THE COMPARATIVE VALUE OF DIFFERENT MOTORS.
So far I have only discussed the navigation of the air by the use of propellers driven by a steam engine. The engines that I employ are what is known as compound engines—that is, they have a large and a small cylinder. Steam at a very high pressure enters the high-pressure cylinder, expands and escapes at a lower pressure into a larger cylinder where it again expands and does more work. A compound engine is more economical in steam than a simple engine, and therefore requires a smaller boiler to develop the same horse-power, so that when we consider the weight of water and fuel for a given time, together with the weight of the boiler and the engine, the engine motor with a compound engine is lighter than a simple engine. However, if only the weight of the engine is to be considered then the simple engine will develop more power per unit of weight than the compound engine. For instance, if, instead of allowing the steam to enter the small cylinder, and the exhaust from this cylinder to enter the large or low-pressure cylinder—which necessitates that the high-pressure piston has to work against a back pressure equal to the full pressure on the low-pressure cylinder—I should connect both cylinders direct with the live steam, and allow both to discharge their exhaust directly into the air, I should then have a pair of simple engines, and instead of developing 363 H.P. they would develop fully 500 H.P., or nearly 1 H.P. for every pound of their weight. I mention this fact to show that the engines are exceedingly light, and that when compared with simple engines their power should be computed on the same basis. It will, therefore, be seen that if we do not take into consideration the steam supply or the amount of fuel and water necessary, the simple steam engine is an exceedingly light motor.
But, as before stated, great improvements have recently been made in oil engines. I have thought much on this subject, and am of the opinion that if one had an unlimited supply of money, a series of experiments could be very profitably conducted with a view of adapting the oil engine for use on flying machines. If we use a steam engine, it is necessary to have a boiler, and at best a boiler is rather a large and heavy object to drive through the air. If we use an oil engine, no boiler is necessary, and the amount of heat carried over in the cooling water will only be one-seventh part of what is carried over in the exhaust from a steam engine of the same power. Therefore, the condenser only need be one-seventh part the size, and consequently should be made lighter with the tubes placed at a greater distance apart, and thus reduce the amount of power necessary to drive the machine through the air. Moreover, the supply of water necessary will be greatly reduced, and a cheaper and heavier oil may be employed, which is not so liable to take fire in case of an accident. It is then only a question as to whether an oil engine can be made so light as to keep its weight within that of a steam motor; that is, an oil engine in order to be available for the purpose must be as light, including its water supply, as a complete steam motor, which includes not only the engine, but also the boiler, the feed pumps, the water supply, the burner, the gas generator, and six-sevenths of the condenser. It requires a very perfect steam engine and boiler, not using a vacuum, to develop a horse-power with a consumption of 11⁄2 lbs. of petroleum per hour; but there are many oil engines which develop a horse-power with rather less than 1 lb. of oil per hour. It will, therefore, be seen that, as far as fuel is concerned, the oil engine has a decided advantage over the more complicated steam motor. Moreover, with an oil engine, the cooling water is not under pressure, so that the waste of water would be much less than with a steam engine, where the pressure is so high as to cause a considerable amount of waste through joints and numerous stuffing-boxes.
The great advances that have been made of late years in electrical science and engineering have led many to believe that almost any knotty scientific question may be solved by the employment of electrical engineering, and a great deal has been written and said in regard to navigating the air by flying machines driven by electric motors.
Before I commenced my experiments, I made enquiries of all the prominent electrical engineering establishments where there was any likelihood of obtaining light and efficient electric motors, and found that it was impossible to obtain one that would develop a horse-power for any considerable time that would weigh less than 150 lbs. Since that time, notwithstanding that a great deal has appeared in the public prints about the efficiency and lightness of electric motors, I am unable to learn of any concern that is ready to furnish a complete motor, including a primary battery, which would supply the necessary current for two hours at a time, at a weight of less than 150 lbs. per horse-power, and as far as I have been able to ascertain from what I have myself seen, I cannot learn that there are any motors in practical use which do not weigh, including their storage batteries, at least 300 lbs. per horse-power. The last electric motor which I examined was in a boat; it was driven by a primary battery which weighed over 1,000 lbs. to the horse-power. From this I am of the opinion that we cannot at present look to electricity with any hope of finding a motor which is suitable for the purpose of aerial navigation.