The miracle of lift

This antiquated observation is obviously fallacious. Otherwise, so the rebuttal goes, if God had intended for man to drive upon the Earth, he would have been provided with little wheels on his feet.

When man observed the flight of birds, he became jealous and attempted to emulate these flying creatures. He created all manner of flapping wing contraptions in an effort to mimic his feathered friends. These were called ornithopters, but none were successful. Ornithopter proponents argued, however, that “Nature must know best,” so experiments with flapping wings should continue. But this argument wasn’t sound. Otherwise, sailing vessels would span the Seven Seas by wiggling their rudders like fish. Flapping wings were eventually discarded in favor of the fixed wing, the heart and soul of the airplane. It is ingeniously designed to create lift yet has no moving parts. Its shapely, sculptured lines miraculously produce lift, but few understand how it does so. There are, however, some concocted explanations.

There is, for example, this amusing fable: Air flowing above a wing has farther to travel (because of camber) than air flowing beneath the wing. Air above the wing, therefore, must travel faster so as to arrive at the wing’s trailing edge at the same time as air flowing beneath the wing.

This is poppycock. How could air molecules flowing above and below the wing have the anthropomorphic intelligence to know they must arrive simultaneously at the trailing edge? The truth is that—because of friction and viscosity—once the airflow divides at the wing’s leading edge, the separated air particles will never again meet unless by coincidence in a typhoon over the South China Sea.

With the help of Sir Isaac Newton and his Third Law of Motion—For every action there is an equal and opposite reaction—aviation pioneers concluded that birds create lift (a reaction) by beating air down (an action). The principle is similar to shooting a rifle. A bullet is fired from the barrel (an action), which causes kickback, an equal and opposite reaction. Ornithopter devotees were on the right track but could not design a pair of wings that could flap sufficiently to force down enough air to lift a man and his machine. Working silently and more efficiently than any ornithopter ever could, fixed wings do much the same thing. They force down quantities of air that in turn causes the reaction we call lift.

To learn how this is accomplished requires traveling a circuitous yet fascinating journey. It begins with a venturi tube and arrives at an aeronautical Shangri-La where the concept of lift becomes clearly understood.

Almost every pilot is familiar with a venturi tube, that hollow tube with the narrow throat (facing page). The fascinating workings of the venturi tube can be partially explained using a flow of water. It’s easy to see that whatever amount of water enters the inlet must come out the other end. If this were not true, the water would have to bunch up in the throat and become compressed. But since water is incompressible, this cannot happen. The same amount of water, therefore, must pass the throat as passes the inlet and outlet. Since there is less space in the throat, the water there must accelerate and travel more speedily.

A more visual example is when you partially block the outlet of a garden hose with your thumb or add a nozzle to the hose. In either case, a venturi type constriction (or throat) is created, and the water escapes much more rapidly than it normally would.

What many do not appreciate is that air and water behave similarly; both are fluids. And since freeflowing subsonic air is also considered incompressible, the same thing happens to it when flowing through a constriction: It accelerates.

Up to now, everything should seem plausible. But now for the question answered only in textbooks. Why does an increase in airspeed within the throat of a venturi tube cause a decrease in pressure? Finding the answer requires a slight detour.

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Air in motion—like any object—possesses kinetic energy. A second form of energy possessed by air is static pressure. An inflated toy balloon is an excellent example of static pressure (energy) being stored. If air inside a balloon is allowed to escape through the neck, the static pressure converts to kinetic energy as the air escapes.

Two important pieces of the lift puzzle state that: (1) energy can neither be created nor destroyed, and (2) air entering a venturi tube consists of two significant forms of energy: kinetic energy (of motion) and static pressure.

The total amount of energy of air entering a venturi tube is equal to the sum of its kinetic energy and its static pressure. As the airflow approaches the venturi’s throat, its velocity increases. This represents an increase in kinetic energy. Yet we learned earlier that energy cannot be created. Has the law regarding the conservation of energy failed? It has not.

What happens is that some of the air’s pressure energy is sacrificed (or converted) into kinetic energy. In this manner, the total energy content of the air remains unchanged. This process of energy conversion is identical to what happens when air escapes from a balloon: air velocity increases and air pressure decreases. Within the venturi tube, static air pressure is sacrificed to accelerate the airflow, resulting in reduced pressure within the venturi’s throat.

