Design and Test of Active Twist Blades for Individual and Local Blade Control

Kurzfassung

Individual- and Local Blade Control (IBC/LBC) for helicopter rotors promises to be a method to increase flight performance and to reduce vibration and noise. Quite a few concepts to realize IBC Systems have been proposed so far. Some of them have already been tested in wind tunnels or on real helicopters. A drawback of all systems that include discrete mechanical components like hinges, levers or gears is their vulnerability in a helicopter environment with high centrifugal loads and high vibration levels. That’s why the idea of using smart materials that are directly embedded in the rotor blade structure is very attractive for this application. Operating as solid state actuators they can generate a twist deformation of the rotor blade without any friction and wear. A promising approach is the use of anisotropic piezoelectric strain actuators embedded in the rotor blade skin. This concept has been intensively investigated in the US. Wind tunnel tests have been performed with 1/6th scaled CH-47 rotor with a radius of 1.397m and a chord of 108mm in a heavy gas me-dium. Investigations on up-scaling have been carried out using a full scale blade segment of a CH-47 blade. Main features of these blades are a D-spar configuration and several plies of Active Fiber Composites (AFC). Motivated by these promising results and the potential benefits, the goal of this work was the development and test of an active twist blade incorporating improved actuation tech-nology and alternative structural concepts to bring this technology a further step forward. At this phase the test are limited to a verification of the actuation system and the structural concepts under centrifugal loads. The intention is to develop the necessary prerequisites for a successful wind tunnel campaign with an active twist model rotor. Because of the large amount of available experi-mental data, the BO105 model rotor was chosen as reference rotor, having a radius of 2m and a chord of 121mm. The baseline design of all blades was a C-spar configuration with an anisotropic blade skin incorporating a single layer of state of the art Macro Fiber Composite (MFC) actuators. The design of the blades was optimized using a finite element code as well as rotor dynamic simu-lations to predict the benefits with respect to vibrations, noise and performance. Several blades with different skin lay-ups, actuator configurations and spar geometry were designed build and tested. The blades were intensively investigated within some bench- and centrifugal tests. To mea-sure the blade deformation the blades were equipped with several sets of strain gauges. An optical measurement system was installed to allow a direct measurement of the active twist angle. The centrifugal test comprised a measurement of the active twist performance at the nominal rotation speed of 1043 RPM at different excitation frequencies from 2/rev up to 6/rev. Finally the blades were compared with respect to their twist performance.