Several considerations regarding the variable blade length rotor

Abstract

Introduction S INCE the revolutionary breakthrough of Cierva and the engineering achievements of Sikorsky, the general layout of the conventional helicopter rotor has not changed much. Though popular and widespread, this arrangement has some major limitations, one of the principal ones being the unsymmetric loading of the rotor disc. Serious difficulties also appear at the tip of the advancing blade (Mach number effects), and at the inner region of the retreating blade (reverse flow). Hence, significant research is still carried out to properly model the blade oscillatory aerodynamics in forward flight, with nonlinear unsteady effects being currently considered using advanced computational techniques.However, the reader's attention is drawn to the fact that there are alternative designs for dealing with some of these problems. One of these alternative designs is the advancing blade concept (ABC) in which two counter-rotating coaxial rotors are used, thus somehow balancing the global loading. However, the ABC design is still not free from Mach number effects on the advancing tip, and needs to deal with the additional constraint of preventing the counter-rotating blades contacting each other while flapping. Another alternative design is the variable length blade (VLB). The increase in tip Mach number on the advancing blade is compensated by the reduction in blade length, with a complementary effect taking place on the retreating blade (cyclic VLB). Or one can decrease the overall rotor diameter as flight speed increases (collective VLB). Such concepts were studied in some depth about two decades ago, and their merits and limitations were highlighted. But, due to technological difficulties at that time, the VLB concept did not pass the research stage. However, with the recent years developments in active control and smart materials technologies, it is the belief of the authors that this alternative design should be given a fresh re-evaluation.