# Helicopter Rotor Thickness Noise Control Using Unsteady Force Excitation
## 3 Validation of Noise Reduction Method  
### Model Rotor and Test System  
Due to the complex flow environment around the helicopter rotor tip, it is important to generate an unsteady exciting force at the tip of the blade while having little influence on aerodynamic performance. In this paper, a model rotor with a trailing-edge winglet was designed to verify the principle of the sound field cancelation. As shown in Figure 6, an active winglet was installed at the trailing edge of the blade tip, and when it deflected, a change in the windward area caused a variation of pressure drag, thereby forming the required unsteady force in the rotor plane. The deflection law of the winglet is \(\theta_{f}=A_{f}\cos\left(H_{f}\psi+\psi_{f}\right)\), where \(\theta_{f}\) represents the winglet deflection angle, \(A_{f}\) is the deflection amplitude, \(H_{f}\) is the frequency harmonic, \(\psi_{f}\) is the initial phase angle, and \(\psi\) is the azimuthal angle. Figure 7 shows the model rotor used in the experiment. The airfoil was OA212, the radius R was 1.27 m, the chord c was 0.13 m, and the linear twist was \(-8^{\circ}\) (from blade root to tip). The trailing-edge winglet had a length of 0.118 m, and the chord was 0.02 m. The winglet was driven by a drive mechanism embedded in the outboard blade to achieve high-frequency rotation in a strong centrifugal force field. In addition, the quality of the drive mechanism was required to be light. The drive mechanism designed in this paper combined the piezoelectric stack and the diamond-shaped amplifier, as shown in Figure 8. When an alternating voltage was applied to the piezoelectric stack, a telescopic motion occurred, which was amplified by the diamond-shaped frame to generate a slight displacement, which was converted into a push-pull motion of the pull rod. Then, the winglet achieved deflection by the bending deformation of the elastic piece. The measurement in the non-rotating condition showed that the amplitude of the winglet deflection angle was of linear dependence to the voltage on the piezoelectric stack, and about 0.8\({}^{\circ}\) corresponded to 10 V voltage change. In the rotating condition, the deflection amplitude was affected by the aerodynamic force and centrifugal force acting on its surface. Due to the lack of angular displacement feedback control in our small-scale rotor model, the deflection angle could not be adjusted in real time, thus there may have been some differences between rotation and non-rotation conditions. This is one of the limitations of present test facilities. To minimize this adverse effect, a lightweight winglet (about 15 g) and a high-power piezoelectric reactor driver were used in the test. The required power of the active winglet can be estimated from \(P=4\times f_{\mathrm{w}}\times C_{\mathrm{pie}}\times V_{\mathrm{pie}}^{2}\), where \(f_{\mathrm{w}}\) is deflection frequency, \(C_{\mathrm{pie}}\) is capacitance of piezoelectric stack, and \(V_{\mathrm{pie}}\) is voltage. Figure 6: The schematic of the rotor with a trailing-edge winglet: (**a**) rotor planform, (**b**) airfoil section, (**c**) exciting force generated by winglet. Taking the maximum power state in the present test as an example (the test cases are seen in Section 3.2), the required power was \(P=4\times 3\times 18\) Hz \(\times\) 40 \(\mu\)F \(\times\) 100 V\({}^{2}\)= 86 W. The rated output power of the piezoelectric reactor was 300W, and it could provide sufficient power, even in the flap in the rotation environment. Therefore, the relationship between the deflection amplitude and voltage in a rotating condition is similar to the non-rotation state. Figure 9 shows the schematic diagram of the test control system. A sinusoidal analogue signal was emitted from the controller when it received the pulse signals from the host computer and the rotor. Then, the signal was amplified by the power amplifier to output alternating voltages to drive the mechanism of the trailing-edge winglet through the current collecting ring of the rotor test rig.