History of aicraft piston engines

In doing so t hey often argued each other into a

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Unformatted text preview: opeller, on toy h elicopters. A helical screw is shown on a DaVinci helicopter drawing of a bout 1500, a nd screw propellers were used on dirigible balloons as early as 1784. An early Langley propeller is shown in figure 68a. T he success of the Wright brothers was in no small degree due to the excellent performance of their two counter-rotating airfoil-section propellers, c hain driven at 8/23 engine speed, or about 380 rpm. They gave serious a ttention to propeller design. Apparently they could get no useful data from marine engineers and had to develop their own theory. In doing so, t hey often argued each other into a reversal of opinion, but finally arrived a t a design which Frank W. Caldwell says ran at near optimum ratio of forward speed to tip speed, and had an efficiency of about 0.70. T he Wright propellers were of 3-ply l aminated wood, very light in w eight. It should, perhaps, have served as a warning to future propeller designers that the first fatal a ccident—the c rash of Orville Wright and L ieutenant Thomas E. Selfridge in 1908, r esulting in the death of Selfridge— w as caused by a propeller failure. A broken blade set up sufficient vibration t o cause the propeller to cut a r udder-bracing w ire, upon which the tail c ame askew and all control was lost. W ooden propellers were universally used from the time of the Wrights' first flight until well after World War I. They were very reliable for the needs of that time, and were superseded only when the requirements for p ower and tip speed exceeded the limits within which a wooden propeller would safely operate. 75 Figure 68.—Propeller d evelopment: a, Langley wood propeller (NASM 1938-56E) of 1893, used on Aerodrome No. 4. b, Curtiss-Reed t wisted-duraluminum propeller (D-6), 1925. c, Hamilton-Standard 2B20 constant-speed propeller, 1946. d, Hamilton-Standard Hydramatic f ull-feathering propeller of 1947. For typical wooden propeller of the period 1910 to present, see figure 50. (Photos, respectively, A-287, A-51875, A-3096, A-3097) 76 M aterials superior to wood were actively sought after World W a r I. F r a n k Caldwell, head of the propeller section at M c C o o k Field ( 1918-1930) a n d later chief engineer at H a m i l t o n S t a n d a r d , was a leader in this field, a n d has given excellent accounts of propeller developments. H e r e there is s pace for only the briefest review. M i c a r t a (canvas laminated with bakelite) was successfully used as a w ood substitute by 1920. I n 1921 Caldwell tested a s teel-bladed p ropeller o n his electric whirling m a c h i n e to twice its rated power. H e then, very i nnocently, presented it to m e for a " r o u t i n e " test on a Hispano-Suiza 3 00-hp engine. After a few minutes at rated power, a blade broke off, c a m e t h r o u g h the control b o a r d between the heads of two operators, climbed a w ooden staircase, a n d went through the roof. T h e engine was reduced to j unk. T h e above incident was an early warning of the i m p o r t a n c e of vibration a n d fatigue...
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