The ability to fly has fueled the imagination of inventors, engineers, artists and poets for centuries. Leonardo DaVinci studied the principles of flight and devised the first architectural blueprints for the earliest helicopter. The Wright Brothers stood on the dunes of Kitty Hawk and watched as their homemade aircraft lifted from the earth and was carried aloft for several hundred feet, braking the barrier between the Earth and the Heavens. In this past century, conventional aircraft have emerged from the imagination to become reality and are now an integral part of our daily transportation system.
But what does it take to fly? How does one break the barriers imposed on all of us by gravity? The ability to fly is the by-product of acceleration and lift. To understand how these two pieces work together, we must first understand how each works separately. We will begin with lift.
To understand lift, we must first understand a scientific principle defined by Daniel Bernoulli which states simply "that an increase of a fluid occurs simultaneously with a decrease in pressure." As it applies to the wing of an airplane, the fluid moving over (and under) the wing is air. When we consider the shape of an airplane's wing as viewed from the wingtip. we will note that it appears to look something like half of a teardrop turned on it's side. Imagine air moving over both halves of the wing. On the top, the teardrop shape of the wing actually creates a greater distance for the air to travel, and the air moves more quickly over the wing. The air moving below the wing (and under the flat half of our "teardrop") moves with a lesser speed as it has a shorter distance to travel. As the air begins at the front of the wing, the higher acceleration of air over the top generates a lesser vertical pressure on the wing than the air on the bottom, which is moving slower, and inversely, the air on the bottom of the wing creates a greater vertical pressure, which literally lifts the wings of an aircraft, allowing it to ascend into the sky.
Now, because an airplane contains a great deal of mass, it is necessary to generate an equally proportional amount of acceleration of air over (and under) the wing to create adequate lift. On a conventional airplane, acceleration is created by use of a propeller (or multiple propellers depending on the size of the aircraft) that literally pull the airplane in the same way that a propeller displaces water on a boat, accelerating it through the water. The propeller pulls the aircraft to an adequate speed that the lifting principles outlined above create enough vertical pressure to lift the aircraft into the air. The propeller continues to pull the airplane to even greater speeds which continue to increase the amount of lift being generated and allows the airplane to move easily through the air.
Another piece of how an airplane flies comes into how it turns. The processing of turning, or in the instance of flight, "banking" the plane involves creating a stall on one wing. As discussed earlier, lift is generated by moving air over the wings of the plane. In level flight, the amount of lift is almost evenly proportional on each wing. When an airplane banks, a flap is raised on the top of one wing which slows the flow of air over the wing and stalls the lift in that wing. The plane effectively "falls" on the side of the stall and begins to turn in that direction. In addition, a tail rudder will turn in the direction the plane is banking and "rudder" the plane in that same direction. Again, using the comparison of a boat rudder moving through water, the airplane's tail rudder creates drag in the air, pulling the airplane in the appropriate direction, thereby causing it to turn.
The last, and most important piece of flying is bring the airplane safely back to the ground. The easiest way to think of landing a plane is creating what I will affectionately call a "controlled stall". The process is actually similar to the principles governing lift. An airplane contains flaps which extend off the tail end of the wing and create drag beneath the wing. By so doing, the air beneath the airplane is stalled to a certain extent, increasing the effects of lift. If we consider Bernoulli's principle again, we remember that "where acceleration decreases, the pressure increases. The end result of such an action is that we can reduce the airplane's speed and still maintain lift. This allows us to decrease the speed of an airplane to a rate that is safe for landing. Once the flaps are extended and the airplane's speed is adequately reduced, a pilot simply needs to line the airplane up with the runway and effectively aim it at the ground in such a way where it gradually approaches the earth. Once the wheel's touch down and the plane has landed, the pilot can apply brakes, increase drag on the wings, and in some instances, even reverse the engines to slow and eventually stop the airplane.
Sounds simple right? The next time you are in an airplane of any type, remember that the entire ability to fly is based on our ability to control air. It's an amazing thing to consider when we look at some of the massive commercial aircraft that exist today. Still, it is important to remember that whether your talking about the Wright Brother's first flights at Kitty Hawk, or the routine traffic of aircraft in our daily lives, the principles of how airplanes fly remain the same.