Vortex development for a 65 degree swept back delta wing with varying thickness and maximum thickness location

Stucke, Jana-Sabrina

Abstract

Low signature, unmanned combat aerial vehicles (UCAV) are foreseen to play an important role in future military missions. Most UCAV concepts feature some sort of delta or lambda type of wing with high or moderately swept leading edge, where the fuselage is part of the lifting surface, thus, forming a blended wing body. These configurations have advantageous characteristics at high speeds and a low radar signature, making them desirable shapes for military purposes. However, blended wing bodies are known for having a serious problem with their longitudinal static stability and their performance at low speeds. Initial wing design or the ability to change the wing shape in flight, by means of wing morphing, could be utilised to overcome these issues. Therefore, with the technology readiness level of morphing configurations increasing, an understanding of the effect of slight changes to the aerofoil design need to be more thoroughly established. Therefore, to understand the change in aerodynamics when altering the thickness/chord ratio (3.4%, 6% and 12%), spanwise thickness distribution and maximum thickness location/chord (30% and 50%) a computational investigation of a 65° delta wing with different profiles of different thicknesses was undertaken. Here, the maximum thickness location was investigated on two biconvex wings with maximum thickness locations at 30% and 50% root chord, whilst a tip taper study was conducted using the NATO’s AVT-113 VFE-2 configuration and a derivation of it. The effect of thickness was investigated on all configurations. Particular emphasis was placed on the longitudinal stability and lift to drag (L/D) ratio as they both impact aircraft design. The former due to its impact on the positioning and size of control surfaces and the latter due to its impact on thrust, maximum range and maximum take-off weight considerations. The investigation was carried out at Mach number 0.1 and Reynolds number of 750,000 with the former being representative for take-off and landing conditions for some military aircraft incorporating slender wing flows. The numerical results were validated for selected configurations in the University of the West of England’s wind tunnel. For the numerical simulations ANSYS FLUENT was used with an unstructured hybrid mesh approach. The steady state runs were conducted using the k-ω Shear Stress turbulence (SST) model with curvature and low Reynolds number corrections.

The study showed that tip taper, thickness and maximum thickness location affect the L/D ratio and longitudinal stability. For the biconvex wings an increase in thickness was found to increase the L/D at higher angles of attack whilst the maximum attainable L/D ratio was found to be a function of both, thickness and maximum thickness location. For wings with mainly flat upper and lower surface L/D decreased with increase in thickness and so did the maximum L/D ratio. This was irrespective of tip taper. The angle of attack at which the maximum L/D was reached moved to a slightly higher angle (α=1°) when thickness was increased on the biconvex configurations, whereby the tapered VFE-2 configuration experienced a shift of up to α= 3°. Having a constant thickness distribution further enhanced this effect moving the angle of maximum L/D from α= 5° to α= 15°. Moving the maximum thickness location forward resulted in an overall increase in L/D for the 6% and 12% configuration whilst not having a significant effect for thin wings, with the same being true for the maximum attainable L/D ratio. The angle of attack at which maximum L/D was reached moved to a lower angle by 1° for the 3.4% and 12% wing whilst being unaffected for the 6% wing. Tip taper showed to improve L/D and maximum L/D irrespective of thickness whilst the angle of attack at which maximum L/D could be achieved moved to lower angles when tip taper was introduced. This effect showed to enhance with increase in thickness.

The stability was evaluated at a lift coefficient C_L=0.5 typical for take-off and landing. It was found that stability improved with increase in thickness indicated by a rearward movement of the aerodynamic centre. These improvements were more significant when maximum thickness location was moved forward and resulted in up to 1.82% increase in static margin. Similar observations were made for the VFE-2 configuration and its adapted model with span taper. Here, the static margin increased by 3.21% when introducing tip taper. This was due to increased rear loading. In particular moving the maximum thickness location forward resulted in a delay in vortex onset, thus, further improving stability. Increase in thickness also resulted in enhanced stability (by up to 6.68%) but was strongly dependent on maximum thickness location and spanwise thickness distribution.

The flow pattern was also affected by increasing thickness, resulting in primary vortex stretching and domination of separation bubbles especially close to the apex. On the biconvex profiles shifting the maximum thickness location rearward helped in maintaining non-linear lift generation even when thickness was increased. It further resulted in delayed onset of vortex breakdown. Increase in thickness on wings with a flat profile was found to cause the formation of an inner co-rotating vortex similar to that observed for delta wings with round leading edges.

The main finding of the study was the demonstration that by alteration of the upper surface, leading edge suction can be recovered despite having a sharp leading edge, a phenomenon normally attributed to wings with round leading edges. This, in combination with the finding that the vortex flow pattern can be altered by reducing it to separation bubbles is not only valuable for future delta wing design but also for the implementation of wing morphing. Here, depending on the flight condition, wing shape could be altered by moving the maximum thickness location or introducing tip taper to get the desired flight performance.

Citation

Stucke, J. Vortex development for a 65 degree swept back delta wing with varying thickness and maximum thickness location. (Thesis). University of the West of England. Retrieved from https://uwe-repository.worktribe.com/output/6012701

• Thesis Type: Thesis
• Deposit Date: Jun 7, 2020
• Publicly Available Date: Oct 13, 2020
• Keywords: CFD, Delta Wing, Vortical Flow, Leading Edge Vortices
• Public URL: https://uwe-repository.worktribe.com/output/6012701
• Award Date: Oct 13, 2020