Aviation Engines.
by Victor Wilfred Page.
PREFACE
In presenting this treatise on "Aviation Engines," the writer realizes that the rapidly developing art makes it difficult to outline all latest forms or describe all current engineering practice. This exposition has been prepared primarily for instruction purposes and is adapted for men in the Aviation Section, Signal Corps, and students who wish to become aviators or aviation mechanicians. Every effort has been made to have the engineering information accurate, but owing to the diversity of authorities consulted and use of data translated from foreign language periodicals, it is expected that some slight errors will be present. The writer wishes to acknowledge his indebtedness to such firms as the Curtiss Aeroplane and Motor Co., Hall-Scott Company, Thomas-Morse Aircraft Corporation and General Vehicle Company for photographs and helpful descriptive matter. Special attention has been paid to instructions on tool equipment, use of tools, trouble "shooting" and engine repairs, as it is on these points that the average aviation student is weakest. Only such theoretical consideration of thermo-dynamics as was deemed absolutely necessary to secure a proper understanding of engine action after consulting several instructors is included, the writer"s efforts having been confined to the preparation of a practical series of instructions that would be of the greatest value to those who need a diversified knowledge of internal-combustion engine operation and repair, and who must acquire it quickly. The engines described and ill.u.s.trated are all practical forms that have been fitted to airplanes capable of making flights and may be considered fairly representative of the present state of the art.
VICTOR W. PAGe,
_1st Lieut. A. S. S. C., U. S. R_.
MINEOLA, L. I.,
October, 1917.
AVIATION ENGINES
DESIGN--CONSTRUCTION--REPAIR
CHAPTER I
Brief Consideration of Aircraft Types--Essential Requirements of Aerial Motors--Aviation Engines Must Be Light--Factors Influencing Power Needed--Why Explosive Motors Are Best-- Historical--Main Types of Internal Combustion Engines.
BRIEF CONSIDERATION OF AIRCRAFT TYPES
The conquest of the air is one of the most stupendous achievements of the ages. Human flight opens the sky to man as a new road, and because it is a road free of all obstructions and leads everywhere, affording the shortest distance to any place, it offers to man the prospect of unlimited freedom. The aircraft promises to span continents like railroads, to bridge seas like ships, to go over mountains and forests like birds, and to quicken and simplify the problems of transportation.
While the actual conquest of the air is an accomplishment just being realized in our days, the idea and yearning to conquer the air are old, possibly as old as intellect itself. The myths of different races tell of winged G.o.ds and flying men, and show that for ages to fly was the highest conception of the sublime. No other agent is more responsible for sustained flight than the internal combustion motor, and it was only when this form of prime mover had been fully developed that it was possible for man to leave the ground and alight at will, not depending upon the caprices of the winds or lifting power of gases as with the balloon. It is safe to say that the solution of the problem of flight would have been attained many years ago if the proper source of power had been available as all the essential elements of the modern aeroplane and dirigible balloon, other than the power plant, were known to early philosophers and scientists.
Aeronautics is divided into two fundamentally different branches--aviatics and aerostatics. The first comprises all types of aeroplanes and heavier than air flying machines such as the helicopters, kites, etc.; the second includes dirigible balloons, pa.s.sive balloons and all craft which rise in the air by utilizing the lifting force of gases. Aeroplanes are the only practical form of heavier-than-air machines, as the helicopters (machines intended to be lifted directly into the air by propellers, without the sustaining effect of planes), and ornithopters, or flapping wing types, have not been thoroughly developed, and in fact, there are so many serious mechanical problems to be solved before either of these types of air craft will function properly that experts express grave doubts regarding the practicability of either. Aeroplanes are divided into two main types--monoplanes or single surface forms, and bi-planes or machines having two sets of lifting surfaces, one suspended over the other. A third type, the triplane, is not very widely used.
