Giant Gasbags: Blimps and Airships
Thanks to companies like Goodyear and a growing number of its competitors, airships are the most recognizable of all the lighter-than-air craft. Blimps have become regular features at sporting events and special occasions all over the world for a couple of reasons.
First, with a maximum airspeed of about 35 miles per hour, blimps can virtually hover over a single location for hours at a time. Second, there’s no more attentiongrabbing billboard than a blimp trumpeting an advertising slogan.
Airships very nearly died out due to public panic after the Hindenburg disaster in 1937 killed 36 people in the air and on the ground. But engineers learned valuable lessons from the accident, most notably to use helium rather than hydrogen gas. Hydrogen gas is explosive, and piping it into a tinder-try wooden framework covered with fabric that burns faster than paper was tempting fate, to say the least.
Engineers found that blimps filled with explosion-proof helium instead of hydrogen produced almost as much lifting power. And they provided enough peace of mind that blimps have become an accepted part of the sports and advertising landscape.
Anatomy of a Blimp
Blimps function much like a series of connected gas balloons with engine – driven propellers to give the whole contraption a little forward speed. Blimps are equipped with controls that enable the pilots to pitch the craft up and down or to yaw it (turn it) left or right.
Inside the taught skin of a blimp are thousands of cubic feet of helium and one or more bags of air called ballonets. A cubic foot of helium, which is a volume slightly larger than that of a shoebox, can lift just over an ounce. That’s not much, but helium’s lifting power lies in large numbers. When you pump 170,000 cubic feet of helium into a blimp’s envelope, you have an airship capable of lifting almost 11,000 pounds. Some airships hold enough helium to allow them to act as airborne cranes.
Here’s how the ballonets and helium envelope work together to keep the blimp flying.
On the ground, high atmospheric pressure squeezes the helium so much that it occupies a smaller area, meaning the blimp’s skin would wrinkle like a prune. After all, blimps don’t have any internal frame. They’re nothing more than fabric bags, keeping their shape only because of the pressure of the helium inside.
To prevent the blimp from wrinkling and losing its shape, large ballonets attached to the inside of the main helium bag are inflated with air, filling part of the vast interior space with air rather than helium. The air-filled pilot-controlled ballonets give the helium a smaller volume to fill while still filling up the familiar cigar shape.
As the blimp climbs to higher altitudes, the atmospheric pressure outside the blimp decreases. As a result, the helium inside the main envelope is able to expand while the ballonets are allowed to collapse so that helium can fill the entire envelope.