From aerial imaging, work-site inspection, to residential package delivery, many industries in the civilian sector are currently using of Unmanned Aerial Vehicles (UAVs) to perform various missions. In mission-driven tasks, these vehicles are flown either in confined spaces or near humans. Not only do they need to be safe and maneuverable, but their performance must also be robust and predictable at the edge of their operating envelope. Due to their size and flight speeds, mini-UAVs operate in the low Reynolds number regime (orders of magnitude 104 to 105). Under these conditions, lift, aerodynamic efficiency, and stall angle of attack are reduced. Therefore, alternative methods to generate high lift and mitigate stall at low Reynolds numbers and at high angles of attack are necessary to achieve mission-adaptability.
Mission adaptability is observed in avian flight as birds engage in many complex maneuvers using the same flight apparatus. It is especially apparent in raptors and birds of prey (e.g. ospreys, eagles, owls, etc.) as these birds’ hunting strategies require high levels of agility. While both bird and mini-UAVs operate under low Reynolds number conditions, the flight envelope of birds far exceeds that of UAVs. In fact, birds morph their wings and feathers according to flight conditions or maneuvers performed. For instance, they have evolved a set of small feathers near the leading edge (LE) of their wings, known as the alula as shown in Figure 1. When deployed, the alula enables the wing to sustain the lift necessary to stay airborne at low speeds and high angle of attack maneuvers such as take-off, landing, perching, and hunting.
We use a bioinspired approach (Figure 2) to develop an adaptive leading-edge device for UAV wings based on the alula feather structure. To date, a static LEAD (Leading-Edge Alula-Inspired Device) has been designed and implemented on a rectangular wing both in a 2D (airfoil) and a 3D (moderate aspect-ratio) configuration. The dimensions and characteristics of the wing and LEAD test sections are inspired by the morphology of high-lift wings (similar to wings of raptors and birds of prey).
We performed a series of wind tunnel experiments to understand the effects of placing the LEAD on the rectangular wing at low Reynolds numbers. 3 LEAD deployment parameters were evaluated: relative angle of attack, tip deflection, and root location along the wingspan. The tests conducted include integrated lift and drag force measurements and wake boundary layer sampling.
Results have shown that the LEAD improves lift to 32% under post-stall and 37% under deep stall conditions compared to baseline. However, a penalty in lift is observed on pre-stall. This indicates that the LEAD is a post-stall device (both in 2D and 3D) and needs to be a deployable device. Despite some drag penalty generated, the wing maintains aerodynamic efficiency when the LEAD is added.
The lift performance wing-LEAD assembly is sensitive to the spanwise LEAD location as shown in Figure 3. The largest improvements are produced when the LEAD is placed near the middle of the semi-span of the wing. The LEAD produces higher percent lift improvements when implemented on a moderate aspect-ratio wing (3D) compared to an airfoil (2D) test section. The performance also depends on deployment parameters and is more sensitive to relative angle of attack and spanwise location than to tip deflection angle.
Hot-wire anemometer measurements show that the LEAD accelerates the flow and reduces the height of velocity deficit in the wake. When the wing is partially stalled, the LEAD reduces the wake velocity deficit and delays flow separation like traditional leading-edge slats. On fully stalled wings, the LEAD does not only act as a leading-edge slat, but also creates the boundary layer fence that prevents the propagation of stall outboard of the LEAD root.
In nature, the alula is an adaptive and flexible high-lift device that is only deployed at steep angles of attack and during flight tasks that require high maneuverability. While we have successfully adapted the alula function to an engineered wing using a static device, future work includes designing a deployment mechanism for the LEAD that can respond to dynamic flow conditions.
Ito,M., C. Duan, and A. Wissa “The Function of the Alula on Engineered Wings: A Detailed Experimental Investigation of a Bioinspired Leading-Edge Device.” Bioinspiration & Biomimetics, 2019 (Under Review)
Ito, M., C. Duan, L. Chamorro, and A. Wissa “A Leading-Edge Alula-Inspired Device (LEAD) for Stall Mitigation and Lift Enhancement for Low Reynolds Number Finite Wings.” Proc. Smart Materials, Adaptive Structures and Intelligent Systems Conf. 2018-8170 San Antonio, TX (Abstract and full draft paper were subject to peer review).
Mandadzhiev, B., M. Lynch, L. Chamorro, and A. Wissa “An Experimental Study of an Airfoil with a Bio-inspired Leading Edge Device at High Angles of Attack,” Smart Materials and Structures, Volume 26, Number 9.
Mandadzhiev, B., M. Lynch, L. Chamorro, and A. Wissa “Alula-inspired Leading Edge Device for Low Reynolds Number Flight,” Proc. Smart Materials, Adaptive Structures and Intelligent Systems Conf. 2016-9210, Stowe, VT.