Adaptive Wingtip Devices for Increased Agility and Maneuverability
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 more, by changing the shape of their wings during the flight in a variety of complex ways. These abilities are not shared with today’s small unmanned aerial vehicles (UAVs).
Image of Harris’s hawk during flight. The primary feathers are encircled in red
Unlike birds, current small UAVs struggle to fly in gusty and turbulent conditions and are mostly relegated to fair weather flight. UAVs lack the agility of birds in environments that are relatively close to ground level, filled with obstacles such as trees and buildings. Increasing the agility and maneuverability of fixed-wing drones, or stability in gusty conditions of quadcopters and similar devices remains challenging. The development of an adaptable wing with morphing wingtips could help alleviate the issues faced by modern UAVs. Facing these challenges requires a multi-disciplinary thought approach, combining ideas from mechanical engineering, electrical engineering, aerospace engineering, and material science.
Inspired by the primary feathers of birds, aircraft designers have implemented devices at the tip of the wing, named wingtips, to reduce the induced drag of the aircraft. Most of the previous studies that investigated aerodynamic benefits of multiple wingtips have concluded that a single wingtip configuration can no improve L/D over the whole range of angle of attack. Hence, to reduce the induced drag at both low and high angles of attack for high efficiency (L/D), at least two different configurations are desired during flight. To achieve two, or more wingtip configurations, there is a need for an adaptive wingtip design that could improve efficiency at both low and high angles of attack. Therefore, this research focuses on developing an adaptive multi-wingtip (AMW) device which yields optimal aerodynamic efficiency (L/D) ratio) across a wide range of angles of attack.
Wingtip Configufations Tests
The configurations of the wingtips suitable for the adaptive multi-wingtip device which yields optimal aerodynamic efficiency (L/D ratio) across a wide range of angles of attacks were tested in the wind tunnel.
Primary degrees of freedom of bird wingtips. Dihedral angle (left), incidence angle (center), gap space (right)
Key design parameters of the system. A) Dihedral angle: Bending angle between the wingtip and the horizontal line of the wing. B) Incidence angle: Twist angle between the wingtip and the chord line of the wing. C) Gap space: Total distance between the wingtips over the tip chord of the wing.
The key parameters in the design of the wingtip configurations were dihedral angle, incidence, angle, and gap space. They were selected based on configurations in the literature that yielded improvements in the aerodynamic efficiency. A summary of the test configurations is shown in the Figure below. The baseline and tip extension were tested to compare the benefits of using the wingtip configurations. The wingtip extension has the same planform area as the sum of the area of the three wingtips.
Summary of the test configurations. 1) Baseline. 2) Tip extension, which has the same planform area of the sum of the three wingtips. 3,4) Planar configuration with and without the gap space. 5,6) Non-planar configuration with and without the gap space.
Wind Tunnel Test
Wind tunnel experiments were conducted to determine the wingtip configurations suitable for the adaptive multi-wingtip system. The wind tunnel experiment setup is shown in the below Figure. The system of the wing and wingtips was attached to the assembly of a rotary table and a 6-axis force/torque transducer. Wind speed was set to 23.0 m/s, which resulted in a Reynolds number of 200,000. The assembly was placed at the sidewall of the wind tunnel. A Velmex 6-axis force/torque transducer was used to perform an angle of attack sweep from -4° to 26°. An ATI Gamma 6-axis force/torque transducer was used to measure the forces acting on the wing.
Schematic of the wind tunnel experiment setup. Velmex B48 rotary table is used for AoA adjustments and ATI Gamma 6-axis force/torque transducer is used for lift and drag measurements at Re = 200,000
Wingtip configurations for high efficiency
Summary of configurations for high efficiency at low (AoA < 4°) and moderate to high (AoA > 4°) angles of attacks
At low angles of attack (AoA < 4°), the planar configuration with 20 % gap shows the highest L/D ratio. At moderate to high angles of attack (AoA > 4°), the non-planar configuration with 20 % gap shows the highest L/D ratio. Therefore, to improve aerodynamic efficiency, the planar configuration with 20 % is needed at low AoA and the non-planar configuration with 20 % gap is needed at the moderate to high AoA.
Wingtip configurations for high efficiency
Summary of configurations for roll control at low (AoA < 4°), moderate (4° < AoA < 10°), and high (AoA > 10°) angles of attack
Aside from the efficiency, the lift results from the experiment show the possibility of roll control due to the lift difference between the configurations. Different wingtip configurations can be used on the left and right wings to achieve a lift difference and create a rolling moment. At low angles of attack (AoA < 4°), the non-planar 20 % gap and planar 20 % gap configurations can be used to create a relative lift difference of 19.5 %. At moderate angles of attack (4° < AoA < 10°), planar no gap and planar 20 % gap configurations can be used to create an average lift difference of 7.14 %. At high angles of attack (AoA > 10°), planar no gap and non-planar 20 % gap configurations can be used to create an average lift difference of 4.53 %. Hence, adapting the shape of the wingtips between three wingtip configurations namely, non-planar 20 % gap, planar 20 % gap, and planar no gap configurations can be used to generate a rolling moment across a wide range of angles of attack.
Adaptation achieved through bend-twist composite and bistable mechanism
A novel morphing concept inspired by dihedral angle, incidence angle, and gap space for the adaptive multi-wingtip system was studied. The below figure shows an adaptive multi-wingtip system concept, where the wingtip gap space is controlled actively using bistable mechanisms, and the dihedral and incidence angles of each wingtip are controlled passively through the composite bend-twist coupling.
Conceptual design of an adaptive multi-wingtip system which the bend-twist coupled composite wingtips are used to passively control the wingtip incidence and dihedral angles and a bistable mechanism to actively control the wingtip gap spacing
Bend-twist coupling composite
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 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 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.
Experimental setup for discrete bend-twist characterization of composite wingtips
The below figure below 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.
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 desired gap spacing between the wingtips will be actively achieved by combining smart actuators with a bistable mechanism. The bistable 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.
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 (i.e. Shore40 is more compliant than Shore60). Each truss mechanism constrained 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 the universal testing machine
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).
Future work includes tailoring the wingtip configurations, bistable mechanism, and composite wingtips to meet the aerodynamic criteria. Actuator selection and wind tunnel testing will be performed to characterize the aerodynamic performance of the adaptive multi-wingtip system in flight-like conditions for symmetric and asymmetric wingtip actuation.