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 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.