The Role of Bioinspired Wingtips in Enhancing Aerodynamic Efficiency and Maneuverability in UAVs
Engineering Challenge
Flight missions involving small-scale uncrewed aerial vehicles (UAVs) increasingly require high levels of aerodynamic efficiency and maneuverability. With limited battery capacity, it is important that UAVs are able to reach their full flight potential even at lower speeds. Additionally, UAVs need to maneuver around obstacles, manipulate payloads, land on uneven surfaces, and fly close to the ground. Aircraft today are designed for specific flight missions and cannot necessarily fly in variable conditions. Improvements to current designs are needed to enable a new generation of UAVs capable of flying a broad range of missions.
Nature’s Inspiration
Birds are highly capable and maneuverable fliers, with the ability to fly at both high and low speeds in a variety of flight conditions. They engage in a multitude of complex flight maneuvers, such as takeoff, landing, gliding, perching, diving, and flying close to surfaces, by changing the shape of their wings during flight. Harris's hawks, in particular, fly in a similar Reynolds number regime to small-scale UAVs, and are able to perform both efficient and agile flight due to their morphology. They have moderate aspect ratio wings, which are associated with maneuverable flight. Though moderate aspect ratio wings are also associated with reduced efficiency, their emarginated wingtips (slotted wingtips) are hypothesized to combat this by spreading vorticity horizontally and vertically. Understanding the flow physics of the wingtip devices, can enable us apply their aerodynamic benefits to the design of UAVs.
Closeup of the primary feathers of a Harris's hawk.
BAM Approach
The Effect of Wingtips on Aerodynamic Forces and Flow Fields
Motivation
An avian-inspired strategy for increasing the aerodynamic efficiency and agility of aircraft involves added wingtip devices to the tip of the wings. Previous studies that investigated aerodynamic benefits of multiple wingtips concluded that a single wingtip configuration cannot improve the lift to drag ratio (L/D) over an entire range of angle of attack. This motivates the need for a passive and adaptive wingtip system that can improve aerodynamic performance throughout the flight envelope.
Experimental Setup
One of the key parameters in the design of the wingtip configurations was the planform area of primaries 7, 8, and 9 of the Harris's hawk, which are those with the most notable emargination and notch. Additionally, the gap space between each wingtip was also incorporated, and in some studies, dihedral angle and incidence angle were also altered. Gap spaces were selected based on configurations in the literature that yielded improvements in aerodynamic efficiency.
Primary degrees of freedom of bird wingtips. Dihedral angle (left), incidence angle (center), gap space (right).
The wind tunnel experiment setup is shown in the below figure, and two types of data are acquired: force and moment data and Particle Image Velocimetry (PIV) measurements.The wing is attached to a Velmex B48 rotary table to perform an angle of attack sweep from -6° to 30° at Re = 200,000 to capture both the pre- and post-stall regimes. It is also attached to an ATI Gamma 6-axis force/torque transducer to measure the forces and moments acting on the wing. To collect PIV measurements, a high speed camera captures particles illuminated by a laser as they flow over the wing.
Schematic of the wind tunnel experiment setup.
Recent Results
The wing with wingtips produces modulation in lift and drag throughout the angle of attack range. Most notably, there is delayed stall with the addition of the wingtips compared to the tip extension wing. These results support current hypotheses suggesting that birds with moderate aspect ratio wings with reduced efficiency compensate by having these wingtips to increase their efficiency while preserving maneuverability.
Coefficient of lift versus angle of attack for a baseline tip extension (yellow) and the wing with wingtips (blue).
By mapping the propagation of stall from outboard to inboard of both wing types it can be seen that the wingtips break up the tip vorticity to alter stall propagation along the wing. The flow is separated for the wing with wingtips outboard of the wing compared to the tip extension, but this is due to the flat plate geometry of the wingtip. The PIV planes taken at the actual tip of the base wing (third row) show that at this point, the flow over the wing with wingtips becomes attached whereas it is separated for the tip extension wing. This could be evidence of the horizontal spreading of tip vorticity by these planar wingtips.
