At any major sporting event there is one thing you are sure to see if the weather is decent. Look, up in the sky, it's a blimp. A gigantic sky whale. Like a helicopter, blimps can provide a hovering platform in the sky that makes them great for aerial photography. But unlike a helicopter, a blimp does its hovering at a much lower fuel cost. Blimps can stay in the air for days if they need to. And don't forget the blimp bonus - there is a huge surface for advertising displays.
The obvious first question is the size. How big is a blimp? They are immense by normal human standards. According to Goodyear, you need to imagine a six-story office building almost 200 feet long to get a sense of a blimp's scale. Or to put it another way, imagine three 18 wheelers parked end-to-end. That handles the length. Then imagine stacking four rows of three on top of each other. A blimp is about 60 feet high, while a tractor trailer is about 13.5 feet high. An un-inflated blimp, including the fabric of the blimp itself, the gondola and the motors, weighs about 13,000 pounds.
A blimp is a giant balloon made out of fabric. There is no internal frame or structure. You might remember the famous film of the Hindenburg exploding, with its immense aluminum framework falling from the sky. A blimp has none of that. It is a big balloon with the gas inside giving it its shape.
That gas is the key to a blimp's ability to float. Blimps are filled with helium, and a lot of it. Think about the normal helium balloon that a child gets at a birthday party. The balloon holds half a cubic foot (14 liters) and can lift about 14 grams. Keep in mind that there are 453 grams in a pound. Therefore, to lift a 50 pound child, it would take about 1,620 birthday balloons.
To get a 13,000 pound blimp off the ground, therefore, you need a whole lot of helium - over 200,000 cubic feet to be exact. That is why blimps are so gigantic compared to an airplane that can carry the same number of people. Of course an airplane can't hover in the air for days at a time either.
The pilot who operates the blimp, plus any passengers, ride in the gondola. Inside, the gondola looks a lot like the cabin of a 8-seater airplane. There are two pilot seats at the front and six passenger seats behind. The controls that the pilot uses look a lot like the controls of an airplane. At the back of the blimp there are big fins that act as the rudder and elevators for steering the blimp. And a blimp has two piston engines spinning normal airplane propellers to move the blimp forward through the air.
You might have noticed that a birthday balloon, once set free, has this tendency to rise until it is out of sight. Essentially a birthday balloon goes up until it bursts. What keeps a blimp from doing that? For one thing, a balloon starts its trip with enough ballast on board to be just slightly heavier than air. If you could put a blimp on a bathroom scale, it would weigh about 100 pounds at takeoff. That keeps it on the ground, but just barely. The slightest gust of wind could kick it into the air. Plus the blimp gets lighter as it burns fuel.
Therefore, inside the blimp there are two large balloons called ballonets. These fill with air under the pilot's control. Since air is a lot heavier than helium, relatively speaking, the pilot puts a lot of air into the ballonets when it is time to descend. To ascend, the pilot lets the air out of the ballonets, allowing the helium inside to uncompress and have its full lifting power. Essentially, the pilot is using big tanks of air as additional, adjustable ballast.
The other big thing with blimps is the advertising. It is possible to paint ads onto the blimp's fabric, and people do, but those signs are invisible at night. Therefore, blimps also have arrays of bright LEDs for night advertising. A blimp becomes a giant floating jumbotron.