CHAPTER X

CONSTRUCTION AND TESTS OF THE LARGE ENGINE

The main requirement in an engine for an aerodrome--aside from reliability and smoothness of operation, which are necessary in an engine for any kind of locomotion—is that it shall develop the greatest amount of power for the least weight. It is, therefore, desirable to reduce the weight and number of parts of the engine to the very minimum, so far as this can be done without sacrificing reliability and smoothness of running. Furthermore, since the strongest metal for its weight is steel, and since the greatest strength of steel is utilized when the stress acting on it is one of tension, it is advisable to design the engine so that the parts which sustain the greatest strains shall be of steel and, as far as possible, meet with strains which are purely tensional ones.

In designing the new engine for the large aerodrome it was, therefore, planned to make it entirely of steel, as far as this was possible. The only parts which were not of steel were the bronze bushings for the bearings, the cast-iron pistons, and cast-iron liners of the cylinders. Previous experience had shown that, while it is possible to use a cast-iron piston in a steel cylinder or even a steel piston in a steel cylinder, provided the lubrication be kept exactly adjusted, yet the proper lubrication of the piston and cylinder of a gas engine is difficult even under the most favorable conditions, owing to the fact that excessive lubrication causes trouble from the surplus oil interfering with the sparking apparatus. It was, therefore, determined not to risk serious trouble by attempting to have the pistons bear directly on the steel walls of the cylinders.

While visiting the French engine builders in the summer of 1900 in the attempt to find one willing to undertake the construction of a suitable engine for the aerodrome, it was pointed out to them that the great amount of weight which they claimed to be necessary for the cylinders, and which they stated made it impossible for them to build an engine which would meet the requirements as to power and weight, could be very greatly reduced by making the cylinders in the form of thin steel shells having cast-iron linings. All, however, to whom this suggestion was made declared that it was impossible to build satisfactory cylinders in this way; some of them even stated that they had tried it and found it impossible to keep the thin liners tight in the steel shells. The difficulty which they had encountered is due to the difference in expansion of the steel and the iron when raised to a rather high temperature by the heat of the explosions, if the cylinders are not well jacketed with water; and if the steel [p235] shells are water jacketed they then do not expand as much as the cast-iron liners, and this causes the latter to become “out of round” because of the compression strains produced in them when trying to expand more than the steel shells. As past experience had shown, however, that it was possible to keep the liners tight in small cylinders, it was believed that by taking proper care in the construction there would be no difficulty in this respect with the cylinders of this larger engine.

In carrying out these plans, however, of making the cylinders of steel, numerous constructional difficulties were encountered which could not be foreseen when the design was made. Had they been foreseen, provision for obviating them could easily have been made. As will be seen from the drawing, Plate 78, the engine cylinders consisted primarily of a main outer shell of steel one-sixteenth of an inch thick, near the bottom end of which was screwed and brazed a suitable flange, by which it was bolted to the supporting drum or crank chamber. These shells, which were seamless, with the heads formed integral, were designed to be of sufficient strength to withstand the force of the explosion in them, and, in order to provide a suitable wearing surface for the piston, a cast-iron liner one-sixteenth of an inch thick was carefully shrunk into them. Entering the side of the cylinder near the top, was the combustion chamber, machined out of a solid steel forging, which also formed the port which entered the cylinder and was fastened to it by brazing. The water jackets, which were formed of sheet steel .020 inch thick, were also fastened to the cylinder by brazing, and it was in connection with the brazing of these water jackets that the first serious difficulty was met in the construction of the engine. In the first place, as the jackets were of an irregular shape and of a different thickness of metal from the walls of the cylinder to which they were joined, the expansion and contraction due to the extreme heat necessary for properly brazing the joints caused such serious strains in various and unexpected directions that it was only by exercising the very greatest care and patience that a completely tight joint at all points of the jacket could be secured. In the second place, the size of the cylinders and the consequently large extent of water-jacket surface, complicated the problem. The maintenance over this large surface of the extreme heat necessary for brazing involved discomfort and, indeed, actual suffering to the person engaged in the work, and much care and skill were demanded in so distributing the heat that the temperature of the surface of the jackets would be uniform enough to prevent serious strains from expansion and contraction. As no workman could be found either competent to do the work or willing to undergo the personal discomfort, the writer was obliged to do all this brazing work himself. Besides the difficulties due to the expansion and contraction of the jackets while they were being brazed, the greatest care had to be exercised to avoid heating the cylinders so hot as to weaken the [p236] joint where the explosion chambers were joined to the cylinders, which, of course, had been brazed before the jackets were fitted to them preparatory to brazing them.

