The research focuses on a delta wing built around the NACA 0012 airfoil profile, a symmetric section that has been a workhorse of aeronautical testing for nearly a century. What distinguishes the configuration is its sweep angle of fifty degrees, which places it firmly in the non-slender category. Unlike the dramatic, needle-nosed slender deltas of supersonic fighter fame, non-slender delta wings sweep at shallower angles and generate lift through a combination of conventional attached flow and leading-edge vortex systems. That hybrid behavior makes them attractive for small unmanned air vehicles, where compact planforms, structural simplicity, and forgiving stall characteristics are prized, but it also makes their ground-effect behavior genuinely complicated to predict.

The experimental campaign was structured around the full range of angles of attack that a small UAV might encounter between a ground run and a stalled climb-out. Force measurements were taken from zero degrees up to forty-two degrees of angle of attack in three-degree increments, a resolution fine enough to capture the gradual evolution of lift and drag as the wing’s vortex system matured and eventually broke down. To simulate the changing clearance of a vehicle during takeoff and landing, the researchers varied the ratio of the wing’s height above the ground plane to its chord length, testing values of 0.1, 0.2, 0.5, and 1. At the lowest ratio, the wing sits a tenth of a chord above the surface, deep inside the ground cushion; at a ratio of one, it is at the edge of where ground influence is typically considered significant.

Ground effect is not a single phenomenon but a bundle of interacting mechanisms. For a conventional wing, proximity to the surface suppresses the downwash behind the wing and reduces induced drag, which is why racing sailplanes and hovering birds seem to glide effortlessly over water. For a delta wing, the picture is richer. The dominant feature of delta-wing aerodynamics is the pair of primary vortices that roll up along the highly swept leading edges, spinning off the edge and coiling above the wing’s upper surface. These vortices generate additional suction on the upper surface, contributing what aerodynamicists call vortex lift, a mechanism famously formalized by Edward Polhamus in the 1960s through his leading-edge suction analogy. How those vortices behave when the ground is close by is the central question of the study.

Force coefficients alone, however, tell only half the story. To understand why lift and drag changed the way they did, Bay and Kara turned to surface oil flow visualization, a classic experimental technique that reveals the topology of flow on a wing’s surface. A mixture incorporating titanium dioxide pigment is painted onto the wing; as air flows over the surface, shear stresses in the boundary layer drag the oil into streaks, and the resulting patterns record where flow attaches, separates, and reattaches. The researchers conducted these visualization experiments from zero to forty degrees of angle of attack in five-degree steps, building a photographic atlas of the surface flow at each ground clearance.

The oil-flow experiments revealed the key flow phenomena governing the wing’s performance. The team identified the onset and progression of vortex breakdown, the dramatic event in which the coherent leading-edge vortex suddenly expands into a turbulent, disorganized mass, causing the suction peak over the wing to collapse and lift to plateau. They also mapped the reattachment region where the swirling vortex flow impinges back onto the wing surface, the footprint of the inner primary vortex, and the delicate surface signatures of the secondary reattachment line and secondary separation line, features that mark the smaller counter-rotating vortices nested beneath the primary system near the leading edge. Flow traces across the surface connected these features into a coherent topological picture at every angle of attack and ground height tested.

This combination of force measurement and flow visualization is what gives the study its practical value. A designer of a small delta-wing UAV can read the force curves to know how much lift and drag to expect during a ground run at a given clearance, and then consult the flow topology to understand the underlying mechanism, whether it is a strengthened vortex, a delayed breakdown, or a shift in the reattachment line. Because the tested height-to-chord ratios of 0.1 through 1 span the realistic range of wheel heights during takeoff rotation and landing flare, the dataset maps directly onto the flight phases the authors set out to investigate: takeoff, landing, and cruise out of ground influence.

What makes the work particularly compelling for the UAV community is that it did not stop at the wind tunnel. The authors also present the results of real flight tests of an unmanned aerial vehicle incorporating this wing, beginning with a real-time ground takeoff and continuing into flight out of ground effect. Flight test data of this kind is rare in academic ground-effect studies, which often remain confined to wind tunnels and computational models. By anchoring the laboratory measurements to the behavior of an actual aircraft leaving the runway, the study provides a bridge between controlled experimentation and the messy, gusty reality of a small vehicle climbing away from the ground.

