RADIAL ADAPTIVE SABER HELICOPTER BLADE: A NEW APPROACH TO REDUCING VIBRATIONS AND NOISE

ABSTRACT

The article considers the theoretical justification for the use of radial-adaptive saber-shaped geometry of the helicopter main blade in order to reduce the level of vibrations and noise without using active systems. The analysis is based on the method of elementary sections of the blade, taking into account the inductive effects and the distribution of circulation along the radius. It was found that changing the saber-shaped geometry and twisting along the radius allows redistributing the lift closer to the blade tip, which reduces the intensity of the vortex wake formation and weakens the BVI effects. As a result, the level of impulse noise and vibration moments decreases, which has a positive effect on the service life of the structure and acoustic comfort. The proposed concept has a high technological applicability for modern and promising propellers, providing a passive reduction in noise characteristics without making the structure heavier. CFD studies and experimental tests are required for final verification of the results.

АННОТАЦИЯ

В статье рассматривается теоретическое обоснование применения радиально-адаптивной саблевидной геометрии несущей лопасти вертолёта с целью снижения уровня вибраций и шума без использования активных систем. Проведённый анализ основан на методе элементарных участков лопасти с учётом индуктивных эффектов и распределения циркуляции вдоль радиуса. Установлено, что изменение саблевидности и скручивания по радиусу позволяет перераспределить подъёмную силу ближе к законцовке лопасти, что снижает интенсивность формирования вихревого следа и ослабляет BVI-эффекты. В результате уменьшается уровень импульсного шума и вибрационных моментов, что положительно сказывается на ресурсе конструкции и акустическом комфорте. Предложенная концепция обладает высокой технологической применимостью для современных и перспективных винтов, обеспечивая пассивное снижение шумовых характеристик без утяжеления конструкции. Для окончательной верификации результатов требуется проведение CFD-исследований и экспериментальных испытаний.

Keywords: aerodynamics, wake vortex, vibrations, rotor, noise.

Ключевые слова: аэродинамика, вихревой след, вибрации, несущий винт, шум.

Introduction: Modern requirements for helicopters focus not only on improving flight performance but also on reducing vibrations, noise, and their impact on the airframe. In the context of growing urbanization and the expanding use of rotorcraft, acoustic footprint and vibration reliability have become key constraints on further industry development. Despite the implementation of solutions such as Blue Edge™, the problem remains relevant, as current approaches are mainly limited to local geometric optimization of blade tip sections. A comprehensive revision of load distribution along the blade radius has not yet been sufficiently studied.

This paper proposes a novel approach based on the radial adaptation of blade geometry, involving gradual variation of blade sweep (sabre shape) and twist along the radius. This method allows for redistribution of circulation and lift, reducing both noise and vibration loads, and opens new opportunities for the development of more efficient and reliable helicopter systems. The theoretical basis relies on classical rotor aerodynamics and circulation models, taking into account vortex wake effects.

The aim of this study is to demonstrate how radial blade geometry adaptation can serve as a simple and passive solution to the persistent problems of vibration and noise.

Relevance of the Problem

Helicopter technology continues to face limitations related to vibrations and noise generated by rotor systems. These issues are especially critical in terms of reducing acoustic signature, ensuring passenger comfort, and extending the service life of the transmission and airframe components. The primary source of noise remains blade-vortex interaction (BVI), which causes acoustic pulses and amplifies vibrations. In light of increasingly stringent regulatory standards and rising demands for reliability and safety, the search for effective solutions remains highly relevant.

Disadvantages of Modern Solutions

Currently, two main approaches are used to reduce vibration and noise: local geometric optimization (e.g., Blue Edge™) and active flow control systems (TEF, HHC). The first approach is limited to modifying the blade tip and does not address the redistribution of aerodynamic loads along the blade radius. The second requires complex mechanical systems, increases weight and cost, and reduces overall reliability. Neither approach provides a comprehensive solution for reducing vibration without relying on additional active components.

Purpose of the Article

The purpose of this work is to provide a theoretical justification for the effectiveness of radial-adaptive saber-shaped blade geometry in reducing vibrations and noise by optimizing the distribution of aerodynamic loads along the radius. It is shown that gradual variation of blade sweep and twist shifts the center of pressure closer to the blade tip, eliminates local circulation peaks, and reduces vortex wake intensity, thereby mitigating BVI effects and decreasing structural impact.

