**1. Introduction**

Men desired to flight since very ancient times being inspired by bird's capability to dominate sky. Nature offers a rich seam of inspiration for a new generation of morphing wing design across a wide range of scales of interest to engineers going from the biggest birds to the smallest insect. For example, birds achieve their wing morphing capability using flexible lifting surfaces, stiffened by hollow bones attached to strong muscle. All the flying creatures of the world show an inherent capacity to adapt, in a fraction of a second, their wing shape as the flight condition changes. A very interesting example may be represented by the perching sequence of an eagle. As reported in [1], birds accomplish changes in wingspan and area by firstly flexing their wings, and then adopting a characteristic M-shape planform with the inner wing section sweeps forward, and the outer section sweeps backwards.

It is noteworthy that "inspiration from nature" is the keywords that lie behind any morphing idea. Many researchers and engineers around the world have been inspired by the multitasking flight capabilities of birds, which tend to cover a broad range of mission phases ranging from slow, near-hover flight to aggressive dives, in order to develop innovative methodologies involved to resolve many technological problems. Just only observing birds and other flying creature wings, it is possible to appreciate the complexity of such systems showing intrinsic capacities to adapt instinctively and immediately to the environment. In particular, birds are able to articulate their wings in a craning motion to vary the dihedral or sweep angles [1], wing area, wing planform, wingspan, and other parameters. These changes allow the bird to quickly adapt between soaring, cruising, and descending flight [1].

The morphing idea was well known by the engineering since the beginning of aviation such as the Wright brothers who built the "first heavier than air aircraft with engine" with twisted wing for roll control. Despite the past century of innovation in aircraft technology, the versatility of modern aircraft remains far worse than airborne biological counterparts. The shape modification accomplished by birds stands as one of the few examples of true morphing. As such, the aircraft engineers worldwide are devoting extensive effort to integrate these concepts in advanced mechanical systems in order to bring morphing technology to the readiness level of a flight vehicle. The key purpose is to realize an innovative device capable to adapt itself to the external environment conditions, by exhibiting an intrinsic multidiscipli‐ nary attitude involving structures, actuation, sensing, and control. In recent years, European community funded many research program involved to improve the morphing structures technology readiness level. SARISTU [2] (acronym of Smart Intelligent Aircraft Structures) was probably the most advanced large-scale integrating project on morphing structures, coordi‐ nated by Airbus, aiming at achieving reductions in aircraft weight and operational costs, as well as an improvement in the flight profile specifically related to aerodynamic performance. Ended in 2015, the project consisted of a joint integration of different conformal morphing concepts in a laminar wing with the aim to improve aircraft performance through a 6% drag reduction inside the lift coefficient range usually devoted to cruise, with a positive effect on fuel consumption. The final product of the project was the first full-scale completely morphing wing tip prototype, ever assembled in Europe, at Finmeccanica Headquarters (Pomigliano, Italy), **Figure 1**. The innovative seamless morphing wing incorporates a gapless morphing leading edge, a morphing trailing edge, and an adaptive winglet.

**Figure 1.** Assembly of the SARISTU morphing wing consisting of different morphing devices [2].

Morphing technology is now approaching the high maturity practices for the integration on real aircraft. *How to adapt* is a problem regarding sensing, actuation, and control laws, which are very critical. Hence, although an animal's wings may be able to change shape in a complex manner, the total number of independently controlled degrees of freedom may not be high. This indicates that a smart structure is built upon relatively simple principles. It will be actuated in one point and, by means of movable structural elements with limited DOF; the movement is transmitted to the whole structure so that the wing will be built to adapt at loading rather than to resist it.

#### **1.1. Actuation systems for morphing applications**

**1. Introduction**

90 Recent Progress in Some Aircraft Technologies

Men desired to flight since very ancient times being inspired by bird's capability to dominate sky. Nature offers a rich seam of inspiration for a new generation of morphing wing design across a wide range of scales of interest to engineers going from the biggest birds to the smallest insect. For example, birds achieve their wing morphing capability using flexible lifting surfaces, stiffened by hollow bones attached to strong muscle. All the flying creatures of the world show an inherent capacity to adapt, in a fraction of a second, their wing shape as the flight condition changes. A very interesting example may be represented by the perching sequence of an eagle. As reported in [1], birds accomplish changes in wingspan and area by firstly flexing their wings, and then adopting a characteristic M-shape planform with the inner

