Structural design and testing of Active Twist Blades – A comparison

Kurzfassung

Individual Blade Control (IBC) for helicopter main rotors promises to be a method to increase flight performance and to reduce vibration and noise. Several structural concepts for the implementation of such a secondary control have been presented in the past. Many of those include discrete me-chanical components like hinges, levers or gears, which are subject to wear. In contrast to that, active twist blades use smart materials, which are directly embedded into the blade structure and – on top – do not show any discrete edges. Several different technologies have been proposed to twist rotor blades. Within the common DLR-ONERA partnership, a project called “Active Twist Bla-de” (ATB) was established in 2004, which investigated different concepts for active twist blades, using the BO105 blade with a cord length of 121 mm and a C-spar as reference. The concept followed by DLR uses a technology consisting of skin integrated piezoelectric patches oriented at +/-45 deg in upper and lower skin, respectively. Specially shaped Macro Fiber Composi-te actuators (MFCs) generate a directed strain in the skin. This is a prerequisite for the shear torsi-on coupling, which causes the blade to twist. In addition to that the twist generation was enhanced by the use of non isotropy in the skin and a variation of the actuator orientation. Several different demonstrators have been designed and built, featuring different degrees of isotropy in the skin and different designs and angles of the actuator. The concept followed by ONERA uses a twistable section closed by actuation (TWISCA), where the actuator is introducing warping into the section. This is done by cutting the section along the radius of the blade and bridging this gap with a shear actuator, which is causing the two sides of the slot to move parallel to one another. Different locations of the slot in the profile were proposed. At first a slotted trailing edge was investigated, where later developments move the slot into the spar region. Specially designed shear actuators – based on MFCs as well – were laid out to intro-duce the warping movement into the structure. For both concepts several 1:2.5 mach scaled demonstrator rotor blades were built which measure 2 m radius, each. Besides intensive lab tests for determining stiffness distribution and active twist for all blades a testing campaign in the DLRs whirl tower facility was carried out, which is especially equipped for testing rotor blades with active elements. The blades were tested in hover conditions, only. Blade tip twist was measured optically for all blades. The active twist angles for quasi static excitation exceeded the minimal demand of ±1.5 deg for all blades. On top of that the blades dy-namic behaviour (fan plot) and active twist under centrifugal loads and quasi static and dynamic excitations were measured. The specific working capability of the designs was derived and com-pared, considering not only the twist at the blade tip, but also the torsional rigidity and the actuator volume. A final discussion of reliability and maintenance issues will close this paper.