Mechanism for Varying the Diameter of Rotors Using Compound Differential Rotary Transmissions

Existing VDR mechanisms have not progressed beyond the ground test experimental stage partially due to their inability to achieve the weight, reliability and maintainability requirements for practical aircraft application. Recent advancements in digital flight control systems and Health Usage Monitoring however can be applied to make a Variable Diameter Rotor (VDR) mechanism practical. Current published VDR design concepts do not integrate with existing rotor hub designs since their mechanisms occupy the space normally used by the rotor hub's structure and constant velocity joint. Many of these designs require a rigid hub configuration to maintain alignment of their VDR mechanism gearing. By using separate differential planetary rotary transmissions at each blade and interconnecting them with flexible timing belts, the mechanism described in this paper integrates with most existing rotor hub design configurations. This paper studies the application of system control and mechanical power transmission technologies to create a practical VDR mechanism for the BA609 three bladed rotor. A historical review of VDR studies and experimentation performed by Bell between 1962 and 1971 provides an introduction to the technical issues facing the development of a practical VDR mechanism design.