PSYCHOLOGY of the public, as well as engineering structure and aerodynamics, is involved in commercial aviation. The public has confidence in metal. With quantity production in view, the author and his associates considered costs of production as related to quantity and also costs of maintenance at airports and in the field, and chose metal as the material of construction. Structural members are fashioned from sheet duralumin rather than from tubes and a type of construction was evolved that can be made with the minimum investment in tools, that is cheap to put together and that can be repaired with the smallest amount of equipment and labor. For compression loads, duralumin has a great deal more strength for a given weight than has steel. It cannot be used, however, for compression members in combination with steel in tension members because of the difference in coefficient of expansion.
USEFUL load-carrying capacity is a measure of the comparative value of two airplanes of the same size, having identical powerplants, speed, rate of climb and other flying characteristics. It seems to be feasible to combine in the same airplane both the greatest ability to carry useful load and the least cost of construction. Blanked and pressed metal work offers substantial advantage to the extent that parts, particularly sub-assemblies, can be made directly by machine in complete units ready to set in the final assembly. The author shows and describes the methods followed by his organization in forming the members, building the frames and assembling the units of metal aircraft. Trusses are blanked and the web members pressed to ¾-circle form. Dies for long members are variable in length by being made in pieces that can be removed or inserted as desired. Flanged-tube sections are employed for truss chords.
MASS production and commercial competition have combined to lend great importance to modifications of motor-vehicle design and so have developed new types of engineers known as tool engineers and production engineers, who take the ideals of automotive engineers and convert them into practicalities, so that the design becomes an ideal manufacturing project that makes it possible to produce a car economically. Special tools are required for many of the machining operations, and for the designing of these the more intelligent and skilled of the workmen are developed into tool-makers for the making of fixtures, jigs, dies, gages, cutters, and punches. Gradually the better tool-makers became tool designers and transferred their work from the bench to the drawing-board, becoming twin brothers of the automotive engineer. Of late, the designing and building of special equipment and allied work have entered into the duties of the tool designer.
NEARLY all steel used in this process of manufacturing frames comes to the plant in the form of strips, which are rolled to remove curvature and inspected automatically for dimensions. All operations and handling are automatic, except pickling, cleaning and oiling the stock and inspecting the assembled frame, until the enameled frame is ready to be shipped. Economical use of the strip steel is dependent upon an offsetting operation that makes the strip conform to the vertical curves desired in the finished frame. With the aid of illustrations, the author follows the fabricating process through the various lines and other units, until a frame is ready for shipment or storage, within less than 2 hr. after it enters the manufacturing line as strip steel.
AN outline is given of the work performed and the method of procedure followed in correlating test results on specimens of heat-treated S.A.E. chromium-vanadium steel 6130 as a basis for revision of the physical-property charts for certain automotive steels. Revision of the charts was proposed by the Iron and Steel Division of the Standards Committee of the Society, and a subcommittee, of which the author is a member, was appointed to carry on the preliminary work of revision. The paper is a report of the results of the tests made. Test specimens of S.A.E. Steel 6130, to be drawn at three different temperatures after quenching, were prepared by four steel manufacturers. These were distributed among 30 cooperating laboratories, which made a series of about 115 tests including complete chemical analysis, tensile-strength, and Brinell, scleroscope and Rockwell hardness tests on the specimens.
MARKED improvement in high-speed high-efficiency engines will be accomplished during the next few years, according to the author. They will have better balance, longer life and greater efficiency, and will develop more power and be more satisfactory to the motoring public. Details of recent developments in this class of engine are given by the author after remarking that the present trend is toward a large number of small changes in design and construction rather than toward radical departures from former design and methods. Mr. Duesenberg comments upon the main features of design of his 91-cu. in. racing-car engine and its parts, and on the troubles that necessitated design changes. The combustion-chamber is stated to be the most important contributor to high efficiency. If the shape of the combustion-chamber, the area of the valves, and the location of the valves and spark-plugs are not right, all the other refinements of detail are of little value.
