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CHAPTER SEVEN POWER AND PROPULSION
The question of power for the propulsion of aeroplanes is one at the present time of the utmost importance. So far, aeroplane progress has been made possible largely through the development of light-weight gasoline motors involved in the growth of the automobile industry. And undoubtedly a most serious obstacle in the way of immediate further progress is the lack of motors still lighter, more efficient, and more reliable than the best of those available. Most aeroplane flights so far made for duration records, for example, have been terminated by motor failure, though close behind this limitation always has been that of fuel radius, which is largely dependent upon the matters of weight and efficiency.
It nevertheless is obvious that many of the best flying machines of the present time might have been flown with much heavier motors than are now used in them—with motors such as have been available for even twenty or thirty years—and there is every reasonable prospect that as progress continues still less power will be required, until ultimately flight may be accomplished by man with even as little power per unit of weight as is demanded for the flight of birds, thus in a manner making the motor problem solve itself—by so reducing the demand for power that almost any sort of a motor will serve the purpose. But it has seemed necessary first to apply the light weight motor as a means of working out the general details of the necessary aerodynamic mechanism, the discovery that heavier and less powerful motors could conceivably have been used at a much earlier period being knowledge secured too late to have been utilized to the utmost advantage in flight development.
AMOUNT OF POWER REQUIRED
It is constantly becoming more evident that the amount of power required to fly an aeroplane can be more and more reduced by improving the efficiency of the aeroplane itself. Thus, in the use of stream-line bodies and other elements, in improvement of the aerodynamic efficiency of wings, and in reduction to a minimum of skin friction, lie the solutions of flight with a minimum of power.
What may be considered the normal type of flight is flight along a level course—the machine neither rising nor falling— and this type of flight is the proper measure of the normal energy requirement.
As has been heretofore explained, such are the efficiencies of aerodynamic reactions that the power required for level flight tends to become extremely low as aeroplanes make progress towards correct designs, just as it is extremely low in the case of nature's mechanism, the bird.
CLIMBING In rising from a lower to a higher level, much more power is required than for level flight, the amount of this maximum energy requirement being, in fact, determined by the climbing ability demanded. This brings to attention a somewhat curious condition, which is that increasing the power and the thrust by which an aeroplane is propelled can not and does not materially increase its speed, but simply forces it to progress on an upward slant, gaining in altitude rather than in rate of travel.
Just as climbing with an aeroplane takes place without material variation in speed, but demands an increase in power, so, conversely, gliding does not ordinarily increase speed, but simply diminishes the demand for power. Thus, if the power is cut off when an aeroplane is in the air, if it is properly designed and properly steered it cannot fall but will commence at once to coast on a slant of air at its normal speed, just as completely and safely under control as in any other condition.
Soaring Flight is a peculiar and as yet unexplained type of gliding flight, concerning both the performance and the mystery of which there is much dispute. Authorities there are, whose practical knowledge is as indisputable as are their theoretical attainments, who stoutly contend that it is going to be possible ultimately to achieve without power something akin to the indefinitely continued soaring flight that is so indubitably established to exist in the case of the larger flying birds. A simpler explanation, but one that unfortunately does not explain all cases if many dissenting authorities are to be believed, is that the soaring bird derives sustention from upward currents of air, caused by wind friction over surface contours or by ascending streams of heated air, permitting the bird, so to speak, to coast indefinitely down an invisible hill that itself rises faster than the bird coasts down.
A question not of less importance than that of the amount of power required to propel an aeroplane is that of the source from which it is to be obtained.
The gasoline engine in certain highly specialized forms being the lightest prime mover known, and it having been developed to high degrees of reliability as an element of motor boats and automobile mechanism, it is the only motor that at present meets any considerable favor or that may be considered to offer material promise for future application to aerial vehicles. Aeroplane engines using gasoline as fuel have been built weighing as little as a pound and a half to the horsepower, and are made of considerable reliability in weights of from two and a half to seven pounds to the horsepower—the latter figure permitting thoroughly adequate water cooling and including the weight of all necessary adjuncts, such as ignition and carbureter equipment, flywheel, radiator, and the like.
