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