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 = cdaV2
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.
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.
Fig. 67.—Relation between air speed and propeller revolutions.
The fact that the speed of the propeller depends on the density of the air has an interesting corollary, which is that a propeller adequate at low altitudes will fail at high ones. The density of the air varies with altitude according to the following figures: