RADIOTELEPHONES FOR AIRPLANES
Then there was a demand for radiotelephone apparatus to be used on airplanes. This was a much more complicated matter and called for a great deal of study. The way in which problem after problem arose and was solved makes an exceedingly interesting narrative. It seemed almost absurd to think that a delicate radiotelegraph apparatus could be made to work in the terrific noise and jarring of an airplane. The first task was to make the apparatus noise-proof. A special sound-proof room was constructed in which a noise was produced exactly imitating that of the engine exhaust of an airplane engine. In this room, various helmets were tried in order to see whether they would be proof against the noise, and finally a very suitable helmet was designed, in which the telephone receiver and transmitter were installed.
By summer-time the work had proceeded so far that an airplane equipped with transmitting-apparatus could send spoken messages to an operator on the ground from a distance of two miles. The antenna of the airplane consisted of a wire with a weight on the lower end, which hung down about one hundred yards from the body of the machine. But a trailing antenna was a nuisance in airplane manœuvers, and it was also found that the helmet which was so satisfactory in the laboratory was not just the thing for actual service in an airplane. It had to fit very tightly around the ears and the mouth, and as the airplane went to high altitudes where the air-pressure was much lower than at the ground level, painful pressures were produced in the ears which were most annoying. Aside from that, in actual warfare airplanes have to operate at extreme heights, where the air is so rare that oxygen must be supplied to the aviators, and it was difficult to provide this supply of oxygen with the radio helmet tightly strapped to the head of the operator. But after considerable experiment, this difficulty was overcome and also that of the varying pressures on the ears.
Another great difficulty was to obtain a steady supply of power on the airplane to operate the transmitting-apparatus. It has been the practice to supply current on airplanes for wireless-telegraph apparatus by means of a small electric generator which is revolved by a little propeller. The propeller in turn is revolved by the rush of air as it is carried along by the plane. But the speed of the airplane varies considerably. At times, it may be traveling at only forty miles per hour, and at other times as high as one hundred and sixty miles per hour, so that the little generator is subjected to great variations of speed and consequent variations of voltage. This made it impossible to produce the steady oscillations that are required in wireless telephony. After considerable experiment, a generator was produced with two windings, one of which operated through a vacuum tube, somewhat like an audion, and to resist the increase of voltage produced by the other winding.
Then another trouble developed. The sparks produced by the magneto in the airplane motor set up electro-magnetic waves which seriously affected the receiving-instrument. There was no way of getting rid of the magneto, but the wires leading from it to the engine were incased in metal tubes which were grounded at frequent intervals, and in that way the trouble was overcome to a large extent. The magnetos themselves were also incased in such a way that electro-magnetic waves would not be radiated from them.
Instead of using trailing wires which were liable to become entangled in the propeller, the antenna was extended from the upper plane to the tail of the machine, and later it was found that by using two short trailing antennæ one from each tip of the wings, the very best results could be obtained. Still another development was to embed the antenna wires in the wings of the plane.
It was considered necessary, if the apparatus was to be practicable, to be able to use it over a distance of two thousand yards, but in experiments conducted in October, 1917, a couple of airplanes were able to talk to each other when twenty-three miles apart, and conversations were carried on with the ground from a distance of forty-five miles. The conditions under which these distances were attained were unusual, and a distance of three miles was accepted as a standard for communication between airplanes. The apparatus weighed only fifty-eight pounds and it was connected with both the pilot and the observer so that they could carry on conversations with each other and could both hear the conversation with other airplanes or the ground. As a matter of fact, airplanes with standard apparatus are able to talk clearly to a distance of five miles and even to a distance of ten miles when conditions are favorable, and they can receive messages from the ground over almost any distance.
A similar apparatus was constructed for submarine-chasers with a standard range of conversation of over five miles. Apparatus was manufactured in large quantities in this country and all our submarine-chasers were equipped with it, as well as a great many of our airplanes and seaplanes, and we furnished radio-apparatus sets to our allies which proved of immense value in the war. This was particularly so in the case of submarine detection, when it was possible for a seaplane or a balloon to report its findings at once to submarine-chasers and destroyers, and to guide them in pursuit of submarines.
The improved audion holds out a wonderful future for radiotelephony. For receiving, at least, no elaborate aërial will be needed, and with a small loop of wire, an audion or two, and simple tuning-apparatus any one can hear the radio gossip of the whole world.