Saturday, August 18, 2007

Flying and the Third Law

Greetings from Cornell University! Haven't really been updating recently, been too busy packing and doing my orientation stuff, but now I finally feel quite inclined to do this.

I don't know if you feel the same thing I do, but every time I take a plane, its take off is always a test of my faith in science and the triumph of human thought. Just think about it: Something like 400 people, sitting in what is effectively a fortuitously well-designed metal tube lifting off from the ground. It defies all human experience and all human logic: it is not surprising that Wilbur Wright told Orville Wright, the two great pioneers of human flight, that man would not fly in a thousand years.

So what exactly makes a plane take off? Well, there have been plenty of misconceptions and poor explanations of the real reasons for lift, and I've also realised that my understanding of it was rather poor before doing a little research for this post. Strangely, the "Bernoulli Principle" that people like to throw around when they explain how an aerofoil generates lift is not really very appropriate.

There are two reasons to explain lift, and the first is pretty straightforward. Imagine hitting a ping-pong ball in a game of table-tennis, or a tennis ball in a game of tennis: if you incline your racket downwards so that it is tilted towards the ground when you hit the ball, you are going to generate a downward force on the ball, and by Newton's 3rd Law, the ball generates an upward force on the bat. This works as well for the airplane wing. As long as it is inclined in an angle to deflect air downwards, lift will be generated!

The other reason is pretty complicated, and you'll just have to take my word for it, because I'm taking the website's word for it! As air passes across the aerofoil, it tends to "stick" to the metal of the aerofoil, and because the aerofoil is curved, the air is forced to curve around the aerofoil. Apparently, when air is forced to make a curve, it will be flung outwards, just like passengers in a car are flung to the side of the car when the car makes a turn. Thus, the air is flung outwards, and because it is stuck to the wing, it pulls the wing along with it.

Sorry for the quick post, but time's really quite tight at the moment. I'll come back with more the next time!


Mr. Liu said...

The Bernoulli's Principle (I'm assuming you already know what it is) is inappropriate here because the assumption behind it is that the two air parcels that pass above and below the aerofoil diverge and then re-converge at th same time, which is not necessarily true. In fact, it is precisely because they don't re-converge at the same time that there is any lift at all!

gonewind said...

The 3rd Newton law may explain what lift the plane up while you ARE flying and actually change the height. But what lift the plane when it takes off ?
And I don't really understand your explaination about Bernoulli's Principle. The air always passes the aerofoil, right ? So does this mean that stream of air create 2 different air parcels all the time ?

Mr. Liu said...

The 3rd Law is actually better at explaining take off: it is simply the angle of attack of the wing that produces a lift.

As to my comment about Bernoulli's Principle, the principle cannot be applied here, because when the air splits into two as it encounters the aerofoil, the two streams of air NEED NOT converge at the same time. This means that we cannot conclude that the top stream travelled faster than the bottom stream, meaning we can't use Bernoulli's principle here.

Hope that's clearer.

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