180 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 27 



eter at the hub end, tapering to 1.25 inches at the end of the blade. The blade 

 was exceedingly stiff as regards pressure produced by thrust, but it was found 

 to be considerably strengthened and made very much safer when guy-wires were 

 added, in the manner explained above. This general type of construction was 

 adhered to in all the future propellers for the aerodrome, though slight mod- 

 ifications, both as to the size of the arms and the number and position of the 

 cross-pieces which formed the framing of the blade, were adopted from time to 

 time. A pair of heavy propellers, 2.5 metre, 1.25-pitch ratio, 36-degree blade, 

 the hubs of which were formed of brass castings, was, however, constructed 

 for experimental purposes, where weighl was not an important factor. 



When these propellers were designed, the calculations as to their size and 

 the horse-power which would be required to drive them at a certain speed were 

 based on the very incomplete data obtained from the various propeller tests 

 conducted during the preceding years. When later calculations were made for 

 them, on the data obtained in the more accurate tests made in the summer of 

 1898, it was found that the power of the engines with which it was proposed to 

 equip the aerodrome would not be sufficient to drive the propellers at anything 

 like the speed which the former calculations had shown would be possible; and 

 that, therefore, either the ratio of the gearing between the propellers and the 

 engine would have to be changed so as to permit the engine to run at a very 

 much higher speed than the propellers, or that propellers, having either less 

 pitch or a smaller diameter, and possibly both, would have to be substituted for 

 these larger ones. 



Since it was easier to change the propellers than to change the gearing, 

 a new set of propellers was designed which were of 2 metres diameter, with a 

 pitch ratio of unity, and with a width of blade of only 30 degrees. It was cal- 

 culated that 20 horse-power would drive these two propellers at a speed of 640 

 R. P. M., when the aerodrome was flying at a speed of 35 feet per second and 

 the propellers were slipping about 50 per cent, this being found to be about the 

 speed at which the engines might be expected to develop their maximum power. 

 As the larger propellers having the brass hubs were thought to be excessively 

 heavy, the hubs weighing 10.25 pounds each, and as any change either in size, 

 pitch, or width of blade necessitated a new set of patterns in case the hubs 

 were cast, it was decided to construct the new hubs of steel tubing. The weight 

 was further reduced by decreasing the size of the wooden arms to 1] inch in 

 diameter at the hub, tapering to 1 inch at the end of the blade. 



After the engine builder in New York had been unable to fulfil his contract 

 on the engine, and it had been condemned, propeller tests were made with the 

 experimental engine built in the Institution shops. These tests showed: First, 

 that the results which mighl be expected from larger propellers could be very 

 safely predicted by extrapolation from the results of the propeller tests of 1898; 