Another great difficulty was that the ring which encircled the cylinder near the middle of its length, and which formed the bottom part of the water jacket, expanded very much more than the cylinder itself, so that, if it was brazed to the cylinder before the jacket was brazed to it, the heat of brazing the jacket to the ring would cause the ring to break loose from the cylinder; while if the ring was not previously brazed to the cylinder, but was brazed after the jacket had been brazed to it, the very much greater heat required for brazing the ring to the cylinder caused the spelter to burn out of the joint between the jacket and the ring. Furthermore, it was found very difficult to braze the two joints at the same time, since in brazing the ring to the cylinder it was best to have the cylinder in an inverted vertical position, so that the spelter could be made to flow evenly around the ring and form a fillet against the wall of the cylinder, while in brazing the jackets to the ring it was best to have the cylinder in the reverse vertical position or lying on its side so that the spelter could properly flow into this joint. Finally, however, after what proved to be most exasperating and tedious work, the five cylinders necessary for the engine were completed and a series of tests was immediately made. During the course of these tests the water circulation became obstructed in several instances, and the consequent high temperature to which the cylinders and jackets were raised caused severe strains in the jackets which, in turn, produced breaks in the brazed joints. These breaks had to be rebrazed, and in brazing them it was necessary in almost every case to remove the cast-iron liners and rebraze the entire jackets from start to finish, as the application of the intense heat necessary for brazing at any one point produced such severe strains that before the break which was being repaired could be completed other breaks developed at various points of the jacket. It was, therefore, necessary to get the whole jacket up to a fairly uniform heat and complete the brazing while it was in this condition, and then keep the whole cylinder at a uniform but gradually decreasing temperature until it had sufficiently cooled off.

On account of these troubles with the water jackets and the cylinders, it was decided to build some extra cylinders, not only because past experience had suggested improvements in detail in the construction of the jackets, which would prevent to a large extent the great troubles which had been met with in the brazed joints, but also to insure having sufficient cylinders to enable the engine to be always in working condition, even though several of the cylinders might be out of commission from slight imperfections in the jackets or at other points. While the construction of these new cylinders involved a repetition of the arduous task of brazing, yet the minor improvements which were introduced [p237] proved eminently successful in providing against future troubles from leaky jackets.

The general form of construction of the engine with the improved cylinders will be readily understood from the drawings, Plates 78–81, in which Plate 78 is a detail sectional view, previously referred to, through one of the cylinders; Plate 79 is an end elevation of the port side, Plate 80 is a plan view, and Plate 81 is an elevation of the starboard bed plate which supports that side of the engine, and by which it was fastened to the aerodrome frame, this view showing particularly the sparking apparatus which was mounted on the bed plate. The engine consists primarily of a single crank shaft provided with a single crank pin, the shaft having bearings in a drum which consists essentially of two heads. Arranged around the crank shaft and attached at equidistant points of the drum are five cylinders. Mounted on the port side of the crank shaft and close to the crank arm is a small gear, which through suitable gears mounted on the port head of the drum drives a double-pointed cam which has a bearing on the exterior of the hub of the drum. The ratio of these gears is such that the cam is driven at one-quarter the speed of the crank shaft, and in the reverse direction. Mounted on the exterior side of the port head of the drum are five punch rods, the upper ends of which are within a sixty-fourth of an inch of being in contact with the exhaust-valve stems of the cylinders, and on the lower end of these rods are hardened-steel rollers which rest on the double-pointed cam--this one cam thus serving to operate the exhaust valves of all five of the cylinders. The port head of the drum is connected to the port bed plate, by which it is supported, by means of a flanged bushing in which are formed tongues and grooves which fit into corresponding grooves and tongues formed in the hub of the drum, it being necessary to have a certain amount of space between this bed plate and the head of the drum to provide room for the exhaust-valve cam and its co-acting punch rods. The starboard bed plate is fastened to the starboard head of the drum by bolts which draw the web of the bed plate against the face of the drum. The sparking gears are driven by means of a gear formed on a sleeve which telescopes over the hub of the starboard drum, and has a bearing thereon, the end of the sleeve terminating in a ring which is fastened to the crank shaft.

