The factors influencing the speed of a dirigible have already been referred to, but it will be apparent that the radius of action is of equally great importance. It is like wise something that has a very direct bearing upon the speed and, in consequence, upon the design as a whole. It is evident that to be of any great value for military or other purposes, the dirigible must possess not only sufficient speed to enable it to travel to any point of the compass under ordinarily prevailing conditions of wind and weather, but it must likewise be able to remain in the air for some time and cover considerable distance under its own power. In fact, one of the chief advantages possessed by the dirigible over the aeroplane at present is its ability to make long sustained flights, while carrying a comparatively large crew and a great deal of extra weight.As is the case in almost every point in the design of the dirigible, conflicting conditions must be rec onciled in order to provide it with a power plant affording sufficient speed with ample radius of action.
Power requirements increase as the cube of the speed, making a tremendous addition neces sary to the amount of power to obtain a dispro portionately small increase in velocity. In this connection there is a phase of the motor ques tion that has not received the attention it merits up to the present time. The struggle to reduce weight to the attainable minimum has made weight per horse power apparently the para mount consideration — a factor to which other things could be sacrificed. But it is quite as important to make the vessel go as it is to raise it in the air, so that the question of total weight per horse power hour will undoubtedly come in for much more attention in future, particularly since weight per horse power appears to have approached so closely the minimum attainable consistent with a due regard for reliability.
The relative importance of these two fac tors may be appreciated from the following example: Assume, for instance, a 100 horse-power motor of a total weight of 1,000 pounds, round figures being chosen merely for the sake of sim plicity. The weight per horse power of such an engine would be 10 pounds. This would not be sufficient data, however, from which the design of a dirigible to employ that motor could be worked out. Pounds per horse power usually refers to a bare engine. The weight of cool ing water, lubricants, accessories, and last, but far from least, that of the fuel must be added. For example, the motor referred to may be assumed to require one pound of fuel and lubricant per horse power to run it at its normal output. This means that it will consume 100 pounds per hour, or for a run of 10 hours 1,000 pounds, and this weight must be added to that of the motor itself in considering the design from the standpoint of radius of action. This factor depends en
tirely upon the efficiency of the motor, while its weight per horse power is a question of its con struction alone. It requires no abstract calcula tion to show that it is quite as possible to have the same number of pounds for the weight per horse power if a very light engine that consumes a great deal of fuel is used as it is with a heavy engine that consumes very little. The diminution of the weight per horse-power hour makes pos sible an increase in the duration of the voyage, which is a very desirable advantage, but as the prime factor is the ability to rise, improvement that involves the addition of weight is closely re stricted by the lifting power available, so that radius of action is governed by numerous con siderations. Take a dirigible with a gas capacity of 12,000 cubic feet, equipped with two 60 horse power motors, giving it a speed of 36 miles per hour. The engines will consume 130 pounds of fuel per hour, and the machine, with six passen gers, will have sufficient lifting capacity to carry 1,300 pounds of gasoline. This would mean traveling for 10 hours, or 5 hours in each direc tion. The radius of action of this dirigible would be 180 miles.
Speed is quite as costly in an airship as it is in an Atlantic liner. To double it the motor power must be multiplied by 8 and the machine must carry 8 times as much fuel. The wind is a serious factor that has to be taken into con sideration. Radius of action as above considered has been based entirely upon traveling in a dead calm. True, where the prevailing wind blew from a certain quarter for a length of time, its favoring influence in going would be neutralized by its resistance in returning, so that the result would be the same, providing the velocity of the wind were not too great to prevent returning at all against it.
When the speed of the dirigible is greater than that of the prevailing wind the dirigible may travel in any direction; when it is consider ably less it can travel only with the wind; when it is equal to the speed of the latter it may travel at an angle with the wind — in other words, tack, as a ship does, utilizing the pressure of the contrary wind to force the ship against it. But as the air does not offer the same hold on the hull of the airship as water does on the sea-go ing ship, the amount of drift in such a manoeu vre would doubtless be excessive and the for ward motion practically nil.