Have you ever paused to consider just how an aircraft weighing hundreds of tons gets itself into the air — and stays there? In this column, I will try to give you a short (and, by necessity, incomplete) answer. There are four aerodynamic forces acting upon an airplane in flight: drag (air resistance or backward force), thrust (the forward force), weight (also known as gravity; the downward force), and lift (the upward force). In order for a plane to take off, its thrust and lift have to overcome its weight and drag. Of these four forces, lift — the force that contradicts gravity and lifts the plane skyward — is the hardest to describe without using a lot of words, diagrams, and a fair amount of math. But I’ll give it a shot.
The first thing I should point out, as strange as it sounds, is that air is a fluid. Like all gases, from a physical and mathematical perspective, the air outside your plane behaves just like water — or maple syrup, for that matter. In fact, aerodynamics tests are sometimes performed underwater.
The shape of an aircraft’s wing, working in combination with the fluid motion of the air, creates most of the lift needed to get a plane in the air and keep it there. Here’s how: The upper surfaces of wings are generally curved, while the lower surfaces are much flatter. As a wing moves through the air, propelled by thrust created by the engines, some air flows over its top and some flows underneath. The air flowing over the curved top is accelerated, which causes the pressure on the top of the wing to drop. The relatively high pressure on the bottom of the wing — caused by the fact that the air on the bottom doesn’t have to travel as fast as the air on the top to meet up again at the back of the wing — forces the wing into the region of reduced air pressure above. We owe our understanding of this phenomenon to an 18th-century mathematician named Daniel Bernoulli, whose “Bernoulli principle” states that as the speed of a moving fluid increases, the pressure within the fluid decreases. Because lift depends on the motion of the air, it increases as the speed of the air increases. It can also be increased (or decreased, as appropriate) by changing the angle of attack, which is the angle the wings make with the airflow, or by just making the wing thicker at its chord. Of course, that increases drag, which is why really fast fighter jets have very thin wings.
That’s an incomplete description of how and why lift occurs, and I’m sure some erudite American Way
readers will come up with ways my explanation could be better. But, at the very least, I hope it gives you a little bit better sense of how we get these giant contraptions we call airplanes up in the air. Of course, once lift has been achieved, the fun has just begun. So in future columns, I will describe how the pilot guides and controls the aircraft by adjusting the pitch, roll, and yaw using ailerons, elevators, and rudders.
I could go on and on, but for now, I want to thank you, on behalf of everyone at American Airlines and American Eagle, for letting us give you a lift today.GERARD J. ARPEY
Chairman & CEO