Since the five connecting rods must center on the one crank pin, the bronze shoes in which they terminate can occupy only a portion of the circumference of the pin, and with the relative proportions which here existed between the length of stroke of crank and the length of the connecting rod, the circumferential width of the connecting-rod shoes was slightly less than sixty degrees, thus leaving uncovered a crank space of about one-sixth of the circumference, which it was necessary to have in order to provide room for the change in relative position of the shoes due to the angularity of the connecting rods. In [p238] the experimental engine the connecting-rod shoes were all given their bearing directly on the crank pin, as heretofore described, being held in contact therewith by means of cone nuts, which were screw-threaded to the crank pin, the taper of the cones permitting adjustment for wear. This method of connecting these parts to the crank pin is the usual plan of connecting three or more connecting rods to one crank pin. So much trouble had been experienced with the water jackets and with minor defects in the experimental engine that no long runs had been possible with it, and consequently no trouble had been experienced because of the small amount of bearing area provided by this method of joining the connecting rods to the crank pin. When, however, the new engine was completed it was found that after working at high power for a few minutes the connecting-rod shoes heated so rapidly that it was impossible to run the engine for more than ten or twelve minutes, the excessive heating of the shoes causing a great diminution in power besides the danger of serious damage if the tests were continued longer. At first this defect seemed almost fatal, as there appeared to be no way of providing sufficient bearing area for the five connecting rods on one crank pin. Happily, however, the writer was able to overcome this defect by an improved design which enables all five connecting rods to operate on the one crank pin, and at the same time provides each with the full amount of bearing area which it would have were it the only connecting rod operating on the crank pin. This arrangement consists essentially of a main connecting rod formed of a steel forging terminating in a sleeve which encircles the crank pin and is provided with a bronze lining for giving a proper bearing surface between the connecting rod and the crank pin, both the steel sleeve and the bronze lining being split, but at right angles to each other, to permit assembling them on the crank pin. This steel sleeve, the upper half of which is formed integral with the main connecting rod is rounded off to a true circle on its exterior circumference, except at the point where the rod joins it. The other four connecting rods terminating in bronze shoes are then caused to bear on the exterior of this sleeve, being held in contact therewith, and permitted to have a sliding motion thereon sufficient to take care of the variation in angularity of the connecting rods, by means of the cone nuts which are screw-threaded to the sleeve and locked thereto by means of the jam nuts, as shown in the drawings. The main connecting rod, of course, acts in the same way as in the ordinary case where each cylinder has its separate crank pin. The other four connecting rods deliver their effort to the crank pin through the sleeve in which the first connecting rod terminates, and they, therefore, do not receive any of the rubbing effect due to the rotation of the crank pin, except that of slipping a very short distance over the circumference of the sleeve during each revolution, the amount of slipping depending on the angularity of the connecting rod. This improved type of bearing was successful from the time of its first trial, and even in later [p239] tests in which the engine was run for ten consecutive hours at full power it showed no signs whatever of overheating. As this new form of connecting-rod bearing for the crank pin had never been tried before, the precaution was taken to leave the threads on the crank pin for the cone nuts, so that if this new bearing should not prove successful the old plan of having the connecting-rod shoes bear directly on the crank pin could be reverted to. These threads are clearly seen in Plate 78 and were never removed from the crank pin, though their removal would have added considerably to the area of the bearing surface of the main connecting rod, had more bearing surface seemed necessary.

The lubrication of the main crank-shaft bearing and of the crank pin was effected by means of a small oil cup, fastened to the port bed plate, which fed oil through a hole in the hub of the drum to a circular groove formed in the bronze bushing in the hub. The crank shaft being hollow, a hole was drilled through it in line with the groove in the bushing, and the oil was then led from the interior of the crank shaft through a pipe connected to the plug in the end thereof, and through a hole drilled in the crank arm to the hollow crank pin. Small holes through the crank pin permitted oil to pass to the exterior thereof and thus oil the bearing of the main connecting rod. Small holes through the sleeve and bushing of the main connecting rod fed oil under the shoes of the other four connecting rods, the small holes being placed in oil grooves formed in the interior of the bronze bushing. The lubrication of the pistons was effected by means of small crescent-shaped oil cups fastened to the outer wall of the cylinders, which distributed the oil equidistantly around the circumference of the pistons, through small tubes which projected through corresponding holes drilled in the cylinder wall. These oil cups for the cylinders were, while small, of sufficient size to furnish a supply for approximately one hour, and were so positioned on each cylinder as to have a gravity feed. It may be mentioned here that while there were many parts of the engine which were of unprecedented lightness there was nothing which excelled these oil cups in this respect, as they were made of sheet steel .003 of an inch thick, riveted and soldered up. The crank-shaft bearing in the starboard drum was oiled from an oil cup mounted on the outside of the bed plate and connected by a pipe to a hole in the inner wall of the drum, which was connected to the oil grooves in the bronze bushing in the hub of the drum.

The first set of pistons for this engine were similar in design to those shown in the assembled drawings, except that they had side walls and heads which were twice as thick as those shown. These lighter pistons were constructed later, and were just as good as the earlier and heavier ones. It will be noted that the pistons have two deep but thin ribs reinforcing the head. The pistons were slightly tapered from the middle, where they were .005 inch smaller than the cylinder bore, toward the outer end, where they were .0075 inch smaller [p240] than the bore. The outer piston ring was .0035 inch narrower than its groove, the second one .003 inch, the third .0025 inch, and the inner one .002 inch narrower than its groove. The rings were bored one-sixteenth inch off center with the exterior surface, and had one-eighth inch diameter of spring. They were of the lap-joint type, with the sides of the laps carefully fitted and only one-sixty-fourth-inch clearance at the ends of the laps to allow for thermal expansion. As no grinding facilities were obtainable in Washington, the cylinders were carefully bored smooth and free from taper, and the pistons were worn in to a perfect fit by running them in by a belt for twenty-four hours, with copious oil supply.

The main connecting rod was 7/8-inch diameter and solid, while the other four were of the same diameter but with a 5/8-inch hole in them. The gudgeon pins in the pistons were hollow steel tubes 7/8-inch diameter and case-hardened, and were oiled entirely by the oil thrown off by centrifugal force from the crank-pin bearing, the oil running along the connecting rods and through suitable holes at the heads into oil grooves in the bronze bushings in these heads.