Dirigible balloons are divided into three cla.s.ses: the rigid, the semi-rigid, and the non-rigid. The rigid has a frame or skeleton of either wood or metal inside of the bag, to stiffen it; the semi-rigid is reinforced by a wire net and metal attachments; while the non-rigid is just a bag filled with gas. The aeroplane, more than the dirigible and balloon, stands as the emblem of the conquest of the air. Two reasons for this are that power flight is a real conquest of the air, a real victory over the battling elements; secondly, because the aeroplane, or any flying machine that may follow, brings air travel within the reach of everybody. In practical development, the dirigible may be the steamship of the air, which will render invaluable services of a certain kind, and the aeroplane will be the automobile of the air, to be used by the mult.i.tude, perhaps for as many purposes as the automobile is now being used.
ESSENTIAL REQUIREMENTS OF AERIAL MOTORS
One of the marked features of aircraft development has been the effect it has had upon the refinement and perfection of the internal combustion motor. Without question gasoline-motors intended for aircraft are the nearest to perfection of any other type yet evolved. Because of the peculiar demands imposed upon the aeronautical motor it must possess all the features of reliability, economy and efficiency now present with automobile or marine engines and then must have distinctive points of its own. Owing to the unstable nature of the medium through which it is operated and the fact that heavier-than-air machines can maintain flight only as long as the power plant is functioning properly, an airship motor must be more reliable than any used on either land or water. While a few pounds of metal more or less makes practically no difference in a marine motor and has very little effect upon the speed or hill-climbing ability of an automobile, an airship motor must be as light as it is possible to make it because every pound counts, whether the motor is to be fitted into an aeroplane or in a dirigible balloon.
Airship motors, as a rule, must operate constantly at high speeds in order to obtain a maximum power delivery with a minimum piston displacement. In automobiles, or motor boats, motors are not required to run constantly at their maximum speed. Most aircraft motors must function for extended periods at speed as nearly the maximum as possible. Another thing that militates against the aircraft motor is the more or less unsteady foundation to which it is attached. The necessarily light framework of the aeroplane makes it hard for a motor to perform at maximum efficiency on account of the vibration of its foundation while the craft is in flight. Marine and motor car engines, while not placed on foundations as firm as those provided for stationary power plants, are installed on bases of much more stability than the light structure of an aeroplane. The aircraft motor, therefore, must be balanced to a nicety and must run steadily under the most unfavorable conditions.
AERIAL MOTORS MUST BE LIGHT
The capacity of light motors designed for aerial work per unit of ma.s.s is surprising to those not fully conversant with the possibilities that a thorough knowledge of proportions of parts and the use of special metals developed by the automobile industry make possible. Activity in the development of light motors has been more p.r.o.nounced in France than in any other country. Some of these motors have been complicated types made light by the skillful proportioning of parts, others are of the refined simpler form modified from current automobile practice. There is a tendency to depart from the freakish or unconventional construction and to adhere more closely to standard forms because it is necessary to have the parts of such size that every quality making for reliability, efficiency and endurance are incorporated in the design. Aeroplane motors range from two cylinders to forms having fourteen and sixteen cylinders and the arrangement of these members varies from the conventional vertical tandem and opposed placing to the V form or the more unusual radial motors having either fixed or rotary cylinders. The weight has been reduced so it is possible to obtain a complete power plant of the revolving cylinder air-cooled type that will not weigh more than three pounds per actual horse-power and in some cases less than this.
If we give brief consideration to the requirements of the aviator it will be evident that one of the most important is securing maximum power with minimum ma.s.s, and it is desirable to conserve all of the good qualities existing in standard automobile motors. These are certainty of operation, good mechanical balance and uniform delivery of power--fundamental conditions which must be attained before a power plant can be considered practical. There are in addition, secondary considerations, none the less desirable, if not absolutely essential.
These are minimum consumption of fuel and lubricating oil, which is really a factor of import, for upon the economy depends the capacity and flying radius. As the amount of liquid fuel must be limited the most suitable motor will be that which is powerful and at the same time economical. Another important feature is to secure accessibility of components in order to make easy repair or adjustment of parts possible.
It is possible to obtain sufficiently light-weight motors without radical departure from established practice. Water-cooled power plants have been designed that will weigh but four or five pounds per horse-power and in these forms we have a practical power plant capable of extended operation.