MOST passenger automobiles are overpowered and probably 80 per cent of such vehicles operate at less than 35 m.p.h. for 90 per cent of the time, according to the author. At 30 m.p.h. an average 3000 to 3500-lb. passenger-car requires from 12 to 15 hp., but the engine carried is capable of developing from 50 to 55 hp. The result is that the car is operated for the greater part of the time at one-third to one-quarter throttle opening. Full power is needed only for accelerating and hill-climbing; during the remainder of the time the excess weight of the engine and other parts must be carried at a loss of efficiency. The author maintains that smaller engines can be used advantageously when equipped with superchargers, the supercharger being used only when excess power is required.
CYLINDER finishing by rough and finish-boring with wide tools, which was thought good enough during the first dozen years of the automobile-production period, was supplanted by reaming and grinding. Later, cast-iron and copper laps were used, but all these methods were slow and did not produce the fine finish for which a demand developed. Experiments were begun about 1920 with the process known as honing. Five years later the company with which the author is connected converted one of its drilling-machines into a single-spindle honing-machine. Other companies made similar conversions. The first honing-head was introduced in 1923. Not until three years ago, however, did honing begin to be regarded as a real production-method possibility. Since then, very rapid progress has been made and numerous improved machines, honing-heads and honing-stones have been produced.
AFTER outlining the history of development of the Packard X engine, the author states the legitimate position in aviation deserved by the water-cooled aviation-engine of this type and predicts large increases in the size, speed and carrying capacity of airplanes within the near future. Passing then to a discussion of the important features of the X-type engine, various illustrations of its parts are commented upon. The cylinders are built-up from steel forgings, with all welds arranged so as to be subjected to no excessive alternating stresses. The novel features of this cylinder design lie in the fact that the valve seats are entirely surrounded by water and that water space is provided above the combustion-chamber and below the top plate of the cylinder. The cylinder-head is extremely rigid, resisting deflection and assuring the maximum integrity of valve seats. The valve ports are machined integrally with the cylinder-head and are not welded thereto as in the Liberty engine.
ECONOMIC factors applying to mass production are dealt with in an endeavor to show how, by following certain laws of manufacturing management based on economic laws, the Ford Motor Co. has attained its very low production costs. Some of these laws, which were put into concrete form as recently as 1926 by L. P. Alford, are quoted, and examples of methods are given to show how they operate.
BY means of the gear-correcting process described, spur and helical gears are corrected to give a high degree of uniformity in spacing and profile so that the gears become practically interchangeable. They acquire a “crown face” which enables them to run with unusual quietness under practical conditions. This is essentially an inspection-correction process, as it automatically finds and eliminates the errors. The lap is the important item in the process. It is of chilled cast-iron, gray cast-iron, or type metal, and is made by casting in a mold around a steel chill cut to approximate the gear to be corrected but has a face-width several times that of the gear. The lap, when completed, looks like a wide-faced internal gear.
GROUND teeth for transmission gears are advocated because they can be made to the same degree of accuracy as the other fine working-parts of a motor-car. The designing engineer is held responsible for conditions unfavorable to the adoption of gear grinding by the production department. Mr. Orcutt believes that cluster gears should be avoided because it is impossible to finish them accurately. Fundamental principles of rigid shafts and correct bearing arrangements are laid down, and the degree of accuracy is specified for the fitting parts. Transmission-case design still needs development and study to avoid resonance. Designs are recommended that will provide ample center distance to avoid pinions with a small number of teeth. The unmodified involute is recommended as the most satisfactory form of tooth. Spigot bearings receive special consideration. Two designs of transmission are submitted, in one of which the spigot bearing is eliminated.