It is noteworthy that "inspiration from nature" is the keywords that lie behind any morphing idea. Many researchers and engineers around the world have been inspired by the multitasking flight capabilities of birds, which tend to cover a broad range of mission phases ranging from slow, near-hover flight to aggressive dives, in order to develop innovative methodologies involved to resolve many technological problems. Just only observing birds and other flying creature wings, it is possible to appreciate the complexity of such systems showing intrinsic capacities to adapt instinctively and immediately to the environment. In particular, birds are able to articulate their wings in a craning motion to vary the dihedral or sweep angles [1], wing area, wing planform, wingspan, and other parameters. These changes allow the bird to quickly

The morphing idea was well known by the engineering since the beginning of aviation such as the Wright brothers who built the "first heavier than air aircraft with engine" with twisted wing for roll control. Despite the past century of innovation in aircraft technology, the versatility of modern aircraft remains far worse than airborne biological counterparts. The shape modification accomplished by birds stands as one of the few examples of true morphing. As such, the aircraft engineers worldwide are devoting extensive effort to integrate these concepts in advanced mechanical systems in order to bring morphing technology to the readiness level of a flight vehicle. The key purpose is to realize an innovative device capable to adapt itself to the external environment conditions, by exhibiting an intrinsic multidiscipli‐ nary attitude involving structures, actuation, sensing, and control. In recent years, European community funded many research program involved to improve the morphing structures technology readiness level. SARISTU [2] (acronym of Smart Intelligent Aircraft Structures) was probably the most advanced large-scale integrating project on morphing structures, coordi‐ nated by Airbus, aiming at achieving reductions in aircraft weight and operational costs, as well as an improvement in the flight profile specifically related to aerodynamic performance. Ended in 2015, the project consisted of a joint integration of different conformal morphing concepts in a laminar wing with the aim to improve aircraft performance through a 6% drag reduction inside the lift coefficient range usually devoted to cruise, with a positive effect on fuel consumption. The final product of the project was the first full-scale completely morphing wing tip prototype, ever assembled in Europe, at Finmeccanica Headquarters (Pomigliano,

wing section sweeps forward, and the outer section sweeps backwards.

adapt between soaring, cruising, and descending flight [1].

The state of the art of high-lift actuation systems of aircraft control surfaces predominantly consists of mechanical transmission shafts moved by rotary or linear hydraulic actuators with common control valves. These architectures assure a synchronous, safe, and reliable deploy‐ ment of all HLD (High Lift Device) but with limited flexibility [3]. The main functionality of the high-lift devices is to provide lift increment at low-speed condition (take/off and landing) so that the clean wing is optimized for the cruise speed regime. There are a lot of HLD on wing aircraft such as plain flaps to fowler flaps with single, double, and even the most complex triple slots (Boeing 747). The design and optimization of high-lift systems are one of the most complex tasks in aircraft design. It involves a close coupling of aerodynamics, structures, and kinemat‐ ics. The evolutionary trend of the HLD has been strongly driven by the dramatic improvement in aerodynamic tools optimization and in computational systems for complex structure simulations (multi-body kinematics). At the early stage, the research of aerodynamics highlift performance (*CLmax*) was achieved by means of multi-slotted experimentally validated twodimensional flap design. These systems allowed to achieve satisfactory performance with penalties in structural complexity and weight and, therefore, in costs that were not sustainable in the current applications. Later on, the improvement in computation fluid dynamics has permitted to carefully optimize flap systems in two-dimensional flow with a clear advantage for fowler mechanism that allowed to reach higher values of maximum lift due to the effect of an increased lifting surface. Such fowler mechanism, on the other side, required even more complex kinematic actuation system due to a combination of two movements: one translation and a rotation. The fowler flap deployment mechanisms were designed using linear or curved tracks in conjunction with revolute joint for the rotation, but unfortunately, the high-lift values achieved were compensated by the relatively high weight penalties introduced by such systems. The reason for such high weight drawbacks was due to very intensive loads to be withstood by track bearings with also subsequent high maintenance costs. More recently, the research for aerodynamic efficiency and reduced weight penalties and complexity has been fostered by large utilization of multi-body system optimization that permitted the develop‐ ment of lighter and more efficient kinematic mechanism such as multi-link system. Such devices permit to match even very complex aerodynamic requirements with relatively structurally efficient system. As a matter of fact, today it seems very difficult to further improve in terms of an optimum balance among aerodynamic, structural weight, and complexity in the

**Figure 2.** Evolutionary trend in high-lift systems [4].

current system, namely A350 or Boeing 767, this appears evident by the flattening of the curve in **Figure 2**.