FACTORS INFLUENCING POWER NEEDED
Work is performed whenever an object is moved against a resistance, and the amount of work performed depends not only on the amount of resistance overcome but also upon the amount of time utilized in accomplishing a given task. Work is measured in horse-power for convenience. It will take one horse-power to move 33,000 pounds one foot in one minute or 550 pounds one foot in one second. The same work would be done if 330 pounds were moved 100 feet in one minute. It requires a definite amount of power to move a vehicle over the ground at a certain speed, so it must take power to overcome resistance of an airplane in the air. Disregarding the factor of air density, it will take more power as the speed increases if the weight or resistance remains constant, or more power if the speed remains constant and the resistance increases.
The airplane is supported by air reaction under the planes or lifting surfaces and the value of this reaction depends upon the shape of the aerofoil, the amount it is tilted and the speed at which it is drawn through the air. The angle of incidence or degree of wing tilt regulates the power required to a certain degree as this affects the speed of horizontal flight as well as the resistance. Resistance may be of two kinds, one that is necessary and the other that it is desirable to reduce to the lowest point possible. There is the wing resistance and the sum of the resistances of the rest of the machine such as fuselage, struts, wires, landing gear, etc. If we a.s.sume that a certain airplane offered a total resistance of 300 pounds and we wished to drive it through the air at a speed of sixty miles per hour, we can find the horse-power needed by a very simple computation as follows:
The product of 300 pounds resistance times speed of 88 feet per second times 60 seconds in a minute ----------------------------------------------------- = H.P. needed.
divided by 33,000 foot pounds per minute in one horse-power
The result is the horse-power needed, or
300 88 60 --------------- = 48 H.P.
33,000
Just as it takes more power to climb a hill than it does to run a car on the level, it takes more power to climb in the air with an airplane than it does to fly on the level. The more rapid the climb, the more power it will take. If the resistance remains 300 pounds and it is necessary to drive the plane at 90 miles per hour, we merely subst.i.tute proper values in the above formula and we have
300 pounds times 132 feet per second times 60 seconds in a minute ----------------------------------------------- = 72 H.P.
33,000 foot pounds per minute in one horse-power
The same results can be obtained by dividing the product of the resistance in pounds times speed in feet per second by 550, which is the foot-pounds of work done in one second to equal one horse-power.
Naturally, the amount of propeller thrust measured in pounds necessary to drive an airplane must be greater than the resistance by a substantial margin if the plane is to fly and climb as well. The following formulae were given in "The Aeroplane" of London and can be used to advantage by those desiring to make computations to ascertain power requirements:
[Ill.u.s.tration: Fig. 1.--Diagrams Ill.u.s.trating Computations for Horse-Power Required for Airplane Flight.]
The thrust of the propeller depends on the power of the motor, and on the diameter and pitch of the propeller. If the required thrust to a certain machine is known, the calculation for the horse-power of the motor should be an easy matter.
The required thrust is the sum of three different "resistances." The first is the "drift" (dynamical head resistance of the aerofoils), i.e., tan [alpha] lift (_L_), lift being equal to the total weight of machine (_W_) for horizontal flight and [alpha] equal to the angle of incidence. Certainly we must take the tan [alpha] at the maximum _K_{y}_ value for minimum speed, as then the drift is the greatest (Fig. 1, A).
Another method for finding the drift is _D_ = _K_ _AV_^{2}, when we take the drift again so as to be greatest.
The second "resistance" is the total head resistance of the machine, at its maximum velocity. And the third is the thrust for climbing. The horse-power for climbing can be found out in two different ways. I first propose to deal with the method, where we find out the actual horse-power wanted for a certain climbing speed to our machine, where
climbing speed/sec. _W_ H.P. = --------------------------- 550
In this case we know already the horse-power for climbing, and we can proceed with our calculation.
With the other method we shall find out the "thrust" in pounds or kilograms wanted for climbing and add it to drift and total head resistance, and we shall have the total "thrust" of our machine and we shall denote it with _T_, while thrust for climbing shall be _T_{c}_.
The following calculation is at our service to find out
_V_{c}_ _W_ this thrust for climbing --------------- = H.P., 550