Since on an engine for an aerodrome the best plan for releasing the exhaust gases from the engine is to get rid of them as soon as possible, so long as they are released behind the aviator and do not interfere with his view in the direction of motion, it was decided to have the gases exhaust immediately from the combustion chambers; but in order to prevent their playing on and heating the main bearing of the crank shaft in the port drum the combustion chambers were each provided with a chamber below the exhaust-valve seat, with a side outlet therefrom. The manifold pipe through which the gaseous mixture was supplied to the inlet valves of the engine consisted of a tube bent to a circle and having five branch tubes, each leading to one of the automatic inlet valves, which fitted removable cast-iron seats fastened by a nut in the upper part of each combustion chamber. The very small amount of clearance between the engine and the frame necessitated that this pipe be cut in three places and joined by flanges in order to properly assemble it on the engine when the latter was mounted in the frame. The carburetor, which was placed near the rear of the aviator’s car, was connected through suitable pipes to this circular inlet pipe, at a point horizontally in line with the center of the shaft. The auxiliary air valve consisted of a sleeve rotatably mounted on the vertical pipe leading from the carburetor to the manifold, holes in the sleeve being brought to coincide more or less with holes in the vertical pipe, by the operator, when more or less air was required or when he wished to vary the speed of the engine. The cooling water for the jackets of the cylinders was led to them through a circular manifold pipe on the starboard side connected by a vertical pipe with the centrifugal pump situated at the lower point of the lower pyramid of the aerodrome frame. The heated water was led from the jackets through another [p241] circular manifold pipe on the port side, through two connections to the radiating tubes at the front and rear, respectively, of the cross-frame. These radiating tubes, which were provided with thin radiating ribs soldered to them, finally led the cooled water to the tank situated in the extreme rear of the aviator’s car, a suitable pipe from the bottom of this tank being connected to the inlet side of the centrifugal pump. The centrifugal pump was driven by means of a vertical shaft connected to the crank shaft through a set of bevel gears which drove it at three times the speed of the engine. The bearings through which these gears were connected were mounted on the port bed plate, and in order to allow for a certain amount of vibration between the engine and the pump this vertical connecting shaft had a telescoping section connected through suitable splines.

The sparking apparatus comprised, first, a primary sparker similar to the simplest form of such devices which have since come into common use, where a cam driven by the engine co-acts with a pawl on the end of a spring, but in this case, as this sparker was used for all five cylinders, the cam was driven at a speed of two and one-half times that of the engine shaft, thus making and breaking the primary circuit five times in each two revolutions of the engine. Second, a spark coil, the primary terminals of which were connected to the primary sparker and to a set of dry batteries. Third, a secondary distributor consisting of a disc carrying a contact brush and driven at a speed one-half that of the engine, this brush being constantly connected through a contact ring to one of the terminals of the high-tension side of the spark coil and running over the face of a five-section commutator, each of the sections of which was connected to a spark plug, the other high-tension terminal of the spark coil being, of course, grounded on the engine frame. This sparking apparatus was first constructed by using blocks of red fibre for insulation. After the engine was completed and was being tested difficulties were met with in the sparking apparatus which at that time appeared inexplicable. After a great deal of annoyance and loss of time it was finally discovered that the red fibre was not as good an insulating medium as it was supposed to be, owing to the zinc oxide used in making it. In damp weather the sparking apparatus absolutely refused to work, and it was found that the moisture in the air caused the zinc oxide in the fibre to nullify its insulating qualities. This trouble, after being located, was cured by substituting hard rubber for the red fibre.

At the time when this engine was built, as well as earlier when the experimental engine was built, it was impossible to procure any wire which had been properly insulated to withstand the high voltages necessary for the connections between the high-tension side of the spark coil and the secondary distributor, and from the secondary distributor to the spark plugs in the cylinders. While at this time this appears a very simple matter, yet the trouble experienced and [p242] the delays caused by the lack of such small accessories which are now so easily procurable were very exasperating, and it was finally necessary to insulate these wires by covering them with several thicknesses of ordinary rubber tube of different diameters telescoped over each other.

In the early tests of this new engine, which were made with it mounted on a special testing frame and delivering its power to the water-absorption dynamometers, the engine was operated without any fly wheels, and, so far as its smoothness of operation was concerned and its ability to generate its maximum power, it did not require any.

After the completion of the tests on the testing frame the engine was assembled in the aerodrome frame, which was first mounted on the floor of the launching car. The car itself was mounted on a short track in the shop, which arrangement provided a smoothly rolling carriage which could be utilized for measuring the thrust of the propellers by merely attaching a spring balance between the rear of the car and a proper holding strap on the track. In the first tests of the engine under these conditions, it was found that while the engine itself did not require any fly wheels, yet the lack of them caused trouble with the transmission and propeller shafts, which, while it had never been anticipated, was easily understood when it was encountered. This difficulty was caused by the “reverse torque,” which fluctuated from a maximum to a minimum five times during each double revolution of the engine, and which set up fluctuating torsional strains of such magnitude in the transmission and propeller shafts that the shafts themselves became exceedingly hot after a few minutes operation of the engine, and under more prolonged periods of operation these fluctuating torsional strains caused a permanent twisting and bending of the shafts. The transmission and propeller shafts were at first made of tubing one-sixteenth of an inch thick, but these were abandoned both on account of the necessity of abandoning the screw-thread method of attaching the flange couplings and gears, and also because these shafts had been designed when it was expected to transmit only twelve horse-power to each propeller, while the increase of power in the large engine necessarily required much stronger shafts. The first shafts which were actually tested in the frame were, therefore, one and one-half inches in diameter by three-thirty-seconds of an inch thick, the tubing having been one-thirty-second of an inch larger originally and turned down to this size to insure a straight shaft. When these shafts twisted under the action of the reverse torque of the engine, a very much heavier set, practically twice as thick, were constructed. When used in the tests these heavier shafts, while much stronger, still showed a large amount of heating due to the fluctuating torsional strains.

