Prerequisites for the closed loop control of an active twist rotor blade

Kurzfassung

The jet smooth ride is still a vision for helicopters. Even though noise and vibration levels were already drastically diminished there is still the need for further reduction. The main rotor as a major source of those disturbances is one factor that hinders the realization of this vision. Individual blade control (IBC) in general has the potential to reduce noise and vibration of rotary wing aircrafts. This is mainly done by preventing blade vortex interactions and generation of aerodynamic forces to cancel out vibrations. The architecture of IBC systems evolved from actuated pitch links to active flaps and active twist. Since the active twist can also be used to adapt the pre-twist of the blade, it is as well possible to improve the hover figure of merit and to increase the maximum take off weight. The active twist blades investigated at DLR (German Aerospace Center) are actuated by piezoelectric Macro Fiber Composite (MFC) actuators that are integrated into the skin of the rotor blades (see Figure 1). However some advantages Actuator Strain gage position Figure 1: Active twist blade 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 feed forward control of the actuation voltage can be realized easily it is quite difficult to find a physical value that is suited to close the loop for a direct control of the twist. As the angle of attack at the blade tip has a big influence on the trajectory of the vortices, and therefore the potential to reduce noise and vibration generated by blade vortex interactions, the tip twist angle of the blade is intended 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 accelerations and the limitations in size. In a first attempt commercial 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 an 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 gage data. The strain gages as well as the actuators were directly integrated into the skin of the rotor blade during the manufacturing process. Theoretically the moments measured by the strain gages can be integrated to a deformation when the distribution of the stiffness is known. The main challenges in the calculation of the tip twist for an active twist rotor blade are the differences in the shear flux distribution that arise when the blade is deformed by the actuators or by other "passive" moments (see Figure 2). Hence the twist can not be predicted without additional information about the actuator state. A method for the compensation of the actuator influence on the torsion strain gage bridges was developed. The efficiency of the algorithm is demonstrated using rotor test data. 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 and April 2009. The German-Dutch Wind Tunnels (DNW) supported this campaign with Stereo Pattern Recognition (SPR) measurements to determine the blade motion. Figure 3 shows the principle setup of the test. One single blade was tested on an articulated rotor hub. The SPR cameras were capturing the position of 28 ultraviolet reflecting markers that were painted on the blade. The blade was illuminated by 20 arrays of triggered ultraviolet LEDs. For an additional optical measurement of the blade tip *(E-mail: Steffen.Opitz@dlr.de) strain gage position profile passive active Figure 2: Active and passive distribution of +45 strain Figure 3: Measurement campaign test setup motion, 2 red LEDs were incorporated at the tips leading and trailing edge. A camera with a shutter time of 10s was used to capture the position of the LEDs. The SPR measurements and the additional optical measurement system served as reference for the strain gage based tip twist calculation. The measurement of deformation using strain gages has another remarkable difficulty. It is a challenging task to incorporate the influence of the centrifugal forces in the calibration of the strain gage bridges. Due to small and unavoidable misalignments in the position of the single strain gages the bridge mean value changes with rising rotor speed. Unfortunately this drift is mixed with signals generated by extension torsion or extension bending couplings. Consequently the static deformation of the blade, caused by the centrifugal forces, is hard to measure with strain gages. As already mentioned the use of acceleration sensors for the prediction of the blade tip twist motion is generally an alternative to strain gage data based methods. Since commercially available sensors did not fulfill the requirements to measure the tip twist a novel sensor concept was investigated. The geometry of the sensor is realized in a way that the cross sensitivity to other accelerations is very small. The signal to disturbance ratio is further improved by the use of a Wheatstone bridge. The sensor can be configured to work as an accelerometer that is sensitive to torsional motions only or as a sensor that takes advantage of the centrifugal field to measure even static twist. An optimization is performed to find the best sensor configuration. 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. Experimental results are used to compare different approaches. Further on the principle of a novel tip twist sensor will be shown and simulation results will be presented.