AIRCRAFT PROPULSION — TAYLOR 283 



power than is required by the supercharger. This engine is standard 

 on the DC-7 and "Super Constellation," the last large piston-engined 

 passenger-transport planes built in this country. 



PROPELLERS 



Gibbs-Smith credits the Chinese with first use of the air propeller, 

 on toy helicopters. A helical screw is shown on a DaVinci helicopter 

 drawing of about 1500, and screw propellers were used on dirigible 

 balloons as early as 1784. 



The success of the Wright brothers was in no small degree due to 

 the excellent performance of their two counter-rotating wooden pro- 

 pellers, chain driven at 8/23 engine speed, or about 380 r.p.m. The 

 Wright brothers encountered many difficulties with propeller design. 

 Apparently they could get no useful data from marine engineers and 

 had to develop their own theory. In developing this theory, they 

 often argued each other into a reversal of)inion, but finally arrived at 

 a design which Caldwell says ran at near optimum ratio of forward 

 speed to tip speed, and had an efficiency of about 0.70. 



The Wright propellers were of 3-ply laminated wood, very light 

 in weight. It should, perhaps, have served as a warning to future 

 propeller designers that the first fatal accident — the crash of Orville 

 Wright and Lieutenant Selfridge in 1908, resulting in the death of 

 Selfridge — was caused by a propeller failure. A broken blade set up 

 sufficient vibration to cause the propeller to cut a rudder-bracing wire. 



Wooden propellers were universally used from the Wrights' first 

 flight until well after World War I. They were very reliable for 

 the duty called for in that time, and were superseded only when the 

 requirements for power and tip speed exceeded the limits within 

 which a wooden propeller would safely operate. 



Materials superior to wood were actively sought after World War I. 

 Frank Caldwell, head of the propeller section at McCook Field (1918- 

 30) and later chief engineer at Hamilton Standard, was the chief 

 agent in this field, and has given excellent accounts of propeller 

 developments. Here there is space for only the briefest review. 



Micarta (canvas laminated with bakelite) was successfully used 

 as a wood substitute by 1920. In 1921 Caldwell tested a steel-bladed 

 propeller on his electric whirling machine to twice its rated power. 

 He then, very innocently, presented it to me for a "routine" test 

 on a Hispano-Suiza 300-hp, engine. After a few minutes at rated 

 power, a blade broke off, came through the control board between 

 the heads of two operators, climbed a wooden staircase, and went 

 through the roof. The engine was reduced to junk. 



The above incident was an early warning of the importance of 

 vibration and fatigue in propeller operation. At my insistence, 

 further "routine" propeller tests on experimental propellers were 



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