www.TeachEngineering.org: Free Curriculum for K-12

From the earliest times, humans have marveled at the flight of birds and have wanted to fly just as they do. According to Greek myth, Icarus flew too close to the sun on wings made of wax, and he fell to his death. Many thousands of years later, it took a series of engineers to make this human dream of flight become a reality.

In his lifetime, Leonardo da Vinci (1452-1519), like other inventors of his time, had a fascination with the flapping flight of birds. He believed that "a bird is an instrument working according to a mathematical law. It lies within the power of man to make this instrument with all of its motions." He designed several ornithopters, which are airplanes with flapping wings. He also designed a glider that resembles modern day hang-gliders.

In 1738, an Italian physicist named Daniel Bernoulli discovered that water and air move faster over curved surfaces than they do over flat ones. He also discovered that as the air or water moved faster, the pressure exerted by the fluid decreased. This critical discovery, known as Bernoulli's Principle, paved the way for modern airplanes.

In 1903, a pair of bike shop owners from Ohio made the world's first powered flight at Kitty Hawk in North Carolina. Orville and Wilbur Wright had studied prior attempts to fly and realized that birds keep their balance by twisting their wings. In 1895, Wilbur designed a kite that reproduced this effect mechanically. Then, for several years after his kite invention, the two brothers experimented with flight using gliders. Wilbur also discovered the principle of combining rudder control with roll for smoother turns.

The atmosphere is a sea of air that is about 80 kilometers thick surrounding our planet. All of that air piled on top of us is pulled down by gravity. In fact, the air pushes against us in all directions. This is air pressure.

The above picture shows a colorful cartoon boy with arrows pointing at him from all directions to represent air pressure pushing on him.

Figure 1. Air pressure acting on a person.
click for copyright

Air is pushing on all people and objects with a force of 14.7 pounds per square inch at sea level. (Disclaimer: With the metric system, pressure is measured in Newtons per square meter, which is called a Pascal. With younger kids, it is easy to use pounds per square inch since the units are more familiar and the numbers are more reasonable. To convert pounds per square inch into Pascals, multiply by 6,895.) As you go higher in elevation, there is less air pushing down, so there is less pressure. The change is not that great until you get very high. Heating the air, cooling it, or causing it to move can change the air's pressure. It is these differences in air pressure that create wind, and make flight possible.

Bernoulli discovered that the faster water moves over a surface the lower the pressure the water exerts on that surface. In the diagram below, water flowing out of a bottle moves faster through the neck than it does in the rest of the bottle. Bernoulli proved that the water pressure on the sides of the neck of the bottle were less than those on the sides of the rest of the bottle.

The above diagram shows a fluid leaving a bottle. The fluid is moving faster at the neck of the bottle than in the rest of the bottle.

Figure 2. Fluid moving out of a bottle.
click for copyright

Bernoulli then found that these same principles apply to air as well. Bernoulli's Principle simply states that the faster a fluid moves over a surface, the less it pushes on the surface.

The above diagram shows three different surfaces. On the first surface, there is no air movement and, therefore, the air pressure is uniform over the entire surface. The second surface has air moving over the top of it, and, therefore, there is less pressure on the top of the surface than on the bottom. The air is moving even faster over the third surface, which means the air pressure on that surface is even lower.

Figure 3. The influence of air movement on air pressure.
click for copyright

Engineers use Bernoulli's Principle when designing and testing airplane wings. They can use this information to determine the amount of lift a wing will produce (how high an airplane can go). Engineers also use their knowledge of these principles for other projects relating to fluid flow. We will learn more about how Bernoulli's Principle is related to flight in Lesson 2 of the Airplanes unit.