Nomenclature
- WBV
-
whole body vibration
- HP
-
helicopter pilot
- LBP
-
low back pain
- QZS
-
quasi-zero stiffness
- TPU
-
thermoplastic polyurethane
1.0 Introduction
Vibration is the oscillatory motion generated by time-varying forces acting on mechanical structures. In helicopters, vibration is unavoidable due to rotating systems, reciprocating components and complex aerodynamic interactions. These vibrations not only accelerate structural fatigue and affect equipment performance, but also pose health risks to pilots, particularly during prolonged exposure.
Helicopter vibrations primarily originate from the main and tail rotor systems, engine components and gearbox dynamics, which together generate both broadband and tonal excitations due to aerodynamic forces and structural imbalances. These vibrations propagate through the airframe and are transmitted to the pilot’s body via the seat and controls. Forward flight introduces strong tonal components, particularly at the main rotor blade passage frequency and its harmonics, which for the UH-60 helicopter is approximately 17.2Hz. To mitigate these effects, helicopter manufacturers utilise structural dampers, elastomeric bearings, tuned mass absorbers and vibration isolation mounts. However, these systems are often designed to protect the airframe and equipment rather than to directly address pilot comfort.
ISO 2631-1 (1997) is the internationally accepted standard for evaluating whole body vibration (WBV) exposure in relation to human health and comfort. It prescribes the use of frequency-weighted root mean square (r.m.s.) acceleration to assess health risks, particularly over the 0–80Hz frequency range. The standard identifies the 4–8Hz band as biomechanically sensitive for seated humans, where resonance effects in the spine and internal organs are most likely to occur. As vibration frequency approaches these resonant ranges, tolerance time decreases and the likelihood of fatigue and discomfort increases. Although the standard does not provide fixed graphical limits, it offers guidance on acceptable exposure durations and acceleration thresholds. These are typically illustrated through boundary diagrams derived from frequency-weighted r.m.s. acceleration limits. Figure 1 illustrates a conceptual boundary diagram derived from this guidance, highlighting the inverse relationship between vibration magnitude and allowable exposure time.
Conceptual vibration exposure boundaries derived from ISO 2631-1:1997 [1]. The figure was developed based on the standard’s guidance on frequency-weighted acceleration levels and allowable exposure durations.
In the vibration measurements conducted in this study, significant amplitudes were observed both at tonal frequencies such as the rotor blade passage frequency (17.2Hz) and within the biomechanically sensitive range of 4–8Hz. While tonal components like 17.2Hz are known to cause discomfort due to their persistent and directional nature, vibrations in the 4–8Hz range are of particular concern because this frequency band coincides with the natural resonance of the human body. According to ISO 2631-1, exposure to WBV within this range significantly increases the risk of fatigue, spinal loading and long-term musculoskeletal disorders. These challenges highlight the need for improved seat-level isolation specifically targeting low-frequency vertical vibrations that conventional passive systems often fail to suppress effectively. The present study addresses this gap by implementing and evaluating a quasi-zero-stiffness- (QZS-) based seat cushion isolator designed to enhance vibration attenuation at these critical frequency bands. Vibration is known to have various effects on exposed flight crew, with low back pain (LBP) being the most common health complaint [Reference Bovenzi2]. According to the Norwegian Institute of Aviation Medicine, over a two-year flight period, helicopter pilots (HPs) were 32% more likely to complain of LBP than fixed-wing pilots [Reference Hansen and Wagstaff3].
To better understand the physiological effects of WBV exposure on the human body, Table 1 presents the resonance frequencies of major body parts as defined in the literature [Reference Chen, Wickramasinghe and Zimcik4]. These ranges are particularly critical for assessing WBV risk under ISO 2631-1. Exposure to WBV at resonant frequencies increases the rate of fatigue development in the back muscles [Reference Pope, Wilder and Magnusson5–Reference Guo, Dong, Yuan, Feng and Fan9].
