Active Suppression of eVTOL Rotor Vibrations via Harmonic Rotor Speed Modulation

Electric vertical take-off and landing vehicles (eVTOL) are now widely regarded as a cornerstone technology for urban air mobility, largely because they promise a combination of distributed propulsion, lower perceived noise, and operational flexibility that conventional rotorcraft hard to achieve. Most contemporary eVTOL designs depart sharply from helicopter architectures, relying instead on multiple rigid rotors driven directly by electric motors. The absence of gearboxes, swashplates, and articulated hubs simplifies the mechanical system and enhances reliability. At the same time, however, this simplification comes at an aerodynamic cost that is only beginning to be fully appreciated. Rigid rotors behave differently once an aircraft leaves hover. During forward flight, the advancing and retreating blades experience markedly different inflow conditions, producing circumferential load asymmetries that cannot be passively relieved. In helicopters, flapping and lead–lag motion naturally redistribute these loads. In most eVTOL rotors, such mechanisms are deliberately excluded. As a result, periodic aerodynamic forces are transmitted almost directly to the hub and, from there, into the airframe. These forces are strongly concentrated at blade-pass frequencies and their harmonics, giving rise to persistent vibrations that accumulate over time. Left unaddressed, they threaten not only structural durability and avionics integrity, but also cabin comfort—an issue that becomes critical for passenger-facing urban operations. Historically, rotorcraft engineers have approached vibration mitigation through either passive or active means. Passive solutions such as tuned absorbers and isolation systems are robust and conceptually simple, yet they inevitably increase mass and are often optimized for narrow operating conditions. Active approaches—including higher harmonic control, individual blade control, and actively controlled flaps—offer far greater authority, but at the expense of added actuators, complex blade hardware, and demanding control requirements. For eVTOL aircraft, which derive much of their appeal from architectural simplicity and efficiency, these trade-offs are especially problematic. Electric propulsion introduces a quietly disruptive possibility. Modern electric motors are capable of rapid, precise speed modulation over a wide frequency range, with a level of controllability that is difficult to achieve using combustion engines. This raises a natural and largely unexplored question: rather than modifying blades or adding hardware, could rotor vibration be addressed by treating rotor speed itself as an active control variable? To this end, new research paper published in Aerospace Science and Technology and conducted by Dr. Kai Guan, Professor Yang Lu, Dr. Changtian Wang, and Dr. Jinru Chen from the National Key Laboratory of Helicopter Aeromechanics at Nanjing University of Aeronautics and Astronautics, the researchers developed an active vibration control framework that uses periodic rotor speed modulation to generate aerodynamic load components capable of canceling blade-pass vibrations. They established an analytical model linking speed fluctuations to hub forces, validated it through high-fidelity simulations, and confirmed its effectiveness experimentally in a wind tunnel. The key novelty lies in exploiting direct-drive electric motors as vibration control actuators, eliminating the need for additional mechanical complexity.

The research team developed an analytical framework capable of capturing how periodic rotor speed modulation influences hub loads. Using blade element momentum theory and assuming rigid, hingeless blades, the authors derived closed-form expressions linking speed fluctuations to additional harmonic components in hub forces and moments. A key outcome of this modeling effort was the demonstration that sinusoidal speed modulation at a chosen harmonic order introduces controllable aerodynamic load components at the same frequency, with amplitudes and phases directly governed by the modulation parameters. The authors conducted numerical simulations using a representative 300-kg-class eVTOL rotor configuration to examine the practical implications of this mechanism. The rotor was discretized along the blade span, and unsteady aerodynamic forces were evaluated using airfoil lookup tables to capture stall and reverse-flow effects encountered in forward flight. Simulations across a range of cruise speeds revealed a consistent pattern: for each operating condition, there existed a narrow combination of speed modulation amplitude and phase capable of nearly canceling the dominant blade-pass vibration component. Under optimal tuning, reductions exceeding 98% were achieved for the primary vertical hub load across forward flight speeds spanning 50 to 150 km/h. They obtained these reductions with relatively small speed perturbations. Even at the highest simulated flight speed, the required modulation amplitude remained only a few percent of the baseline rotor speed. Beyond the dominant vertical load component, the authors observed secondary but non-negligible effects on other hub force and moment components. In particular, oscillations in counter-torque were also reduced, often to a comparable extent as the primary vertical vibration, while changes in lateral forces and rolling or pitching moments remained relatively small. This balance is important. It suggests that the control strategy selectively targets the most energetically significant vibration modes without introducing large unintended couplings elsewhere in the load spectrum.

Afterward, the authors designed a small-scale rotor test rig specifically to examine speed modulation under controlled inflow conditions. The use of a direct-drive servo motor was central to the setup, as it allowed the prescribed speed fluctuations to be imposed with tight control over amplitude and phase. Hub loads were captured using force–torque sensors with sufficiently high temporal resolution to resolve blade-pass harmonics. Rather than focusing on a single operating point, the experiments spanned multiple wind speeds, collective pitch angles, and modulation parameters, enabling a systematic assessment of both performance and sensitivity.

The team found at the primary blade-pass frequency, vibration reductions exceed 90% were repeatedly achieved for both vertical force and counter-torque and they were consistently across a range of operating conditions. When the analysis was extended to include multiple harmonics simultaneously, the control authority naturally weakened but remained substantial. Across the first nine blade-pass components, overall vibration reductions on the order of 65–70% were obtained, indicating that the method retains effectiveness even when the objective shifts from single-frequency cancellation to broadband mitigation. One particularly interesting observation was that speed modulation altered portions of the higher-frequency load content as well. This points toward interactions with unsteady aerodynamic effects—possibly dynamic stall or wake modulation—that are not fully captured by simplified harmonic arguments.

Taken together, Professor Yang Lu and his colleagues show that hardware already present in electric propulsion systems can be repurposed to perform active vibration mitigation, simply by rethinking how rotor speed is used. From an engineering perspective, this is a natural fit for electric aircraft. Direct-drive motors are inherently capable of fast torque response and fine speed control; exploiting these properties for vibration suppression avoids the need for additional actuators, blade-mounted devices, or complex hub mechanisms. The payoff is a control strategy that adds functionality without adding mass, mechanical fragility, or maintenance burden. Moreover, the authors provided a physically transparent explanation of why the method works by framing vibration reduction in terms of harmonic superposition and phase cancellation. This kind of clarity matters, especially during early design phases, when designers rely on simplified models to navigate large configuration spaces. The fact that effective control can be achieved with relatively small speed perturbations further suggests that practical implementation would not impose excessive electrical or mechanical stress on the propulsion system. Looking beyond eVTOL platforms, the implications are broader. As electric propulsion continues to penetrate helicopters, tilt-rotors, and unmanned aerial systems, rotor speed modulation may evolve into a general-purpose tool for load and vibration management. Ultimately, by transforming rotor speed from a fixed operating parameter into an active control variable, this work points toward a quieter, smoother, and more efficient future for electric vertical flight without sacrificing the simplicity that makes eVTOL aircraft attractive in the first place.