With aeroplanes of one-half the width of those I employed, and with a velocity twice as great, the angle could be much less, and the advantages of continually running on to fresh air would be still more manifest. With a screw thrust of 2,000 lbs., the air pressure on each square foot of the projected area of the screw blades is 21·3 lbs., while the pressure on the entire discs of the screws is 4 lbs. per square foot, which would seem to show with screws of this size, that four blades would be more efficient than two.
Fig. 85.—One pair of my compound engines. This engine weighed 310 lbs. and developed 180 H.P., with 320 lbs. of steam per square inch.
The engines, as before stated, are compound ([Fig. 85]). The area of the high-pressure piston is 20 square inches, and that of the low-pressure piston is 50·26 square inches. Both have a stroke of 12 inches. With a boiler pressure of 320 lbs., the pressure on the low-pressure piston is 125 lbs. to the square inch. This abnormally high pressure in the low-pressure cylinder is due to the fact that there is a very large amount of clearance in the high-pressure cylinder to prevent shock in case water should go over when the machine pitches; moreover, the steam in the high-pressure cylinder is cut off at three-quarters stroke, while the steam in the low-pressure cylinder is cut off at five-eighths stroke. If we should compute the power of these engines with the steam entering at full stroke, without any friction, and with no back pressure on the low-pressure cylinder, the total horse-power would foot up to 461·36 horse-power at the speed at which the engines were run—namely, 375 turns per minute. If we compute the actual power consumed by the screws, by multiplying their thrust, which is probably 2,000 lbs. while they are travelling, by their pitch, 16 feet, and this by the number of turns which they make in a minute, and then divide the product by 33,000,
2,000 × 16 × 37533,000 = 363·63,
we find that we have 363·63 horse-power in actual effect delivered on the screws of the machine, which shows that there is rather less than 22 per cent. loss in the engines, due to cutting off before the end of the stroke, to back pressure, and to friction. The actual power applied to the machine being 363·63 horse-power, it is interesting to know what becomes of it. When the machine has advanced 40 miles (which it would do in an hour), the screws have travelled 68·1 miles (375 × 16 × 605,280) = 68·1; therefore, 150 horse-power is wasted in slip, and 213·63 horse-power consumed in driving the machine through the air. Now, as the planes are set at an angle of 1 in 8, the power actually used in lifting the machine is 133·33, and the loss in driving the body of the machine, its framework and wires through the air is 90·30 horse-power.
| Power | lost in | screw slip, | 150 | H.P. | |
| „ | „ | driving machinery and framework, | 80 | ·30 | „ |
| „ | actually consumed in lifting the machine, | 133 | ·33 | „ | |
| Total power delivered by the engines, | 363 | ·63 | „ | ||
THE ADVANTAGES AND DISADVANTAGES OF VERY NARROW PLANES.
Fig. 86.—The path that the air has to take in passing between superposed aeroplanes in close proximity to each other. By this arrangement the drift is considerably increased.