Upon calculation it was found that by providing specially light fly wheels the major portion of this reverse torque could be eliminated for a less increase [p243] in weight than would be occasioned by sufficiently increasing the thickness of the transmission and propeller shafts to safely stand it. Since it was desired to concentrate as much as possible of the weight of the fly wheels in the rims, the idea at once suggested itself of building them up like a bicycle wheel by means of tangent spokes. Two steel automobile-wheel rims were therefore procured thirty-three inches in diameter, and these were provided with tangent spokes connected to special steel hubs fitted to the crank shaft of the engine. The rims themselves not being quite heavy enough, and constructional reasons necessitating their being at different distances from the center of length of the crank pin, the extra weight which it was desired to give to these rims was provided by means of steel wire wound tightly around and fastened to the rims, the weight of each rim being made inversely proportional to its distance from the center of the crank pin. The first spokes which were used for these wheels were standard bicycle spokes three-thirty-seconds of an inch in diameter, but these were soon found to be entirely too weak to withstand the sudden strains due to the rapid starting of the engine. They were therefore replaced by standard spokes one-eighth of an inch in diameter, but these also proved too weak and were later replaced with special spokes made in the shop out of No. 10 coppered-steel wire, which by test was found to have a tensional strength of 2192 pounds. As these steel rims were only one-sixteenth of an inch thick and had not been made exactly true, but had been straightened before being used, it was found that they very quickly went out of shape under the strain due to the centrifugal force at high speeds, and also when the engine was suddenly accelerated. As long as they did stay true, however, it was found that they were sufficiently heavy to provide all of the fly-wheel effect it was necessary to have in order to eliminate all trouble from the reverse torque.

After further consideration, it was decided that the only means of constructing a fly wheel which would have a stiff rim and at the same time would not be heavier than the steel ones, which had been found adequate, was by perpetrating what would at first sight appear to be an absurdity. A new set of rims for the fly wheels was made by constructing them of an aluminum casting, the section of the rim being U-shaped. After machining these rims and assembling the fly wheels with them, it was found that they were many times stiffer than the previous steel ones of the same weight, and after this change no further trouble was experienced in keeping the fly wheels perfectly true, even under the most severe strains. In fact, on one occasion when the engine broke loose from the propellers, it ran to a speed, which, while not exactly known, yet reached the limit of the tachometer, which was 2000 R. P. M., without injury to the fly wheels.

It will be recalled that in starting up the engine on the quarter-size model, the initial “cranking” necessary with a gasoline engine was accomplished by [p244] having two of the mechanics turn the propellers. While this same plan might have been followed in the case of the large aerodrome, yet it would have involved some danger to the mechanics and would also have left the aviator without any means of restarting the engine should it for any reason stop while in the air. Believing it to be very important to provide means for enabling the aviator to restart the engine in case it stopped in the air, the writer devised the starting mechanism shown in the drawings, Plates 78 to 80. Fastened by tongues and grooves to the port side of the engine crank shaft, just outside of the bed plate, is a worm wheel, on the hub of which is mounted the bevel gear which drives the water-circulation pump through the bevel pinion, as already described. Mounted on the web of the bed plate are two brackets, in which the shaft for the starting crank is journaled, this shaft passing forward and downward through the front of the cross-frame of the aerodrome, where it is journaled in a bracket secured to the brace tubes thereof. At the front or lower end of the shaft a crank handle is connected thereto by a ratchet mechanism. The upper end of the starting shaft, between the bearings of the two supporting brackets, is tongued and grooved, and slidably mounted thereon with co-acting grooves and tongues is a worm screw which, in the position shown in Plates 79 and 80, is in gear with the worm wheel just described. However, when the worm screw is slid along on the shaft until it is against the upper bracket it is out of gear with the worm wheel. Mounted in the interior of the tubular starting shaft is a spring-pressed pawl plug, not shown, but which projects through one of the tongues on the shaft near the upper bracket. If the worm screw is slid up against this upper bracket, this pawl catches in a radial hole in the worm screw and holds it in this position out of gear with the worm wheel. Connected to this pawl plug and passing longitudinally through the center of the shaft is a wire which terminates in a button just at the end thereof. By pulling on this button the operator may release the worm and thus permit it to slide downward so that when the starting crank is turned in a clockwise direction the worm will screw itself into gear with the worm wheel, and any further turning of the starting crank will cause the worm to force the worm wheel, and, consequently, the engine shaft, around in a clockwise direction. As soon as the engine gets an explosion the worm wheel slides the worm along against the upper bracket, where the spring pawl catches and holds it till it is again released by the operator as before.

