The lifting forces on a hot air balloon are primarily the result of which condition?

The lifting forces on a hot air balloon are primarily the result of which condition?

• A. Interior air temperature greater than ambient temperature
• B. Internal pressure exceeding envelope pressure
• C. Wind speed behaving like a Venturi effect
• D. Propane burn rate increasing buoyancy

Answer: (A)

The lifting force in a hot air balloon comes from buoyancy due to the density difference between the air inside the envelope and the air outside. When you heat the air inside, its density drops, so the air inside weighs less for the same volume. The balloon displaces a certain amount of outside air, and the buoyant force equals the weight of that displaced air. If the total weight of the balloon system (envelope, basket, passengers, and the lighter interior air) is less than the weight of the outside air that would occupy the same volume, there’s a net upward force. So interior air temperature higher than the ambient is what creates the lift. The internal pressure is near ambient because the opening at the bottom allows air to flow in and out; the pressure difference isn’t the main driver of lift. Propane burn rate is the energy source that heats the air, but the fundamental cause of lift is the resulting temperature (and thus density) difference. Wind effects or Venturi-like pressure changes don’t provide the sustained buoyancy needed for lifting.

The lifting force in a hot air balloon comes from buoyancy due to the density difference between the air inside the envelope and the air outside. When you heat the air inside, its density drops, so the air inside weighs less for the same volume. The balloon displaces a certain amount of outside air, and the buoyant force equals the weight of that displaced air. If the total weight of the balloon system (envelope, basket, passengers, and the lighter interior air) is less than the weight of the outside air that would occupy the same volume, there’s a net upward force. So interior air temperature higher than the ambient is what creates the lift.

The internal pressure is near ambient because the opening at the bottom allows air to flow in and out; the pressure difference isn’t the main driver of lift. Propane burn rate is the energy source that heats the air, but the fundamental cause of lift is the resulting temperature (and thus density) difference. Wind effects or Venturi-like pressure changes don’t provide the sustained buoyancy needed for lifting.