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.