This starting mechanism was a success from the first, and the engine was never started up in any other way. With an aerodrome having the qualities of automatic equilibrium, which the Langley machines have, it was felt very certain that by this mechanism the engine could be easily restarted while in the air, in case it was inadvertently stopped. [p245]

The reason for building the engine with five cylinders instead of some other number, and for arranging them radially on a central drum using only one crank pin may not appear quite obvious. The advantages gained by such a construction, however, are very great, and may be briefly summed up as follows:

First, since in a gas engine of the four-cycle type there is only one explosion in each cylinder every two revolutions, and the crank shaft and crank pin therefore are loaded only one-quarter of the time for each cylinder, it is obvious that by having four cylinders arranged radially around a central drum the load on the bearings of a single crank shaft and crank pin may be kept very uniform. However, with four cylinders thus arranged it is impossible to have the cylinders explode and exert their effort on the crank at uniform intervals in the cycle, it being necessary to have the cylinders explode in the order of 1, 3, 4, 2, 1, etc., thus giving intervals between explosions of 180 degrees, 90 degrees, 180 degrees, 270 degrees, etc., or to have them explode in the order of 1, 3, 2, 4, 1, etc., thus giving intervals of 180 degrees, 270 degrees, 180 degrees, 90 degrees, etc. On the other hand, with any odd number of cylinders the explosions will occur at equal intervals in the cycle. With three cylinders they will explode in the order of 1, 3, 2, 1, etc., or at equal intervals of 240 degrees, while with five cylinders they will explode in the order of 1, 3, 5, 2, 4, 1, etc., or at equal intervals of 144 degrees. It is therefore seen that there is a great advantage in smoothness of operation and uniformity of torque of the engine through having an odd number of cylinders instead of an even number.

Second, it is readily apparent that the greater the number of cylinders, provided the number is an odd one, the more uniform the torque will be, and it would seem at first that seven cylinders would therefore be better than five, since the uniform intervals between explosions with seven cylinders would be only 103 degrees (approximately). The advantage gained, however, through seven cylinders instead of five is largely, if not completely, counterbalanced by the added number of parts and the difficulty of providing sufficient circumferencial width for the connecting-rod shoes on the crank-pin bearing, even with the improved construction of this bearing already described. There is considerable fluctuation of the torque in each revolution of the engine with five cylinders, but this fluctuation of torque is more easily smoothed out by the use of very light fly wheels than by increasing the number of cylinders, and thus adding to the complication of the engine.

Third, the strongest point in favor of the radially arranged cylinders is the reduction in weight and complication which it permits. The crank shaft is reduced to the very minimum, there being only one crank pin with two main bearings which can, without any difficulty whatever, be kept absolutely in line with each other and thus prevent binding and loss of power. Again, the use of a single-throw crank not only reduces the cost and weight of the crank [p246] itself, but makes it very much less liable to damage; long crank shafts with several crank pins being frequently twisted by improper explosions in the cylinders. The supporting drum or crank chamber is likewise reduced to the very minimum, both in weight and simplicity, the drums being perfectly symmetrical with no lost space either inside of them or on their exteriors. The cam mechanism for operating the valves is reduced to a simple ring carrying (for a five-cylinder engine) a double-pointed cam and journaled on the exterior of the hub of one of the drums, the cam being driven by a train of gears journaled on studs mounted on the drum, and co-acting with a gear fastened to the crank shaft against the crank arm.

The radial arrangement of the cylinders is thus seen to give not only an engine with the smallest number of parts, each of which is as far as possible worked to a uniform amount during each complete revolution of the crank shaft, but it also gives a very compact and readily accessible mechanism with its center of gravity coincident with its center of figure, and with the liability of damage to it, in case of a smash of the vehicle on which it is used, reduced to the minimum from the fact that the greatest weight is located at the strongest part.

Fourth, and of almost as great importance as the reduction in weight which the five-cylinder radial arrangement permits, is its unusual qualities as regards vibration. Since these five-cylinder engines were built by the writer a very thorough treatment of their properties as regards balancing has been given in a treatise on the balancing of engines,[44] so no discussion of the mathematical formulæ involved in a study of the question of the inherent balancing properties of these engines will be here given. It is sufficient to call attention to the fact that in an engine having five cylinders arranged radially, all of the reciprocating parts are balanced for all forces of the first, second and third orders. As it is only the reciprocating parts which give any trouble in balancing any engine, the unbalanced rotating parts being readily balanced by placing an equal weight at an equal distance from the center of rotation, and on the opposite side thereof, it is readily seen that the properties of balancing which are inherent in this type of engine are unusual. A six-cylinder engine having a six-throw crank shaft is not nearly so thoroughly balanced as this type having its five cylinders radially arranged, for in the latter case all the moving parts are in one plane, while in the former case the moving parts are in six separate and parallel planes, and there is consequently considerable longitudinal vibration which can never be overcome. While this is true as regards the vibration due to moving masses, it is still more impressively true as regards vibration due to reaction arising from the force of the explosions in the engine cylinders, especially when the engine is running slowly and having heavy explosions.

