# Review of Control Technologies for Quiet Operations of Advanced Air-Mobility
## 4 Active Noise Control (ANC) Technology
### On-Blade Actuation  
The second type of ANC is the modification or suppression of the sound generation, is the case where a modification is done on the blade to alter the radiation impedance of the original noise source for acoustic mitigation. Most active rotor noise control ideas fall under this mechanism or classification. Several decades of research and studies were directed towards BVI noise reduction, vibration reduction, and performance enhancement [43; 44; 45]. On-blade active rotor control methods such as higher harmonic control (HHC), individual blade control (IBC), trailing edge flap (TEF), blowing on or near the rotor blade have been tested as potential ways of altering the wake structure, and subsequently the reduction of the BVI effects [44]. Higher harmonic control system is an active vibration control mechanism where the rotor blades are oscillated at higher harmonics of the rotational speed. Although originally devised as a method for vibrational reduction, it was suggested as a potential method for reducing BVI in the late 1990s [45]. However, it was determined that when implemented, fuselage vibration levels were drastically increased when driven at low BVI noise frequencies [46; 47]. This can be reduced by applying the excitation at a higher harmonic frequency, which ultimately led to the development of the individual blade control methods. eVTOLs are generally scaled down variants of rotorcrafts with multiple rotor systems. Applying this method of active rotor control might prove quite deleterious as the vibrations or excitation of the oscillations from all the rotors might introduce some structural issues and may reduce the level of comfort in the fuselage/cabin. Several active control research have been done particularly for helicopter rotors with little to none done for small-scale rotors for UAVs. However, we review a few techniques and their feasibility and successes. [PERSON] et al. [48] developed a low-frequency controller geared towards reducing helicopter rotor BVI noise. The utilize an active twist rotor concept via torque load distributions at 2/rev frequency. They apply this methodology numerically to a scaled Bo-105 4-bladed helicopter main rotor, with a radius of 2 m, a constant chord of 0.212 m, linear twist of -8 degrees, and a rotational speed of 109 rad/s. The general Kussner-Schwarz theory is employed to determine the unsteady aerodynamic loads associated with the profile downwash. Aeroelastic computations are performed to account for the blade deformation and its effects on the wake. The wake inflow is predicted using a free-wake boundary element solver. They perform open-loop control using the active twist, which showed that a 2/rev actuation was effective in reducing the BVI sound pressure level by about 2 dB. However, this also resulted in an increase in vibratory loads and low frequency noise of about 3 dB. Using the microphones below helicopter and at the skid ends (Figure 8a), a closed loop controller was developed which resulted in a maximum BVI sound pressure level reduction of 6 dB at the retreating side. One of the earliest proven tests of the concept of on-blade actuation for harmonic noise control was the full-scale \"Boeing SMART\" rotor [49]. They utilized an active trailing edge flap for the blade noise control. The results showed that placing the controls near the tips is an effective thickness noise control, with the total thickness noise reduced to about 50% in amplitude. More recently, [PERSON] and [PERSON] [43; 50; 51] performed experiments with a 1/7 th scale rotor, where they used tip mass ejection to attempt to reduce specifically the in-plane thickness noise. They implement this by extracting out the mass flow rate equation and comparing with the Ffowcs-Williams Hawking equation ([PERSON], 1969).