The use of active trailing edge flaps on rotors may lead to performance benefits as well as noise and vibration reduction. In this work, the HMB CFD solver is used, and the trailing edge flaps are modelled using a combination of surface and mesh deformation. Starting from a baseline blade design, the flap is first assessed using dMdt computations, that account for the simultaneous variations of pitch and Mach around the azimuth. It was shown that enhanced lift was obtained while inspection of the moment coefficient showed negative damping for the flap for a limited set of conditions. Due to the 2D formulation, dMdt computations are fast to perform and can be used to inform codes predicting the rotor performance. The flap was then assessed in hover, and only allowed for limited improvement in blade performance at high thrust. In forward flight, the flap was actuated in a 1-per-rev fashion and was found to have a strong effect on the loads on the retreating side. The effect on the moments was even stronger. The flight envelope of the blade was explored, and clean and flapped cases were compared. The most noticeable changes occur at high and medium thrust. The CFD method was found to be efficient and robust, without any substantial penalties in CPU time over the tested conditions. NOMENCLATURE cChord length

This paper presents aerodynamic optimisation of tiltrotor blades with high-fidelity computational fluid dynamics. The employed optimisation framework is based on a quasi-Newton method, and the required high-fidelity flow gradients were computed using a discrete adjoint solver. Single-point optimisations were first performed, to highlight the contrasting requirements of the helicopter and aeroplane flight regimes. It is then shown how a trade-off blade design can be obtained using a multi-point optimisation strategy. The parametrisation of the blade shape allowed to modify the twist and chord distributions, and to introduce a swept tip. The work shows how these main blade shape parameters influence the optimal performance of the tiltrotor in helicopter and aeroplane modes, and how a compromise blade shape can increase the overall tiltrotor performance. Moreover, in all the presented cases, the accuracy of the adjoint gradients resulted in a small number of flow evaluations for finding the optimal solution, thus indicating gradient-based optimisation as a viable tool for modern tiltrotor design. overall profile drag coefficientbaseline rotor torque coefficient Greek Symbols INTRODUCTIONThe aerodynamic design of tiltrotor blades is a challenging task, requiring the best compromise in performance between hover and propeller modes [1,2]. In hovering flight, the blade aerodynamics is characterised by strong interaction with the rotor wake, resulting in a significant effect of the induced drag on the total drag [3]. The propeller mode, on the other hand, is dominated by strong compressibility effects, especially at high advance ratio, resulting in a prominent contribution of the profile and wave drag components [4]. As a consequence, to accurately capture the effect of the blade shape on the optimal rotor design, the use of high-fidelity flow models is required. Unlike in the case of helicopter and propeller blades, the aerodynamic optimisation of tiltrotor blades has not been the subject of considerable research. The present work analyses the contribution of the main blade shape parameters to the optimal performance of the tiltrotor using high-fidelity computational fluid dynamics (CFD). It also demonstrates the use of gradient-based optimisation and the discrete adjoint for the efficient design of tiltrotor blades. Aerodynamic optimisation needs large computational resources, since each design point requires the solution of a set of partial differential equations. The choice of the optimisation algorithm is therefore crucial. Broadly speaking, the optimisation algorithms can be classified in gradient-based or gradient-free methods. Gradient-based methods usually require a limited number of flow evaluations [5], and this makes them particularly attractive for complex aerodynamic optimisation problems. They need, however, the computation of flow derivatives with respect to the design variables, which can be a very expensive task, unless the adjoint method is used. Also, gradient-based methods are local in nat...