The usual practice in balancing the rotating parts of an engine is to attach [p247] balance weights to the crank arms which are prolonged beyond the center of the crank shaft and on the opposite side from the crank pin; the radius of rotation of these balance weights being made approximately equal to the radius of the crank pin. But aside from the constructional difficulties which would be introduced, it was seen that if this plan was followed in this engine it would require a very large additional weight. Since the amount of this weight could be diminished in exact proportion to the increase of the radius of rotation of the balance weights, it was at first decided to attach the weights to the rims of the fly wheels, the relative amount of weight attached to each wheel being inversely proportional to its actual longitudinal distance from the crank-pin center. It was very soon found that the attachment of these balance weights to the fly wheel caused excessive strains on the rims of the wheels, thereby causing them to go out of line. In order, therefore, to keep the amount of balance weight small by carrying it at a considerable distance from the center of the shaft, the weights were finally arranged as clearly shown in the drawings, Plates 78 to 80. There it is seen that the main portion of each of the balance weights consists of a flat arm bolted between the flanges which couple the transmission shafts to the engine shafts. The flat arm terminates in a lozenge-shaped lug, additional weight being provided by a plate fastened to one end of a tube, the other end of which terminates in a collar fastened around the transmission shaft. The tube is inclined at an angle of about thirty degrees with the flat balance arm, thus acting as a brace to prevent the balance arm from wobbling, the plate on the bracing tube being fastened to the lozenge-shaped lug by means of small bolts.

The tabulated statement of the weight of this large engine is given below. From this it will be readily seen that the net weight of the engine proper is 124.17 pounds. The fly wheels were in no way necessary to the engine itself, but were used solely for the purpose of smoothing out the torque of the engine so that the transmission shafts and propeller shafts might be kept down to the very minimum in weight. Including the two fly wheels, the weight is 140 pounds.

Including the 20 pounds of cooling water the total weight of the power plant is 207.47 pounds. Without flywheels the total weight is 191.64 pounds.

The construction of this large engine was completed in December, 1901, and the first tests of it were made in January, 1902. As already stated, these first tests were made with the engine mounted on a special testing frame and delivering its power to two water-absorption dynamometers, no fly wheels being used, as none were required. Later, when it became necessary either to use fly wheels or to greatly increase the weight of the transmission and propeller shafts, in order to overcome the reverse torque, the two light fly wheels were added, and another series of tests was made of the engine on its testing frame. The arrangement of the engine, dynamometers, and accessory [p248] apparatus is clearly shown in Plates 82, 83 and 84. The engine ran in a clockwise direction, as viewed in Plate 82. ‹AA› are the fly wheels; ‹BB› the balance weights; ‹CC› the dynamometer shafts, on which are fastened the rotor plates which revolve inside of the dynamometer drums, ‹DD›, between stator plates fastened therein. The drums have a hub on either side, by which they are supported, these hubs being journaled on ball-bearings in the pedestals resting on the wooden framework. The rotor plates do not touch the stator plates in the dynamometers, but drag on the water with which the drums are partially filled, and thus tend to cause the drums to revolve around with them. The torque on each drum is measured by means of a rope, not shown, fastened into the hook at the top of the drum, the rope being given a partial coil around the drum and passing off tangent thereto at the horizontal diameter is fastened to a pair of spring scales hung from the ceiling vertically above the point of tangency. The scales and ropes were unfortunately not in position when these photographs were taken, but the arrangement of them should be readily understood. As the friction of the rotor plates on the water heats it in exact proportion to the amount of power absorbed, the small amount of water in the drums would be soon converted into steam unless continually renewed or cooled. When the rotor plates are revolving the centrifugal force keeps the water pressed toward the circumference of the drum, and the friction at any speed is dependent on the area of the rotor plates in contact with the water. The horse-power required to revolve the plates at any definite speed can therefore be controlled by having an outlet for the water at the proper radial distance from the shaft. The water from the water mains is led through the upper vertical pipe and allowed to flow into the funnel, and thence into the drum near the center where the centrifugal force throws it to the circumference of the drum. The lower vertical pipe is connected to the drum at a suitable radial distance from the center, and the heated water thus passes through this pipe and into the lower funnel connected to the sewer. By the use of the funnels the drums are allowed to rock sufficiently to exert their pull on the spring scales without being affected by the supply and exhaust of water.

The water for cooling the cylinders is led from the bottom of the tank ‹E› to the circulating pump ‹F›, supported in a discarded lower pyramid of the aerodrome frame, the pump being driven by the small vertical shaft, as already described. The water, after passing through the pump and the engine cylinders, is led back to the upper part of the tank. By suitable connections to the water mains and sewer, the water in the tank is kept at any desired temperature. The gasoline supply tank is seen on the left-hand side of the testing frame, as viewed in Plate 82, the carburetor being placed below it and the gas supply pipe from the carburetor passing through the gasoline tank. Instead of jacketing the carburetor, a grid formed of thin copper tubes is supported just above the multitude of small air pipes leading into the carburetor, and some of [p249] the hot water from the engine is by-passed through this grid and thus warms the air as it passes into the carburetor. The small pipe that by-passes this water through the grid is seen connected to the outlet water pipe just above the cylinders, a small butterfly valve in the outlet pipe enabling the amount of heated water passing through the grid to be controlled. The return from the grid is by means of the small pipe leading to the top of the large water tank. The tachometer, which gives instantaneous readings of the speed of the engine, is seen at ‹G›, where it is at all times in full view of the operator.

