=Performance and Efficiency Data.=—The first step in deciding upon methods of power drive, and indeed in deciding whether power drive is feasible at all, is to assemble definite data as to the power required to drive representative cameras. Approximate figures for some of the cameras described in previous chapters are: L camera, 26 watts, deRam, 60 watts, “K” film, 30 watts. These requirements—not exceeding ⅒ horse power—are insignificant in comparison with the total of 100 to 400 horse power available for all purposes from the plane's engine. _Propeller characteristics._ Data on the performance of small propellers are somewhat meagre. However, the results of the rather extensive researches on large ones, suitable for driving planes, may be applied, with proper reservations, to give a fair guide to the study of the application of small propellers for driving plane auxiliaries. The first factor to be considered is the thrust or _head resistance_ offered by a propeller to motion through the air. This varies as the _square of the velocity_, as the _density of the medium_, and as the _area of the body_ projected normally to the wind, the formula being _T_ = _cdaV_^2 where _T_ = thrust, _d_ = density, _a_ = area, _V_ = velocity. Data on the L camera propeller are shown in Fig. 66, where its thrust both when free and when loaded with the camera is given, as well as that of a solid disc of the same diameter as the propeller. For this propeller, which is double-bladed, and six inches in diameter, _cda_ = .000275 with the load on. The total thrust amounts to only about three pounds when the plane velocity is 100 miles per hour. The head resistance of the whole plane is a matter of hundreds of pounds, so that the propeller resistance is quite negligible. [Illustration: FIG. 66.—Wind propeller data.] The next factor is the speed of revolution of the propeller, expressed in revolutions per minute. This varies with the design—the number of blades, their area, and pitch. For a given design the speed of revolution is _directly proportional to the speed of motion through the air_, and to _the density of the air_. Representative data for the L camera propeller are shown in Fig. 67. It will be noted that the speed goes up to 8000 for 120 miles per hour air speed. This illustrates the necessity for great strength to withstand centrifugal force. Propellers should be constructed of tough material, and subjected to whirling tests up to speeds considerably in excess of any the plane will attain in any maneuver. At low speeds the linear relationship fails, as a critical velocity is reached—about 3500 r. p. m. for this propeller—where it refuses to turn. [Illustration: FIG. 67.—Relation between air speed and propeller revolutions.]