Resonant frequencies of human body parts
Advanced biodynamic modeling efforts have significantly advanced our understanding of how seated humans interact with rotorcraft vibration. Ma et al. [Reference Ma, Li, Cao and Zhou10] developed a nonlinear biodynamic model coupling an anthropomorphic test device with seat dynamics to predict injury risk under vertical impact conditions, providing valuable insights for airworthiness certification. Desai et al. [Reference Desai, Papaioannou and Happee11] introduced a computationally efficient multibody dynamics model that accurately captures vibration transmission through the seated human body, enabling detailed assessments of biodynamic behaviour across multiple axes. Complementing these approaches, Tamer et al. [Reference Tamer, Zanoni, Cocco and Masarati12,Reference Tamer, Zanoni, Cocco and Masarati13] presented a comprehensive numerical framework integrating helicopter structural dynamics, seat motion, ocular tracking and instrument panel vibrations to quantify the impact of cockpit vibration on instrument reading capability and situational awareness, and subsequently proposed a generalised vibration exposure index that incorporates seat, controls, visual system, and foot vibration pathways for a holistic assessment of pilot exposure. These integrated biodynamic and structural modeling efforts form the foundation for the QZS-based seat cushion isolator evaluated in the present study, ensuring that both physiological and cognitive vibration effects are addressed.
There are many studies in the literature that have measured vibration according to the ISO 2631-1 standard and as a result of these studies, vibration is associated with LBP [Reference Lis, Black, Korn and Nordin14–Reference Kumar19]. In HPs, the prevalence of LBP is most strongly associated with increased daily WBV exposure; more chronic LBP is more strongly associated with total flight time [Reference Bongers, Hulshof, Dljkstra, Boshuizen, Groenhout and Valken20, Reference Byeon, Kim, Jeong, Sim, Kim, Choi, Im and Kim21]. The prevalence of LBP in HPs has been shown in several studies [Reference Hansen and Wagstaff3, Reference Van Leusden, Prendergast and Gray22–Reference Bowden28]. Although it is stated that WBV alone does not cause LBP, WBV is considered to increase LBP according to the studies reviewed. Between 1992 and 2025, systematic reviews and meta-analyses at the top of the hierarchy of evidence also show that workers exposed to WBV have an increased risk of LBP compared to unexposed groups [Reference Burström, Nilsson and Wahlström29–Reference Bainbridge, Moutsos, Johnson, Mcmenemy, Ramasamy and Masouros31]. In a study showing that musculoskeletal disorders arise from multiple causes; vibration, demanding posture, inadequate organisational support, injustice and poor organisational culture significantly increased miners’ risk of work-related musculoskeletal disorders [Reference Zhang, Yin, Li and Tong32].
Seventy to 80% of aviation accidents can be attributed to human factors in the causal chain, and 88% of these have been found to be caused by issues with situational awareness [Reference Feng, Liu, Wanyan, Dang, Wang and Qian33]. Fixed seating position, exposure to low-frequency vibration and ergonomically inadequate seat designs adversely affect spinal health and lead to chronic musculoskeletal disorders. This is not only limited to physical discomfort, but also has a negative impact on pilots’ attention, decision-making and reaction times. Exposure to high-acceleration vibration and its consequences, such as spinal disorders and discomfort, can pose serious performance and safety risks for the human factor.
According to MIL-STD-1472G aircraft seat cushion definition and requirements, wool is used for the comfort and long-term flight capability of HPs, and breathable cloth is used as a covering material. In addition, this hybrid seat cushion is intended to utilise the vibration absorption properties of wool material [Reference Marul and Karabulut34]. The designed hybrid vibration isolator seat cushion is expected to reduce the vibration acceleration transmitted from the seat to the HP.
Recent developments have emphasised the need for passive vibration reduction strategies not only at the airframe level but also at the pilot–seat interface. Tamer et al. [Reference Tamer, Muscarello, Masarati and Quaranta35] introduced a comprehensive simulation framework incorporating structural dynamics, seat models and biodynamic representations to predict pilot exposure to vibration under various absorber configurations. Their findings highlight that passive vibration mitigation elements can significantly influence ride quality when tailored to the biodynamic response of the seated occupant. This underscores the importance of designing isolation systems that are specifically tuned to human resonance characteristics – particularly in the 4–8Hz range – as targeted by the QZS-based seat cushion isolator in the present study. Such integration of biomechanical and structural perspectives offers a pathway toward more effective vibration attenuation solutions for rotary-wing platforms.
