Lighter-than-air flight uses buoyancy to lift a craft into the sky instead of relying mainly on wings moving through air. Hot-air balloons and airships float because the gas inside them has a lower density than the surrounding air. This makes the upward buoyant force greater than, or equal to, the downward weight of the craft.
Understanding this idea connects aviation to density, pressure, temperature, and forces.
Key Facts
- Buoyant force equals the weight of displaced air: F_b = rho_air V g
- A craft rises when F_b > W, floats level when F_b = W, and descends when F_b < W
- Weight is the total downward force: W = mg
- Heating air lowers its density, so a hot-air balloon can produce more lift
- Helium airships use a gas with lower density than air to create buoyant lift
- Net lift can be estimated by F_net = F_b - W
Vocabulary
- Buoyancy
- Buoyancy is the upward force a fluid exerts on an object that is surrounded by or immersed in it.
- Density
- Density is mass per unit volume, often written as rho = m / V.
- Envelope
- The envelope is the large fabric or outer gas container of a balloon or airship.
- Ballast
- Ballast is extra weight that can be carried or released to help control altitude.
- Airship
- An airship is a powered lighter-than-air aircraft that can be steered through the air.
Common Mistakes to Avoid
- Thinking hot air balloons rise because hot air pushes upward by itself. The real cause is buoyancy, because heated air inside the envelope is less dense than cooler outside air.
- Forgetting to include the weight of the basket, passengers, fuel, and envelope. Lift must support the entire craft, not just the gas inside it.
- Assuming airships and airplanes fly by the same main force. Airplanes mostly use aerodynamic lift from wings, while airships get most of their lift from buoyancy.
- Confusing helium with hydrogen as if they are equally safe. Hydrogen gives strong lift but is flammable, while helium is nonflammable and much safer for flight.
Practice Questions
- 1 A balloon displaces 900 m3 of outside air with density 1.2 kg/m3. Using g = 9.8 m/s2, what is the buoyant force on the balloon?
- 2 An airship has a total weight of 80,000 N and a buoyant force of 95,000 N. What is the net upward force, and will it rise, sink, or float level?
- 3 Explain why a hot-air balloon pilot can make the balloon rise by turning on the burner and descend by letting the air inside cool.