284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



made in a specially constructed "bombproof" shelter. All metal 

 propellers of that time (1921-22) failed, with murderous results to 

 the engine (pi. 24, fig. 1). In one case the whole assembly of crank- 

 shaft, rods, and pistons was pulled out and thrown 20 feet from what 

 remained of the engine and stand. 



Subsequent metal propeller development involved careful attention 

 to vibration problems. The Keed type, using a twisted aluminum 

 plate as a base (pi. 24, fig. 2), was one of the early successful designs. 

 Later, the manufacturers of metal propellers developed elaborate 

 equipment and procedures for the measurement and suppression of 

 blade vibration. 



The need for pitch control was realized in the 1920's, and some early 

 designs of controllable pitch propellers were tested, usually with dis- 

 astrous results because of mechanical weaknesses. Variable pitch 

 became essential with the advent of the high performance airplane, 

 the DC-3 being an early example. To make a long story short, the 

 following is the sequence of important propeller developments with 

 approximate dates : 



1921 Aluminum blades, fixed pitch (Reed). 



1923 Aluminum blades, adjustable pitch. 



1931 Hollow steel blades. 



1929 Controllable pitch, 2-position. 



1935 Automatic, constant speed. 



1938 Feathering. 



1945 Reversible and feathering. 



Both hydraulic and electric pitch control were used until after the 

 last war. Now hydraulic control and aluminum blades are standard 

 on piston engines, with a few exceptions. Plate 25, figure 1, shows an 

 example. 



REDUCTION GEARS 



The Wrights, with their chain drive, were evidently aware that the 

 optimmn speed for engines is not usually that for propellers. Even 

 before the Wrights, most experimental airplanes (Stringfellow, 

 Maxim, Langley) had belt- or gear-driven propellers, although the 

 drive ratio for steam engines was usually up rather than down. 



Direct propeller drive, with the propeller mounted on the crank- 

 shaft, is attractive for its simplicity and reliability, and was used by 

 most of the early flyers after Wright and up to the start of World War 

 I. An exception was the early Renault air-cooled V-8 (pi. 7, fig. 1), 

 the propeller shaft of which was an extension of the camshaft (or vice- 

 versa) and ran at half crankshaft speed — a ratio which has been 

 widely used since. 



Other geared engines which appeared for use in World War I in- 

 cluded the R.A.F. (copy of the Renault), the 8-cylinder-ui-line Mer- 