To address these issues at the seat interface level, this study proposes and evaluates a novel hybrid seat cushion isolator designed to reduce vibration transmission to the pilot. The isolator incorporates a QZS metamaterial structure composed of thermoplastic polyurethane (TPU), engineered to attenuate low-frequency vertical vibrations. Metamaterials are typically periodic structures engineered to exhibit unique dynamic properties not found in natural materials. Unlike conventional isolators, the QZS design shifts the system’s natural frequency to a lower range, increasing isolation efficiency without sacrificing static stiffness. The prototype integrates a pressure-distributing upper plate and utilises wool padding to meet comfort and breathability standards in accordance with MIL-STD-1472G. The vibration reduction performance of the QZS-based cushion was tested under four real flight regimes in a UH-60 helicopter and compared to the original seat cushion.
In addition to conventional passive and semi-active isolation methods, recent research has introduced alternative mechanical solutions such as strut-mounted inerters for helicopter vibration attenuation, targeting similar low-frequency domains [Reference Tamer, Masarati, Zilletti and Bottasso36]. Unlike such mass-damper-based systems, the present study proposes a QZS metamaterial cushion isolator designed for direct pilot-seat interface, offering simpler integration and passive performance enhancement.
Advanced biodynamic modeling has played a central role in understanding how seated humans respond to vibration in rotorcraft environments. Zanoni et al. [Reference Zanoni, Cocco and Masarati37] performed a multibody dynamics analysis of the upper body under vibration, providing detailed insights into torso and spine behaviour during rotorcraft–pilot interaction. Similarly, Tamer et al. [Reference Tamer, Zanoni, Muscarello, Cocco, Quaranta and Masarati38] reviewed biodynamic modeling approaches to evaluate rotorcraft ride comfort, highlighting the significance of including accurate human body representations in comfort-oriented seat designs. These studies support the rationale behind incorporating biodynamic principles into the QZS-based seat cushion, which aims to mitigate harmful accelerations while preserving comfort in vibration-sensitive frequency ranges.
2.0 Methodology
In the Sikorsky UH-60 helicopter, as in other helicopters, the vibration acceleration value in the z-axis is greater than the x and y axes. If the high intensity of vertical vibration is felt by the pilots and flight crew, and during periodic maintenance, accelerometers mounted on the pilot seats are used to measure cabin vibration values such as main rotor, tail rotor and engine vibration values. Aircraft maintenance manuals do not contain procedures for the measurement of vibration values transmitted to pilots and flight crew from surfaces such as seats, flight controls and floors, and therefore the vibration exposure values of HPs are not known. Since vibration values transmitted on HPs are not measured, daily or cumulative vibration dose values according to ISO 2631-1 standard or European Directive cannot be calculated, and it is not known in which risk zone the vibration exposure is located.
A three-axis (x, y, z) vibration measurement mount, shown in Fig. 2, was designed to calculate the WBV in HPs. The three-axis accelerometers were mounted on the chest area of the HP to measure the WBV transmitted to the pilot from various helicopter surfaces. The acceleration values received were measured and evaluated in r.m.s. in the range 0-80Hz as specified in ISO 2631-1 standard.
Three-axis accelerometers (x, y, z) and mounting plate.
Vibration measurements were performed in accordance with ISO 2631-1:1997, which defines the methodology for assessing WBV and specifies the x, y and z axes as the fore-aft, lateral and vertical directions, respectively. For each axis, the frequency-weighted r.m.s. acceleration a w was calculated using Equation (1), which corresponds to the standard’s recommended method:
\begin{align}{a_w} = {\left[ {\frac{1}{T}\mathop \smallint \limits_0^T {a_w}^2\left( t \right)dt} \right]^{\frac{1}{2}}},\end{align}
where a w (t) is the frequency-weighted acceleration and T is the measurement duration in seconds. According to ISO 2631-1, different weighting filters and multiplying factors are applied depending on the axis of measurement. For the z-axis (vertical direction), the Wk weighting filter was used, with a multiplying factor of 1.0, to account for the human body’s greater sensitivity to low-frequency vertical vibrations. For the x-axis (fore-and-aft) and y-axis (lateral), the Wd weighting filter was applied, each with a multiplying factor of 1.4, as specified in the standard. Measurements were taken in the 0–80Hz frequency range. Although ISO 2631-1 provides a method for combining multi-axis accelerations into a single equivalent value, in this study each axis (x, y and z) was analysed independently to evaluate the specific effectiveness of the isolator cushion in each direction.
