Military Aeroplanes; An Explanatory Consideration of Their Characteristics, Performances, Construction, Maintenance and Operation, for the Use of Avia

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1916 edition. Excerpt: ...to express the Efficiency of a propeller of definite shape and section for any combination of values of (1) The velocity of the aeroplane in feet per second, v, (2) The revolutions per second of the engine, n, (3) The diameter of the propeller in feet, d. The speed thru the air of the tip of the blade is determined in feet per second by the circumference = 7rd, and the number of times a second it covers this distance = n. The quantity v/xnd is the actual relation between the "tip speed" of the propeller and the speed thru the air of the entire aeroplane. Let us say, briefly, that it has been "discovered" that this relation definitely determines the efficiency of any particular blade. Our data on the engine gives us n, which is taken in this example as normally 1200 r. p. m. =20 r. p. s. The diameter, d, in this example is 8 feet. For any speed of the aeroplane, v, therefore, we can compute v/nd, and on the Efficiency chart, p. 97, read % efficiency of the propeller. Knowing the horse power of the engine for the given value of n, we readily determine the actual horse power available from the propeller. As an example, at 60 m. p. h. speed, v = 60 X 1.47 = 88 feet per second, n = 20, and d = 8, whence v/nd =.55. Reading on the first chart p. 97, we find that for v/nd =.55, propeller efficiency = 76%. On the second chart, p. 97, it is seen that at 20 revolutions per second, or rather 20X60 = 1200 r. p. m., the engine may be expected to give 88 h._ p. Our propeller Power Available, therefore, is 76% of 88 = 67 h. p., and is so plotted on the Power chart for 1200 r. p. m., on p. 96. In the same way all the other points, not only for this same curve but for values of r. p. m. = 800, 1000, etc., are plotted, and we thus obtain...