Morphing Wing Concept in the Aviation Domain

The word « Morphing » comes from the verb « to morph », which refers to a continuous shape of a body, changed under its actuation system influence. In the aeronautical domain, « morphing » is defined as « a set of technologies that increase a vehicle’s performance by manipulating certain characteristics to better match the vehicle’s state to the environment and task at hand » (Weisshaar, 2006).

Traditional aircraft with fixed wing geometries offer have high aerodynamic performance over a fixed range, and for a limited set of flight conditions. They are designed for a specific mission, for which they are quite efficient. However, for a set of different flight conditions, they give poor aerodynamic performances and sub optimal fuel consumption efficiency, and so are not advantageous for civilian aircraft (Barbarino et al., 2011; Weisshaar, 2006). This actually represents a weakness for aircraft, as flexibility and multirole capacities are two important aviation considerations.

Morphing aircraft, in particular those equipped with adaptive wings, have the ability to vary their geometry according to external changes in aerodynamic loads, resulting in improved aerodynamic performance at various flight conditions (Smith et Nelson, 1990) and thereby offering more efficient fuel consumption. Changing the wing geometry of an aircraft for each flight condition may require complex adaptive wing technologies, but the aerodynamic performance gained would not be negligible (Stanewsky, 2001).

The morphing of aircraft is not a new concept in the aviation industry. Technologies such as variable sweep, retractable landing gear, retractable flaps and slats, and variable incidence noses have already been investigated and analyzed as part of the morphing aircraft concept. However, most of the time, their use has not been wide spread because of the associated penalties in terms of cost, complexity or weight, even if the penalties could sometimes be overcome by the benefits (Weisshaar, 2006).

Following the huge technological advances in smart and adaptive structures, other aspects of the morphing concept, such as changing the wing surface area to control the airfoil shape, are becoming popular research topics.

The state of art of Smart Structures and of the integration of various systems have been summarized (Chopra, 2002; Murugan et Friswell, 2013). Generally, a smart structure includes four technologies: « smart material actuators », « sensors », « control strategies » and « power conditioning electronics ». The integration of smart structures in the aerospace domain is becoming common and is rapidly expanding. A « smart structure » should have the capability to respond to changing external environment conditions. In an aircraft, this capability translates into the ability to change shape in order to more efficiently withstand different load conditions.

The geometrical parameters of aircraft influenced by morphing solutions can be classified into three categories: out of plane transformation, planform alteration and airfoil adjustment (Barbarino et al., 2011).

Morphing airfoil technology has become a dominant topic, surpassing the interest in the planform or out-of-plane methods. The design of a morphing airfoil technology means that the focus is on the wing skin (called « morphing skin ») change, which must be soft enough to allow shape changes, while being stiff enough to withstand aerodynamic load, and to keep the desired airfoil shape (Thill et al., 2008). The CRIAQ MDO 505 project is part of this recent wave of technological research projects focused on the morphing airfoil design.

Although morphing technologies are relatively new as research areas in aerospace, the design of changing wing planforms is as ‘old as motorized flight itself » (Thill et al., 2008). This section will review some examples of morphing technologies’ beginnings in the aerospace field.

In 1903, the aviation industry was officially marked by the beginning of controlled human flight. But even before that date, the first shape changing aircraft had already been experimented on, by Clement Ader in 1873, in France. He further proposed a wing morphing design and in the 1890’s, he developed other ideas for aviation (Weisshaar, 2006).

Not long afterwards, other variable geometry technologies were experimented on in France. Along the same lines, G.T.R Hill designed a tailless monoplane aircraft called the Pterodactyl IV, which offered a variable sweep angle range of between 4 and 75 degrees that was enabled by a mechanical worm wheel arrangement driving hinged wings capable of changing the sweep angles in flight. The Pterodactyl was flight tested for the first time in 1931.