The vibration isolator seat cushion we developed for the Sikorsky UH-60 helicopter has a hybrid design and consists of four layers. A QZS design, known in the literature for damping low-frequency vibrations, was used as the main and bottom layer of the isolator [Reference Xu, Zhang, Zhou and Lou39–Reference Zhou, Wang, Xu and Bishop42]. The QZS structure was developed using a combination of positive and negative stiffness elements. Negative stiffness refers to a mechanical behaviour where an element temporarily resists an applied force by displacing in the opposite direction, resulting in a region with a negative slope in its force–displacement curve. In this design, each cell contains laterally positioned thin beams designed to buckle under axial load, thereby generating a negative stiffness effect in the vertical direction. Positive stiffness was provided by six-legged spherical structures within a rigid hexagonal frame. As shown in Fig. 3, these components are combined to form individual QZS cells. The cells were fabricated using TPU 95A material via 3D printing and assembled into a honeycomb configuration, as shown in Fig. 4. Each cell was physically bonded to its neighbours using TPU-based adhesive, ensuring mechanical continuity while maintaining the metamaterial’s flexibility and vibration isolation performance. When vibrations in a certain direction transmitted from one system to another system are met with nearly zero stiffness, this minimises the transmission of vibration energy in that direction to other system parts or the environment. The QZS structure is in direct contact with the seat surface. The isolator cushion was produced using TPU 95A material with passive vibration damping properties via 3D printing and includes a total of 57 QZS cells. Each cell structure and its assembly are illustrated in Figs 3 and 4, respectively.
QZS single-cell structure.
Honeycomb structure designed for the UH-60 helicopter pilot seat by combining QZS isolator cells.
To enhance damping behaviour, the spherical elements were printed with asymmetrical wall thicknesses of 3.5, 3.0 and 2.5mm, inspired by the work of Chen et al. [Reference Chen, Tan, Hu, Zhu, Wang, Wang, Jin and Wu43], aiming to improve the frequency-dependent response of the isolator. On the QZS honeycomb structure, a 1.2mm thick sheet metal plate and a 12mm thick polyurethane sponge were added on top of it in order to distribute the HP weight evenly to the isolator cells (Fig. 5).
QZS isolator with plate and polyurethane foam added.
Wool was added as the last layer, giving the QZS isolator the shape of the original UH-60 cushion. Figure 6(a) and (b) shows the final shape and cross-sectional area of the cushion. The layers are wool, polyurethane foam, thin plate and QZS isolator from the user contact surface downwards.
Final form of the UH-60 HP seat cushion with QZS isolator (a) cross-sectional area, (b) whole shape.
The UH-60 HP seat cushion with QZS isolator installed on the HP seat of the UH-60 helicopter is shown in Fig. 7.
View of the QZS isolator cushion on the UH-60 HP seat.
Three-axis accelerometers mounted on the HP chest area.
Axes in sitting position according to ISO 2631-1.
Three-axis accelerometers were mounted on the chest of a 75kg HP to measure WBV transmitted through the seat, as illustrated in Fig. 8. The direction of vibration transmission and body axis orientation was defined according to the ISO 2631-1 standard, as shown in Fig. 9. Vibration measurements were performed under real in-flight conditions using the original cushion of the UH-60 helicopter and were then repeated under identical flight regimes with the isolator seat cushion incorporating a QZS isolator. For each axis (x, y and z), acceleration values were recorded in the 0–80Hz frequency range and computed as r.m.s. values in accordance with ISO 2631-1. Measurements were taken during four distinct flight regimes: on the ground with the power control lever in the fly position, in hover, in 100-knot cruise flight and on final approach before landing. All vibration measurements were performed on a single HP to ensure consistency and minimise biological variability during comparison between the original and the QZS-based isolator cushion. While this limited statistical generalisation, it allowed for direct isolation of the cushion’s performance. The change in vibration levels between the two cushions was evaluated for each flight regime and axis to determine the amount of vibration reduction or increase.
