Motivation and previous work at the LARCASE

Aeronautical transport has evolved at a very rapid pace; the traffic has increased by threefold over the last decade, and by 2025 it is expected to double its current level. An approximate 3% increase in the number of passengers is expected annually, reaching approximately 1 billion passengers by 2016. The percentage of global CO2 emissions from the aviation industry is 2%, based on the most recent statistics, as shown in Figure 1.1. Air travel has become the most popular means of long-distance transportation over the last several decades. According to the Annual Report of the International Air Transport Association (IATA), which is composed of 290 member airlines, 4.1 billion passengers travelled on scheduled flights during the year 2017, which is 280 million more than in 2016. The growing number of flights is contributing to the continous increase of the aeronautical industry’s share of global CO2 emissions. In addition, rising fuel prices have become another important concern for airline operators and the aviation industry in general.

Both climatic and financial concerns have led to academic and industrial collaborations around the world to develop advanced aircraft systems to address these global concerns. State-of-the-art aircraft still have conventional hinged flight control surfaces. Conventional hinged control surfaces can be optimized for certain flight phases, but they produce drag due to gaps between the main body of the wing and its adjacent control surfaces. Unnecessary drag on the airfoil results in degraded aircraft performance and increased fuel burn by adding to CO2 emissions and increasing fuel costs. Aeronautical engineers have always strived to develop aircraft systems that can mimic the bird flight. However, these attempts have not reached the required level of technology readiness so they can be installed on commercial passenger aircraft due to weight and safety issues raised by certifying authorities. Neverthless, with the emerging technologies, research teams around the world are working to develop new aircraft systems that would be more fuel efficient and climate friendly. With the advent of smart materials and shape-memory alloys, researchers are trying to replace conventional aircraft wings with morphing wings. Morphing wings structures involve the use of shape memory alloys, flexible materials which change their shape according to temperature variations. Another technology which has revolutionized many other industries, including aviation, is power electronics. This technology, has led to a new research area in aeronautical industry, known as more electric aircraft. In aircrafts, four different powers are extracted from engine: mechanical, pneumatic, electrical and hydraulic. The more electric aircraft goal is to use only one source of power, “electrical power”.

The electrical power can be used for various purposes like cabin air-conditioning or actuator control. The flight of birds in nature has always inspired scientists, that resulted in the development of aircraft. From the development of the first aircraft, aeronautical engineering researchers have taken their inspiration from bird flight to improve the efficiency of overall aircraft design. The concept of a morphing wing is not new. The Wright brothers were able to roll their aircraft by twisting the wing and by using wires that actuated directly. Conventional flight control surfaces, such as flaps and slats, have been successfully incorporated to control aircraft during various flight conditions, but their aerodynamic efficiency was suboptimal. Historically, morphing wings have been associated with various kinds of complexities such as increased weight as well as cost and safety issues. Over the last few decades, morphing wing technology has not been enough successful in making significant improvement in aerodynamic efficiency. Most of the large shape modifications over the last several decades have been carried out in military aircraft. During recent years, the researchers’ focus has also shifted towards UAVs due to their less-strict certification requirements. New developments in the various fields, such as materials and actuation devices may apply to morphing wing developments. Current morphing wing research is a multidisciplinary field, as researchers can choose from a variety of materials, actuation devices and sensors. Some of the more important objectives in morphing wing design are the type of morphing, the time when the shape change occurs, and the choice of materials, actuation devices and sensors.

Morphing wing classification

Morphing wings can be classified into three categories: planform, out of plane and airfoil adjustment. Planform morphing can be achieved by varying the span, chord and sweep of the morphing wing either individually or in combination. Planform morphing results in modification of the aircraft aspect ratio, which has a direct effect on the lift to drag ratio. From aerodynamics point of view, increasing the aspect ratio will result in increased range and endurance. A larger span results in a broader range and improved fuel efficiency, however it reduces the manoeuvrability. Wing out of plan morphing is achieved by combination of twist, span wise bending and dihedral gull. Dihedral gull is the angle between the wing root and the wing tip, dihedral wings provide the capability to enhance the flight control and performance of the aircraft; in case when the wing is at a lower angle than the wing root than its known as the “anhedral wing”. Twist is added to the wing in order to distribute the lift over the wing with an aim to increase its flight performance. Simillary the aim of spanwise bending is also to improve the flight performance, however fewer studies have been carried out on this subject (Barbarino et al., 2011).

Developments in the field of smart technologies such as microelectronics, support hardware, actuators and sensors have led to breakthroughs in many scientific disciplines. These developments have the potential to advance the edges of aircraft technology in various aspects, including safety, environmental compatibility and affordability. At NASA, the goal of the Aircraft Morphing program is to develop smart devices based on active component technologies. The ultimate aim of the research is the development of self adaptive flight, which will lead to improved aircraft efficiency. Many of these research endeavors ended up not being qualified for the real aircraft, mainly due to the high cost of implementation or because of the fact that the overall benefit was too small to conduct the system into production. One of the key enabling technologies is the control system design, which is helpful in the mathematical modelling and feedback control in the Aircraft Morphing program. Once the choices of actuator and sensor type have been made, the next key decision was to determine the number of actuators and their locations (NASA website). Various types of actuators, including electromechanical, hydrauic, pnematic and piezo electric actuators have been used for applications ranging from flaps/Slats, ailerons, rudders and spoilers to landing deployment and retraction control. The following section explains the various types of actuators and controllers used for morphing applications.

Types of actuators and control techniques Pierre and Jacques Curie first discovered the piezoelectric effect in 1880. Piezo is a greek word which means pressure. When pressure is applied on certain materials they generate electricity and vice versa when electricity is applied to them they change their shape. Thus piezoelectricity is a phenomenon which relates electrical and mechanical systems (Inman, d. et al, 1998). Macro Fiber Composites (MFC) are materials whose properties can be modified by an external stimulas in order to meet certain objectives. These are the kind of materials which can sense their envirnment and accordingly change their physical characteristics, furthermore when voltage is applied to them, they deform and change their shape. Based on the electrode patern inside the material, it either elongates or contracts. NASA has used MFC for alleviating uncontrolled vibrations and unsteady aerodynamics. At virginia tech a hinged trailing edge was replaced with the deformable surface, and hence the camber could be changed continously by embedding the MFC actuator in the wing surface (Kelvin, P et al, 2014).In the past few decades Macro Fiber Composite have been used widely, because they provide high actuation and structural stability.

A common drawback with the piezo ceramic actuators is that they may require high voltages in the range of 1.8 KV to 10 KV, however the current drain is extremely low which results in small power consumption. MFC are made up of piezo ceramic fibers embedded in a polymer matrix (Bilgen, O. et al, 2010). Patches of MFC were bonded to a wing and voltage was applied to change the camber of the wing, it was concluded that MFC are one of the best means of shape modifications for structural and aerodynamic applications. Some of their useful characteristics are high flexibility and large displacements( Bilgen, O., Alper et al, 2010). Shape memory alloys are used to design morphing wing actuators. A morphing wing actuator varies the camber of the wing, which causes the same effect as a mechanical flap, and reduces the overall drag caused by the discontinuous parts of the conventional flap control surface. Various tests, including material property and actuation characteristics tests were performed; these led to the selection of Flexinol wire as an appropriate actuator for the morphing wing. An Hardware In the Loop (HIL) interface based on Matlab/Simulink was used to analyze the displacement response of the actuator to a commanded input current (Misun et al, 2014).