Design & modelling of a composite rudderless aeroelastic fin structure

This thesis presents the study of a gapless and rudderless aeroelastic fin (GRAF) to enhance the directional stability and controllability of an aircraft. The GRAF concept was proposed and developed in the wake of previous research, targeted to improve flight performance and manoeuvrability, and to reduce fuel consumption and airframe weight. The study involved the subjects of aerodynamics, structural design and analysis, and flight mechanics. The work includes conceptual design, structural modelling, aeroelastic analysis and flight performance evaluation of a GRAF variant designed for a small subsonic Unmanned Aerial Vehicle (UAV). The Eclipse UAV, a platform designed by part time students at the Department of Aerospace Engineering of Cranfield University, was chosen as a case study. A new approach to design a more effective fin with an unconventional structural layout and novel techniques which have not been investigated in previous research is proposed. Despite the GRAF planform being similar to classical fin-hinged rudder configurations, it is provided with a flexible gapless control surface, kept as one continuous piece and integrated with the fin primary structure. With its fixed root and rudderless feature, the GRAF adopts an original method of operation. Its way of working relies upon an unconventional technique of combining morphing technology and aeroelastic effect. The morphable configuration is twisted to gain an aeroelastically beneficial effect to enhance the efficiency and manoeuvrability of the aircraft. This warping capability of the fin is the key role player enabling the GRAF surface to seamlessly generate the required aerodynamic forces. Unlike the conventional structures designed to be as rigid as possible to withstand the external loads, the GRAF will exploit its structure‟s flexibility to use the aeroelastically induced twist deformations for a self-adaptive warping behaviour and improve flight dynamic response and performance. In order to ensure the above features are achievable in practice, further study on the structural configuration was conducted. To achieve performance improvement, together with the original structural layout and aeroelastic effect exploitation, another three novel key components are investigated, proposed and introduced in the GRAF model. A structurally integrated actuation system, termed L-shape stringers device (LSS), is designed to transform actuator axial forces in spanwise distributed bending moments, to create seamless deformations of the trailing edge (TE) section. An innovative trailing edge joint, namely the swivel edge closure, is specifically designed to enhance the mobility and degrees of freedom of the trailing edge box. It is a revolutionary concept which, by virtually interrupting the structural integrity of the closed TE section, allows relative translation and rotation of the TE panels. Finally, it is the novel concept of the slot-connection that, whilst appearing to clamp the GRAF structure inside the slot, actually enables the design to increase the twist angle at the tip of the fin without overstressing the materials. In order to enhance the GRAF efficiency, a tailored design of the fin structure was conducted. A novel internal structure configuration integrated with the key components has been designed to be connected to a flexible cladding skin, rotating ribs and a load-carrying tubular beam all of which constitute the primary parts of the GRAF model. With the ultimate goal of a lighter tail version, the entire design has been made by using composite, light frames, in an engineering trade-off of stiffness, elasticity, weight and cost of both glass and carbon fibre laminates. The analysis via 2-D aerodynamic codes and FEA was conducted to assess and validate the GRAF model and the obtained performance. Static linear elastic analysis has been carried out to verify the structural layout of the novel design subject to strength and stiffness criteria in addition to the fin warping and cambering capabilities. Also an investigation of aeroelastic stability related to steady and unsteady aerodynamic conditions has been carried out during the model analysis phase. The study has shown that although the GRAF divergence and flutter margins are slightly smaller than those of the conventional fin, the design and performance requirements are satisfied within the very challenging objective of a lighter vertical tail structure.The dynamic analysis study has also demonstrated the beneficial effect obtained by damping yawing oscillations when such a self-adaptive structure, compared to a rigid one, can be operated under cross wind circumstances. The manufacturing feasibility and assembly of the GRAF structure has been explored with the construction of a 1:1 scale model of the fin prototype. The model has been used as concept demonstrator to assess the functionality of the introduced technical novelties, the ease of manufacturing and the structural weight of the final assembly.