Scientific Novelty

The scientific novelty of this research lies in the development and theoretical justification of a fundamentally new blade geometry with radial adaptation of shape along the entire radius, in contrast to existing solutions focused solely on blade tips. For the first time, blade sweep and twist are considered as continuous functions of the radius, aimed at redistributing aerodynamic loads and optimizing circulation. The possibility of reducing vibration moments and impulse noise without the use of active systems is demonstrated through modification of the phase characteristics of the vortex wake. This work establishes a solid foundation for further research through CFD simulations and experimental validation.

Materials and Methods

One of the fundamental principles of the aerodynamics of rotating blades, including the helicopter rotor, is the relationship between the flow circulation and the lift force generated. This relationship is expressed in the Zhukovsky theorem, which establishes a direct proportion between the lift force, the density of the medium, the flow velocity and the circulation:

L' = ρ · V · Γ,

where L' is the lifting force per unit length, ρ is the air density, V is the undisturbed flow velocity, Γ is the circulation.

For rotating blades, the speed at each section depends on the radius:

U(r) = ω · r,

where U(r) is the velocity at radius r, ω is the angular velocity of rotation. Thus, circulation and lift increase towards the tip, which determines the load distribution along the radius.

According to Blade Element Theory, a blade is represented by a collection of small sections, each of which creates lift:

ΔL = 0.5 ρ U(r)^2 c(r) CL(r) Δr,

where c(r) is the profile chord, CL(r) is the lift coefficient, Δr is the radius element.

The lift force increases proportionally to the square of the speed, which requires optimization of geometry and aerodynamics along the radius to reduce negative effects. In rectangular blades, the main load is concentrated at the radius of 0.7–0.75, which increases vibrations of the structure [2, pp. 15–17]. Radial adaptation allows redistributing the load closer to the tip, reducing vibrations and the likelihood of BVI effects [3, pp. 6–7].

Circulation is directly related to the vortex wake, which affects the flow around the blades and the acoustics of the propeller [4, pp. 27–31]. Uneven loading increases vortex formation, causing noise and vibration effects [5, pp. 68–71].

Thus, circulation models and load distribution analysis form the basis for modern design methods. Radial adaptation of the saber shape allows for effective circulation control, vibration and noise reduction.

Modern optimization of blade shape increasingly requires changing not only the overall geometry, but also its parameters along the radius. The concept of radial adaptation is promising, which involves a smooth change in the saber shape, twist, and chord from the root to the tip, which allows you to control the load distribution, affecting the lift and dynamics of the structure.

In traditional rectangular and fixed blades, the load is concentrated in the zone of 0.7–0.75 of the radius due to the increase in speed (U = ω r). This leads to peak values of circulation, fixing the center of pressure in this zone, which increases the vibrations of the structure [2, pp. 15–17].

Radial adaptation involves changing the sabre shape along the radius. Instead of the traditional deflection only at the tip, a gradual change in angle along the entire length is proposed, creating a more uniform distribution of circulation, eliminating local peaks and reducing the amplitude of oscillations.

Along with the sabre shape, it is important to vary the twist of the profile along the radius. Twist regulates the local angle of attack depending on the flow velocity, ensuring uniform distribution of lift and preventing flow separation. Correctly selected twist promotes a smooth increase in lift along the radius, improving the aerodynamic quality of the blade [3, pp. 6–7].

This approach shifts the center of pressure closer to the tip, distributing the load evenly. This reduces moments created by aerodynamic forces and reduces the amplitude of oscillations of the carrier system. As a result, vibrations are reduced, the service life of the structure and flight comfort are increased due to reduced fatigue loads [4, pp. 27–31].

Radial adaptation of the shape not only reduces vibrations, but also weakens the BVI (blade-vortex interaction) effect. A more uniform distribution of lift and optimal profile twisting reduce the intensity of vortex formation and spread the zones of vortex interaction with subsequent turns of the blade out of phase. This reduces pulse acoustic effects, which are the main source of noise in modern propellers [5, pp. 68–71].

Thus, radial adaptation, which involves varying the sabre shape, twist and other parameters along the radius, is an effective tool for improving the aerodynamic quality of the rotor, reducing vibrations and noise, without degrading the overall efficiency of the system.

