Fig. 190.—A Model Airship
The covering is to be yellow Japanese silk, proofed with varnish diluted with 20 per cent. of linseed oil and 10 per cent. of turpentine; two coats should be given before the fabric is applied, and two afterwards. A covering strip of fabric should be glued over all seams to make the envelope as impervious as possible. And where the outriggers pass through further pieces should be glued over, and flanged on to the tube itself, being afterwards well doped. It is well to impress here the importance of making the envelope as gasproof as possible, and although a fabric entirely impervious has yet to be invented, it is possible, with care, to reduce loss of gas by percolation to a very low figure indeed.
A Lucas cycle valve is soldered into the brass end-cap constituting the rear of the envelope, for the purpose of inflation, which is effected in the following manner: A T-piece is fixed to a cycle pump, and a pipe from the gas container to the T-piece. A further tube is connected from the remaining arm of the T-piece to the valve. To inflate the envelope, release the pressure from the container until the pump handle is forced out to its full extent, and then shut off pressure and force the pump down, thereby causing ingress of gas to the envelope. Continue thus until the envelope “swells.” Of course, this method could be adopted if hydrogen from the gas jet is used, although this does not possess the lifting capacity of hydrogen procured from the balloon manufacturers.
In conclusion, it may be pointed out that the weight of the complete model inflated should not exceed 2 lb. A view of the appearance of the model in flight is given by [Fig. 190].
For those who wish to design model airships of their own it may be stated that coal-gas lifts about 35 lb. per 1,000 cub. ft., or half the weight lifted by an equal volume of hydrogen. A small model would not be very successful, as the volume, and hence the lifting power, decreases as the cube of the diameter. Thus assuming a model to be built of half the dimensions given in this chapter, the weight of it could only be ½ × ½ × ½ = ⅛ of the original model, and it would be found difficult to work to this limit. The writer would point out that little success can be expected from an airship of such small dimensions, as the following elementary calculation will show. It takes 35,000 cub. ft. of hydrogen to support 1 ton. Then 1 cub. ft. of hydrogen supports ²²⁴⁰/₃₅₀₀₀ lb. Now, the cubic contents of a model dirigible, we will assume, is
| 22 | 5 | 5 | 36 | 1 | |||||
| — | × | — | × | — | × | — | × | —— | = cub. ft., |
| 7 | 2 | 2 | 1 | 1728 |
and therefore the total weight it is capable of supporting is
| 22 | 5 | 5 | 36 | 1 | 2240 | ||||||
| — | × | — | × | — | × | — | × | —— | × | ——— | = .026 lb., or .4 oz. |
| 7 | 2 | 2 | 1 | 1728 | 35000 |
From this it will be seen that it is extremely improbable that a model can be built to this weight.