Physics of Flight | Research & Encyclopedia Articles

Physics of Flight

Any object moving through space, above the ground and within our atmosphere can be thought of as being in flight. Birds, planes, helicopters, Frisbees, and baseballs, if moving through the air, are all subject to the physics of flight. The physics of flight, or aeronautics, is similar to other branches of physics in that it is all about the balancing of forces. A body in straight and level flight can be said to be in equilibrium.

Four forces act upon a body in flight; two forces aid flight and two oppose it. Forward momentum is provided by thrust. Upward movement, away from the ground, is provided by lift. Air resistance is called drag, and the tendency of an object to fall toward Earth is gravity.

Thrust is the motive power that propels a flying object forward. An airplane's engine, a baseball pitcher's arm, a bird's thoracic muscles; all these are providers of thrust. Thrust can be divided into two categories. Ballistic thrust is the expenditure of energy early in the flight, with inertial momentum carrying the object the rest of the way. Bullets, rockets, and the thrown baseball all fly ballistically. The other type of thrust is generated in-flight, and continuously. Birds generate their thrust with every flap of their wings and airplanes generate it with either the turning of propellers or the combustion of jet fuel.

Drag is the force that opposes thrust. Anyone who has ever stuck an arm out of a car window has experienced drag. In its simplest form, drag is friction. Drag can be reduced greatly with streamlining, but it can never be eliminated entirely. At very high speeds, drag presents itself as an actual compression of the air layer along the leading edge of the flying body. This form of drag is responsible for, among other things, shock waves and sonic booms.

Gravity is probably the most familiar force. Newtonian physics tells us that gravity is the attraction between two bodies, proportional to their mass. Observation tells us that gravity seeks to pull flying bodies out of the air. Gravity is the aeronautic force that opposes lift.

The most elusive of the aeronautic forces is lift. Since lift opposes gravity, it follows that lift can be quantified based on the mass of the object in flight. The minimum amount of lift needed to get an object off the ground is equal to the object's own weight. This concept is easy enough to apply to lighter-than-air flight, but becomes somewhat counterintuitive when one considers the airfoil, or wing.

It is generally agreed that what supplies lift beneath an airfoil is an effect of the Bernoulli principle. Daniel Bernoulli discovered that the pressure of fluids, which in this case includes air, is proportional to velocity. Slower moving fluids have higher pressures than do faster moving fluids. Thus, if air passing under a wing is moving slower than the air passing over it, lift is created and the airplane flies. The question is, why is there a differential in velocities of the air over and under the wing?

The most commonly held theory regarding lift involves the shape of the wing. Most aircraft have wings that are thicker toward the leading edge of the wing, and are slightly curved along the top. This shape causes the airflow along the top of the wing to have a further distance to travel than air along the bottom. The air along the bottom, since it has a shorter distance to travel, is moving more slowly, and thus has greater pressure. It is this rise in air pressure below the wings, goes the theory, that produces lift.

One of the problems with this theory is that it does not explain how paper airplanes, balsa wood models, or other flying bodies with flat wings produce lift. In the flat-wing model, the relative velocities of air above and below the wing are presumed to be equal, and their respective pressures are likewise equal. One theory seeks to explain this by suggesting that "trailing vortices" formed at the wing tips cause a pressure differential, resulting in lift. Another supposes that lift is nothing more that the equal-but-opposite reaction to downwash created at the wings' trailing edges.

It is interesting to note that, nearly 100 years after the first manned heavier-than-air flight, physicists are still not in agreement as to what makes it possible. One explanation may be that lift is provided by a combination of the three recognized, and perhaps even other unrecognized phenomena. Regardless, it is clear that powered flight is here to stay, is governed by the laws of physics (whether we fully understand them or not), and needs further investigation.