. A passenger jet with engines but no wings would not take off, it would just move forwards.
So, based on these two misconceptions about flight it seems every ignoramus in the world has come out of the woodwork with bad analogies and obsessions with wheel friction. These things are all irrelevant to the need for lift, not thrust, to get the plane airborne. It's really distressing that the take off people simply won't read the explanation after explanation of how lift works.
6:43 PM, December 12, 2006
A lot of the people on those blogs are getting bogged down with the idea that the plane would somehow necessarily move forward anyway, despite the problem's injunction against this.
There was an idea for a deep space probe that was kicked around a while back that used an ion drive. The idea was to push a very small amount of gas through an electrified grid, and their interaction would cause a small amount of thrust. The resulting acceleration in a vacuum was equal to the weight of a single sheet of paper on Earth. The engine was designed to run out of fuel after about 2 years, but with constant, unimpeded acceleration over that entire period the final speed achieved was a healthy fraction of the speed of light (1/16?).
The treadmill would be similar to this in that all of the acceleration produced by the engines would be absorbed by the treadmill needing to go faster and faster to keep up. If the wheels were frictionless then it would be just like applying all that thrust in a vacuum, and the speeds are going to add up quickly.
But, of course, as long as the treadmill did actually keep up there would be no lift and no take off. But if that level of acceleration was applied for, say, a month, the treadmill might end up needing to go faster than light, which would make this problem just that much stupider.
-JE
7:58 PM, December 12, 2006
Well, a 747 has a .27 thrust to weight ratio. If my calculations are correct (I don't get to say that too often), it would take 36 years for the treadmill to approach the speed of light. As long as the treadmill kept up there would be no take off, but I think it's not seeing the crazy-ass speeds involved that is (in part) fueling all the misdirected blather on the other blogs.
-JE
8:52 PM, December 12, 2006
Ok... One try. Just one.
The treadmill cannot keep the plane from moving, friends. It has only the rolling resistance of the tires to affect the body of the plane with, which the engines can already overcome handily...
Sorry, guy, I dig your blog, but this one's measurable. Look up the rolling resistance of the tires - which does not increase with rotational velocity - and look up the thrust of the engines. If the engine thrust exceeds the rolling resistance of the wheels (which we know it does, since it takes off on a normal runway)...
Planes already take off on a treadmill - the earth, that's travelling 1000mph at the equator.
Ok, i'm done.
11:20 PM, December 12, 2006
I think the only way that a plane could take off on a treadmill is if the treadmill were situated in a wind tunnel.
1:55 AM, December 13, 2006
Steve,
It's not about thrust, it's about lift. Thrust is meaningless in this problem.
It's so funny that every single time you get a lift off answer they're talking about thrust or wheels or whatever, these things have nothing to do with flight.
Lift, lift, lift. Yes the earth is like a giant conveyer belt, but again this is besides the point. If this fact mattered at all the way you think it does then planes would be unable to hold still ever, they'd just take off from a standstill all the time.
The issue is lift. Thrust provides the plane with forward momentum that allows the wings to slice through the air. Without air over the wings? No flight, unless we're talking about a thrust-only plane like an F16 which is more or less a rocket with wings.
10:02 AM, December 13, 2006
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