Although the generally accepted spheres of usefulness of the airship and the airplane are usually based on their comparative ranges of operation and their speeds, the suitability of either of these types for a given purpose is primarily dependent on two classes of factors, those fundamentally dissimilar and those roughly similar. Conclusions as to relative usefulness should be based on a consideration of the dissimilar characteristics, which include aerodynamic efficiency, size and comfort.
Aerodynamic efficiency governs range and, since it determines fuel consumption, influences the cost of operation. The size required depends on the paying loads that are available for carrying. Comfort concerns passenger-carrying only.
As the propeller efficiency, rate of fuel consumption and ratio of weight of fuel carried to gross lift are similar in both types of aircraft, the range must depend on the L/D factor, that is, the ratio of gross lift to thrust. Although it is not customary to apply this ratio to airships, comparative curves for the airplane and the airship on this basis show a surprising superiority for the airship, not only at speeds of 60 or 70 m.p.h. but at higher speeds that have not yet been attained. In the matter of fuel consumption, an airship of 150-ton capacity traveling at 70 m.p.h. requires only one-quarter the fuel per ton-mile, and at 105 m.p.h. only one-half the fuel per ton-mile that is needed to propel 1 ton of airplane 1 mile at either speed.
Partly nullifying the advantage of the airship because of lower fuel-consumption has been the waste of lifting-gas that occurs in maintaining equilibrium when the weight has been reduced by the consumption of fuel. Notwithstanding this waste the airship can still compete with the airplane on an equal basis in the matter of fuel consumption at a speed of 70 m.p.h.; but through water recovery and hydrogen burning the waste is no longer necessary.
With regard to size, consideration is given to the effect of dimensional laws on the dead-weight of both types of aircraft. Assuming that the dead-weight of airplanes increases with the increase in size under the 3/2 law, the further assumption is made that 10 tons is the weight at which the law begins to operate; in airships, the factors operating are different from those of airplanes and do not become effective until sizes far in excess of those contemplated at present are reached.
Asserting that airplanes and airships of the same size are not likely to come into direct competition, the author then examines the fields in which each type is likely to prove useful.
Among the conditions affecting the comfort of passengers are found to be the relative amounts of space available on the two types of craft, and the various motions encountered, such as rolling, pitching, bodily vertical motion, and vibration.
But the observations made from the standpoints of aerodynamic efficiency, size and comfort are said to be subject to modification by additional factors, of which the most important are (a) initial percentage of useful load, (b) initial cost per unit of gross weight, (c) relative operating-costs, (d) insurance and safety, and (e) rate of depreciation, each of which is discussed in detail; and reference is made to the causes leading to the Shenandoah disaster.
The conclusions reached are that, on account of aerodynamic superiority, the airship will be used wherever possible for carrying passengers in comfort, but a large field exists in which it cannot be used because of insufficient loads; that the airship will continue to be a long-range, the airplane, a short-range vessel, but these distinctions will be affected largely by the volume of traffic available; that although the cost of transportation per ton-mile is greater by airplane than by airship, this fact will not seriously limit the use of the airplane within its own field, the comparative magnitudes of the fields of heavier-than-air and lighter-than-air operation being similar to those of rail and water transportation; and that, inasmuch as the two types are complementary, the successful operation of either one will react to the benefit of the other.