Blimps fly by using the principle of buoyancy, where they are filled with a gas lighter than air, typically helium, which provides lift by making the blimp positively buoyant. Here's a detailed explanation of how blimps fly:

• The blimp's large envelope is filled with helium gas, which is much lighter than air. This makes the blimp buoyant, allowing it to float upwards like a balloon.
• The envelope is a durable, airtight fabric bag that holds the helium. It can hold tens or hundreds of thousands of cubic feet of helium.
• Inside the envelope, there are ballonets—air-filled bags that can be inflated or deflated to control the blimp's buoyancy and maintain its shape.
• To take off, the pilot vents air from the ballonets, increasing the volume of helium, making the blimp lighter than air (positively buoyant), so it rises.
• The pilot controls ascent and descent by adjusting the air in the ballonets. Adding air to ballonets makes the blimp heavier and it descends; releasing air makes it lighter and it ascends.
• The blimp has engines and propellers that provide thrust, allowing it to move forward, reverse, and maneuver like an airplane.
• Flight control surfaces such as rudders and elevators control the direction and angle of flight.
• Unlike airplanes, blimps do not rely on wings for lift but on the helium's lift in the envelope.
• The pilot maintains neutral buoyancy during flight by balancing helium and air pressure inside the envelope and ballonets, allowing stable cruising.
• The engines typically provide speeds around 30 to 70 mph, and the blimp can hover or cruise for long periods.

In summary, a blimp flies by using helium gas to float in the air, controlling its altitude and stability by adjusting air inside ballonets, while engines and control surfaces steer and propel it forward.