In addition to the objective measurements, five other HPs participated in non-instrumented test flights using the isolator cushion. These sessions involved no vibration sensors and were conducted solely to collect subjective feedback regarding perceived comfort and vibration levels compared to the original cushion. The participants were asked to report whether they experienced any discomfort or improvement when using the isolator cushion.
3.0 Results
The vibration measurements of the Sikorsky UH-60 helicopter original cushion, and the vibration isolator cushion we developed were compared in the frequency range of 0–80Hz and at frequencies of 4–8Hz, which is the resonance frequency of the human body. The measurements were made on the same helicopter, in the same flight regimes, under the same weather conditions and on the same pilot. To quantitatively compare the performance of the isolator cushion with the original cushion, a metric called vibration gain-loss percentage was used. This value represents the percentage change in the r.m.s. acceleration between the two cushions and is calculated as follows according to Equation (2):
\begin{align}Vibration\;Gain - Loss\;\left( \% \right) = \frac{{{a_o} - {a_c}}}{{{a_o}}} \times \;100\end{align}
where a o is the r.m.s acceleration measured using the original cushion, and a c is the r.m.s. acceleration measured using the isolator cushion. A positive result indicates a reduction in vibration (gain), while a negative result indicates an increase (loss).
Acceleration–frequency spectrum of the original cushion during ground regime (0–80Hz).
Figures 10 and 11 show the acceleration–frequency spectra measured in the x, y and z axes during ground operation using the original and isolator cushions, respectively. These graphs illustrate the vibration distribution across the 0–80Hz range and highlight the tonal and low-frequency components of helicopter-induced vibration. Figure 12 presents the vibration gain–loss percentages over the full 0–80Hz spectrum, showing average reductions of 80.46%, 5.05% and 80.37% in the x, y and z axes, respectively. Figure 13 focuses on the 4–8Hz frequency band, which is particularly important due to its alignment with human biomechanical sensitivity. In this range, the isolator achieved significant reductions of 79.17%, 90.00% and 75.70% in the x, y and z axes, respectively. Additionally, at the main rotor blade passage frequency of approximately 17.2Hz, reductions of 50.00%, 42.86% and 97.78% were observed in the respective axes. These results indicate that the isolator cushion is highly effective at attenuating both biomechanically critical frequencies and dominant tonal components during ground operation.
Acceleration–frequency spectrum of the isolator cushion during ground regime (0–80Hz).
Ground regime x, y, z axes vibration gain-loss percentages (0–80Hz).
Ground regime x, y, z axes vibration gain-loss percentages (4–8Hz).
Figures 14 and 15 present the acceleration–frequency spectra measured in the x, y and z axes during hover flight using the original and isolator cushions, respectively. These spectra illustrate the distribution of vibration amplitudes across the 0–80Hz frequency range. A comparison between the two reveals that the isolator cushion provides noticeable attenuation, particularly in the 4–8Hz band, which is known to have the greatest effect on the human body due to biomechanical sensitivity. Figure 16 shows the vibration gain–loss percentages calculated over the full 0–80Hz range, with average reductions of 17.68%, 39.24% and 5.54% in the x, y and z axes, respectively. Figure 17 focuses on the gain–loss percentages in the 4–8Hz band, where higher reductions of 48.89%, 38.06% and 44.17% were achieved in the corresponding axes. However, at the main rotor blade passage frequency (17.2Hz), which represents a critical tonal excitation, an increase in vibration was observed: +28.57% in the x-axis, +50.00% in the y-axis and no change (0.00%) in the z-axis. These results demonstrate the isolator’s effectiveness in reducing low-frequency vibrations relevant to human comfort, while also revealing its limitations in certain tonal components.
Acceleration–frequency spectrum of the original cushion during hover flight (0–80Hz).
Acceleration–frequency spectrum of the isolator cushion during hover flight (0–80Hz).
Hover flight regime vibration gain-loss percentages (0–80Hz).
