Trial of Maxim's Steam Flying Machine
Scientific American—September 15, 1894 [From Engineering, London.]

On Tuesday, July 31, for the first time in the history of the world, a flying machine actually left the ground, fully equipped with engines, boiler, fuel, water, and a crew of three persons. Its inventor, Mr. Hiram Maxim, had the proud consciousness of feeling that he had accomplished a feat which scores of able mechanics had stated to be impossible. Unfortunately, he had scarcely time to realize his triumph before fate, which so persistently dogs the footsteps of inventors, interposed to dash his hopes. The very precautions which had been adopted to prevent accidents proved fatal to the machine, and in a moment it lay stretched on the ground, like a wounded bird with torn plumage and broken wings. Its very success was the cause of its failure, for not only did it rise, but it tore itself out of the guides placed to limit its flight, and for one short moment it was free. But the wreck of the timber rails became entangled with the sails, and brought it down at once. The machine fell on to the soft sward, embedding its wheels deeply in the grass, and testifying, beyond contradiction, that it had fallen and not run to its position. If it had not been in actual flight, the small flanged wheels would have cut deep tracks in the yielding earth.

The Maxim flying machine is a large braced structure formed of steel tubes and wires, and is exceedingly stiff for its weight, which is about 8,000 lb., including men and stores, At its lower part it carries a deck, on which the crew stand, where, also, the boiler, steering wheel, and reservoirs of water and gasoline are mounted. At a height of some 10 feet above the deck come the engines, each of which drives a screw propeller of 17 feet 10 inches diameter and 16 feet pitch, working in air. Above the propellers is the great aeroplane. Smaller aeroplanes project out, like wings, at the sides, the extreme width being 125 feet and the length 104 feet. There are five pairs of wings, as shown in the illustration, but the intermediate three pairs are not always used, and at the time of the accident these were not in place. At that time the area of the aeroplanes, was 4,000 square feet. With all the planes in position, the total area is 5,400 square feet. Forward and aft of the great plane are two steering planes, carried on trunnions at the sides, and connected by wire strands with a drum on the deck. By turning this drum the steering planes can be simultaneously tilted to direct the machine upward or downward, or to keep it on an even keel.

The chief interest centers on the boiler, as, unless this be made exceedingly light, it is hopeless to expect that the machine will soar. There is a very close resemblance between the Thornycroft boiler and Maxim's boiler. In each case there are two wing drums, connected by a large number of curved tubes with a steam and water drum, and there are also downcomers to facilitate the circulation. The casing is also made of straight tubes. In the boiler of the flying machine a feed heater is placed over the steam drum, but it is not shown in the engraving. The feed heater is constructed of steel tubes three-sixteenths inch bore and one-twelfth inch thick; the water is pumped through It at a pressure 30 lb. higher than the pressure in the boiler, and is delivered through an injector-like nozzle into the top of the downcomer pipe. The incoming water delivers its surplus energy to the surrounding liquid, creating a rapid and powerful current in the pipe, and consequently maintaining an active circulation in the small tubes in which the steam is generated. The feed pumps are placed on the deck beneath the engines, and are of variable stroke, so as to be adapted to the needs of the boiler. As they work at high speed, the valves are of large diameter—larger than that of the plungers. Pounding is prevented by a rubber bag on the suction and spring pistons on the discharge. The total quantity of water in the boiler only amounts to 200 lb., so that it is necessary that the amount of feed should be accurately adjusted. There is a very ingenious water level indicator. A small pipe is led in a loop from front to back and from back to front of the furnace. It is then taken to the steam and water drum, and led backward and forward through that in the same way, below the water line. The whole is filled with water, and forms a closed circuit having two loops—one in the furnace and one in the water. Now, so long as the upper loop is in the water the pressure does not rise greatly beyond that in the boiler, because the heat taken up in the furnace is conveyed, by the circulation, to the water in the drum. But if the water level falls in the drum, then there is no outlet for the heat; the pressure, consequently, rises most rapidly, and shows itself on a gauge attached to the pipe. By this most ingenious device an open-faced pressure gauge is substituted for the usual gauge glasses. The weight of the boiler, with casing, feed water heater, dome, and uptake, is 904 lb.; with burner and water it is 1, 200 lb. The heating surface is about 800 square feet, and the flame surface 30 square feet.

The fuel burned in the boiler is gasoline, of a specific gravity of 72 degrees Baume. It is carried in a copper vessel on deck, and is pumped through a vaporizer into the furnace. The pipe from the pump is led into a vessel having a large gasoline burner beneath it. In this vessel the spirit attains a pressure of 50 lb. on the square inch, and a corresponding temperature, in which condition it is, of course, highly inflammable. The gas which it gives off is conducted by a pipe, passing through the furnace, to a jet, like that of a Bunsen burner, at the front of the furnace, and in rushing through it, induces a powerful draught of air, with which it mixes. The combined charge passes through hollow fire bars, pierced on the upper surfaces with fine holes, and burns in 7,650 separate flames. The arrangement is so powerful that the pressure in the boiler can be raised from 100 lb. to 200 lb. in a minute. The air supply can be regulated at will, while the expenditure of gasoline automatically adapts itself to the needs of the boiler. The pressure of the gasoline vapor acts on a ]ever, which is balanced by a spring. If the feed is greater than the consumption, the pressure on the ]ever puts a pawl in gear with a ratchet wheel, and, through intermediate mechanism, works a block along a slotted arm to reduce the throw of the gasoline feed pumps. If the feed is too small, the opposite effect is produced, and the throw of the pump increased.

There are two screws, each driven by a separate compound engine, having cylinders 5.05 inches and 8 inches in diameter by 12 inches stroke. The steam is distributed by means of piston valves having 3 inches stroke, and operated by eccentrics. The exhaust steam is delivered into the air, but Mr. Maxim informs us that he used successfully an air condenser. This seems to be a necessity, because the supply of water would prove a serious load. Even to drive 100 horse power would require some 2,500 lb. of water per hour, which would be a considerable addition to a lengthy trip, especially if undertaken for warlike purposes in a hostile country.

To supplement, or replace, the safety valve, by-passes are provided so as to allow live steam to pass directly to the low pressure cylinders. Instead of blowing off into the air, the steam is blown past the high pressure cylinders, and the fall in pressure is made to do work on the exhaust from the high pressure cylinders, drawing the steam from the high pressure cylinders and driving it into the low pressure cylinders. The boiler will make more steam than the engines can take in the usual way.

The boiler pressure, when running, is 320 lb. per square inch, giving in the high pressure cylinder a differential pressure of 195 lb. and in the low pressure cylinder 125 lb. The cut-offs are respectively 0.75 and 0.625 of the strokes. In the high pressure cylinder there is a very large clearance, designed to prevent injury from water in case the machine should pitch. The actual horse power delivered to the screws is 363 when the engines are running at 375 revolutions per minute. Of this, we are informed by Mr. Maxim, 150 horse power are expended in slip, 133 horse power in actual lift on the aeroplanes, and 80 horse power in driving the machine, with its frames and wires, through the air. The thrust of the screws, when the machine is moored, is 2,100 lb., and when it is running it is 2,000 lb. We give these figures as they were supplied to us, omitting decimals. The total lift is something over 10,000 lb. at a speed of forty miles an hour and with the aeroplanes making an angle of about 7.25 degrees with the horizontal.

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