Helicopters are indispensable in our society and save countless lives when deployed as rescue helicopters. At the same time, they are very loud, especially during the landing approach. In the STAR (Smart Twisting Active Rotor) project, the German Aerospace Center (DLR) is working with other project partners from the U.S., France, the Netherlands, Japan, and South Korea to increase the performance of rotor blades while simultaneously reducing their noise emissions and the vibrations they generate.

During hover flight and at maximum speed, helicopters require a great deal of power and exhibit high vibration levels, particularly during slow, fast, and maneuvering flight. This could be significantly improved if the rotor blades could adapt statically and, above all, dynamically to the respective aerodynamic flight conditions.

Active warping of rotor blades without mechanical components

In this project, DLR researchers from the Institute of Flight Systems and the Institute of Lightweight Structures are investigating actively warpable rotor blades with piezoceramic actuators integrated into the blade skin, which deform when an electrical voltage is applied. In this way, the rotor blade is twisted—statically under DC voltage and dynamically under AC voltage—as if an artificial muscle were working within the blade. “What makes this approach unique is that the active torsion of rotor blades requires no mechanical elements and is only minimally influenced by the centrifugal forces acting on the rotor blades,” explains Prof. Dr. Berend Gerdes van der Wall, project leader at the Institute of Flight Systems Engineering.

Test campaign in the large low-speed wind tunnel at DNW

After extensive preparation, a four-blade rotor equipped with active twisting and measuring four meters in diameter was tested for the first time worldwide in the large low-speed wind tunnel at the German-Dutch Wind Tunnels (DNW) in the Netherlands. The three-week measurement campaign took place at the end of 2025 under the leadership of DLR in close collaboration with all project participants from NASA, the U.S. Army, ONERA, DNW, JAXA, KARI, and Konkuk University. During the tests, noise reductions of up to seven decibels were measured during the landing approach, which corresponds to more than a halving of the perceived noise. Vibrations were reduced by more than half, while the rotor’s efficiency under high loads was increased.

Results can be applied to various scenarios

“As part of the measurement campaign, we were able to successfully test the concept in a realistic test environment. The results show that not only was efficiency increased, but noise and vibration were also significantly reduced,” says van der Wall. In addition to rotor forces, moments, and power, the data collected includes blade movements, their deformations and loads, surface pressures, acoustic measurements, as well as flow field and boundary layer measurements. This information enables comprehensive validation of computational programs. Furthermore, the results can be applied to various scenarios, ranging from conventional helicopters to high-speed configurations and urban air mobility concepts.

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