Aerodynamic Analyses Of Skins For Morphing Wings
espite years of research and development in aviation, nearly all aircraft’s wings are designed according to the same model of a rigid structure equipped with few discrete control surfaces. This design performs well at a single flight regime and suffers degradation in its aerodynamic properties at other regimes. For that reason, and with the advancements that have been witnessed in the field of smart material and adaptive structures, a biomimetic concept that challenged this rigid model has been revived. This concept is morphing aircraft. Aircraft morphing technology enables a single air vehicle to undergo substantial geometric changes in-flight, with the purpose of increasing efficiency, versatility, and mission performance.
One of the most challenging tasks in this research topic is to design, implement and test a morphing skin that can compromise between flexibility to ensure low actuation requirements, and high stiffness to carry all the aerodynamic loads. While several studies have focused on the mechanical designs of morphing skins, studying the aerodynamic effects of these models have been largely ignored. Infinitesimal discontinuities resulting from the overlapping panels of sliding skins, and small ripples of stretchable skins can affect the stability of the viscous boundary layer which dictates the overall aerodynamic performance.
In this thesis, the aerodynamics of three types of morphing skins are studied. The first two fall under the category of sliding morphing skin where several rigid panels overlap and slide against each other during morphing. During morphing, the panels can either have backward-facing steps between them, gaps, or both. Despite their geometric similarity, the aerodynamics of backwardfacing steps are quite distinguished from panels with gaps between them.
An aerodynamically less invasive design for morphing skin is the stretchable skin. This design consists of two layers, an underlaying supporting structure, and an outer sealant layer. The underlaying layer is manufactured from a modified zero-Poisson ratio cellular structure with major ribs extending along the chord-wise direction, and minor ribs joining these major ribs. The outer layer consists of a highly anisotropic elastomer composite with Silicone rubber used as the matrix
material and reinforced with carbon fibers along the chord-wise direction. During morphing, the flexible skin forms wrinkles along the main direction of morphing which significantly affect the aerodynamic properties of the morphing wing.
The aerodynamics of each one of these three designs is numerically studied using the high fidelity commercial computational fluid dynamics (CFD) code FLUENT 15.0. The first design with the backward-facing steps distributed along the chordwise direction of the airfoil showed to have a degrading effect on the aerodynamics of the wing. Regardless of the step size, the boundary layer experiences a transition from laminar to turbulent state at the step edge. At Re = 5.7e6, M = 0.2, and α = 2.5°, a drop of 21.1% in value of the lift coefficient and an increase of 120.8% in the drag coefficient were observed in case of a step located at 25% of the chord length on the upper surface of the airfoil. These effects can be mitigated by either shifting the step location towards the trailing edge or decreasing the step depth. For a step located on the lower surface of the airfoil, the lifting forces increased by at least 11% due to decreasing the airfoil thickness on the pressure side. The drag coefficient also increased by 63.46% for a step located at 25% of the chord length on the lower surface of the airfoil. This value decreases to 25.96% when the step is shifted to 75% of the chord length. A degraded near stall behavior was observed for a step located at either side of the airfoil. The separation of the flow at the step edge promotes early stall of the airfoil. These results show that distributing the sliding panels of the morphing skin in a sequential order is not an aerodynamically viable solution.
When the panels are separated with gaps, the aerodynamic performance was found to be different. It was observed that in some cases, the boundary layer maintained its laminar state while travelling over the cavity, and in other cases, the boundary layer experienced a transition over the cavity vicinity. To investigate the cavity parameters that influence the transition of the boundary layer, a parametric study is performed over a wide range of flow conditions and cavity dimensions. It is found that the boundary layer by-passes the cavity and maintains its laminar state when 𝐿/𝜃 ∗√𝑅𝑒𝜃~ < 600, where L is the cavity length, 𝜃 is the momentum thickness of the boundary layer, and 𝑅𝑒𝜃 is the Reynolds momentum thickness based 𝜃 on at the cavity leading edge. This formula is used to design an airfoil with sliding morphing panels that have the same performance as the clean airfoil, making this design a favorable design for morphing skins.
The third design is the flexible morphing skin. With an underlaying supporting structure and a highly anisotropic flexible outer layer, the flexible skin fulfilled all the kinematic and structural requirements of a successful morphing skin. However, during morphing, wrinkles are formed on the upper and lower surfaces of the morphing wing. Regardless of its location, shape or size, the introduced wrinkles alter the boundary layer state and behavior which significantly affect the overall aerodynamic performance of the morphing wing. The aerodynamic effects of introducing a single wrinkle to the upper surface of a morphing wing is numerically studied. At Re = 5.7e6, M = 0.2, and α = 0°, results showed that introducing a single wrinkle to the upper surface of a NACA 2412 airfoil has dropped the lift coefficient by as much as 34.7% of the clean airfoil value, increased the drag coefficient by 267.9% and dropped the lift-to-drag ratio by 75.6% when the wrinkle is at the first quarter of the chord length. Shifting the wrinkle towards the trailing edge of the airfoil has mitigated these effects but did not eliminate them.
By comparing the results of the three designs, the sliding morphing skin with its panels arranged in a staggered manner has fulfilled the structural and kinematic requirements of a morphing skin while maintaining an aerodynamic performance similar to the clean airfoil if designed properly