The Four Forces

Four opposing forces act on a plane during flight: lift, weight, thrust, and drag. When an airplane is flying straight and level, lift works upward and is opposed by weight, which acts toward the earth. Thrust is the propelling force that gives an airplane the

speed required to stay aloft, and it is opposed by drag, the force which tends to slow down an airplane. Let’s examine each of these forces a little more closely.

Lift: The Gift of Newton and Bernoulli

Lift is the force that makes flight possible for any aircraft that relies on an airfoil, including airplanes, gliders, and helicopters. (You’ll learn in Chapter 11, “Up, Up, and Away: Hot-Air Balloons,” that balloon and blimp flights are made possible by buoyancy.) Simply put, lift is the force that pushes an airplane upward.

Lift is not produced by a single force or property. Instead, it is a combination of two forces that work together. We’ll look at one component of lift that can be characterized by a law of physics first written down by Newton, and another, more subtle form that was the brainchild of a lesser-known physicist named Daniel Bernoulli.

For Every Action…

The Four Forces

By the Book

An airfoil is a surface that generates an aerodynamic force because of its shape. A wing is an airfoil, as is a horizontal and vertical stabilizer, and even the ргореІГег.

The Four Forces


This discussion of lift and other aerodynamic forces is going to stop short of the extreme detail that pilots and engineers like to indulge in. Those who crave the mathematical minutiae can find plenty of excellent resources in Appendix C, “Recommended Reading.’

Sir Isaac Newton, the eighteenth-century philosopher, mathematician, physicist, and man for all seasons, put down in writing one of the laws of motion that explains the first element of lift. To paraphrase, Newton wrote that every action causes a reaction of equal force and in an opposite direction.

To understand what this means in terms of airplane flight, think of a wing as a simple flat metal plate. If we place that flat plate in a steady stream of moving air, such as in a wind tunnel, and position the plate so that it is perfectly streamlined in the wind, the air above and below the plate will flow past at the same speed and no lift will be created. But if we tilt the plate upward in the air stream, some of the air will strike the bottom of the plate and deflect downward. In Newton’s words, the plate has “acted” on the air stream by changing its direction. The “reaction” is a force pushing in the opposite direction, upward. Voila—lift.

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