It should now be easier to understand why airspeed and air pressure are so closely related and why an increase (or decrease) of one results in a decrease (or increase) of the other. This relationship between airspeed and static pressure was discovered by Daniel Bernoulli, an eighteenth-century Swiss mathematician, and is known as Bernoulli’s Principle.

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The figure above shows the airflow pattern about a wing. Notice that the wing’s cambered (curved) upper surface is shaped like the bottom half of a venturi tube. The upper half of this imaginary tube is simply the undisturbed, horizontal airflow at some distance above the wing.

Notice what happens to the air flowing over the wing’s upper surface. As it enters the venturi constriction formed by the wing’s camber, air accelerates just the way it does when passing through a conventional venturi tube. The result is a corresponding decrease in pressure along the upper surface of the wing.

This reduced air pressure is often and erroneously called suction. The amount of pressure reduction is actually quite small. A fully loaded Cessna 177 Cardinal, for example, has a gross weight of 2,500 pounds and a wing area of 172.4 square feet. Dividing the weight by the wing area results in the Cardinal’s wing loading of 14.5 pounds per square foot. In other words, each square foot of wing lifts 14.5 pounds. Since there are 144 square inches in a square foot, we see that each square inch of wing creates only 0.1 pound (or less than 2 ounces) of lift.

It seems logical that the relatively high-pressure air beneath the wing would attempt to flow to the area of reduced pressure above the wing. After all, this is what happens in the free atmosphere. Air flows from high pressure to low. But in the case of an airplane, a wing separates the areas of high and low pressure, and the wing rises into the low-pressure area above it. (Some of the relatively high-pressure air does curl around the wingtip to “fill the low” above the wing. This curling of air about the wingtip breeds that hazard known as the wingtip vortex and is responsible for induced drag.)

This explanation of lift could end at this point but would leave the serious student short of his destination.

Notice how the airflow in Figure 3 completes its journey across the wing. It flows not only rearward but also downward. This action is called downwash. Remember the lessons of Sir Isaac Newton and the birds? When air (or anything else for that matter) is deflected downward, there must be an equal and opposite reaction. This reaction to downwash is that misunderstood force called lift. By adding together the vertical component of force with which each particle of air is deflected downward, the total would exactly equal the lift being created by the entire wing.

Additional lift can be created only by increasing the downwash of air behind the wing. Aerodynamicists are aware of this and try to get as much air above the wing as possible because all of it will ultimately be directed downward from the wing’s trailing edge. This is accomplished by an ingenious application of the principles already discussed.

The low-pressure area above the wing attracts the air approaching the wing’s leading edge. As a result, air from ahead of the wing flows not only from in front of the wing but also from below it (see Figure 3). This upwash increases the mass of air flowing above the wing, and therefore, the downwash behind it.

Parenthetically, there is a stagnation point at the wing’s leading edge above which air flows above the wing and below which it flows under the wing.

When the wing is flown at a large angle of attack, a more highly constricted venturi tube is created by the wing. This further increases airspeed and reduces static pressure above the wing. Greater quantities of air are attracted over the wing’s leading edge and, as a result, considerably more downwash is created. Total lift is increased. (The reaction produced by downwash is particularly significant considering that each cubic yard of sea-level air weighs 2 pounds.)

When air strikes the bottom of the wing (particularly during flight at large angles of attack), it creates greater downwash and increases total wing lift. This deflection of air from the bottom of the wing explains the flight of a kite and the skimming of water skis. Air (or water) is deflected downward, causing an upward reaction. This is why it can be said truthfully that—given enough power—anything can be made to fly, even the proverbial barn door.

Anyone who doesn’t comprehend the tremendous force created by aerodynamic downwash has only to stand beneath a hovering helicopter. This downward blast of air is precisely what occurs during fixed-wing flight. The rotors of a helicopter create lift identically to the manner in which a fixed-wing creates lift. The only difference is that helicopter wings rotate and create relative wind without movement of the helicopter. Fixed-wings encounter relative wind only when the airplane is in motion.

We have learned much from the birds, but we still watch them with envy. There is always more to learn.

www.BarrySchiff.com