lift performance (*CLmax*) was achieved by means of multi-slotted experimentally validated twodimensional flap design. These systems allowed to achieve satisfactory performance with penalties in structural complexity and weight and, therefore, in costs that were not sustainable in the current applications. Later on, the improvement in computation fluid dynamics has permitted to carefully optimize flap systems in two-dimensional flow with a clear advantage for fowler mechanism that allowed to reach higher values of maximum lift due to the effect of an increased lifting surface. Such fowler mechanism, on the other side, required even more complex kinematic actuation system due to a combination of two movements: one translation and a rotation. The fowler flap deployment mechanisms were designed using linear or curved tracks in conjunction with revolute joint for the rotation, but unfortunately, the high-lift values achieved were compensated by the relatively high weight penalties introduced by such systems. The reason for such high weight drawbacks was due to very intensive loads to be withstood by track bearings with also subsequent high maintenance costs. More recently, the research for aerodynamic efficiency and reduced weight penalties and complexity has been fostered by large utilization of multi-body system optimization that permitted the develop‐ ment of lighter and more efficient kinematic mechanism such as multi-link system. Such devices permit to match even very complex aerodynamic requirements with relatively structurally efficient system. As a matter of fact, today it seems very difficult to further improve in terms of an optimum balance among aerodynamic, structural weight, and complexity in the

92 Recent Progress in Some Aircraft Technologies

**Figure 2.** Evolutionary trend in high-lift systems [4].

From the previous graph, it is evident that today's high-lift system are moving toward the development of innovative mechanisms with continuous curvatures, leading to the removal of gaps in order to obtain the same performance with the less deflections. In other words, this means implementing morphing concepts, as highlighted in the graph reported in **Figure 3**.

**Figure 3.** Simplification of the high-lift actuation systems over the last few decades.

Additionally, flap mechanisms must be reliable and fail-safe. In order to not violate safety needs, the driving idea is to elude a multitude of links and joints in series, where high load concentrations are located; because the failure of any one of which could either locks up the flap, make it collapse. There are many type of flap mechanism that are largely investigated in [4, 5]. The actuation scheme of the Airbus A340 and its extraction device are depicted in **Figures 4** and **5**. The central hydraulic power control unit (PCU) supplies the power necessary to deflect the flap panels on each wing. A mechanical transmission shaft transmits the mechanical power to the rotary actuators, which move the flaps on the tracks. This shaft system consists of gearboxes necessary for larger direction changes as well as system torque limiters, wing tip brakes, universal joints, plunging joints, and spline joints to accommodate wing bending and temperature effects. The high-lift system is controlled and monitored by two slat-flap control computers (SFCC) using sensor information from several analogue and discrete sensors. This type of mechanical transmission shaft system consists of a high number of components with different part numbers and requires high design-engineering and installation effort.

**Figure 4.** Global scheme of the inboard and outboard A340 flap actuation system [3].

**Figure 5.** A340 flap mechanism based on the link/track architecture [5].

In contrast to the previous mechanism, the flap deployment system of the Boeing 767 (**Figure 6**) is based on a limited number of links in order to create an articulated quadrilateral or more complex hexagonal chain.

**Figure 6.** Boeing 767 flap system: cruise position (a) and landing configuration (b) [5].

Recent development programs at Airbus and Boeing extend the functional capabilities of the flap systems. The A350 XWB as well as the B787 high-lift systems design will incorporate additional functionalities that provide aircraft performance optimization. Additional func‐ tionality is achieved with an evolution of the traditional mechanical transmission shaft system and additional active components [6]. The A350's flaps are a very simple "drop-hinge" design with a single slot between the trailing edge of the spoiler and the leading edge of the flap. As the flap extends, the spoilers deflect downwards to control the gap and optimize the high-lift performance of flap. It constitutes a multipurposes high-lift system with augmented function‐ alities, and furthermore, it is a lightweight structures thanks to its low complexity link-based kinematic. This can be summarized in the next **Figures 7** and **8**.

**Figure 7.** A350 XWB (Extra-Wing Body) flap in cruise condition [6].

**Figure 4.** Global scheme of the inboard and outboard A340 flap actuation system [3].

**Figure 5.** A340 flap mechanism based on the link/track architecture [5].

**Figure 6.** Boeing 767 flap system: cruise position (a) and landing configuration (b) [5].

complex hexagonal chain.

94 Recent Progress in Some Aircraft Technologies

In contrast to the previous mechanism, the flap deployment system of the Boeing 767 (**Figure 6**) is based on a limited number of links in order to create an articulated quadrilateral or more

Recent development programs at Airbus and Boeing extend the functional capabilities of the flap systems. The A350 XWB as well as the B787 high-lift systems design will incorporate additional functionalities that provide aircraft performance optimization. Additional func‐

**Figure 8.** A350 XWB (Extra-Wing Body) with A/B and tab deflection for roll control maneuver [6].

Moreover, for the first time, the flap system will have the both the capability for differential inner and outer settings as well as a variable camber function. The design is composed of a gearbox with a motor installed between the outer and inner flap that enables a differential control of the relative angle in order to shift inboard the resultant lift for a less bending moment. Furthermore, both inner and outer flaps can be moved together during the cruise to optimize the wing's camber for each phase of the flight and use the polar of drag to its most efficient configuration [6].

