3. Spring motors.
4. Electric motors.
These may first be considered largely from the descriptive standpoint, leaving questions of performance and efficiency for separate treatment.
Power may be derived directly from the engine through a flexible shaft, similar to that used for the revolution counter. This source of power is inherently the most direct and efficient, since the engine is the seat of all the lifting and driving energy of the plane. There is no loss through transformation into other forms of energy, such as electrical; or by the use of more or less inefficient intermediary apparatus, such as wind propellers. Against the direct drive of the camera from the engine may, however, be urged that the usual distance between engine and camera is too great for reliable mechanical connection, as by flexible shafting. Objections also arise from the standpoint of speed. This cannot be controlled by the camera operator; and varies over too wide a range, as the engine changes from idling to full speed, to fit it for automatic camera operation. The first objection may be met by that combination of methods of power drive which consists in transmitting the power electrically; that is, by letting the engine operate a generator from which cables run to a motor close to the camera. This method, of course, sacrifices efficiency, and it breaks down when the engine speed drops below the speed necessary to generate the requisite voltage. This defect may in turn be met by floating in storage batteries, which brings up the whole question of electrical drive, to be treated presently. While use of the engine for direct drive or for generating electric current has not been adopted in the American service, it is known that some German planes were supplied with electric current in this way.
Coming next to the wind motors, these possess one very great merit: they utilize a motive power that is always present as long as the plane is in motion through the air. On the other hand, the process of using the main propeller of the plane to pull another smaller propeller through the air appears a roundabout way to utilize the driving power of the airplane engine. Yet on the whole it is probable that some form of propeller or wind turbine is the simplest and most convenient device we have for the operation of airplane auxiliaries. As long as the amount of power required is small, such inefficiency as is inherent in its use is offset by its convenience and reliability. An advantage of the propeller is that its speed is almost directly proportional to that of the plane through the air, a desirable feature in automatic cameras provided the proportionality is under control. Yet it is just in this matter of varying the speed at will that the propeller presents difficulties, to be met only by additional mechanisms for gearing down or governing. Propellers have the practical disadvantage that they present an easily bent or broken projection to the body of the plane (Figs. [83] and [84]). The strength of small propellers for operating auxiliaries is never so much in question with reference to their resistance to whirling and thrust of air as it is to their ability to withstand the inevitable knocks and careless handling that will fall to their lot. The propeller bracket is just what the pilot is looking for to scrape the mud off his boots before climbing in.
The wind turbine has the advantage over the propeller that its speed can be varied rather simply by exposing more or less of its face to the wind. A turbine fitted with an adjustable aperture for admitting the wind is shown in Fig. [64], in connection with the type K automatic film camera. The turbine has the advantage of being compact and lying close against the body of the plane. In the form figured, altogether too much head resistance is offered—just as much for low as for high speeds—but with proper design this need not be the case. It is, moreover, quite too small to give the needed speed regulation, as it only begins to operate near its full opening.
Fig. 64.—U. S. Type “K” (Folmer) automatic film camera, with wind turbine and Venturi tube.
Fig. 65.—Type “K” camera, open, showing suction plate.