In the realm of modern drone technology, small and medium-sized Unmanned Aerial Vehicles (UAVs) dominate low-altitude airspace, with vertical takeoff and landing (VTOL) capabilities being a critical feature. Various VTOL methods exist, including multirotor, tail-sitter, tilt-rotor, and hybrid-wing designs. Among these, multirotor drones, particularly quadcopters, are prevalent in consumer-grade UAVs due to their ability to provide lift for VTOL, making them suitable for applications like last-mile logistics, close-range reconnaissance, short-duration material delivery, and river pollution monitoring. However, multirotor UAVs primarily generate lift via rotors, leading to limited endurance and range. Tail-sitter VTOL aircraft position propulsion systems at the rear, transitioning from vertical ascent to level flight, but they often have small payloads and are optimized for cruising. Tilt-rotor designs involve complex aerodynamic coupling during transition, while hybrid-wing configurations combine multirotor and fixed-wing advantages, using rotors for VTOL and a wing for cruise, though the rotors add dead weight and drag, reducing efficiency.
Current research in drone technology focuses on flight control, fault handling, formation flying for multirotors; trajectory control and stability for tail-sitters; aerodynamic characteristics and automation for tilt-rotors; and control systems and structural design for hybrid-wing UAVs. Despite these advances, there is a gap in integrating overall layout, aerodynamic shaping, and structural design to optimize performance. To address this, we propose an elliptical hybrid-wing UAV that merges multirotor and fixed-wing structures, enhancing structural strength, lift during level flight, and enabling capabilities like continuous water sampling, river inspection, and emergency material delivery on water surfaces. This design leverages the VTOL benefits of multirotors or tail-sitters while improving cruise efficiency and high-angle-of-attack performance.
Our elliptical hybrid-wing UAV features an elliptical structure comprising upper and lower wings with a central straight fixed-wing, all using the NACA4412 airfoil. We developed two configurations: Design A and Design B, with a conventional fixed-wing UAV (Design C) for comparison. Design A integrates the elliptical wings with a vertical tail through blended wing-body design, placing four rotors at junctions between the elliptical wings, fixed wing, and vertical tail. The horizontal and vertical tails use the symmetric NACA0012 airfoil for stability and control. Key parameters include a fixed-wing span of 1.36 m, a leading-edge sweep of 12°, an aspect ratio of 13.3, and an elliptical wing with a major axis aligned to the fixed-wing span (73.5% of span). The fuselage is 1 m long, accommodating components like motors, electronic speed controllers, and batteries. Design B modifies the tail by integrating the horizontal tail with the elliptical wing tips and shifting the fixed wing forward by 1.45 times its chord length to enhance lift and reduce interference.
To analyze the aerodynamic characteristics, we employed numerical simulations using the K-Ω-SST turbulence model in FLUENT software. The computational domain was set with a semi-model unstructured grid, refined at wing junctions and trailing edges to capture high-angle-of-attack flow features. The grid dimensions extended 10 body lengths ahead, 25 behind, and 10 span lengths above, below, and sideways, with polyhedral surface and hybrid volume grids totaling 6.86 million, 6.06 million, and 4.54 million cells for Designs A, B, and C, respectively. We simulated steady, incompressible flow at a cruise speed of 60 m/s and an altitude of 103 m, corresponding to typical low-altitude operations. The Reynolds-averaged Navier-Stokes (RANS) equations and continuity equation were solved with no-slip wall boundaries. Convergence criteria were set to residuals of 10^−6 for pressure, velocity, and temperature. Angles of attack ranged from -4° to 16° in 2° increments to study lift-drag properties, pressure contours, and streamlines.
The lift coefficient \( C_l \), drag coefficient \( C_d \), and lift-to-drag ratio \( K \) are defined as:
$$ C_l = \frac{L}{qS} $$
$$ C_d = \frac{D}{qS} $$
$$ K = \frac{C_l}{C_d} $$
where \( L \) is lift, \( D \) is drag, \( q = \frac{1}{2} \rho v^2 \) is dynamic pressure, \( \rho \) is air density, \( v \) is freestream velocity, and \( S \) is the reference wing area. For Designs A and B, \( S \) includes the projected area of the elliptical hybrid wing to account for lift contributions, whereas Design C uses the fixed-wing area alone.
