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Military


Airship Scale - Bigger is Better

One very important feature of lighter-than-air craft which confers upon them a superiority over heavier-than-air craft, namely, that an increase in the size of airships gives greatly improved efficiency. The reason why in an airship the efficiency ratio is improved with an increase in size, whereas in the airplane it remains at best stationary, is due to the fact that the airship derives its lift from volume, while the airplane derives its lift from area. Now, in either case what may be termed the unit of lift, cubic feet of volume or square feet of area, exerts only a definite lifting effect; in the airship this is determined by the specific gravity of hydrogen or helium, and is therefore immovable, whereas in the airplane it depends upon progress in design, and is thus still open to improvement.

This fact may, in a certain measure, work out in favor of the airplane, but there are other factors which actually limit the practicable size of heavier-than-air craft. One of these is that while the lift of an airplane theoretically varies with the square of linear dimensions, the weight of the wing structure per unit of area does not remain stationary with an increase of wing area, but increases, because the larger wing structure must be made proportionally strong. Consequently, a size is eventually reached where every additional square foot of surface weighs as much as it can lift, so that beyond this size there will occur a marked reduction in efficiency.

The strength of similar structures is inversely proportional to their linear dimensions. This rule applies equally to airships and airplanes, but while the lift varies as the cube of dimensions in lighter-than-air craft, and as the square in heavierthan-air craft, an increase in lift is attained in airships with much less increase of dimensions than in airplanes. In an airship, when the lift is increased four times, the length of the structure is only increased by the cube roor of four, that is, approximately one and a half times, so that the weight of the structure will be only four times greater for the same proportionate strength, which is to say that the weight of an airship is, for the same structural strength, directly proportional to the total lift.

Another technical advantage afforded by the airship is that the so-called coefficient of tractive resistance, that is, the ratio of the weight of machinery to total lift, obviously decreases with an increase of size, because the lift varies as the cube of the linear dimensions, while the resistance of the air and the brake horsepower vary as the square of the dimensions, all other things being equal. This explains why the performance of the latest Zeppelins, which had almost trebled in size since 1914, had so marvelously improved.

For war purposes, the value of an airship depends upon its speed. A greater distance can be traversed, with increased speed, before gas leakage or dispersion forces a return to the supply base; in other words, a greater radius of action is gained. Moreover, an airship of great speed is not so dependent on meteorological conditions. A war engine should not be the slave of weather conditions. Increased speed increases security in permitting easier escape from pursuers.

The best way to increase the speed of the Zeppelin is to increase its volume. The greater lifting power permits heavier motors. Air resistance does not increase in the same ratio, because it varies with the square of the balloon's dimensions, whereas the ascensional force varies with the cube. So when the length and width are doubled the resistance of the air is quadrupled, while the ascensional force is increased eight times.

A large balloon loses gas more slowly than a smaller one, because the loss is a function of the surface and not of the volume. Besides increased speed, the super-Zeppelin gains in weight-carrying capacity, and can therefore transport more projectiles and more combustible material-a gain in destructive power and in radius of action.

These are the principal reasons why the Germans constructed such enormous dirigibles. The dimensions of these airships were comparable to those of the Lusitania.





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Page last modified: 11-07-2011 15:29:14 ZULU