The airplane may or may not be the most life-changing invention of the 20th century; arguments can plainly be made for all manner of other innovations, including antibiotic drugs, the computer processor and the advent of wireless global communications technology. Yet few of these inventions, if any, carry both the visual grandeur and the innate human spirit of daring and exploration as does the airplane.
The bulk of a typical plane is largely indistinguishable from other large-scale passenger vehicles; it consists of a tubelike compartment in which passengers, the people in charge, and other transported items sit. Also, most planes have wheels; most observers wouldn't site them as a primary feature, but most planes couldn't take off or land without them.
Clearly, however, the main physical feature that makes an airplane immediately identifiable its its wings. To some extent, the supporting structures you will also read about add to an airplane's characteristic appearance, but the wing is somehow the most compelling; despite its deceptively basic appearance, the airplane wing is a genuine marvel of engineering as well as indispensable to life in modern civilization.
Aerodynamically Active Parts of Airplane
Airplane control requires not only lift (much more on that later) but also vertical as well as horizontal steering and stabilizing equipment. The following applies to a standard passenger-style airplane; clearly, no one design of an airplane, or for that matter a passenger jet plane, exists. Think of the physics, not the specific ingredients.
The tube, or body, of an airplane is called the fuselage. The wings are attached to the fuselage at a point about halfway along its length. The wings themselves have two sets of movable components on the back; the outer set are called ailerons, while the longer, inner ones are simply called flaps. These change the roll and the drag of the aircraft respectively, aiding in steering and slowing the plane. The wing tips often have small movable winglets, which decrease drag.
The tail parts of a plane include horizontal and vertical stabilizers, the former mimicking tiny wings in orientation and boasting elevator flaps, and the latter including a rudder, the airplane's primary means of altering horizontal course. An airplane that only had an engine and wings but no rudder would be like a powerful car with no steering wheel, and it doesn't take a physicist or professional race-car driver to spot the problems here.
The History of the Airplane Wing
Orville and Wilbur Wright are credited with making the first successful flight, in 1903 in North Carolina, U.S.A. As you perhaps surmised, they were not mere daredevils who threw together a slapdash contraption from a motor and some lightweight planks and made a go of it, one that happened to work in their favor. On the contrary, they were meticulous researchers, and they understood the wing would serve as the critical aspect of any successful aeroplane flying mechanism. ("Aeroplane" is a quaint but lovable term in the aviation world.)
The Wrights had access to wind tunnel data from Germany, and they used this in the formulation of wings for the gliders that preceded their instantly famous 1903 motorized version. They experimented with different wing shapes, and discovered that ones with wingspan-to-wing-breadth ratios within a close range, and near 6.4 to 1, seemed ideal; that this is a nearly perfect aspect ratio has been borne out by modern engineering methods.
A wing is a kind of airfoil, which is the cross section of anything of interest to engineers in the realm of fluid dynamics, such as sails, propellers and turbines. This representation is helpful in solving problems because it offers the best visual representation of how a plane rises and how this can be modulated through different wing shapes and other features.
Basic Aerodynamics Facts
Perhaps in school, or merely by watching the news, you've seen or heard the term "lift" in reference to flight. What is lift in physics? Is lift even measurable quantity, or does it map on to one?
Lift is, in fact, a force, one that by definition opposes an object's weight. Weight in turn is the force produced as a result of gravity's effects on objects with mass. To achieve lift is to essentially counteract gravity – and gravity "cheats" in this vertical tug-of-war, because it never rests!
Lift is a vector quantity, like all forces, and thus has both a scalar component (its number, or magnitude) and a specified direction (usually including two dimensions, labeled x and y, in introductory-level physics problems). The vector is drawn acts through the center of pressure of the object, and is directed perpendicular to the direction of fluid flow.
Lift requires a fluid (a gas or mixture of gases, such as air, or a liquid, such as oil) as a medium. Thus neither a solid object nor a vacuum serves as a hospitable flying environment; the first of these is intuitively obvious, but if you ever wondered if you could steer a plane in outer space by manipulating its wings or tail, the answer is no; there is no physical "stuff" for the plane parts to push against.
Everyone has watched the eddies and currents of a river or stream, and pondered the nature of fluid flow. What happens when a river or stream suddenly becomes much more narrow, with no change in depth? The river water flows past far more quickly as a result. Higher speeds mean more kinetic energy, and increases in kinetic energy rely on some input of energy into the system in the form of work.
Concerning fluid dynamics, the key point is that the pressure P will drop in rapidly moving fluids of density ρ, including air. (Density is mass divided by volume, or m/V.) The various relationships between the kinetic energy of a fluid (1/2)ρv2, its potential energy ρgh (where h is any change in height over which a fluid pressure difference exists) and total pressure P is captured by the equation made famous by the 18th-century Swiss scientist David Bernoulli. The general form is written:
Here g is acceleration owing to gravity at Earth's surface, which has the value 9.8 m/s2. This equation applies to countless situations involving the flow of water and gases and the movement of objects in fluids, such as airplanes zipping through the air of the sky.
The Physics of Airplane Flight
In considering the airplane wing, the last term in Bernoulli's equation can be dropped because the wing is treated as being at a uniform height:
You should also be aware of the continuity equation, which relates pressure to cross-sectional wing area:
Combining these equations shows how lift force is produced. Critically, the pressure differential between the top of the wing and the underside is the result of the different shapes of the respective sides of the airfoil. The air above the wing is allowed to move faster than the air underneath, which results in a sort of "sucking pressure" from above that opposes the weight of the plane.
The forward movement of the plane itself, of course, is what creates the movement of the air; the plane's horizontal velocity is created by the thrust of its jet engines against the air, and the resultant opposing force exerted against the craft in this direction is called drag.
- Thus a summary of the upward, downward, forward and backward forces on an airplane and its wings as seen from one side are lift, weight, thrust and drag.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.