You know from reading the previous page or from completing the experiment ‘Inflating Balloon’ found under the titleAir has Temperature that warm air rises. Why does this happen? Well, the molecules of warm air have more motion, push out with more force and take up more volume than colder air. Now, by increasing the volume of air that has the same amount of mass, you decrease the density of that air. Remember, the mass is the same but the volume has increased. The force pushing up, the buoyant force is greater than the force pushing down, the force of gravity. This buoyant force pushes the warm air upward through the colder air.
Balloons (hot air) and blimps (helium) fly because the warm air inside the balloon has a lower density than the air surrounding the balloon. It is lighter! The balloon becomes more buoyant and floats up into the sky.
Hot air rises. This is the power behind hot air ballooning. By increasing the temperature of the air in the balloon envelope, letting it cool, or maintaining it, the pilot is able to climb higher, descend (lower), or fly level.
Hot air balloons have three main parts: the envelope to contain the heated air, the propane burner used to heat the air, and the basket to carry the pilot, the fuel and any passengers.
1. Why does a hot air balloon rise?
2. Why is it better to launch a hot air balloon on a cold day than on a warm day?
3. Why is hot air less dense than cold air?
4. Why does the hot air balloon fly differently with ballast (weight) added?
5. Why do hot air balloon pilots avoid windy days?
Make Your Own Hot Air Balloon: All you need is a thin plastic bag. You want the bag to be thin so that it is light. Decorate the bag if you wish. Leave the bottom edge open. If you need any weight (ballist), use paper clips. Perhaps you would like to add a small basket to the bottom to make your hot air balloon look more realistic. When ready, get your heat source (hair drier) and fill the balloon. When the hot air balloon is full of hot air then let it go.
We have lift-off.
Blimps or airships are similar to Hot Air Balloons as they too use gas to lift off and float in the air. These air ships however are quite different as they move forward by their own power (like airplanes). They can also remain in the air for long periods of time and in one place. They have the ability to be stationary while in the air. Just like a helicopter.
If you are interested in more airship information then How Stuff Works has a good blimp section. click here
Why Can A Bird Fly?
The Four Forces
Before we get really involved with flight we need to take a look at the four forces: lift, weight (gravity), drag and thrust. These four forces act with each other and against each other.
Lift is the force that causes a plane to rise. It is the wings of the airplane that give it lift. Lift is caused by air movement and air pressure. If you are not sure how this works, here is a quick review of Bernoulli's Principle, which explains how air pressure produces lift.
Weight: The earth's gravity pulls down on the plane. This is the plane’s weight. The lift needs to be greater than the weight of the plane if the plane is to take off.
Thrust is the force that moves the airplane forward. Engines provide thrust for airplanes, unless of course it is a glider. The engine's thrust keeps the plane moving forward and the air flowing over the wings keep the plane in the air.
Drag is caused by the wind or air against the plane. Drag can slow a plane down. The shape of the airplane is important in telling how much drag there will be. The more streamlined the plane is the less drag it will have. An aerodynamic plane has little drag. If there is more thrust than drag then the plane speeds up. If there is less thrust than drag, the plane slows down. If they are the same then the plane flies at a constant speed..
Review of Bernoulli's Principle
The air above the wing moves faster than the air below it. Slower air has higher pressure than faster air, so the air pressure pushing up on the bottom of the wing is greater than the pressure pushing down. When this happens the wing moves up and we have lift.
The shape representing wings (purple colour) is calledairfoil.