ScienceSunday: Flying on an Airplane
Over the past week, I've spent approximately 16 hours on airplanes. Naturally, in the tedium of being trapped inside one, I became curious how they worked. I mean, I know there's something about aerodynamics and lift and something else, but somehow this giant hollow bullet with wings is able to fly safely and smoothly across an ocean, despite weighing quite a lot, so there must be some pretty serious physics involved. However, when you break it down, the fact that an airplane can fly is determined by only four forces, in two pairs: weight and lift, and drag and thrust.
First, let's look at weight and lift. We're all pretty familiar with the concept of weight, at least on some level. Every object weighs something, like your 45 pound checked bag that you have to get onto the airport scale. The airplane, too, has weight, and quite a lot of it: the weight of the machine itself, the weight of fuel, your 45 pound checked bag and 15 pounds of carry-on, plus your body weight, plus the luggage and bodies of the other 100+ people on your flight. The final weight varies based on the type of aircraft, the distance to fly, and even the elevation of the runways on either end, but it adds up quickly to a lot of weight. This weight is the product of the mass of the loaded aircraft, times the acceleration due to gravity (9.8 m/s2 on the planet Earth).
Lift is the force that counterbalances that weight. In order for an aircraft to take off, the lift must exceed the weight, and it must equal the weight to stay at a constant elevation. Once the plane is in the air, lift is the only thing keeping it up, since there is no upward resistance from the ground acting on the plane. The shape of an airplane's wings provides this lift to the plane. Air flows faster over than the top than the bottom, meaning there is less air pressure on the top of the wing, buoying it up, due to the curved shape of the wings. This is an oversimplification: the exact physics at play are much more theoretically complex (see this NASA site for a more in-depth look). However, the math works out in any case, and the point is that the plane is designed such that, while moving forward, this lift is quite sufficient to keep the machine in the air, in the vast majority of cases.
The fact that the plane does move forward is due to its thrust. This is what the fuel and engines are for, is providing thrust. It's a pretty intuitive force: you prove a push in one direction, and the place goes in the opposite direction. This is the same force that rockets use: the flaming expulsion towards the ground thrusts the rocket into the air. The reactive force to this thrust is drag, which is friction in this case. It slows the airplane down, trying to make it move more slowly, until it reaches zero. But so long at the thrust is greater than the drag, the plane will continue to fly forward.
This sort of aerodynamics is pretty simple, once you break it down, but the implications are pretty amazing. The properties of lift, weight, drag, and thrust obviously give us everything that flies, like airplanes, helicopters, even hot air balloons and space rockets. Aerodynamics also have implications in water, which is just a different fluid. Thus, it affects boats and submarines as well. Even the design of your car is based on aerodynamics, which can improve your gas mileage by reducing the amount of drag of the car, so you get the most thrust out of your fuel. If you're interested in more about these forces, you can check out the NASA Kids page and the HowStuffWorks article, or just do a Google search on aerodynamics.