John P. Crotty
Since the first part of the word airplane is air, it is important for us to take a brief look at the makeup of air itself. The ancient Greeks believed that air was one of the four basic elements from which all things were formed; the other elements were earth, fire and water. Each element had a quality associated with it. Air was cold; fire was warm. Earth was dry and water was moist.
Today’s definition of air is quite different. Air is the gaseous mixture which surrounds the earth. Air is invisible, colorless, tasteless and odorless. Yet, air is as much a part of the earth as is the land and the sea. Since air is a gas, it can change its shape when it is under pressure. Because gases lack strong molecular cohesion, air will adjust its shape to completely fill any container into which it is placed.
Because we can not see air, we sometimes don’t realize that it exists. Here are four simple demonstrations involving air to show your class.
When we drink all of the liquid from a glass, we say that the glass is empty. It is actually full of air. Turn the glass upside down; push it down into a pan of water. The water will not rise up all the way inside the glass because of the air inside.
Another everyday example of air is drinking through a straw. As you suck on the end of the straw, you remove the air from it. The liquid is then forced into the straw because of the air pressing down on the liquid.
To establish that air has weight, weigh a light bulb. In order for the light bulb to work, nearly all the air was pumped out. Now, make a small hole in the light bulb to let the air in. Because air has flowed in, the light bulb is heavier.
To show air resistance, drop a sheet of paper. Record the time that it takes to flutter to the ground. Now, ball up the sheet of paper. Drop the paper ball. It will fall much more rapidly to the ground since there is less air underneath this shape to resist the downward motion.
The layer of air that surrounds the earth is called the atmosphere. The atmosphere is composed of 78.09 percent nitrogen, 20.95 percent oxygen and .93 percent argon. The other .03 percent consists mainly of carbon dioxide, helium, hydrogen, krypton, methane, neon, ozone and xenon. The air in the lowest layer of the atmosphere also contains water vapor. Since some of these elements are heavier than others, the heavier elements, such as oxygen, are found closer to the surface of the earth. The lighter elements are found in the higher regions. Most of the oxygen is found below 35,000 feet altitude.
Air is in constant motion. Air moves parallel to the surface of the earth and also up and down. The differences in the weight of the air in different places causes air movement. The sun heats the earth’s surface. The air that touches the surface then becomes heated. Air expands and becomes lighter when it is warmed. This warm air then rises and mixes with the air which is higher up. Some surfaces heat faster than others. Heavier air from surrounding areas fills the space vacated by the heated air forcing the light air upward.
Descartes said that the earth is at the bottom of an ocean of air. Although air is very light, it has a mass. Thus it is affected by the law of gravity and has a weight. Since air is a gas, this weight can be expressed as pressure. At sea level air exerts a pressure of 14.7 pounds per inch. The higher up in the “ocean” you go, the less air you have on top of you. Less air means less weight which means less pressure. At 18,000 feet air pressure is only one-half what it is at sea level.
The other thing to remember about air pressure is that since air is a gas, the pressure is exerted equally in all directions. A nice experiment on barometric pressure is to take a glass of water and fill it to the rim. Put a cardboard on it. Holding the cardboard, turn the glass over. With one hand on the glass, take your other hand off the cardboard. The liquid stays in the glass! Air pressure holds the cardboard up.
If your students remain skeptical, don’t be upset. Since air is invisible, it has taken man a long time to recognize that it causes resistance. In fact, Leonardo da Vinci was the first to suggest that air offered resistance to motion. He predicated an expected increase in density in front of a moving body. A hundred years later Galileo stated that air resistance was proportional to velocity. He concluded this on the basis of an experiment using two pendulums that were started at different heights. Then in 1690, Christian Huygens performed experiments that showed that air resistance was proportional to the square of velocity. A little later, Newton calculated the resistance of moving bodies in a fluid. He assumed that resistance was primarily a function of density, velocity and shape. He felt that other fluid properties like viscosity would have a small effect. In 1713 one of Newton’s assistants, Roger Cotes, stated that over the front half of a body the resistance is due to momentum transferred to the fluid.
It was Daniel Bernoulli, an eighteenth century Swiss mathematician, who explained in 1738 how the pressure of a moving fluid varies with its speed. Gas is a subset of fluids. Bernoulli stated that an increase in the speed of movement would cause a decrease in the fluid’s pressure. This is exactly what happens when air passes over the curved top of an airplane wing.
Now that we have looked at the substance that planes fly in, we,11 move on to the forces that act on an airplane in flight. There are four forces which can be grouped into two sets of opposite forces. The first set is thrust and drag. The second set is lift and weight.
Thrust is the force which pushes the plane forward through the air. As the plane flies, it pushes air out of its way. When an airplane takes off or speeds up, thrust is greater than drag. When the plane levels off at constant speed, thrust and drag are equal. If thrust were always greater, the plane would always be accelerating.
Drag is the resistance the air exerts on the forward motion of a plane. Many diagrams show drag as an anchor. The amount of drag depends greatly upon the shape of the plane. The wings, fuselage and other protruding parts disrupts the airflow around the plane.
Weight is the combined load of the airplane and every thing in it. Weight pulls the plane down because of the force of gravity. An airplane must overcome the earth’s downward pull to leave the ground and stay in the air.
Lift is the upward force that counteracts gravity and keeps a plane in the air. The wings produce lift as a result of the difference in the speed of the air over the wing versus the speed under the wing. A plane flies when the lift is greater than the planes total weight.
Lift is produced in two ways. One way is by the direct pressure of the air against a tilted wing. Air in motion exerts a pressure against any object that it strikes. Stick your hand out of a moving car window, close your fingers, and hold your palm perpendicular to the ground. Air will press against your hand and force your arm back against the door frame.
The other way that lift is created and this is the “magic” that allows planes to fly is Bernoulli’s principle. We have already stated how Bernoulli showed that as the speed of a fluid increased, the pressure of the area it occupied decreased. When an airplane wing, an airfoil shape that is curved on the top and flat on the bottom, moves through the air, the speed of the air on the top of the wing is greater than the speed of the air below it. The speed of the air on top is greater because the wing is curved and the air has farther to travel. This means that the pressure above the wing is less than the pressure below it. The higher pressure below the wing generates lift.
Stick your hand back out the car window. This time angle your palm into the wind. Your arm will now move to the top corner of the door frame. You have created a low pressure above your hand which caused the lift.
