Development of active twist rotors at the German Aerospace Center (DLR)

Abstract

Helicopter main rotors are characterized by complex unsteady aerodynamic conditions, which are causing vibrations and noise in and around rotary wing aircrafts. The aerodynamic conditions also cause increased drag, which leads to higher fuel consumption. Even modern helicopters still suffer from these drawbacks. This is why there are many efforts to influence the flow conditions by passive and active means. Active means have the advantage to adapt to varying demands, which can change significantly. A first attempt is to perform a sinoidal variation of the pitch angle of the blades with a frequency that is an integer multiple of the rotor frequency, the so called higher harmonic frequencies. This can be done by additional displacements of the swash plate or by a variation of the pitch link length using actuated pitch links. Since both designs have several drawbacks, one of the most promising approaches is the realization of a secondary control via the deformation of individual blades. Such an actuation can be realized by blade flaps or by the integration of piezoelectric actuators in the blade itself, which causes the blade to twist. At the DLR such twist blades have been investigated intensively. A series of blades has been built using thin skin integrated actuators. This Paper gives an overview of all active twist blades with skin integrated actuators that have been designed and manufactured at the DLR so far. Different design philosophies have lead to different geometrical setups. A comparison of the blades power consumption for given control laws for noise and vibration reduction is also given. Finally, a unique testing technique for nondestructive measurement of mass distribution is discussed in this paper. are characterized by complex unsteady aerodynamic conditions, which are causing vibrations and noise in and around rotary wing aircrafts. The aerodynamic conditions also cause increased drag, which leads to higher fuel consumption. Even modern helicopters still suffer from these drawbacks. This is why there are many efforts to influence the flow conditions by passive and active means. Active means have the advantage to adapt to varying demands, which can change significantly. A first attempt is to perform a sinoidal variation of the pitch angle of the blades with a frequency that is an integer multiple of the rotor frequency, the so called higher harmonic frequencies. This can be done by additional displacements of the swash plate or by a variation of the pitch link length using actuated pitch links. Since both designs have several drawbacks, one of the most promising approaches is the realization of a secondary control via the deformation of individual blades. Such an actuation can be realized by blade flaps or by the integration of piezoelectric actuators in the blade itself, which causes the blade to twist. At the DLR such twist blades have been investigated intensively. A series of blades has been built using thin skin integrated actuators. This Paper gives an overview of all active twist blades with skin integrated actuators that have been designed and manufactured at the DLR so far. Different design philosophies have lead to different geometrical setups. A comparison of the blades power consumption for given control laws for noise and vibration reduction is also given. Finally, a unique testing technique for nondestructive measurement of mass distribution is discussed in this paper.