The idea of humans flying is potent stuff. It's an urge that's existed probably as long as people have paced the earth. Ancient scrolls talk about flying in religious tones. Medieval times saw people jumping off towers and drawing fanciful flying machines. But controlled flight of the merest form—balloons aside—didn't come until the end of the nineteenth century, when Otto Lilienthal made his famous glider flights.
What most probably don't realize, however, is that Lilienthal's first aerodynamic concepts leaned heavily on bird flight. In this sense, Otto fell victim to what had been an ages-old affliction: the idea that man could fly if only he could properly imitate the flapping action of bird wings. Before he hit on the rigid-wing glider designs he's famous for, Lilienthal even went so far as to study each and every known bird, looking for relationships between wingspan and weight, and number of flaps versus distance of gliding flight.
It wasn't until the Wright brothers came along that we first began to truly understand how to make an aircraft fly. As it turns out, nature does provide some aerodynamic guidelines and parallels, but flapping wings sure aren't one of them. Airplane structures, such as rudders, stabilizers, and strakes, however, do have functions analogous to those performed by the caudal, dorsal, and pectoral fins of most fishes.
In your first pilot training sessions, you're bound to learn about the four forces that act on an airplane in flight. These four forces are lift, weight, thrust, and drag. Too bad the old-timers didn't know what you'll know after just two hours.
Lift is produced by the wings and acts in an upward direction. Wings are able to create lift by accelerating air over their top surfaces, which are curved expressly for that purpose. As the oncoming air—called the relative wind—strikes a wing's leading edge, it splits and travels aft until meeting again at the trailing edge. The airfoil's curve guarantees that the air flowing over the top surface travels faster than the air passing beneath the wing. It's this extra speed that creates a zone of low pressure air—suction, if you will—atop the wing. Meanwhile, the air flowing beneath the wing is of relatively higher pressure. And it's this pressure differential that generates the airplane's lift. It works the same way with bird wings, but there's a big difference that we'll explain shortly.
As a child, you've probably conducted a crude experiment that illustrates the principle of lift. Who hasn't thrust an outstretched hand out a car window, palm down, with thumb and forefinger into the onrushing wind, and felt the hand rise? Here, there is also a relative wind (created by the car's forward motion) and an airfoil (created by the tapered thickness of the hand).
Weight acts against lift and works in a downward direction. To stay in level flight, you have to keep enough air moving over the wings to exactly match the weight you're lifting—the airplane itself, plus its occupants, fuel, and any baggage.
Thrust is created by the propeller or, in the case of a jet, the outflow of a turbine engine. Drag is thrust's enemy, so the pilot has to apply enough engine power to overcome it. Drag comes in many forms, and it can add up quickly. There's the drag caused by the airframe itself (form drag), the drag caused by air turbulence at the fuselage juncture of the wings and tail (interference drag), the drag caused by cooling ducts (cooling drag), drag caused by antennas and other protrusions (parasite drag), and drag that's even caused by lift (induced drag). So it follows that the draggier the airplane, the more thrust will be required.
In steady flight, lift, weight, thrust, and drag all balance each other. If you want to climb, descend, or turn, you simply make adjustments to the four forces using cockpit controls.
Let's say you want to climb. You add back-pressure to the control yoke, which raises the elevators at the tail's trailing edge. Then you add power, which increases thrust. Both these actions increase wing lift above the value needed for level flight. Now you have more lift than weight, so up you go.
Want to descend? Reduce power and use the elevators to lower the nose slightly. Now there's not enough lift or thrust to maintain level flight, and the resulting added weight and drag make the airplane lose altitude.
Turns are made with the help of ailerons, which create differential lift and drag. These control surfaces are at the trailing edges of both wings and are connected to the pilot's control yoke or stick. For a right turn, turn the yoke (or pull the stick) to the right and the right aileron deflects upwards; the left aileron moves down. The upraised right aileron creates drag on that wing. This slows the right wing's travel through the air. The extra drag on that side causes the right wing to drop slightly, and the airplane rolls to the right. Left turns work just the opposite. When flying straight and level, the ailerons are in trail and even with the wing trailing edges.
The art of flying, then, is learning how to smoothly manipulate engine power (thrust) and the aircraft control surfaces to make the airplane go where you want it to go. Back to the question: How come flapping flight won't work for us? Flapping wings on the downstroke would create lift, but on the upstroke, the drag penalty is ferocious. Add the weight of the articulation system, and even the most furious flapping would be futile. Birds solve the problem by having a built-in, super- lightweight articulating system that guarantees airflow over the top of their wings at all times.
Preserving adequate airflow over the upper wing surfaces is very important for both birds and airplanes. If high-velocity air can't make the journey across the top of the wing, an aerodynamic stall will occur. In other words, the wing stops producing lift. It "stalls"—stops flying—and the airplane will descend. However, it doesn't necessarily fall out of the sky. All that's needed to recover from an aerodynamic stall is to get a sufficient quantity of air moving across the wing again.
To recover from or, better yet, avoid a stall, you'll be taught several tactics. One of them is to maintain enough airspeed so there's plenty of air moving over the wing. Like riding a bicycle, flying an airplane means having enough forward motion to ensure a stable, controllable ride.
Come to think of it, learning to fly an airplane is a lot like learning to ride a bicycle: Once you learn, you never forget. Maybe the Wright brothers were the first to discover this truism. After all, they started out in the bicycle business.