These dynamometers proved to be excellently suited for the testing work, and far ahead of anything else the writer has ever found for engine testing. Since the power required to rotate the rotor plates, with a uniform amount of water in the drums, varies as the cube of the speed, it is readily seen that it is impossible for the engine to race or injure itself by running away, as frequently happens where there is no engine governor and Prony brakes are used to measure the power.

In the early tests the engine was never allowed to develop more than 40 horse-power, as it was feared that by letting it develop more, which it was clearly seen to be capable of, it might be injured and cause a delay in the tests of the aerodrome. In the second series of tests it was allowed to develop 51 horse-power at 935 R. P. M., but it was not thought to be advisable to let it run at maximum power for more than an hour, for the same reason as before. In the summer of 1904, after it was seen that there was no immediate possibility of securing funds for continuing the tests of the aerodrome, it was planned to enter the engine in the competitive tests at the St. Louis Exposition, where a prize of $2500 was offered for the lightest engine for its power. As the conditions specified in this competition required that the engine run at its maximum power for one hour, and that this be followed by a durability test of ten hours’ continuous running, it was decided to make some durability tests of the engine before taking it to St. Louis. In these tests, the engine was run on three separate trials for a period of ten hours[45] with a constant load of 52.4 horse-power at 950 R. P. M. Even in these long durability tests the engine and the dynamometers both worked so smoothly and evenly that the engine did not vary its speed more than ten revolutions per minute, and the pull on the spring scales varied less than ten pounds in the entire ten hours. Considerable correspondence was had with the officials of the St. Louis Exposition regarding the entrance of the engine in the competition, in order to make sure that suitable facilities for conducting the tests had been provided. After receiving assurance that everything necessary had been provided, the engine and its testing dynamometers were boxed for shipment to St. Louis and arrangements were just being completed for their transportation when the following telegram was [p250] received from the director in charge of the aeronautical department of the Exposition: “On account of lack of competition engine tests abandoned.” As the main object of entering the engine in the competition was to insure for it an unquestioned record of its performance it was decided to reassemble it in the testing frame in Washington and invite some engineers of prominence to witness and certify to its performance, but on account of the lack of funds for meeting the expenses incident to such a series of tests as it was planned to make this was never done.

In the tests which were witnessed on April 26, 1902, by Captain I. N. Lewis, Recorder of the Board of Ordnance and Fortification, the engine was held down to a pull of 200 pounds on a 13-inch lever, when running at 1000 revolutions per minute. In the later tests in May, 1903, which were witnessed by Captain Gibson, who was then Recorder of the Board of Ordnance and Fortification, and Mr. G. H. Powell, the Secretary of the Board, the engine was allowed to work at a pull of 265 pounds on the 13-inch lever arm at a speed of 950 revolutions per minute. In the tests made in August, 1904, the engine was run for ten consecutive hours[46] at a pull which varied from 263 to 271 pounds, or an average of 267 pounds, on a 13-inch lever, with the speed varying from 945 to 955 revolutions per minute, thus showing 52.4 horse-power at the average speed of 950 R. P. M.

DETAILED WEIGHT OF NEW LARGE ENGINE.

Name of Weight in part. grammes.

Crank shaft 5,225 Connecting rods (total) 5,005 Pistons-- No. 1 1,652 No. 2 1,647 No. 3 1,655 No. 4 1,660 No. 5 1,646

Cylinders-- No. 1 (including exhaust and inlet valves, oil cups, etc.) 4,768 No. 2 (including exhaust and inlet valves, oil cups, etc.) 4,685 No. 3 (including exhaust and inlet valves, oil cups, etc.) 4,638 No. 4 (including exhaust and inlet valves, oil cups, etc.) 4,637 No. 5 (including exhaust and inlet valves, oil cups, etc.) 4,796

Port crank chamber drum, including cam, cam gears, punch rods, etc. 5,225 Starboard crank chamber drum 3,440 Spark plugs (5) 450 Outlet water pipe 450 Inlet water pipe 360 Inlet gas manifold 1,700 Primary and secondary sparkers and wires 512 Balance arm with braces for same--starboard 1,040 Balance arm with braces for same--port 1,067 ------ Total 56,323 = 124.17 lbs. Starboard fly wheel 3,946 Port fly wheel 3,234 ------ Total weight of engine and fly wheels 63,503 = 140.00 lbs. Spark coil and batteries 6,800 Carburetor 3,751 Inlet gas pipe from carburetor to manifold 756 Gasoline tank 1,004 Water tank 717 Water circulating pump and shaft 807 Radiator 7,700 ------ Total weight of power plant 85,038 = 187.47 lbs.

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