Lighter-than-air Craft more than a Bag of Hot Air
A fairly common sight around the urban areas of Alaska is that of brightly colored hot air balloons drifting serenely along above the scenery. It all looks so easy that one is tempted to think that anybody could be a balloonist. It's not quite that simple.
As Professor James B. Tiedemann of the University of Alaska points out in The Northern Engineer magazine, control of even a simple hot air balloon presents a number of problems not apparent to the casual observer. While some control over the path of a free balloon may be exercised by seeking altitudes at which the wind is blowing in a favored direction, the burner does not immediately control the vertical speed or acceleration of the balloon. Considerable practice is required before the pilot can anticipate its motion far enough in advance to exercise smooth (or any) control. Landings are frequently slow-speed crashes, which is why the baskets are made of shock-absorbing wicker.
There are additional problems that the would-be balloonist might not anticipate. For instance, while rising hot air "thermals" are eagerly sought by glider pilots (and eagles) to gain altitude, the exact opposite may hold true for a hot-air balloonist. Because warm air is less dense, which is what causes a hot-air balloon to go up in the first place, the balloon becomes less buoyant, some of the density difference between the air trapped inside the balloon and that outside is lost. If the thermal is hot enough, even though the surrounding air is rising, it can cost the balloon its upward motion, so that it sinks instead.
Although the days of the Zeppelins have passed, lighter-than-air craft remain in the public eye because of their use in televising sporting events. The Goodyear blimp, though, is a far cry from the Zeppelins of World War I and the ensuing magnificent airships of the Hindenburg class. The Zeppelins had a rigid airframe containing many independent gas cells. The much smaller Goodyear blimp, and others like it, have no rigid structure inside the gasbag, and rely on gas pressure to hold the bag in shape.
The primary difference between the hot-air balloon and the blimp is that the blimp is powered. An unpowered gasbag must change its altitude by dropping ballast, venting gas or, in the case of a hot-air balloon, warming the air in the bag with the burner to rise or releasing hot air to reduce altitude. Powered blimps have other options.
Goodyear non-rigids begin their flights heavier-than-air, making a short takeoff run to gain speed and then nosing upward to develop lift. Using the short, stubby gasbag as a lifting wing permits extreme nose-up attitudes without risk of a stall, often unnerving passengers--especially those who are fixed-wing pilots. Once airborne, the airship becomes progressively lighter as fuel is consumed, so that modern blimps land at nearly neutral buoyancy. If expensive helium is not to be wasted by venting in order to return to the ground, blimps must keep their trips fairly short.
Because the Allies would not sell helium to Germany, the Germans were forced to rely on hydrogen as the lifting gas in their Zeppelins, Hydrogen, being the lightest element, is the ultimate lifting gas and, being flammable, can be used for fuel as well. Its flammability, of course, is what led to the destruction of the Hindenburg at Lakehurst, New Jersey, in 1937.
But, as Tiedemann points out, most people forget that the majority of the Hindenburg passengers survived. He further notes that British pilots found Zeppelins surprisingly difficult to shoot down until special incendiary bullets were developed. They often returned home safely with their hydrogen gas bags riddled with bullet and shrapnel holes. With this in mind, he asks the rhetorical question if passengers might not feel safer beneath a cloud of hydrogen that burns with an upward-radiating flame than surrounded by a sea of jet fuel that turns to instant napalm in an accident.