It remains to discuss if, as the complexity level of the actuation mechanism seems to reduce, the promise of morphing aircraft will become feasible within the next few years. If so, how morphing devices will be actuated?

The next technological challenge, envisaged in the context of more or all-electric aircraft, will be to replace the heavy conventional hydraulic actuators with a distributed spanwise arrange‐ ment of smaller electromechanical actuators (EMAs). This will bring several benefits at the aircraft level: firstly, fuel savings. Additionally, a full electrical system reduces classical drawbacks of hydraulic systems and overall complexity, yielding also weight (-15%) and maintenance benefits. Lack of supply buses, improved torque control, enhanced efficiency, removal of fluid losses and flammable fluids are only some of the benefits that can be achieved. On the other hand, a general limit of electro-mechanic actuators is the possibility of jamming failures that can lead to critical aircraft failure conditions. **Figure 9** shows a practical compar‐ ison between the aircraft torque shaft configuration and a distributed actuation arrangement suitable for a morphing trailing edge device.

**Figure 9.** Distributed concept versus concentrated actuation concept.

The simultaneous need for monitoring target morphed shapes, actuation forces, and flight controls along with the counter-effects of aerodynamic loads under aircraft operating condi‐ tions suggests the use of a ground-based engineering tool for the physical integration of systems. The most suitable to optimize and validate such systems including electromechanical component such as actuators and flight controls is the "Iron Bird." The basic scheme of an Iron Bird suitable for the integration of different morphing systems is depicted in **Figure 10**. It includes different morphing devices installed on an aeroelastically reasonable aircraft wing box as well as the basic equipment needed to carry out "hardware in the loop simulations." Such a concept may be used to demonstrate advanced control technologies in a modular multilevel design that provides the robustness and the flexibility of a real aircraft integration. Manufacturing, assembly, and integration issues including electrical and flight control may be extensively addressed in relation to the actual configuration of the aircraft. It is the perfect tool to confirm the characteristics of all system components or to discover an incompatibility that may require modifications during early development stages, and thereby, it accelerates the transition to test in a relevant environment. Additionally, failures and mitigation actions introduced in the systems can be studied in full detail and recorded for analysis using such a dedicated testbed.

**Figure 10.** Representative scheme of an Iron Bird tool suitable for testing morphing devices.

The "Iron Bird" for testing morphing wing architectures enables test engineers to evaluate the real-time capabilities of morphing devices with the purpose of:


ment of smaller electromechanical actuators (EMAs). This will bring several benefits at the aircraft level: firstly, fuel savings. Additionally, a full electrical system reduces classical drawbacks of hydraulic systems and overall complexity, yielding also weight (-15%) and maintenance benefits. Lack of supply buses, improved torque control, enhanced efficiency, removal of fluid losses and flammable fluids are only some of the benefits that can be achieved. On the other hand, a general limit of electro-mechanic actuators is the possibility of jamming failures that can lead to critical aircraft failure conditions. **Figure 9** shows a practical compar‐ ison between the aircraft torque shaft configuration and a distributed actuation arrangement

The simultaneous need for monitoring target morphed shapes, actuation forces, and flight controls along with the counter-effects of aerodynamic loads under aircraft operating condi‐ tions suggests the use of a ground-based engineering tool for the physical integration of systems. The most suitable to optimize and validate such systems including electromechanical component such as actuators and flight controls is the "Iron Bird." The basic scheme of an Iron Bird suitable for the integration of different morphing systems is depicted in **Figure 10**. It includes different morphing devices installed on an aeroelastically reasonable aircraft wing box as well as the basic equipment needed to carry out "hardware in the loop simulations." Such a concept may be used to demonstrate advanced control technologies in a modular multilevel design that provides the robustness and the flexibility of a real aircraft integration. Manufacturing, assembly, and integration issues including electrical and flight control may be extensively addressed in relation to the actual configuration of the aircraft. It is the perfect tool to confirm the characteristics of all system components or to discover an incompatibility that may require modifications during early development stages, and thereby, it accelerates the transition to test in a relevant environment. Additionally, failures and mitigation actions introduced in the systems can be studied in full detail and recorded for analysis using such a

suitable for a morphing trailing edge device.

96 Recent Progress in Some Aircraft Technologies

**Figure 9.** Distributed concept versus concentrated actuation concept.

dedicated testbed.

**•** validating the electrical consumption of each actuation system, in stationary and dynamic conditions, and the required command to A/C surface in each test case.
