No more suitable covering than gold-beater’s skin exists. If the model is of the rigid type, then the covering should be stretched over the framework that imparts the ichthyoid shape to the envelope. Strips of the fabric must be attached with mucilage over all seams to make the envelope as impervious as possible. It is worthy of note that full-size airships have as many as three or four coverings to eliminate loss of hydrogen by escape through the pores of the fabric. If, however, the reader contemplates building a model of the nonrigid type, a wooden hull should be cut and the fabric fitted up to this. The hull should represent the shape of the inflated envelope.
Compressed air also lends itself to model airship propulsion.
In conclusion, a word of warning: do not place the model near any fire, gas, match, etc., as any small leaks may cause an explosion.
In building model airships, it is advisable to remember that by doubling the diameter of the envelope we get four times the capacity and hence four times the lift for only double the weight. This will be seen from the following calculation:
Lift of airship 3 ft. long 2½ in. diameter equals, as we have just seen, ·026 lb.
By doubling the diameter the calculation becomes:
| 22 | 5 | 5 | 36 | 1 | 2240 | ||||||
| — | × | — | × | — | × | — | × | —— | × | ——— | = ·052 lb. |
| 7 | 1 | 1 | 1 | 1728 | 35000 |
From this it will be seen that if it is desired to lift a certain weight with a lighter-than-air craft, the minimum capacity should first be calculated and the diameter (which should always be as large as possible) then varied to obtain the required capacity.
Although it has been stated that the lift of hydrogen is 70 lb. per 1,000 cub. ft., it will be in order to explain two important considerations. It will be clear that, firstly, a full-size balloon must be inflated to a much higher pressure than a model, owing to the heavier mass of material to be forced out to form. As a direct adjunct to this fact it will be seen that percolation will be high. Consequently the lift weight ratio 70: 1,000 is somewhat a low estimate for a model, since, firstly, it will only require to be inflated to less than half the pressure of a full-size balloon; and, secondly, percolation will be considerably less. The writer intentionally gave the full-size limit in order that, should the builder’s model fail to come within the prescribed limit of 2 lb., its flying capability would not be appreciably impaired.
Further, the envelope is kept to its shape by a framework inside, and is consequently full of air. Now if we start pumping hydrogen in it will make the contrivance heavier instead of lighter, because no outlet is provided for the escape of air. This could easily be remedied by standing the model on its end with the cycle valve at the top, with a valve or tap on the bottom brass cap. Open this bottom tap to allow the air to escape, and allow the hydrogen to fizz in through the cycle valve. Hydrogen will not escape through the bottom tap until all the air has been displaced. If the hydrogen cannot be had at a high enough pressure to operate the valve itself a valve should not be used.