(AVIATION, September 1, 1920, p. 88)

Steam Motors for Aircraft

Germany and England have produced experimental steam motors for aircraft that have at least shown promise in their trials and it is high time that the United States produces the finished product, especially considering that we already lead in this type of motor as applied to automobiles. In order that this motor may be of value however it must not only equal existing internal combustion engine performance to be acceptable but must supersede that performance. It is hoped that inventors and those who are technically familiar with steam engine performance will contribute their experience to the solution of this problem. As late as 1913 the consideration of steam for powering airplanes was met with candid disfavor, but owing to the rapid progress made in aeronautics since that time it is believed that the time is ripe to make such overtures as would meet with reasoning. Simultaneously the developments in steam engines and boilers have been so progressive as to suggest that the impracticabilties formerly pointed out may have been almost entirely eradicated. Below one hundred horsepower there would seem to be little demand for the steam aeronautical motor but for power over that it is felt that there is a large field of utility. As multi-motored planes become more frequent the limitations imposed formerly by the amount of fuel and water will not seriously impede the radius of action, but this point must be watched carefully in the design of such a motor even now and the maximum efficiency procured. The question of weight of motor and boiler is today not of such vital importance even as is this problem of fuel and water, for the design and progress in the former two essentials have come to a point of easy adaptability.

Naturally high pressure steam will have to be used, and the efficiency of such an engine is most marked in its non-condensing designs to which airplane uses will be undoubtedly limited. In that case the steam consumption will have to be reduced to the minimum and high steam pressure carried. This will involve a problem of securing the same efficiency as is obtained in engines operating against low steam pressures. With ordinary steam engines there is a great difference of temperature in the cylinders which causes additional cylinder condensation. Multiple cylinders or multiple expansion engines have heretofore cared for this somewhat, but condensation in this fashion offsets the benefits of high pressure. The problem of cylinder condensation however has been satisfactorily solved so that high pressure steam can be used in one cylinder without loss due to the wide range of temperature.

This is accomplished by allowing the exhaust steam to remain in the cylinder near the head where it is superheated by the jacket in the head and then when compression increases the temperature of the steam in the cylinder the walls are simultaneously heated to a temperature above that of the entering steam so that there is no initial condensation. This status obtains throughout expansion also. The single acting steam engine is particularly adapted for this purpose as a type primarily due to the fact that it eliminates stuffing boxes and simplifies this problem of expansion in that there is a gradual difference in temperature between the bottom and the top of the cylinder. Simplification is also achieved in this design in the valve motion and obviation of the reversal of strains, both of which become serious problems at high speed.

Probably the most advantageous arrangement for a steam aeronautical motor will be found to be that of three cylinders set at 120 deg. in the one plane. In this design a good turning torque is obtained with a minimum of base while the valve motion is very simple. The latter consists of one cam directly on the main driving shaft driving the valve push rods. Exhaust is cared for in allowing the piston to uncover a port at the bottom of the stroke. A variable shut-off is accomplishable by tapering the cam. While this design offers considerable head resistance it seems to be more reliable than others and with the boiler will compare favorably with other designs and motors.

An engine designed to develop 200 hp. at 900 r.p.m. on 400 lb. to 500 lb. of steam pressure ought not to weigh more than 400 lb. and the condenser with it ought not to weigh more than 100 lb. Such an engine would require about 1 1/4 lb. of liquid fuel per horse power hour and 14 lb. of water. Condenser efficiency is so high today together with the conditions of flying which are favorable that at least 80 per cent of the water ought to be reclaimed.

The boiler, which will be considered in a later article, must owercome all the problems of pressure decrease which include the rising boiling point, increasing pressure per square inch and so forth. In general it is felt that the reliability of this means of propulsion will appeal heavily and with the advent of all-metal airplanes is sufficiently attractive to warrant trial although it should be undertaken by someone who is familiar with aeronautics as well as a specialist in steam engineering. - Air Service News Letter.

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