Propeller Sense

I: PRINCIPLE OF THE VARIABLE-PITCH PROPELLER I f we think of any little bit of a propeller we sec at once that it moves in a corkscrew path. Suppose that in the time in which the propeller makes one complete revolution it moves forward 10 feet—a likely enough figure on die climb—and suppose that the little bit w r e have our eye on is 3 feet 6 inches along the blade from the axis of rotation (the propeller shaft, if you prefer). This little bit describes, in one turn, a circle of radius 3 feet 6 inches and therefore circumference 22 feet. Let us set this ABout. is the circumference des­cribed and BC the forward movement, and the resultant movement is along AC. To get the real corkscrew you must wrap this sheet-of-paper triangle around cylinder. We shall call the angle CAB the Angle of Advance. Now a corkscrew bores its way into a cork simply according to its pitch, or angle of advance of the helical coil of wire, point foremost, following the line of least resistance. If we think of a propeller as a set of three—or four—knives which we suppose are cutting their throughway the air alike knife going into cheese, following the line of least resis­tance, we shall have a precise analogy in the corkscrew. We do not care much what pitch a corkscrew has, within reason but w redo want to be able to choose the corkscrew paths of our propellers over quite a wide range of pitch. We may want to climb at 150 m.p.h. at 2,400 r.p.m. (5^ feet per engine rev), and to fly level at 250 m.p.h. at the same r.p.m. (91 feet per engine rev). We don’t want to have to start our take-off at zero r.p.m., nor to have the engine fly to pieces at umpty hundred r.p.m. in a steep dive. So far we seem to have found that we do need a variable corkscrew for our aircraft. 0024001502400250 We do seem to want a variable corkscrew for our aircraft
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