Saturday, August 21, 2010

Hello boys and girls! Welcome to Flight 101!

I think it’s time we all got on the same page when it comes to understanding how a plane stays in the air.

Those are pretty nifty things, aren’t they? Airplanes? Did you ever wonder how it is they stay up there? Even though they are heavy as hell, and should come crashing down? Well.. No, it’s not magic, and it isn’t big J up in the sky giving a helping hand, it’s not even 'cause of all the people in the various houses of prayer throwing their hands up and chanting.. Nope. It’s actually just physics, geometry, and fluid mechanics at play.

This is a simplified explanation, 'cause it can get pretty messy and heavy on the math, but the concept itself is simple, and I think it’s worth knowing. Plus you will totally be able to impress the person seated near you on your next plane ride. *Note: you should probably avoid using this as a pick up line. Results may vary.

So, how do planes stay in the air? Lift! Lift is what we call the upward force that combats gravity, and keeps the plane afloat in the sea of gaseous molecules that make up our atmosphere. In order for Lift to be produced, the air and the plane must be in motion relative to each other.

Let’s take as an example, a 2D slice of a wing. Here we have a free body diagram of all the forces the wing is subjected to. I’ll only talk about Lift in more detail though.


Lift = Upward force keeping you in the sky

Weight = Mass times Acceleration (in this case 9.8m/s^2 due to gravity), the force pulling you down to the down to the to the flo’. (Did you catch that one? Luda)

Thrust = The power behind your machine that propels you in the direction you desire.

Drag = The force that slows you down, same reason why when you try to angrily throw a non crumpled piece of paper, it don’t really go nowhere.

Basically, if you think about it, you want there to be more force pushing up on the plane, than pushing down on it. Right? So how does that happen?

Physics and fluid dynamics tell us that when a fluid flow, in our case, the air, encounters a solid body, or an obstacle, the flow is diverted to follow the shape of the body encountered. The part of the flow diverted around a larger obstacle will move faster than the part of the flow diverted around a smaller obstacle. Wings are designed to take advantage of this natural occurrence, and the geometry is selected such that the top of the wing will be that larger obstacle.

So now we know the velocity of the air flow over the wing is higher than the velocity of the flow under it.

A really smart Fluid dynamics dude named Bernoulli, came up with a principle back in the 1700's. It states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure. And a decrease in the speed of a fluid occurs simultaneously with.. you guessed it, an increase in pressure.

Fluid speed up, pressure down. Fluid speed down, pressure up.

Keep in mind that Pressure, is simply Force per unit area. So when the fluid flows faster over the wing, it's producing less force per unit area. When the fluid is moving slower under the wing, it's producing more force per unit area.

Since we know that the upper portion of the wing is a greater obstacle to the flow, and flow will therefore be moving faster there... annnnd we also know that since the flow is moving faster, there will be a decrease in pressure.. which would lead to a larger force in the UP direction, we know this will result in.. LIFT!

Here is a really cool animated pic that shows you exactly how the steamlines (flow) move before they encounter the obstacle (wing), how they traverse it, and where they end up once they've passed it. You can see that the part of the flow traveling over the wing exits on the other end long before its counterpart streamline that was diverted under the wing.



Flow around an airfoil: the dots move with the flow. Note that the velocities are much higher at the upper surface than at the lower surface. The black dots are on timelines, which split into two — an upper and lower part — at the leading edge. The part of a timeline below the airfoil does not catch up with the one above. Credit: Wikipedia
Hooray! You have now passed Flight 101.

Ok, yes it may be slightly more complex than this, and in fact, engineers and scientists still argue about the semantics of how lift is actually produced -

"It is amazing that today, almost 100 years after the first flight of the Wright Flyer, groups of engineers, scientists, pilots, and others can gather together and have a spirited debate on how an airplane wing generates lift. Various explanations are put forth, and the debate centers on which explanation is the most fundamental."

--Dr John D. Anderson, Curator of Aerodynamics at the National Air and Space Museum, and fellow Florida Gator!


But you get the idea. I hope.

For more info check out what Wikipedia has to say about Lift Force.
Also check out UK Open University First Flight