Stall propagation over the wingspan for the wing with wingtips (blue, left column) and the tip extension (yellow, right column). PIV planes are taken further inboard of the wings with each row in descending order.
An Adaptive Multi-Wingtip System
Motivation
To change the shape of the wingtips throughout an angle of attack range, an adaptive multi-wingtip system concept was developed. The wingtip gap space is controlled actively using bistable mechanisms, and the dihedral and incidence angles of each wingtip are controlled passively through composite bend-twist coupling.
Experimental Setup
The bend-twist shape adaptation of the composite wingtips is varied by the aerodynamic loading experienced by the wingtip, allowing for passive control of wingtip dihedral and twist angle. The composite wingtips were printed using a Stratasys Objet printer with Tango Black Plus as the matrix material and Vero White Plus as the fiber material. The composite was fabricated with 8-ply laminate with a 30° fiber orientation angle. To characterize the bend-twist coupling of the composite laminate wingtip, a point load was applied at the free end of a clamped-free composite wingtip as shown in the figure below. A two-camera stereovision system (3D DIC) was used to capture the resulting deflection of the wingtip in the z-direction.
Conceptual design of an adaptive multi-wingtip system.
Experimental setup for discrete bend-twist characterization of composite wingtips.
More recently, the composite wingtips have been manufactured using an Ultimaker printer with TPU 95A as the matrix material and PLA Transparent as the fiber material. Placing these wingtips in the wind tunnel, an avian-like deflection was observed, as shown in the figure below.
The desired gap spacing between the wingtips are actively achieved by combining smart actuators with a bistable truss mechanism. This mechanism allows for the efficient use of smart actuators by reducing the power required to change and maintain the spacing between the wingtips. The material properties of the beam and the material composition of the joints at the end (joints 1 and 2) were varied to determine their effects on the bistability of the truss mechanism.
(left) Multilayer composite wingtips in an exploded view. (right) Composite wingtips showing bend-twist behaviour.
The truss mechanisms were 3D printed from the Stratasys Object materials Shore40, Shore60, and Shore85, obtained by mixing various percentages of Vero White Plus and Tango Black Plus, with a higher numerical identifier representing a less compliant material. Each truss mechanism contained two bistable beam elements supported by rigid Vero White Plus support plates to ensure that all deformation and energy storage occurred in the bistable elements. A universal force testing machine was used to record the force-displacement behavior of all truss material combinations in compression.
Progression of a truss mechanism compressed by a universal testing machine.
The figure to the right shows the dihedral and twist response of the composite wingtip for applied loads ranging from 50 g to 400 g. Twist angles from 1.7° to 8.3°, and dihedral angles from 5.1° to 38.2° were achieved with the composite wingtip at the range of applied loads. The results show the feasibility of using a composite wingtip to achieve the desired bending and twist of the wingtip to design wingtip configurations for improved aerodynamic performance.
Recent Results
Bend-twist coupled response of the wingtip (8-ply laminate with 30° fiber orientation) composite layup for discrete applied loads ranging from 50 g to 400 g.
The results from the compression test showed that as the compliance of the bistable element decreased (from Shore40 to Shore85), the force required to transition the truss mechanism between the first and second stable positions increased along with the strain energy in the mechanism. However, the force required to return to the initial position decreased, making the mechanism less bistable. These results demonstrated that for a given truss geometry, varying the compliance of the bistable element expanded the design space of the mechanism actuation force and bistability. Additionally, for fixed beam compliance, more compliant boundary conditions resulted in a lower actuation force and higher required force to return to the initial configuration. Alternatively, less compliant boundary conditions resulted in higher actuation forces and a truss combination that did not exhibit bistability (the case where both joints were less compliant than the beam element).
Acknowledgments