Hover flight regime vibration gain-loss percentages (4–8Hz).
Figures 18 and 19 show the acceleration–frequency spectra measured in the x, y and z axes during 100-knot cruise flight using the original and isolator cushions, respectively. These graphs illustrate how vibration is distributed across the 0–80Hz frequency range during steady forward flight. Figure 20 presents the vibration gain–loss percentages over the full 0–80Hz range, where vibration levels increased by 51.92% in the x-axis and 30.33% in the y-axis, while a reduction of 31.67% was observed in the z-axis. Figure 21 focuses on the 4–8Hz band, a range known for its impact on pilot comfort. In this frequency range, the isolator cushion achieved reductions of 57.92% in the x-axis and 39.77% in the z-axis, while a slight increase of 1.85% was observed in the y-axis. At the main rotor blade passage frequency of approximately 17.2Hz, vibration decreased by 48.08% in the x-axis, while increases of 13.75% and 7.52% were observed in the y and z-axis, respectively. These results suggest that while the isolator provides effective attenuation in the vertical direction at lower frequencies, its performance is limited in certain directions and tonal frequency components during cruise conditions.
Acceleration–frequency spectrum of the original cushion during 100-knot cruise flight (0–80Hz).
Acceleration–frequency spectrum of the isolator cushion during 100-knot cruise flight (0–80Hz).
One hundred-knot cruise flight regime vibration gain-loss percentages (0–80Hz).
One hundred-knot cruise flight regime vibration gain-loss percentages (4–8Hz).
Figures 22 and 23 display the acceleration–frequency spectra measured in the x, y and z axes during the final approach flight regime, using the original and isolator cushions, respectively. These graphs illustrate the frequency-dependent vibration distribution in the final phase of flight. Figure 24 shows the vibration gain–loss percentages over the full 0–80Hz range, where vibration levels increased by 36.16% in the x-axis and 30.38% in the y-axis, while a reduction of 19.68% was observed in the z-axis. Figure 25 focuses on the 4–8Hz band, which is biomechanically critical for pilot comfort. In this range, vibration increased by 52.37% in the x-axis, showed no change in the y-axis and decreased by 64.89% in the z-axis. At the main rotor blade passage frequency of approximately 17.2Hz, the isolator resulted in a 15.00% reduction in the x-axis, a 29.85% increase in the y-axis and a significant 61.11% reduction in the z-axis. These results indicate that while the isolator remains effective in mitigating vertical vibrations at specific frequencies during final approach, its performance in horizontal directions and within certain frequency bands may be inconsistent or limited.
Acceleration–frequency spectrum of the original cushion during final approach flight (0–80Hz).
Acceleration–frequency spectrum of the isolator cushion during final approach flight (0–80Hz).
Final approach flight regime vibration gain-loss percentages (0–80Hz).
Final approach flight regime vibration gain-loss percentages (4–8Hz).
Average values were taken for all four flight regimes in the frequency range of 0–80 and 4–8Hz and total gain rates were calculated separately. In the 0–80Hz range, a decrease of 2.52%, 34.32% and 4.11% in the x and z-axis, respectively, and an increase of 4.11% in the y-axis were observed throughout the entire flight. In the 4–8Hz range, a decrease of 33.4%, 31.59% and 56.13% was observed in the x, y and z-axis, respectively (Fig. 26).
Gain-loss percentages in all flight regimes (0–80Hz, 4–8Hz).
