All through this development the designers had to confine their activities within limits set by the producers of the aircraft. This in itself created some puzzling problems. For instance, a constant current of electricity must be supplied to heat the filaments of the vacuum tubes and to operate the transmitter. A simple way to provide this current would seem to be to connect a dynamo with the driving shaft of the airplane engine, but the airplane constructors would not allow any such connection with the engine. Current could be supplied from storage batteries, but the planes were already loaded down with all the gear they could carry, and the use of heavy batteries was out of the question. Therefore it was the task of the phone designers to supply a dynamo plant that would not add appreciably to the weight of the plane. This was done by installing on the outside of the plane a wind propeller, which was driven by the rushing air and had power enough to turn the dynamo.
The dynamo must deliver a constant and unvarying voltage to the radio phone, if its operation is to be possible, yet a wind propeller on the airplane would be driven by air rushing by at speeds varying from 90 or 100 to 160 miles per hour, the latter figure being the speed of a diving plane. This meant that the wind propeller, and hence the armature of the dynamo, would revolve at a speed varying from 4,000 to 14,000 revolutions per minute. It would seem to be impossible to procure current at a constant rate from a dynamo varying so widely in its speed of operation; yet one of the experts engaged in this enterprise solved the problem, and the dynamo thereafter performed always in a most steady going and dependable manner.
Incidentally as a sort of by-product of the undertaking the special transmitter and helmet may be employed as a means of communication between the pilot and observer in a two-seated machine. When the helmet is used for this purpose, the wireless is not employed at all, but the head sets are connected by wires so that notwithstanding the fact that one can not hear himself talk because of the noise on the plane the pilot and observer can converse over the telephone with ease. Then at any time by throwing a switch they can connect themselves with the radio apparatus and talk with the men in another plane 3 or 4 miles away or to their squadron headquarters on the ground.
One good result of the airplane telephone was to speed up the training of aviators in this country and to make that training safer. But the primary object of the wireless phone was to make it possible for the leader of an air squadron at the front to control the movements of his men in the air. For this purpose extra-long range was not required, and the distance over which the machines could talk was purposely limited to 2 or 3 miles so that the enemy could not overhear the conversation except when the planes were actually engaged in fighting each other.
The Navy made use of the wireless telephone sets in the seaplanes, and here the range of the equipment was made greater. The Navy also adopted a modified form of the set for the 110-foot submarine chasers. The subchasers hunted the submarines in packs, and by means of the radio telephone the commanders of the boats kept in constant touch with each other, thereby greatly increasing the effectiveness of their operations.
Altogether there were produced for the Army airplanes about 3,000 combined transmitting and receiving sets of the radio telephone and about 6,500 receiving sets alone.