From this short statement it is evident that, by a comparatively small increase in volume, the lifting capacity of an airship is enormously increased, and it is in this subject that the airship possesses such undoubted advantage over the aeroplane. In the heavier-than-air machine there is no automatic improvement in efficiency resulting from greater dimensions. In the airship, however, this automatic improvement takes place in a very marked degree; for example, an airship of 10,000,000 cubic feet capacity has five times the lift of the present 2,000,000 cubic feet capacity rigid, but the length of the former is only 1.7 times greater, and therefore the weight of the structure only five times greater (1.7); that is, the weight of the structure is directly proportional to the total lift. Having seen that the total lift varies as the cube of the linear dimensions while air resistance, B.H.P.--other things being equal--vary as the square of the linear dimensions, it follows that the ratio "weight of machinery/total lift" decreases automatically.
In comparing the different methods of transport for efficiency, the resistance or thrust required is compared as a percentage of the total weight. The result obtained is known as the "co-efficient of tractive resistance." Experiments have shown that as the size of the airship increases, the co-efficient of tractive resistance decreases to a marked extent; with a proportionate increase in horse-power it is proportionally more economical for a 10,000,000 cubic feet capacity rigid to fly at 80 miles per hour than for a 2,000,000 cubic feet capacity to fly at 60 miles per hour.
As the ratio "weight structure/total lift" is in airships fairly constant, it follows that the ratio "disposable lift/total lift" increases with the dimensions.
It is therefore obvious that increased benefits are obtained by building airships of a larger size, and that the bigger the ship the greater will be its efficiency, providing, of course, that it is kept within such limits that it can be handled on the ground and manoeuvred in the air.
The proportion of the useful lift in a large rigid, that is the lift available for fuel, crew, passengers, and merchandise, is well over 50 per cent when compared with the gross lift. When the accompanying table is studied it will be seen that with airships of large capacity the available lift will be such that considerable weights of merchandise or passengers can be carried.
Capacity in Gross Lift Length Diameter
cubic feet in tons in feet in feet
2,000,000 60.7 643 79
3,000,000 91.1 736 90.4
4,000,000 121.4 810 99.5
5,000,000 151.8 872 107.2
6,000,000 182.2 927 113.9
7,000,000 212.5 976 119.9
8,000,000 242.8 1,021 125.5
9,000,000 273.3 1,061 130.4
10,000,000 303.6 1,100 135.1
In airships of their present capacity, in order to obtain the greatest amount of lift possible, lightness of construction has been of paramount importance. With this object in view duralumin has been used, and complicated girders built up to obtain strength without increase of weight. In a large ship with a considerable gain in lift, steel will probably be employed with a simpler form of girder work. In that way cheapness of construction will be effected together with increased rapidity of output, and in addition the strength of the whole structure should be increased.
The rigid airship of 10,000,000 cubic feet capacity will have a disposable lift of over 200 tons available for fuel, crew, passengers, and merchandise in such proportions as are desired. The endurance of such a ship at a cruising speed of 45 miles per hour will be in the neighbourhood of three weeks, with a maximum speed of 70 to 80 miles per hour, and a "ceiling" of some 30,000 feet can be reached. This will give a range of over 20,000 miles, or very nearly a complete circuit of the globe.
For commercial purposes the possibilities of such a craft are enormous, and the uses to which it could be put are manifestly of great importance. Urgent mails and passengers could be transported from England to America in under half the time at present taken by the steamship routes, and any city in the world could be reached from London in a fortnight.
In the event of war in the future, which may be waged with a nation situated at a greater distance from this country than was Germany, aircraft Of long endurance will be necessary both for scouting in conjunction with our fleets and convoy duties. The British Empire is widely scattered, and large tracts of ocean lie between the various colonies, all of which will require protection for the safe-guarding of our merchant shipping. The provision of a force of these large airships will greatly add to the security of our out-lying dominions.