268 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 



which in itself is quite flexible. This flexibility took the place of the 

 articulation which was the secret of success in later full-sized 

 machines. Also, in a small model rough action would be unnoticed, 

 since there is a very great difference between observing the perform- 

 ance of a model, and actually being in a machine yourself. This 

 model flew remarkably well and encouraged him to proceed with 

 further experiments. 



It was the next machine, employing hinged joints at the attach- 

 ment of the blades to the rotor hub, which proved really successful. 

 The construction permitted the blades to flap upward and downward 

 during rotation, so that each vane automatically took the correct 

 effective angle of incidence at all times. For the first time, the 

 problem of gyroscopic force, which is a very powerful influence to 

 work against, was nullified. 



Besides the immediate and great improvement in the performance 

 of the rotor, this articulation solved a number of other besetting 

 problems. It made autogiration possible at angles of attack where 

 otherwise the rotation of the rotor would discontinue. In wind 

 tunnel tests which Cierva conducted in Spain it was found that the 

 rotor would continue to turn at every angle. This however would 

 not be so if the blades were rigidly attached to the rotor hub. 



Secondary articulation, permitting variation of the position of 

 the blades with regard to one another along the circle which they 

 describe, was another important step. This gave smooth action to 

 the rotor, which before was rough. Further success came with the 

 addition of the fixed wing, which improves efficiency in the present 

 state of the art of the Autogiro. Before that time, Cierva had 

 utilized ailerons on outriggers, as it had been found necessary to 

 have some type of lateral control, although he had flown the 

 machines with neither fixed wings nor ailerons. 



Another improvement in eificiency came with the use of the geared 

 motor. The gears in this case refer only to reduction of propeller 

 speed since the rotor depends entirely on the action of the air 

 during flight. A large slow turning propeller gives much greater 

 efficiency at slow forward speeds than a smaller one turning at 

 engine speed. The geared propeller therefore increases greatly the 

 efficiency of the Autogiro since its best angle of climb is at a much 

 slower forward speed than the airplane. 



A step toward making the Autogiro practical is the self-starting 

 tail invented during the past year by Cierva. This is a boxlike bi- 

 plane structure by which the slipstream can be diverted upward 

 while the machine is at rest on the ground, thus bringing the rotor 

 to the necessary speed for take-off. Before the introduction of this 

 feature, it had been necessary to taxi slowly for some time about the 

 field until this rotational speed was attained. 