Examples and Data concerning various established aeroplane power plants are much more informing than any merely general consideration of the subject.
By far the most successful as well as one of the lightest aeroplane motors at present in use is the Gnome—a revolvingcylinder type, all of the parts of which are machined from bars of high-grade alloy steel. This motor in its different models weighs from two and two-tenths pounds to four pounds to the horsepower, and is capable of running for at least as many as ten hours without stopping, constantly delivering its maximum power output.
Despite the fact that when first placed upon the market it was regarded as an exceedingly freakish variation from accepted automobile practice, the Gnome motor has vindicated the originality of its designers by securing for aeroplanes in which it has been installed practically every aeroplane record of importance within two years from its introduction. It possesses, however, serious disadvantages in combination with its important advantages. Most valuable of the advantages are the very considerable flywheel effect due to the large revolving mass, the immunity from vibration due to the fact that none of the parts really reciprocate except relatively to one another, and extreme lightness and effective air cooling, due to the fact that the whole construction especially lends itself to these desiderata without sacrificing strength or reliability.
The most objectionable features of the Gnome motor are the gyroscopic effect due to the revolving mass, the amount of power required to revolve it against the air resistances, the great space it occupies, and the excessive oil and fuel consumption.
Contrary to common opinion, the cooling of the Gnome motor is not so much due to the revolving of the cylinders through the air as it is due to a wasteful internal scavenging effect of the incoming charges through the valves in the pistons, whereby the usual tendency to overheat in these elements is largely counteracted. Another condition that contributes to the effective cooling is the very low compression employed, disadvantageous otherwise, however, in that it manifests itself in a power output not over fifty percent of that commonly obtained from the same cylinder capacity in ordinary automobile engines.
In accounting for the cooling efficiency of this revolving motor, it is usual to consider the path of the cylinder a circular path, which is the condition only when the machine is not progressing through the air. But with the normal forward movement in flight of a definite number of feet to each revolution of the motor and propeller, the path of the cylinders through the air becomes a helical path—around a helix so elongated as to add but a small percentage of the distance the cylinders would travel did they not revolve.
Especially clever in the design of the Gnome motor is the quite original use of a "master connecting rod," to the big end of which those of the other pistons are linked in such a manner that, while their angularity is slightly increased, there is afforded the very great advantage that the big ends of all the rods except the master rod become rocking rather than rotating bearings, and permit that of the master rod to be provided with a large ball bearing of a type that reduces friction and minimizes wear.
The apparently paradoxical fact that the pistons and connecting rods of the Gnome motor do not reciprocate but do really revolve will be understood when it is considered that the crankpin is stationary, so that the pistons, linked to it by the connecting rod, are compelled to revolve in a circle around it. This circle is eccentric to that around the main bearing of the crankshaft, about which the cylinders revolve, wherefore there is a relative reciprocation between the cylinders and pistons, due to their traveling two eccentric circles, but no actual reciprocation with reference to anything external to the motor.
With the exception of the revolving motors, there are no conspicuous examples of successful air cooling in aeroplanemotor practice, with the possible exception of the Renault motors, which are air cooled by powerful blasts of air forced over the cylinders by a centrifugal blower.
Typical of a more conservative school of aeroplane-engine design are such motors as the Wright, the Panhard, the Curtiss, the Antoinette, and other fairly close approximations to what is standard in automobile engineering. In these water-cooled motors a high degree of reliability and durability is sought rather than extreme light weight, but it is only as aeroplane efficiencies become higher and the consequent demand for power lessens that motors as heavy for their power as these really can come into their greatest fields of usefulness.
As in automobile engineering, a common type of watercooled aeroplane motor is that with four vertical cylinders, but preferred to this where greater power is required is the "V-shaped," eight-cylinder construction, involving two rows of four cylinders each, placed at ninety degrees to each other, thus permitting the use of an ordinary four-cylinder crankshaft for the eight cylinders, and allowing especially neat, compact, and fairly light weight construction.
Another engine type of much promise, and of some present application, is the horizontal-opposed motor with two cyl