To analyze the distribution of aerodynamic load and assess the effect of adaptation on the aerodynamic and acoustic behavior of the propeller, a technique based on the theory of angular momentum in combination with the theory of elementary sections of the blade (Blade Element Momentum Theory, BEMT) is used. This approach allows calculating the distribution of lift and taking into account the main inductive effects.

In BEMT, the blade is considered as a collection of small sections, each of which is analyzed in isolation. For each section, the local lift is determined based on its geometry, flow velocity, and angle of attack. Induced velocities due to circulation are taken into account as a correction to the undisturbed flow velocity.

The lifting force on a section of radius r and length Δr is determined by the formula:

ΔL = 0.5 ρ U(r)^2 c(r) CL(r) Δr,

where ρ is the air density, U(r) = ω · r is the velocity at the radius, c(r) is the profile chord, CL(r) is the lift coefficient, Δr is the radius element.

The complete circulation is taken into account through the inductive velocity Vi according to the momentum model:

Vi = T / (2 · ρ · A · V),

where T is the propeller thrust, A is the area of the swept circle, V is the speed of translational motion or vertical draft.

The method allows us to specify the angle of attack taking into account the induced speed, correct the distribution of lift and calculate the resulting propeller load.

Particular attention is paid to the influence of radial adaptation on the wake vortex and BVI. Smooth changes in the sabre shape and twisting change the phase characteristics of the circulation along the radius, forming a more uniform and weakened wake vortex. Instead of a concentrated vortex ring at a radius of 0.7–0.75R, typical for straight blades, the distributed geometry creates a less pronounced wake elongated along the radius [3, pp. 6–7].

BEMT allows to construct a vortex wake diagram, determining the trajectory of vortices and their phase position relative to subsequent turns of the blade. This makes it possible to assess the influence of geometry on the level of impulse noise and vibrations. It is known that the noise peak occurs when a vortex hits a blade in the active circulation zone, and changing the phase interaction allows to reduce its intensity [5, pp. 68–71].

The applied BEMT model taking into account radial adaptation allows quantitative and qualitative assessment of the aerodynamic load redistribution, the effect on the vortex wake, BVI interaction and the expected acoustic characteristics of the propeller. This approach is effective at the design stage and creates a basis for further CFD studies and experiments.

Results and Discussion

Theoretical analysis has shown the influence of radial adaptive geometry on the load distribution along the radius and the formation of a vortex wake, which determines the acoustics of the rotor. In rectangular blades and with a constant sabre shape, the load peak is concentrated at a radius of 0.7–0.75 due to the increase in speed (U = ω r) and insufficient twisting of the profile. This causes an intense vortex ring, which provokes BVI, noise and vibrations [2, pp. 15–17; 5, pp. 68–71].

Radial adaptation with a gradual change in the sabre shape and twisting gives a different distribution of lift: the peak load is shifted closer to the tip, and becomes more uniform along the radius. In a number of configurations, the lift increases almost linearly, eliminating local circulation peaks and weakening vortex structures [3, pp. 6–7].

Additionally, radial adaptation spreads the load zones along the radius in phase, changing the structure of the vortex wake. Instead of a concentrated ring, a system of vortices with lower intensity, stretched in radius and phase, is formed, which reduces the probability of their intersection with the active circulation zone. This reduces the severity of BVI. By analogy with Blue Edge, the effect can reach a noise reduction of 3–4 dB [3, pp. 6–7; 4, pp. 27–31]. The expected result can be comparable or higher, since the effect extends over the entire radius of the blade.

In addition to reducing the acoustic load, the redistribution of the lifting force improves vibration characteristics. The shift of the center of pressure to the tip reduces aerodynamic moments and the amplitude of the structure's vibrations. Radial-adaptive geometry reduces noise and vibration, increasing the service life and flight comfort.

The results confirm the effectiveness of the proposed approach and the need for further CFD studies and experiments to clarify the quantitative characteristics and confirm its practical feasibility.

One of the key effects of radial adaptive geometry is the shift of the pressure center closer to the blade tip, which directly affects the dynamic behavior of the propeller. In traditional rectangular blades, the load center is formed at a radius of 0.7–0.75 due to the increase in flow velocity and the lack of geometry adaptation. This creates significant aerodynamic moments that cause vibrations of the hub, shaft, and transmission, reducing the service life and reliability of the structure [2, pp. 15–17].

