The French hit on an apt phrase to distinguish them, dividing aircraft into those which are lighter than the air, such as airships, and those which are heavier than the air, like airplanes.
Airships are literally lighter than air. So are all free balloons, used for training and racing, and all anchored balloons, such as the observation balloon widely used in the last war and the barrage balloons of the present war.
The airship goes up and stays up because the buoyancy given by its lifting gas makes it actually lighter than the air it displaces, and even with the load of motors, fuel, equipment and passengers, must still use ballast to hold it in equilibrium.
The airplane, on the other hand, is heavier than the air. Even the lightest plane can stay up only if it is moving fast enough to get a lifting effect from the movement of air along the wings, similar to that which makes a kite stay up. A kite may be flown in calm weather only if the one who holds the cord keeps running. On a windy day, the kite may be anchored on the ground, and the movement of the wind alone will have sufficient lifting effect. So powerful are these air forces that a plane weighing 20 tons may climb to an altitude of 10,000 feet if its speed is great enough, and its area of wing surface broad enough to produce this kiting effect.
But an airplane can remain aloft only as long as it is moving faster than a certain minimum speed. Cut the motors, or even throttle down below this stalling speed, and the plane will start earthward.
The airship needs its motors only to propel it forward. It can cut its speed, even stop its engines, and nothing happens. It retains its buoyancy, continues to float. The airplane’s lift is dynamic, that of the airship is static.
The airship has some dynamic lift, also, because its horizontal fins or rudders, and the body of the airship have some kiting effect in flight. The blimp pilot, starting on a long trip, will fill up his tanks with all the fuel the ship can lift statically, then take on another 2,000 pounds, taxi across the airport till he gets flying speed and so get under way with many more miles added to his cruising speed.
This dynamic lift however, while useful in certain operations is still incidental. Primarily the airship gets its lift from the fact that the gas in the envelope is much lighter than the air.
Hydrogen is only one-fifteenth the weight of air, helium, the non-inflammable American gas, is a little heavier, about one-seventh. The practical lift is 68 pounds to the thousand cubic feet of hydrogen, 63 pounds in the case of helium.
Lighter-than-air ships are of three classes, rigid, semi-rigid and non-rigid. The rigid airship has a complete metal skeleton, which gives the ship strength and shape. Into the metal frame of the rigid airship are built quarters, shops, communication ways, even engine rooms in the case of the Akron and Macon, with only the control car, fins, and propellers projecting outside the symmetrical hull. The lifting gas is carried in a dozen or more separate gas cells, nested within the bays of the ship.