The main objective of this paper is to describe a methodology to be applied in the preliminary design of a tiltrotor wing based on previously developed conceptual design methods. The reference vehicle is the Next-Generation Civil Tiltrotor Technology Demonstrator (NGCTR-TD) developed by Leonardo Helicopters within the Clean Sky research program framework. In a previous work by the authors, based on the specific requirements (i.e., dynamics, strength, buckling, functional), the first iteration of design was aimed at finding a wing structure with a minimized structural weight but at the same time strong and stiff enough to comply with sizing loads and aeroelastic stability in the flight envelope. Now, the outcome from the first design loop is used to build a global Finite Element Model (FEM), to be used for a multi-objective optimization performed by using a commercial software environment. In other words, the design strategy, aimed at finding a first optimal solution in terms of the thickness of composite components, is based on a two-level optimization. The first-level optimization is performed with engineering models (non-FEA-based), and the second-level optimization, discussed in this paper, within an FEA environment. The latter is shown to provide satisfactory results in terms of overall wing weight, and a zonal optimization of the composite parts, which is the starting point of an engineered model and a detailed FEM (beyond the scope of the present work), which will also take into account manufacturing, assembly, installation, accessibility and maintenance constraints.

The tiltrotor wing structure is one of the most critical and heavily investigated structures in design due to the fundamental need to consider the interactions between the wing, pylon, and rotor systems to achieve aircraft aeroelastic stability. Indeed, in high-speed forward flight, wing flexural and torsional stiffness have fundamental roles in pitch-whirl stability. Another specific concern of tiltrotors is dynamic mode placement; it is necessary to properly place wing bending modes away from prop-rotor forcing frequencies. The main aeroelastic stability and dynamics requirements and the wing design process flow for the next generation civil tiltrotor are presented in this work. In this context, the use of composite materials plays a fundamental role in the attempt to satisfy the requirements, with the perpetual aim of minimizing the structural weight. An overview of the idealized and adopted models for strength, aeroelasticity, and whirl flutter analysis is provided. The primary focus was on the aeroelastic tailoring process. To satisfy, at the same time, all of the structural dynamic and aeroelastic stability requirements, the best compromise, with an acceptable weight penalty, was the mixture of two methodological solutions: adding unidirectional tape in the zones of the upper and lower skins for flexural out-of-plane frequency and adding a proper number of ±45° fabric layers at the locations of the skin with the highest value of strain energy for in-plane torsional modes. The results show that the proposed method based on modal strain energy analysis enables a tiltrotor aeroelastic tailored wing design. It can be easily employed in similar applications (e.g., vehicle scale-up/down) with the advantage of using the stiffness requirements derived directly from the aeroelastic ones (i.e., structural frequencies). The specific wing achieved aeroelastic clearance by adding only 2.7% of extra mass.

This paper is focused on structural scalability studies of a new generation of civil tiltrotor wingbox structures. Starting from a reference wingbox, developed under the H2020 Clean Sky 2 NGCTR-TD T-WING project, a geometric scaling was performed to upscale the concept up to a larger class tiltrotor named “NGCTR”. Given the wing and the wingbox geometry, a multi-objective optimization, based on genetic algorithms, was performed to find for the NGCTR, among different materials and layups, the best composite wing in terms of weight that satisfies stiffness and crash requirements. The crash requirement plays an important role in regards to wing weight performance. It was found that not all materials investigated in this study succeeded in satisfying both stiffness and crash requirements. The results in terms of minimum structural mass as the target of the optimization process show that the mass ratio of the optimized up-scaled wing is near the geometrical scale factor: 1.58 vs. 1.29. Furthermore, the solution found by the optimizer NGCTR upscaled wing is comparable with other tiltrotor data coming from a literature study. The difference in terms of the ratio between wing structural weight and tiltrotor MTOW is Δ% = +1.4: an acceptable small overestimation of weight compared to a design, optimization, and scalability method that is easily adaptable and effective. The study presented in this work is, in fact, part of a broader activity on scalability and constitutes its first phase, based on low-fidelity models. The scalability study will continue with a further phase (indicated as “phase 2”), in which more reliable models will be set up, allowing a better estimation of the wing’s structural weight and further optimization. The results shown in this manuscript concern phase 1 only and can be considered a starting point at the System Requirements Review level of the up-scaled wing. This phase allowed for a fast exploration of the available solutions by making a first assessment of the main requirements and by aiding in the material choice at the very beginning of the design.

Multirotors are gaining great importance in the layout of innovative and more agile mobility. In this framework, a possible solution to developing an aircraft complying with the stringent size requirements characterizing this type of application may be a coaxial rotor configuration. To exploit several possibilities linked to coaxial rotors, a scaled experimental model is designed to evaluate the performances of the counter-rotating propeller system, specifically regarding the distance between the two propellers. Both thrust and noise are considered as parameters of interest. Two brushless motors are deployed, whereas the propellers’ angular velocity, in terms of rounds per minute (rpm), is controlled by an external control system. Tests are conducted on both single isolated propellers as well as on the counter-rotating system: the two propellers and their respective motors are characterized regarding the thrust. Furthermore, a comparison with a numerical model is performed. Noise evaluation on the single propeller shows a motor contribution prevalence at a low rpm range (1140–1500 rpm) and a propeller prevalence for angular velocities higher than 1860 rpm. By varying the distances between the propellers, a sensitivity analysis is performed with the aim of identifying the optimum configuration, taking into account both noise and thrust performances.

Pressure load measurements in operative conditions represent a fundamental parameter for improving the knowledge of the in-flight forcing generation and transmission. Furthermore, it is a fundamental step for assessing the validity and confidence of numerical methods able to predict such pressure loads, usually generated by propulsion and turbulent boundary layer. The experimental in-flight measurements and analysis and the correlation with the numerical methods are activities required as a starting point for studying the transmission mechanism through the fuselage which is responsible for the vibration and noise level which can be measured inside and for identifying potential solutions for their mitigation. The in-flight measurements have been carried out on a Leonardo Helicopter tiltrotor with the main goals to perform the validation of an experimental setup in a fully new environment and the assessment of a data analysis procedure according to the objectives of the Clean Sky 2 Fast Rotorcraft Project T-WING (CS2, Clean Sky 2 joint undertaking third amended bi-annual work plan and budget, 2018–2019).