HELICOPTER. A flying machine whose supporting surfaces are rotated mechanically, being equivalent to an air-screw (or air-screws) so arranged as to give an approximately vertical thrust. Since such a system does not derive its lift from a motion of the whole machine through the air (contrast both the ordinary aero plane and the GYROPLANE, q.v.) it is in principle capable of hov ering, and of rising or descending vertically. Schemes for heli copters are found throughout the ages-old literature of flying, but there are only rare records of successful flight of more than a comparatively few minutes. The device is in fact extravagant of power. In the light of modern knowledge one horse-power can not be expected to lift more than about 15 lb. The whole of the power of a modern commercial aeroplane arranged as a helicopter would be needed merely to sustain it, leaving nothing to raise or propel it.
The most hopeful line of development for the helicopter is along the lines of the gyroplane. A mechanical drive to the rotat ing wings will enable the gyroplane to rise from the ground after a shorter run than is necessary for the "autogiro," though it is unlikely that, for a given expenditure of power the rate of climb will be thereby improved. It may be possible to dispense with the driving air-screw of the gyroplarie, devoting the whole power to rotating wings and obtaining the thrust required for forward mo tion by inclining their axis. The essential difficulty is in the con trol, and as with the aeroplane success will not be attained until the problem of control has been solved. Failure of the power plant will force a helicopter to descend, but need not result in a catastrophe. Given effective control over the inclination of the axis of the wings, all the ordinary manoeuvres of a normal aero plane about to land may be regarded as within its powers, with the additional possibility of a steep final descent at a slow speed.
It is unlikely that the helicopter, even as visualized above, will equal the aeroplane in performance, or in the proportion of its total weight available for load. If it ultimately displaces the aero plane for certain purposes, it will be entirely on account of the gain in safety in which the practical realization of its essential feature—lift not dependent on forward speed—will result.
(W. S. F.)