The broader context of the research is a growing body of literature on wings in ground effect, a field that stretches from the wing-in-ground-effect vehicles studied extensively by Kirill Rozhdestvensky to recent experimental work on non-slender delta wings by groups including Tumse and colleagues, and Koçak and Yavuz, who examined both static and dynamic ground boundary conditions. Much of the earlier delta-wing ground-effect work used flat-plate or thin airfoil sections; building the wing from a full NACA 0012 profile with realistic thickness adds a layer of fidelity relevant to actual vehicle design. Prior studies have also shown that thickness-to-chord ratio itself alters non-slender delta wing aerodynamics, which makes the choice of a standard airfoil section a meaningful one rather than a mere convenience.

For the emerging fleet of small drones, eVTOL demonstrators, and micro air vehicles that spend much of their operating lives near surfaces, the message of this research is that ground effect is not simply a bonus of extra lift. It reshapes the vortex system that defines delta-wing flight, moves the boundaries of stall and breakdown, and alters the drag budget in ways that depend intimately on height, angle of attack, and planform geometry. By pairing precise force measurements across a dense grid of angles of attack and ground clearances with titanium dioxide surface visualization that exposes the underlying flow structures, and by closing the loop with real flight tests, Bay and Kara have delivered a reference dataset and a physical explanation that should help engineers design delta-wing unmanned aircraft that behave predictably in the most demanding moments of flight: the seconds spent leaving the ground, and the seconds spent returning to it.

Subject of Research: Ground effect aerodynamics of a NACA 0012-based non-slender delta wing for unmanned air vehicles

Article Title: An Experimental Investigation of Aerodynamic Performance of an NACA 0012-Based Non-slender Delta Wing Under Ground Effect

Article References: Bay, A. E., & Kara, T. (2026). An Experimental Investigation of Aerodynamic Performance of an NACA 0012-Based Non-slender Delta Wing Under Ground Effect. International Journal of Aeronautical and Space Sciences. https://doi.org/10.1007/s42405-026-01197-5

Image Credits: AI Generated

DOI: 10.1007/s42405-026-01197-5

Keywords: delta wing, ground effect, NACA 0012, unmanned aerial vehicles, vortex breakdown, aerodynamic performance, surface oil visualization, titanium dioxide, wind tunnel testing, leading-edge vortices, takeoff and landing, flight tests

The research, published in the journal Aerospace Systems, was carried out by Pavel Makeev of the Department of Aeronautical Engineering at the Moscow Aviation Institute. The work revisits and modernizes a concept first embodied in the tail rotor scheme of the TsAGI-1EA, an experimental helicopter built in the Soviet Union in the 1930s. The idea is deceptively simple: instead of mounting a single multi-bladed rotor on one side of the tail boom, the coaxial tail rotor places a pair of two-bladed rotors on the same axis but on opposite sides of the boom, one to the right and one to the left. This arrangement gives the rotors a substantial axial separation along the axis of rotation, and it is precisely that separation, the study finds, that unlocks an aerodynamic benefit familiar from coaxial main rotor systems.

To evaluate the concept, Makeev used a free-wake model, a computational approach that tracks the vortical wake shed by each blade as it moves through the air rather than imposing a prescribed wake geometry. The free-wake method is particularly valuable for coaxial configurations, where the wake of one rotor travels directly into the path of the other and the resulting blade-vortex interactions strongly influence both performance and loads. The simulations focused on hovering flight, the aerodynamically demanding condition in which a rotor must generate maximum thrust for the least possible power, and the rotors were analyzed in isolation, without the complicating influences of the main rotor wake or the helicopter airframe.

The coaxial tail rotor was benchmarked against two established alternatives: the conventional four-bladed tail rotor used on serial production helicopters and the so-called scissors tail rotor, a promising design in which blade pairs are mounted at an angle to one another on the same hub. The baseline axial spacing between the rotational planes of the coaxial blade pairs was set at 0.8 rotor radii, a value chosen because it is approximately equal to the doubled side spacing of a conventional tail rotor relative to the helicopter’s longitudinal axis. Both co-rotating and counter-rotating versions of the coaxial pair were examined, along with two different control strategies: one in which both rotors operate at the same blade pitch angle, and one in which both rotors are trimmed to produce the same thrust.

