Measurement of the dynamic tip twist angles of an Active Twist model scale rotor blade

Kurzfassung

Ever since the early days of helicopter flight vibrations and noise have been issues. One major source for helicopter noise and vibration is the main rotor. Individual blade control (IBC) in general tackles noise and vibration of rotary wing aircrafts. The architecture of IBC systems evolved from actuated pitch links to active flaps and active twist. The active twist blades investigated at DLR (German Aerospace Center) are actuated by piezoelectric Macro Fiber Composite Actuators that are integrated into the skin of the rotor blades. Unfortunately some advantages of the active twist concept like: distributed actuation without big local strains or no moving components are also causing some challenges in the control of the actuation. Even though a feedforward control of the actuation voltage can be realized easily it is quite difficult to find a physical value that is suited to close the control loop. As the angle of attack at the blade tip has a big influence on the trajectory of the vortices the tip twist angle of the blade is identified to be controlled. But since the twist is generated over the whole span of the blade, it is not possible to measure the actuation amplitude at a discrete hinge as for an active flap. The fact that the active twist blades investigated at DLR are model scale blades causes additional challenges due to the high centrifugal forces and the limitations in size. Within this paper the requirements on a measurement system for the detection of the tip twist angle of an active twist blade in model scale will be elucidated. As the next step in the development of the active twist blades at DLR are wind tunnel tests, the system should work at least under wind tunnel forward flight conditions. First two optical measurement methods for the detection of the tip twist at one azimuth position will be introduced and compared. Those techniques are serving as a reference later on. From the intention to control the tip twist in a closed loop, arises the need for a continuous measurement of the tip twist angle. In a first approach accelerometers were used to detect the flap wise accelerations at the leading and trailing edge of the blade tip. The results of those measurements will be compared to the optical reference and the limitations of this principle will be shown. Another way to estimate the motion of the blade tip is the use of strain gauge data. Theoretically the moments measured by the strain gauges can be integrated to a deformation when the distribution of the stiffness is known. The database that is needed for these investigations was established during an extensive measurement campaign performed in the DLR rotor test facility in Braunschweig between January 2009 and April 2009. The challenges related to the analysis of this data for an active twist rotor blade will be pointed out. A possible solution will be investigated and restrictions on the applicability of this measurement method will be made.