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Published online by Cambridge University Press: 28 July 2016
In 1915, when dealing with various problems connected with Lifting Screws, the author desired to obtain a general idea as to the performance of a propeller when it was moved at various speeds forwards and backwards along its axis of rotation, and to express this information in some simple way, so that the approximate performance of any similar propeller working in a fluid medium of any density might readily be deduced.
If, when comparing the performance of similar propellers working at the same slip ratio, the resistances due to skin friction and viscosity are assumed to follow the velocity squared law, and change of density in the neighbourhood of the propeller is neglected, there are six main factors to be considered.
1 As far as the author could discover, no tests at negative speeds of advance had previouslybeen made.
2 As propellers are now (1929) usually tested in wind tunnels, this conclusion was of considerable importance.
3 It will be noticed that the points so reduced lie fairly well together, thus justifying, over a range of thrust of one to four, the approximate assumptions made in deriving equations (1) and (2) below.
4 At zero thrust, the experimental mean pitch multiplied by the revolutions equals the speed of advance.
5 If it is assumed that the forces are proportional to areas, densities and squares of velocities. It should be noted that these assumptions are only approximately true. (See Appendix I.)
6 C = 15.
7 Read as “ function four.”
8 Nos. 1 and 2, Fig. 6.
9 For positive speeds of advance, which are those usually dealt with, these curves are very : easily calculated and plotted.
10 For the propellers used 9in. pitch is approximately equivalent to a value of 6 of 30 deg., 6in. to 21.5 deg., 4.5in. to 16 deg., 3in. to 11 deg. and 1 1/2in. to 5.5 deg.
11 The direction of rotation of all the propellers shown in this figure was contra-clockwise.