Drag reduction: As the tiltrotor turns

Organised around six major platforms, Clean Sky will lead to the development of in-flight and ground demonstrators. Among these Integrated Technology Demonstrators (ITDs), the Green Rotorcraft platform is specifically devoted to helicopters and tiltrotor aircraft. Their operations are expected to grow sharply in the future to meet the increasing transportation demands.

By tilting the rotor nacelles perpendicular or parallel to the flight direction, the tiltrotor offers the capability to take-off and land like a helicopter and to cruise like an aeroplane.

The EU-funded project DREAM-TILT (Assessment of tiltrotor fuselage drag reduction by wind tunnel tests and CFD) aimed to perform tests and run simulations to assess the aerodynamic characteristics of fuselage components.

Considering the different operational modes for a tiltrotor, the primary drag sources are located at the front fuselage, the wing root fairings, the landing gear sponsons and the rear empennages. The Green Rotorcraft Consortium 2 (GRC2) had previously identified optimised shapes of these components that contribute to decreasing aircraft drag and enhancing aerodynamic efficiency.

To this purpose, the team used computational fluid dynamics (CFD) coupled with innovative design methodologies based on multi-objective evolutionary algorithms. Through wind tunnel tests, DREAM-TILT assessed fuselage components of the most recent European civil tiltrotor concept based on ERICA architecture. Specifically, project partners determined drag reduction with respect to the baseline configuration.

All components were evaluated in a wind tunnel test campaign to obtain an accurate drag breakdown and identify their individual contribution to overall fuselage aerodynamic performance. In addition to global force measurements, researchers carried out additional flow visualisation runs to better understand flow mechanisms responsible for the benefits observed for new drag reduction configurations.

Three velocity components of the flow field were provided for localised validation of CFD tools adopted in the optimisation process. Next, scientists used numerical models already tested and validated to perform a series of calculations of the aerodynamic performance of the optimised ERICA fuselage at full scale. The results were compared against wind tunnel data to assess the impact of shape optimisation, including also rotor effects.

The benefits from the aerodynamic optimisation of tiltrotor components have already been translated into a drag reduction equal to -4.5 % at full-scale conditions. DREAM-TILT demonstrated the effectiveness of the CFD-based approach for optimisation of tiltrotor components, opening the way to design of a more environmentally friendly tiltrotor. Decreasing fuselage drag will have major implications in terms of efficiency and fuel consumption.