Aircraft technical Basics: Introduction to Airplanes - Navy Training Courses Edition of 1944: Chapter 2 - Why Airplanes Fly

CHAPTER 2 WHY AIRPLANES FLY

AERODYNAMICS

WHAT makes it possible for man to fly? How can a contraption of wood and metal and fabric transform earthworms into creatures that can out-wing the birds of the air ? The answer is locked up in the science of AERODYNAMICS.

Actually, aerodynamics is just two Greek words spliced together. "Aer" means air. "Dyne" means force, or power. Teamed together they mean "the force or power of the air."

As a kid, you probably flew a kite. If so, you learned one of the basic lessons in aerodynamics. A. kite is really quite a scientific toy. When a breeze is blowing the kite stays up by itself. But if the air is still you have to run with the kite to keep it flying. In both cases there is ONE important similarity. Either the air moves with respect to the stationary kite, or the kite moves with respect to the still air.

Aerodynamics has to do with the forces produced by the relative motion between air and any other object. An airplane, which is fundamentally a glorified kite, is supported in flight by the forces of the air in motion across its wing surfaces.

It's easy to understand how the forces of the air in motion operate if you remember a few basic facts about AIR. First of all, air is a substance - like cheese, or gravel, or water - and therefore has weight. It is a fluid, hence it will flow when moderate pressure is applied. It is a gas and can be compressed into a smaller space by pressure.

The deeper you go below the surface of the ocean the greater the pressure becomes, because of the weight of the water overhead. Well, there is a "sea" of air - about 500 miles deep - surrounding the earth. The greater the depth from the outer surface of the "air sea," the greater the pressure of the air. Thus, at ocean level on the surface of the earth, the air pressure is 14.7 pounds per square inch under average conditions. At this level, the air is more dense than the air above it because the weight of the "air sea" presses down on it. Figure 2 will help you to picture this quickly.

Figure 2.-High and low air pressure zones of the "air sea.""

Since air pressure is equal in all directions, you can't feel it when you are standing still. But stick your hand out the window of a moving auto-mobile. You feel the force of the air immediately, even though there is no wind blowing. You and the automobile, however, are moving in relation to the still air, and a pressure is built up in front of your hand, tending to push it back. That push is called IMPACT PRESSURE.

Other forces are also at work. Your motion through the air creates a partial vacuum behind your hand, REDUCING the pressure on that side. The result is similar to what would happen if you leaned against a wall and it were to collapse suddenly. The pushing force (You) would still be there, but the resisting force (THE WALL) would be removed. As a result, you'd tend to fall toward where the wall was before it collapsed.

Now, take a look at an airplane wing. Its surfaces furnish the support for the airplane in flight. A wing can't be as thin as a kite, since then it wouldn't have enough strength to hold up the weight of the airplane. If you were to cut a vertical slice out of a wing, you'd get an AIRFOlL SECTION, like one of those in figure 3. Notice that it's curved on top, and maybe on the bottom.

The curvature of airfoil surfaces is called the CAMBER. The top camber of a wing airfoil is always POSITIVE - that is, its central part bulges out-ward. The bottom camber may be either positive or negative - bulging outward or inward. In many cases, there is no bottom camber at all - the bottom being flat.

The front of the airfoil section is the LEADING EDGE, and the rear is the TRAILING EDGE. The distance between the leading edge and trailing edge is called the CHORD. And the term ANGLE OF ATTACK is used to describe the angle between the wing chord and the direction of the relative wind. In a moment it will be clear why these definitions appear here.

Figure 3.-Typical airfoil sections.

An airplane wing obtains its lifting power from the action of the relative wind on the airfoil surfaces. The ability of an airfoil-shaped wing to lift is determined according to a principle set down by a scientist named Bernoulli. This principle is util ized in another device known as the venturi tube, and an understanding of how a venturi tube works will help to clarify the whole idea. A venturi tube is simply a short piece of tubing which is reduced in diameter near its midpoint. When a fluid passes through such a constricted tube, THE SPEED OF' THE FLUID IS GREATEST - AND THE PRESSURE IS LOWEST - AT THE NARROWEST PART, OR THROAT.

That's fine ! But what does it have to do with an airplane wing? Just about everything, as far as its LIFT is concerned. A look at figure 4 (A, B, and C) will help you visualize the similarity between an airfoil section and one-half of a venturi tube. Remember, air is a fluid, and acts accordingly. In (A) and (B) you'll notice that the path of the air is straight along the center line of the

Figure 4. (A, B and C) How the venturi-tube principle causes airfoil. lift; (D, E and F) the effect of angle of attack on lift.

tube, but that the air particles next to the walls follow a curved path. In (C) the top half of the tube has been lifted off and taken away.

The remaining lower half of the tube resembles those cambered airfoil sections you saw in figure 3. You'll recall that the fluid passing through the throat of the venturi tube moves at its fastest rate - but exerts its smallest pressure - at the narrowest part. Well, air passing over the top of an airfoil's upper surface speeds up in the same way. And here's the PUNCHLINE of the story! The air pressure on the top of the airfoil, therefore, is LESS than the pressure on the bottom, and the wing is pushed upward.

Actually, this tendency of the wing (to be pushed upward because of reduced pressure above it) provides by far the biggest part of the lift which keeps an airplane in the air. If the wing is TILTED upward in the path of the relative wind, impact pressure on the bottom will also contribute something to the lift. But under many conditions of airplane operation, impact pressure is negligible.

The propeller of an airplane has one main function - to pull or push the airplane through the air so that the relative wind will maintain the craft in flight. Whenever possible, pilots also take advantage of any of nature's own wind by making their take-offs into the wind, thereby gaining additional lift.

You'll recall that the angle of attack was defined as the angle between the wing chord line and the direction of the relative wind. By looking at figure 4 (D, E, and F) you'll see airfoils at three different angles of attack. As the angle of attack is increased, the lift is also increased - up to a certain point. The increase in lift with each in-crease in the angle of attack is due to two things.

FIRST, the impact pressure on the bottom surface of the wing is greater. SECOND, the venturi-tube effect on the wing's upper surface is greater. When the angle of attack becomes too great, how-ever, the air no longer flows smoothly over the top of the wing and the lift begins to decrease.

The angle of attack up to which lift increases and above which lift decreases is called the BURBLE POINT, or STALLING ANGLE.

The speed at which an airplane travels affects lift directly, because it effectively changes the velocity of the relative wind. With the angle of attack remaining the same, an airplane traveling 200 miles per hour has FOUR times as much lift as it would have at 100 miles per hour.

The density of the air also helps to determine the lifting power of an airplane wing. Thus, with other factors being the same, an airplane wing has more lift at sea level - where the air is most dense - than at higher altitudes where the air is more rarefied. Density of the air, however, varies considerably with temperature and humidity as well as with altitude. As air becomes warmer or more humid, its density decreases. So, in hot and humid weather considerably more speed is needed to lift an airplane into the air than is required in dry and cool weather.