Page:Popular Science Monthly Volume 91.djvu/563

 Popular Science Monthly

��547

��So little trouble was taken to adapt the airplane fuselage to the gas envelope that not even a motor-driven blower was pro- vided for the airbag in the first Blimps. Part of the airplane propeller's slipstream, caught in a hose, sufficed.

The hybrid "Blimp" has shown itself incredibly superior to anything in its own class. It has a speed of forty-five miles an hour and a radius of action of several hundred miles. Its lift control approaches the seventy -mile -an -hour Zeppelin's, be- cause the "Blimp's" surface is relatively greater, compared with its weight, than a Zeppelin's. Moreover, for the same speed the dirigible with a greater relative surface has more grip on the air in rising or descending by reason of the airplane effect of its gasbag than if it had a surface smaller in proportion to its weight.

y Pressure relief valve

Ballonet

��The United States has also built "Blimps." The main original features of the American "Blimps" is the addition of a blower driven by a cycle-motor and of a second airbag or ballonet with valves to shift the air at will from bag to bag.

As a result, the ship can be "trimmed" (that is, its flotation forward and aft can be varied at will by driving the gas where there is less air) even while it is at rest and the elevator or vertical rudder is powerless. The blast of the propeller will probably be used to inflate the ballonets, that being safer than a separate motor. There has been added an efficient device for anchoring the vessel safely in a storm. The equivalent of life belts, in the form of kapok buoys are fastened above the airplane's floats. Hence the entire craft can rest lightly on the water, supported by its gas. Enough water and sand ballast are carried to permit the craft to rise the better part of a mile; the safe altitude is given as one 1/4 miles, but with the help of the powerful airplane action of the craft itself this may be doubled.

���A ballonet is simply an air-bag within the main balloon. When the dirigible descends and the gas contracts the airbag is blown up with air by a little motor in the car below. Thus what buoyant gas remains is compressed] and made to restore the envelope to its normal shape

���Beginning Has Proved

��dirigibles is the erratic changing of the lifting force of the hydrogen gas with which they are inflated. When- ever the sun disappears, the gas cools and shrinks. When the vessel enters a cold stratum of air the gas shrinks, to expand again upon reaching a warmer

��layer. But even the old slow dirigibles could add or subtract much weight to or from their lift by reason of their airplane action. As this action increases proportionately with the square of the speed the fast dirigible of today compensates easily for such variations

�� �