Unit 8 Lighter-than-air aircraft

All aircraft are designed to fly through the air. But not all of them fly the same way. In fact, there are two types of aircraft. One is called “lighter-than-air.” The other is known as “heavier-than-air.” A lighter-than-air craft is able to float. It can become lighter in weight than the air around it. A circus balloon is a simple example of this type of craft. Before it gets filled, the balloon doesn’t move. Its weight keeps it down. But fill the bag with helium gas, and the balloon lifts up. Why does this happen? The reason is that helium is much lighter than air.

Why Kites Fly

A heavier-than-air craft is different. It always stays heavier than the air around it. This type of craft flies for another reason. Its surfaces cause moving air to lift up the craft. A simple heavier-than air craft is a kite. Kites come in many shapes and sizes. But they are all made to lift up in the wind. The kites people around the world fly for fun are named after the kite bird. This type of bird is also found all over the world.

Hot-Air Balloons

Most of us have seen them. Big and colorful, hot-air balloons carry people for long rides through the air. How do they do it? These lighter-than-air craft work in several ways. First, every hot-air balloon has an air-filled bag, or envelope. Because of its large size, the bag can move aside a lot of air. But air moved to another place pushes back. In fact, this displaced air presses so hard that it keeps the bag afloat. In other words, the bag is buoyant. But the basket tied to the bag stays on the ground. Its cargo of equipment and people is heavy.

To make the hot-air balloon go up, the pilot turns on a burner under the bag. The burner creates a flame. Hot air quickly rises into the huge bag. This heats up the air molecules inside. They start moving faster. Also, some of them escape through the bottom of the bag. That leaves fewer molecules inside. They are farther apart, too. The inside air, then, is less dense than the air outside, and it weighs less. The result? The hot-air balloon rises. To lower the hot-air balloon, the pilot turns off the burner. The air inside the bag gets cooler. That means the air molecules slow down. They also move closer together. Soon there’s room for outside air to re-enter the bag. More air molecules inside make the air in the bag denser—and heavier. So the craft comes down.

Floating Airships

Hot-air balloons can fly, but they cannot be steered. They simply float in whatever direction the wind takes them. An airship is a lighter than-air aircraft, too. But unlike a hot air balloon, the airship has an engine and fins. These allow the ship to be steered in the direction where the pilot wants it to go. The envelope of an airship is long and rounded. Usually helium gas issued to inflate it. The helium is sealed inside. Since helium is lighter than air, the envelope stays afloat.