Effect of the Backward-Facing Step Location on the Aerodynamics of a Morphing Wing

Abstract

Over the last decade, aircraft morphing technology has drawn a lot of attention in the aerospace community, because it is likely to improve the aerodynamic performance and the versatility of aircraft at different flight regimes. With the fast paced advancements in this field, a parallel stream of research is studying different materials and designs to develop reliable morphing skins. A promising candidate for a viable morphing skin is the sliding skin, where two or more rigid surfaces remain in contact and slide against each other during morphing. The overlapping between each two panels create a backward-facing step on the airfoil surface which has a critical effect on the aerodynamics of the wing. This paper presents a numerical study of the effect of employing a backward-facing step on the suction side of a National Advisory Committee for Aeronautics (NACA) 2412 airfoil at a high Reynolds number of 5.9 × 10 6 . The effects of the step location on the lift coefficient, drag coefficient and critical angle of attack are studied to find a favorable location for the step along the chord-wise direction. Results showed that employing a step on the suction side of the NACA 2412 airfoil can adversely affect the aforementioned aerodynamic properties. 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. However, these effects are mitigated by shifting the step location towards the trailing edge. Introducing a step on the airfoil caused the airfoil's thickness to change, which in turn has affected the transition point of the viscous boundary layer from laminar to turbulent. The location of the step, prior or post the transition point, has a noteworthy effect on the pressure and shear stress distribution, and consequently on the values of the lift and drag coefficients.

Passive Flow Control of Rotary-Wing and Fixed-Wing Aircraft Airfoils Via Surface-Based Trapped Vortex Generators

“…Boroomand and Hosseinverdi [24] and lately Mishriky and Walsh [25]. They have all reported that the aerodynamic outcomes from stepped airfoils are strongly dependent on the baseline airfoil, i.e., it differs from one baseline airfoil to another, a fact that supports the findings of [21] and [22] above.…”

“…Yet, lift was significantly increased when using the proposed new concepts on the lower surface of the airfoil.For all the newly proposed concepts (2 nd , 3 rd and 4 th generations), as much as 44% increase in the lift-to-drag ratio was achieved regardless of whether the geometry modification was implemented on the upper or lower surface of the airfoil.On the other hand, the conventional stepped airfoils (i.e. the 1 st generation), with a surface modification on the upper surface failed to increase the lift-to-drag ratio, indicating that what worked at low speeds, (gliders and RC airplanes[17][18][19][20][21][22][23][24][25]), might not be suitable for high Mach number and high Reynolds number transonic flows. This was the main reason behind seeking alternative designs to the stepped airfoils.…”

Fixed and rotary wing transonic aerodynamic improvement via surface-based trapped vortex generators

“…In addition, the turbulence resulted from the aerodynamic discontinuity will either move stall to higher angle of attacks or will produce less aggressive stall than the original airfoil. The use of Fertis' 24 airfoil, which itself was inspired by Kline's ''stepped airfoils'', received a noteworthy attention in the past decades, several computational and experimental studies have been published with encouraging results [25][26][27][28][29] but none of these examined the feasibility of stepped airfoils during transonic conditions, i.e. for controlling the occurrence of shock waves on airfoils.…”

Effect of Step Depth and Angle on the Aerodynamics of a Sliding Morphing Skin

“…In all configurations, the step will be located at mid-chord length of the airfoil. For the effect of the step location, the reader can refer to [9]. The main stream fluid is directed at an angle of attack of 2.5° and treated as an ideal gas at 300 K, with its viscosity modeled using the Sutherland three coefficient method.…”

“…From the aerodynamic perspective, the interface between the sliding segments is regarded as backward-facing step incorporated along the chord-wise direction of the airfoil. In a previous study [9], the authors have studied the aerodynamic effect of employing a backward-facing step on the upper surface of a NACA 2412 airfoil. A correlation between the step location the aerodynamic properties was established to show that the step has decreased the lifting capability, increased the drag forces and lowered the critical angle of attack of the stepped airfoil.…”