PROVISION is made, in the piston and rings described by the author, for an adequate flow of heat from all parts of the piston-head to the cylinder-wall by means of adequate cross-section of aluminum alloy in the head and a tongue-and-groove type of piston-ring structure which provides a greater amount of surface than is usual for heat transfer. A labyrinth oil-seal is provided which aids heat transference and prevents leakage past the piston-rings, and the heat transfer is said to be such that the heat does not destroy the oil seal between the piston and the ring. Charts are included that show the effects in reduced temperatures, oil consumption and gas leakage with the construction described. Attention is given also to a skirt construction most suitable to use with the piston-head and rings described.
NEARLY all the aircraft propellers used by both the Army and the Navy are of the detachable-blade type. The Navy has found it necessary to make its own designs and to furnish the propeller manufacturers with finished detail drawings. The author lists the sources from which data can be obtained and shows a chart from which can be found a diameter and setting of a pair of detachable blades that will give reasonably good performance for nearly any horsepower, revolutions per minute and airspeed commonly used with the direct-drive type of propeller. Discrepancies between model tests and wind-tunnel tests are cited, and the author then considers the subject theoretically. Substitute propellers are next considered, and also the strength of propellers.
FOLLOWING a brief outline of the development of aircraft propellers and a statement of the most important fundamentals of propeller design, the authors discuss the problem of propellers for use on geared-down engines, this being the installation of reduction gearing between the crankshaft of the engine and the propeller hub when the increase of airplane-performance characteristics more than offsets the added complication of the installation. The advantages and the disadvantages of using reduction gearing are considered. Concerning the installation of reduction gears, the authors state that the decision whether to use gears or not must result from a compromise between the gains and the losses involved and the amount of net gain depends largely upon the particular engine and airplane combination and its designed performance.
A NUMBER of the more important commercial alloys having aluminum as their base are discussed by the author, who points out their main physical characteristics and outlines methods which can be used in their fabrication, indicating in a general way which alloys are best suited to various aircraft-engine requirements. Tables are given showing chemical compositions and physical properties, including a table of physical properties of various casting alloys at elevated temperatures. Special-purpose alloys are commented upon, and also a new aluminum alloy for pistons which is beginning to find commercial application and possesses properties particularly desirable in aircraft engines. Recent developments in magnesium alloys and their application to aircraft-engine design are specified, tables of physical properties are given, and comments are made on the characteristics of the material as compared with aluminum alloys.
IF the costs of almost any group of manufacturers who market the same product are analyzed, two kinds of differences will be detected, according to the author. The real differences in costs arise from superior management, higher productivity, and better disposition and utilization of capital. The accidental differences result from the failure of manufacturers to include in cost records all of the proper legitimate items of expense. Confining his treatment of the subject to an analysis of the depreciation of plant and equipment, the author states that depreciation is a decline in the value which is certain to occur as a result of wear and tear and gradual obsolescence. It is caused by the possession and use of an asset, and is therefore a part of the cost of production. The accountant attempts to recover depreciation loss in the value of the capital assets by charging it into the cost of production.
PRODUCTION of parts in lots, rather than continuously, may sound like a throwback in the automotive industry, but analysis shows that forgings, stampings, body parts and hardware, replacement parts and other parts are made in lots even in large-production manufacturing organizations. Formulas presented by Professor Raymond determine the size of lot that can be manufactured most economically, and show when the change should be made to continuous production. Consideration is given even to such factors as cost of the space for finished stores and return on the investment in finished parts. The lots indicated are not absolute quantities but are designated in the form of economic ranges that are practicable until there is a marked change in sales or other conditions. The formulas can also be applied to help determine the type of handling equipment that will be most economical to use.
CONVEYORS and handling systems often are planned and installed after a building is erected. The Pontiac plant, described in this paper, is an exception because it was designed without limitations as to space and for a definite production program. With the aid of photographs and floor plans on which the positions from which the photographs were taken are indicated, the complete production line of the plant is shown in detail. The order of assembly and the points at which various units are applied to the chassis are shown; also the locations of the storage spaces for many of the parts and the provisions for transporting them to the assembly line. Among the striking features of the chassis-assembly line is a hump, midway of the length of the building, which raises the chassis to the mezzanine level to allow passage underneath.