DYNAMIC BLADE SELECTION - a procedure to get inter-changeability of the modern EC135 - Main Rotor Blades

The EC135, a light weight helicopter with a take-off weight of 5800 lbs, has a bearingless main rotor with 4 fibre composite blades (Ref. 1). In order to reach full interchangeability of production blades, a new simplified dynamic selecting procedure has been established, which allows to avoid using master-blades. This procedure consists in measuring the inplane and out-of-plane location of the blade tip during test runs on the whirlstand. Corrective means like pitch link length and chordwise balancing masses allow to fit the blade behaviour versus rotor thrust to a calibrated electronic master. The sensor signals caused by the passing blades during whirlstand running are measured with a ground-based triangular optical system installed below the rotor disc. Computerized data processing determines the conformity of the single blades with respect to given tolerances. Considerable cost savings are achieved because of fewer test runs on the whirlstand and deletion of primary and secondary master-blades. The operators have the benefit of complete interchangeability of the main rotor blades.