The 30-foot diameter air propeller was mounted on a horizontal shaft. The propeller was of the double kind built up of solid drawn steel tubes tied and strutted together. The propeller was designed to give a thrust or lifting power of 600 lbs., and having a factor of safety of 3½, it weighed 120 without the canvas, the idea of the double form or superimposed structure was for reasons of strength.

It is on the same principle as a lattice or “ warren ” girder supported at one end and loaded at the other, the members of one blade being in compression and the other in tension.

This propeller was tested, and it gave way with 600 lbs. on the extreme top or 1,200 in propeller, a deflection curve was taken which showed that even with 600 at the top it was well within the limits of elasticity.