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Published online by Cambridge University Press: 14 September 2016
In spite of the rapid development of aeroplanes there are many experimenters interested in the helicopter; it will perhaps be useful to consider a few theoretical points in connection with this type of machine upon which recent research has thrown a stronger light.
Propeller Thrust with Full Slip.—The principal question that concerns us is the possible value of the propeller thrust, and it should, at once be noted that it is probable that much larger values are obtainable for this than is possible in the case of the propellers of an aeroplane. Each horse-power employed by the propeller produces 550 ft.-lbs. of work per second.
Note on Page 109 * The power employed for producing a thrust is 
of the total power employed.
Note on Page 110 * A similar result has been obtained by Lanchester for the aeroplane, but in a slightly different way.
Note on Page 111 * This is a check on the values of ξ and c, and can be compared with Tumbull's results. I have taken c as 3 according to Eiffel's results. Lanchester makes it rather less for small aspect ratio.
Note on Page 111 † This result is arrived at as follows:—The air enters the aerocurve with a velocity v(2πrnt) and moves upwards in the direction a. It is then deflected downwards with an unaltered velocity in the direction β so that the change of momentum is 2v.sin½(α + β) = approximately v.(α + β). This reaction is inclined to the perpendicular to the direction of motion at an angle ½(β−α), so that its component parallel to the line of motion is v½(β 2−α 2) approximately. Assuming the curve deals with the same bulk of air as the aeroplane, and that there is about the same amount of negative field behind it, the equations become as above.
Lanchester proceeds a little differently and assumes fairly perfect stream-line motion; the results do not greatly differ.