Radial adaptation by increasing the saber shape and twisting redistributes the load to the tip (more than 0.85 R), reducing aerodynamic moments and local circulation peaks [3, pp. 6–7]. This reduces the amplitude of oscillations and vibration loads on the body and transmission, increasing the service life and flight comfort.

In classic configurations without radial adaptation, such effects are achieved only with the help of active dampers or reinforcement of the structure, which increases the mass and complicates the scheme. The proposed concept achieves the result exclusively by geometric means, without additional systems.

Traditional vibration reduction solutions involve rectangular blades with minimal sabre-shaped blades or the Blue Edge™ concept with a deflected tip. Rectangular blades are easy to manufacture but retain the disadvantages of vibration and noise, as the peak load remains at the radius of 0.7–0.75, contributing to vortex shedding and BVI [2, pp. 15–17].

Blue Edge™ partially spreads the wake vortex by deflecting the tip and twisting the last 15–20% of the radius. This reduces impulse noise by 3–4 dB and vibrations in a number of modes [3, pp. 6–7; 4, pp. 27–31], but the effect is limited to the tip area, while the central part of the blade remains unchanged.

Unlike Blue Edge™, radial adaptation along the entire length of the blade ensures uniform load distribution from the root to the tip. This eliminates load peaks, shifts the center of pressure closer to the tip along the entire radius, reducing vibrations and noise not locally, but along the entire length of the blade.

In addition, the scheme does not require complex profiling of the tips, simplifying production and allowing it to be integrated into both new and modernized propellers. Unlike active systems, shape adaptation is a passive solution and does not require energy consumption.

Thus, compared to traditional solutions, the radially adaptive saber blade provides a deeper optimization of load distribution, reducing vibrations and noise along the entire length of the blade while maintaining structural simplicity.

Discussion

The proposed concept of a radially adaptive saber-shaped blade has a high potential for use on both modern and future rotorcraft. Unlike traditional solutions — rectangular blades or partial adaptation of the Blue Edge™ type — it allows for a deeper optimization of the aerodynamic load distribution along the entire radius.

The relevance of this geometry increases in the context of modern requirements for reducing noise, vibrations and increasing the service life of structures. Transferring the load to the tip reduces vibration moments and the intensity of vortex formation, which has a positive effect on the acoustics of the propeller. The absence of the need for active control or complex design solutions simplifies implementation both for new propellers and during modernization [3, pp. 6–7].

Particularly promising is the use in medium and heavy transport helicopters, where vibration reduction is directly related to the service life of units, comfort and flight safety.

The main advantage of radial adaptation is the reduction of vibrations and noise solely due to geometry, without the use of active devices such as HHC or TEF, which require complex electronics, energy and increase mass [4, pp. 27–31]. This passive solution is based on the redistribution of aerodynamic loads in natural flow.

The absence of complex control elements simplifies operation, reduces the probability of failures and increases reliability. Radial adaptive geometry is an economically and technologically advantageous solution for serial production.

Despite the theoretical justification, further refinement of the effects is required using CFD (Computational Fluid Dynamics). This will allow for nonlinear effects of flow interaction, the influence of end velocities, and also to refine the structure of the vortex wake in non-stationary modes [5, pp. 68–71].

Further studies should consider the acoustics of the proposed geometry, including aeroacoustics modeling, as well as experimental testing on benches or full-scale installations. In addition, it is necessary to consider the impact on the strength, dynamic and vibration resistance of the hub assembly design, which requires an integrated approach involving related disciplines.

Conclusion

The conducted theoretical analysis confirmed the efficiency of the radially adaptive saber-shaped blade for optimizing the aerodynamic characteristics of the propeller. Gradual variation of the geometry along the radius ensures uniform load distribution, eliminates local circulation peaks, reduces vibration moments and weakens vortex formation, which provokes BVI effects.

Unlike local solutions (Blue Edge™) and active damping systems, the proposed concept reduces vibrations and noise without complicating the design, additional energy costs and control, which makes it promising for new and modernized machines.

Despite the preliminary nature of the work, further CFD studies, acoustic calculations and experimental tests are necessary to refine the parameters. However, it is already clear that radial adaptation opens up new possibilities in the development of helicopter system aerodynamics, offering a practical solution to one of the most persistent problems in the industry - noise and vibration.

This concept is not just an engineering solution, but a challenge to outdated approaches, aimed at the future of more efficient, reliable and safe helicopter technology.

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