Passionately Curious – Taking Flight

Passionately Curiousjustin_howplanesfly

Well, that’s my article for today. Enjoy the rest of the week guys.

I’m pulling your leg, of course. Today I’ll be taking a look into how the hell a giant lump of metal, weighing in at around 200 tons, can take off and just chill up high in the atmosphere pretending it’s a bird. Let’s fly.

Today, we won’t need any crazy kind of physics. No quantum mechanics or relativity involved here – really, all we need is Newton’s third law and some fairly simple mechanics. For those of you who have forgotten them since your days of high-school level physics, I’ll reiterate them here:

  1.     An object moving at a constant speed upon which no force is acting continues to move at that same speed.
  2.     F = ma – or in other words, force equals mass multiplied by the acceleration.
  3.     Every action has an equal and opposite reaction.

Now, to be fair, the first two are relatively simple. If you’ve still got difficulty remembering how that worked exactly (or you’ve simply never really been able to grasp them at all) I’ll defer you to the Wikipedia article, which goes a little more in depth on them. The last one, however, does require a bit of clarification.

Imagine standing on an extremely slippery sheet of ice. So slippery, in fact, that we can say that we have absolutely zero friction with the floor. Now imagine what happens when we push ourselves away from a wall: we start sliding away from the wall. We exerted a forward force on the wall, which resulted in us being accelerated backwards. Another scenario: we’re still on the ice, but this time it’s a filing cabinet we’re pushing against, which is free to slide over the ice like we are1. So, what happens when we push it? We’re still exerting a forward force on the cabinet, and so we’re being accelerated backwards. But unlike the wall, which was immovable and absorbed our exerted force like a sponge2, the filing cabinet goes flying in the direction we pushed it!

This is the basic principle by which jet engines work: we burn the fuel, and eject it together with air with great speed backwards – thus applying an equal force that throws the engine forward.

“But wait,” you might argue. “How is that going to help me keep a 200 ton aircraft floating in the sky? The engines point to the front – they’re just helping the thing go forward.”

Hold on to your horses, I’ll get to that in a minute. You see, for the aircraft to be able to go up, it first needs to have some forward speed. Why? Just have a look at the diagram below. wingThe wings are curved in such a way that the air that flows underneath it is pushed downward, while the air above is hardly affected. So, by the very same principle that allows the plane to go forward, it also manages to leave the ground behind! Because the air is pushed downward, the plane is in response pushed upwards. Because the engines are pushing the plane forward at such a ridiculous speed (900 km/hour for the average Boeing 747), there are massive amounts of air constantly being funnelled underneath the wings, providing the necessary lift to stay up in the air.

This is also why you may recall that planes (usually) have plates at the end of the wings that can move up and down: by being in a downward position, they increase the pressure that the air underneath the wing exerts on the plane, pushing it even higher. When they’re folded upwards, they decrease the pressure from below, while simultaneously receiving some additional pressure from the air above – the plane is pushed downwards slightly.

And guess what – that’s really all there’s to it. Of course, there is all the mechanical minutiae like how we actually reach such speeds to manage to get the darned thing off the ground, or how to construct the wings in such a way to optimize their lift, but that’s all details. The big trick that keeps it all up in the air? Newton’s third law. Or, in the words of TARS, the robot from Interstellar: “The only way you humans have ever figured out to get anywhere is by throwing things away.” Still – throwing things away is what eventually got us soaring with the eagles. I think sir Newton would approve.

Paul

Footnotes

1. Passionately Curious does not condone undue violence against innocent filing cabinets. This one had it coming, though.
2. Technically you’d be exerting a forward force on the Earth, but since the Earth is so immensely heavy compared to the teeny tiny amount of force you’re exerting on it, nothing much happens.

References:
http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion
http://www.grc.nasa.gov/WWW/k-12/UEET/StudentSite/dynamicsofflight.html

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