The simulation results reveal significant differences in aerodynamic performance. The lift coefficient curves show that the elliptical hybrid-wing designs substantially increase lift linearity and maximum lift compared to the conventional fixed-wing. Design B exhibits the highest lift curve slope of 0.1198, followed by Design A at 0.08138, and Design C at 0.05541. This represents a 116.19% improvement for Design B and a 46.87% improvement for Design A over Design C. At an angle of attack of 14°, Design B achieves a lift coefficient of 2.03318, Design A 1.5284, and Design C 1.1957, indicating enhancements of 70.03% and 126.17%, respectively. The elliptical wings delay stall, with Designs A and B maintaining attached flow up to 16°, while Design C stalls at 12° with significant separation.
| Design | Lift Curve Slope | Max \( C_l \) at 14° | \( C_d \) at 2° | Max \( K \) |
|---|---|---|---|---|
| A | 0.08138 | 1.5284 | 0.0123 | 11.1609 |
| B | 0.1198 | 2.03318 | 0.0125 | 13.0778 |
| C | 0.05541 | 1.1957 | 0.0098 | 14.2105 |
Drag coefficients are higher for Designs A and B due to the additional surfaces and interference. At low angles of attack (below 6°), drag differences are minimal, but beyond 6°, drag increases more rapidly for the hybrid designs. The lift-to-drag ratio \( K \) is lower for Designs A and B in most ranges, with Design B achieving a maximum of 13.0778 at 2° angle of attack, a 17.17% improvement over Design A’s 11.1609, but still below Design C’s 14.2105. This trade-off is acceptable given the enhanced lift and stall performance, which are crucial for VTOL transitions and high-angle-of-attack maneuvers in drone technology applications.
Pressure contour and streamline analyses provide insights into the flow behavior. At a cruise angle of attack of 2°, high-pressure zones on Designs A and B are concentrated on the lower surfaces of the elliptical wings and fixed wing, contributing to lift. Design B’s forward-shifted fixed wing and integrated horizontal tail add lift areas, as seen in the expanded high-pressure regions on the upper wing’s lower surface. Streamlines remain attached with minimal interference, though rotor mounts cause vortices that increase drag. For Design C, the flow is simpler, with high pressure only on the fixed wing’s lower surface. At 14° angle of attack, Designs A and B maintain laminar flow on the upper surfaces of the elliptical wings, with no separation, whereas Design C exhibits significant flow separation on the fixed wing’s upper surface, leading to stall. The elliptical wings in Designs A and B also modify the flow approaching the horizontal tail, reducing its effective angle of attack and improving control authority during high-angle-of-attack flight.
The aerodynamic advantages are further illustrated by examining cross-sectional flow at 0.2 m from the fuselage axis. At 2° angle of attack, Design A shows high pressure on the lower elliptical wing, while Design B has additional high-pressure areas on the forward fixed wing and upper wing’s trailing edge. Both designs maintain stable flow over the tail. At 14°, Design A’s lower elliptical wing remains a primary lift source, with the fixed wing and upper wing contributing; Design B’s forward fixed wing enhances lift distribution. The horizontal tail in both designs experiences corrected flow, avoiding separation, whereas Design C’s tail is affected by separated flow from the fixed wing. This demonstrates the elliptical hybrid-wing’s ability to improve overall flow management in Unmanned Aerial Vehicle designs.
To validate these findings, we constructed prototypes of Designs A and B using carbon fiber for high-strength components like spars and fuselage, and balsa wood for skins to facilitate repairs. Flight tests were conducted in various environments, including riverbanks, lakes, and hilly areas, to assess VTOL, transition, and cruise performance. The UAVs demonstrated stable vertical takeoff and landing, with ascent to 8-15 m in 20-40 seconds, and smooth transition to level flight in 3-6 seconds. Cruise flights at 50-85 m/s and altitudes of 20-60 m showed excellent controllability at high angles of attack (15°-22°), with no stall incidents. The elliptical hybrid-wing provided sufficient lift for low-speed endurance and reliable hovering over water for sampling, confirming the design’s practicality for applications like multi-point water quality monitoring and emergency delivery.
In summary, the elliptical hybrid-wing UAV effectively combines the benefits of multirotor VTOL and fixed-wing cruise, addressing limitations in existing drone technology. Numerical simulations and flight tests confirm enhanced lift coefficients, delayed stall, and improved flow characteristics, making it suitable for demanding missions. Future work will focus on optimizing wing parameters and tail designs to reduce drag and increase lift-to-drag ratios, further advancing Unmanned Aerial Vehicle capabilities for diverse operational scenarios.