The headline result concerns hover efficiency, conventionally measured by the Figure of Merit, a dimensionless ratio that compares the ideal power required to produce a given thrust with the actual power consumed. For the coaxial counter-rotating configuration with 0.8R axial spacing operating under the equal-thrust strategy, the Figure of Merit increased by up to 12.2 percent relative to the conventional four-bladed rotor, while power consumption fell by approximately nine percent. That is a substantial gain for a component whose aerodynamic losses translate directly into fuel burn and payload capacity. The equal-thrust control strategy consistently outperformed the equal-pitch strategy, indicating that allowing each rotor of the pair to find its own pitch while matching thrust output extracts more of the available aerodynamic benefit.

The direction of rotation mattered as well. Counter-rotating pairs, in which the upper and lower rotors spin in opposite directions, delivered better performance than co-rotating pairs. This finding echoes the established behavior of coaxial main rotors, where counter-rotation allows the swirling wake of one rotor to partially cancel the swirl of the other, reducing rotational losses in the slipstream and improving the distribution of induced velocity across the rotor disk. The study also documented the shapes of the rotor wakes, the detailed flow patterns, and both distributed and total aerodynamic characteristics, providing a physical picture of how the separated blade planes interact through their vortex systems.

Perhaps the most intriguing result concerns how the axial spacing between the two rotors shapes the benefit. Makeev extended the analysis across a range of spacings from 0.6R to 2.0R and found that the maximum increase in Figure of Merit, reaching 15.8 to 16.2 percent, occurs at spacings of 1.4R to 1.8R. Beyond that range, the positive effect begins to decline. This non-monotonic behavior suggests there is an optimal window in which the wake of one rotor is far enough from the other to avoid destructive interference yet close enough for the two vortex systems to interact favorably. In practical terms, the axial separation inherent in the coaxial tail rotor arrangement makes it possible to maximize the positive aerodynamic effect that has long been recognized in coaxial main rotor research, where high Figure of Merit values at hover have been documented since the 1990s.

The implications extend beyond raw efficiency. Because the coaxial arrangement places rotors on both sides of the tail boom, it offers the potential for increased yaw control authority, which is directly relevant to flight safety. Loss of tail rotor effectiveness, whether caused by main rotor wake interference, crosswind conditions, or low-speed maneuvering, remains a serious hazard, and a tail rotor with higher aerodynamic efficiency and greater thrust capability for the same power budget could enlarge the safety margin in precisely the flight regimes where conventional designs are most vulnerable. The concept also fits into a broader research effort spanning variable-speed tail rotors, advanced blade tip shapes, and optimized twist distributions, all aimed at squeezing more performance from this hardworking component.

Makeev is careful to frame the findings as preliminary. All results were obtained for an isolated tail rotor, and real helicopters present a far messier aerodynamic environment. The wake of the main rotor sweeps past the tail, the airframe sheds its own vortices, and in forward flight the tail rotor operates in oblique flow rather than in the clean axial conditions of hover. Two open tasks stand out: quantifying how the main rotor wake and airframe modify the coaxial tail rotor’s performance, and assessing the drag penalty of the coaxial hub assembly at forward flight, since two hubs and their linkages inevitably create more parasite drag than a single conventional hub. The final choice of parameters, the study concludes, must account for the specifics of the helicopter on which the concept is to be installed.

Even with those caveats, the study makes a compelling case that a design idea nearly a century old deserves a second look with modern computational tools. A tail rotor that delivers more than twelve percent better hover efficiency and nine percent lower power consumption, with the potential for stronger yaw control and improved safety, would be a meaningful advance for an industry under pressure to reduce fuel consumption and emissions. As eVTOL aircraft and next-generation helicopters bring coaxial configurations back into the mainstream, the coaxial tail rotor may find itself moving from a 1930s curiosity to a standard feature of tomorrow’s rotorcraft.

Subject of Research: Numerical analysis of the hover aerodynamic performance of a coaxial helicopter tail rotor using a free-wake model

Article Title: Numerical study of the aerodynamic performance of the coaxial tail rotor concept in hover

Article References: Numerical study of the aerodynamic performance of the coaxial tail rotor concept in hover. (n.d.). https://doi.org/10.1007/s42401-026-00549-z

Image Credits: AI Generated

DOI: 10.1007/s42401-026-00549-z

Keywords: helicopter, coaxial tail rotor, hover, free-wake model, aerodynamic performance, Figure of Merit, rotorcraft, tail rotor efficiency, blade-vortex interaction, yaw control, computational fluid dynamics, aerospace engineering