Mechanics of pressure-adaptive honeycomb and its application to wing morphing

Abstract

Current, highly active classes of adaptive materials have been considered for use in many different aerospace applications. From adaptive flight control surfaces to wing surfaces, shape-memory alloy (SMA), piezoelectric and electrorheological fluids are making their way into wings, stabilizers and rotor blades. Despite the benefits which can be seen in many classes of aircraft, some profound challenges are ever present, including low power and energy density, high power consumption, high development and installation costs and outright programmatic blockages due to a lack of a materials certification database on FAR 23/25 and 27/29 certified aircraft. Three years ago, a class of adaptive structure was developed to skirt these daunting challenges. This pressure-adaptive honeycomb (PAH) is capable of extremely high performance and is FAA/EASA certifiable because it employs well characterized materials arranged in ways that lend a high level of adaptivity to the structure. This study is centered on laying out the mechanics, analytical models and experimental test data describing this new form of adaptive material. A directionally biased PAH system using an external (spring) force acting on the PAH bending structure was examined. The paper discusses the mechanics of pressure adaptive honeycomb and describes a simple reduced order model that can be used to simplify the geometric model in a finite element environment. The model assumes that a variable stiffness honeycomb results in an overall deformation of the honeycomb. Strains in excess of 50% can be generated through this mechanism without encountering local material (yield) limits. It was also shown that the energy density of pressure-adaptive honeycomb is akin to that of shape-memory alloy, while exhibiting strains that are an order of magnitude greater with an energy efficiency close to 100%. Excellent correlation between theory and experiment is demonstrated in a number of tests. A proof-of-concept wing section test was conducted on a 12% thick wing section representative of a modern commercial aircraft winglet or flight control surface with a 35% PAH trailing edge. It was shown that camber variations in excess of 5% can be generated by a pressure differential of 40 kPa. Results of subsequent wind tunnel test show an increase in lift coefficient of 0.3 at 23 m s −1 through an angle of attack from −6 • to +20 • .

Novel structure with negative Poisson’s ratio and enhanced Young’s modulus

“…Murray et al [37,38] filled honeycombs with a lossy polymer in order to achieve a high stiffness structure with higher damping. What is more, Vos et al [39] filled hexagonal cells with pneumatic bladders for trailing edge morphing.…”

Elastic shape morphing of ultralight structures by programmable assembly

“…Flexible mechanical systems, such as morphing wings, have been proposed to adapt wing geometry to changing flight conditions [4], seeking to increase performance at a range of air-speeds [5], reduce vibrations [6], increase maximum lift [7], decrease drag [8], and augment control of the vehicle [9]. However, scalable manufacturing and integration with traditional flight systems remain an open challenge [10].…”

“…Some proposed adaptive aerostructures leverage planar configurations that have much higher stiffness across an orthogonal out-of-plane axis that is oriented to maintain stiffness in one or more dimensions while allowing orthogonal dimensions to retain low stiffness for passive elastic behavior or ease of actuation. Example technologies include specialized honeycombs [8], corrugated designs [12], and custom compliant mechanism designs such as those developed by Kota et al [13]. Planar designs generally choose a single loading plane to achieve airfoil camber morphing, span-wise bending, or span extension.…”

A Survey of Subscale Aircraft Primary Flight Control Actuator Dynamic Response Characteristics

“…By using natural examples, the world's highest mass-specific actuator energy density actuator class was being developed as shown in Figure 4. [47][48][49][50] By using Pressure Adaptive Honeycomb (PAH), it was shown that the amount of work achieved per kg added to the aircraft for flight control can be maximized if pressure adaptive honeycomb actuators are used. Because these actuators use only FAR-25 certifiable materials operating strictly in the "infinite life" stress-strain zone on S-n curves, they are inherently built to be compatible with certifiable aircraft.…”