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Integrated multi-functional morphing aircraft technologies

Publication . Santos, Pedro Daniel da Rocha; Gamboa, Pedro Vieira; Silva, José Miguel

In the past years, the development of morphing wing technologies has received a great deal of interest from the scientific community. These technologies potentially enable an increase in aircraft efficiency by changing the wing shape, thus allowing the aircraft to fly near its optimal performance point at different flight conditions. This thesis explores the development, analysis, building and integration of two new functional Variable-Span Wing (VSW) concepts to be applied in Remotely Piloted Aircraft Systems (RPAS). Additional studies are performed to synthesize the mass of such morphing concepts and to develop mass prediction models. The VSW concept is composed of one fixed rectangular inboard part, inboard fixed wing (IFW), and a moving rectangular outboard part: outboard moving wing (OMW). An aerodynamic shape optimization code is used to solve a drag minimization problem to determine the optimal values of wingspan for various speeds of the vehicle’s flight envelope. It was concluded that, at low speeds, the original wing has slightly better performance than the VSW and for speeds higher than 25 m/s the opposite occurs, due to the reduction in wing area and consequently the total wing drag. A structural Finite Element Model (FEM) of the VSW is developed, where the interface between wing parts is modelled. Deflections and stresses resulting from static aerodynamic loading conditions showed that the wing is suitable for flight. Flutter critical speed is studied. FEM is used to compute the VSW mode shapes and frequencies of free vibration, considering a rigid or the real flexible interface, showing that the effect of rigidity loss in the interface between the IFW and the OMW, has a negative impact on the critical flutter speed. A full-scale prototype is built using composite materials and an electro-mechanical actuation system is developed using a rack and pinion driven by two servomotors. Bench tests, performed to evaluate the wing and its actuation mechanism under load, showed that the system can perform the required extension/retraction cycles and is suitable to be installed on a RPAS airframe, which has been modified and instrumented to serve as test bed for evaluating the prototype in-flight. Two sets of flight tests are performed: aerodynamic and energy characterization. The former aims at determining the lift-to-drag ratio for different airspeeds and the latter to measure the propulsive and manoeuvring energy when performing a prescribed mission. In the aerodynamic testing, in-flight evaluation of the RPAS fitted with the VSW demonstrates full flight capability and shows improvements produced by the VSW over a conventional fixed wing for speeds above 19 m/s. At low speeds, the original wing has slightly better lift-to-drag ratio than the VSW. Contrarily, at 30 m/s, the VSW in minimum span configuration is 35% better than the original fixed wing. In the other performed test, it is concluded that the VSW fitted RPAS has less overall energy consumption despite the increased vehicle weight. The energy reduction occurs only in the high speed condition but it is so marked that it offsets the increase in energy during takeoff, climb and loiter phases. Following the work on the first VSW prototype, a new telescopic wing that allows the integration of other morphing strategies is developed, within the CHANGE EU project. The wing adopted span change, leading and trailing edge camber changes. A modular design philosophy, based on a wing-box like structure, is implemented, such that the individual systems can be separately developed and then integrated. The structure is sized for strength and stiffness using FEM, based on flight loads derived from the mission requirements. A partial span, fullsized cross-section prototype is built to validate the structural performance and the actuation mechanism capability and durability. The wing is built using composite materials and an electromechanical actuation system with an oil filled nylon rack and pinion is developed to actuate it. The structural static testing shows similar trends when compared with numerical predictions. The actuation mechanism is characterized in terms of actuation speed and specific energy consumption and it was concluded that it functioned within its designed specifications. A full-scale prototype is later built by the consortium and the leading and trailing edge concepts from the different partners integrated in a single wing. Wind tunnel tests confirmed that the wing can withstand the aerodynamic loading. Flight tests are performed by TEKEVER, showing that the modular concept works reliably. From the previous works, it is inferred that morphing concepts are promising and feasible methodologies but present an undesired mass increase due to their inherent complexity. On the other hand, mass prediction methods to aid the design of morphing wings at the conceptual design phase are rare. Therefore, a mass model of a VSW with a trailing edge device is proposed. The structural mass prediction is based on a parametric study. A minimum mass optimization problem with stiffness and strength constraints is implemented and solved, being the design variables structural thicknesses and widths, using a parametric FEM of the wing. The study is done for a conventional fixed wing and the VSW, which are then combined to ascertain the VSW mass increment, i.e., the mass penalization of the adopted morphing concept. Polynomials are found to produce good approximations of the wing mass. Additionally, the effects of various VSW design parameters in the structural mass are discussed. On one hand, it was found that the span and chord have the highest impact in the wing mass. On the other hand, the VSW to fixed wing ratio proved that the influence of span variation ratio in the wing mass is not trivial. It is found that the mass increase does not grow proportionally with span variation ratio increase and that for each combination of span and chord, exists a span variation ratio that minimizes the mass penalty. Using the VSW to fixed wing ratio function, the mass model is derived. To ascertain its accuracy, a case study is performed, which demonstrated prediction errors below 10%. Although the mass model results are encouraging, more case studies are necessary to prove its applicability over a wide range of VSWs. The work performed successfully demonstrated that VSW concepts can achieve considerable geometry changes which, in turn, translate into considerable aerodynamic gains, despite the increased weight. They influence all aspects of the wing design, from the structural side to the actuation mechanisms. The parametric study summarizes the mass penalties of such concepts, being successful at demonstrating that the mass penalty is not straightforward and that a careful selection of span, chord and variable-span ratio can minimize the mass increase.

2018-04Doctoral thesis

Open access