4.0 Discussion and Conclusion
In this paper, r.m.s. acceleration is calculated at all frequency values in the range of 0–80Hz and comparisons are made between the original cushion and the isolator cushion in different flight regimes. It was aimed at reducing the transmission of the vibration generated in the helicopter to the HP, and a hybrid seat cushion isolator was produced inspired by the QZS isolators in the literature. In the study, only the QZS isolator layer could not be compared to the original cushion as it would reduce the HP comfort and hinder flight safety. Therefore, polyurethane foam and wool materials were used in the isolator cushion to ensure both HP comfort and comfortable and safe flight for a long time according to MIL-STD-1472G. The reason for the use of polyurethane foam and wool is that in literature, thin polyurethane and wool materials have passive vibration damping properties and are also thought to provide HP comfort for long flights. Vibration measurements were made on an HP weighing 75kg. In addition, the produced cushion was assessed on various flights by five different HPs by conducting a survey. All five HPs reported that vertical vibration was noticeably reduced, especially in the final approach sections of the flights. HPs did not report any negativity in terms of comfort. In the vibration measurements, it was observed that the vibration transmitted on the HP was at higher accelerations at low frequencies and that the z-axis had the highest vibration acceleration values in all regimes except the 100-knot cruise flight regime. No vibration acceleration above 0.001m/s2 was observed on the HP in the 40–80Hz range. The original cushion showed a vibration acceleration above 0.5m/s2 in 16 cases (Figs 10, 14, 18 and 22), while the isolator cushion showed acceleration above 0.5m/s2 in 8 cases (Figs 11, 15, 19 and 23). This study demonstrated that the QZS isolator dampens low-frequency vibrations. The dominant excitation frequencies in forward flight include the main rotor blade passage frequency and its harmonics, which for the UH-60 helicopter are approximately 17.2 Hz and its multiples. These frequencies are responsible for tonal vibration peaks that propagate through the structure and can be especially difficult to isolate due to their coherent and directional nature.
The results of a systematic review and meta-analysis of randomised clinical trials by S. Daniel et al. [Reference Zhou, Wang, Xu and Bishop42] suggest that exercise combined with training (carrying, lifting, lumbar health and anatomy, etc.) may reduce the risk of LBP, while exercise alone may reduce the risk of LBP attacks and LBP-related sick leave, at least in the short term. Current evidence suggests that training, back belts, shoe insoles and ergonomics alone do not prevent LBP. The topic of low back pain and low back injuries is an overly broad research topic and not due to a single cause. This study aims to minimise the risk of vibration-induced low back pain and low back injuries. It is predicted that vibration-induced low back pain will decrease with the reduction of vibration transmitted to the HPs; however, in order to be able to investigate whether there is a change in pain after prolonged use of the isolator cushion, especially in HPs with low back pain, and to obtain results, HPs should use the isolator cushion for a period of time and report their pain levels. While the isolator cushion demonstrated effective vibration reduction in the z-axis across all flight regimes, slight increases in the x and y axes in the 0–80Hz range were observed in certain conditions. This behaviour is likely related to the directional specificity of the cushion, which was primarily designed and optimised to reduce low frequency vertical vibrations – the dominant axis for WBV exposure in helicopter pilots. Consequently, its performance in the x and y directions may be limited. However, as the vibration amplitudes in these directions were generally much lower than in the z-axis, these increases are not expected to significantly affect overall pilot comfort or vibration exposure. This observation suggests a potential direction for future design improvements aimed at enhancing multidirectional isolation capabilities.
A comparative analysis of all flight regimes revealed that the isolator cushion’s vibration attenuation performance varies with both frequency range and flight condition. In the ground regime, substantial vibration reductions were observed across all axes, particularly in the z-axis and in the biomechanically sensitive 4–8Hz range, indicating optimal isolator performance in static conditions. During hover, the isolator also demonstrated effective attenuation in the 4–8Hz band, though tonal excitation at 17.2Hz led to increased vibrations in the x and y axes. In the 100-knot cruise regime, while the isolator provided moderate vertical (z-axis) attenuation at lower frequencies, it failed to suppress – and in some cases amplified – horizontal vibrations, particularly at the rotor blade passage frequency. Finally, in the final approach regime, the isolator achieved significant vibration reduction in the z-axis, including at 17.2Hz, but showed increased or unchanged vibration levels in the x and y axes, especially in the 4–8Hz band. These findings highlight that the isolator is most effective in mitigating vertical low-frequency vibrations, especially under less dynamic flight conditions, but exhibits directional and frequency-dependent limitations in suppressing horizontal and tonal excitations during more complex manoeuvers.
Total daily vibration dose values and total vibration exposure values of the HPs were not calculated in this study. Calculating the daily vibration exposure (A(8)) and vibration dose value (VDV) values of Sikorsky UH-60 HPs is a different type of study that should be performed to determine whether they are within the limits of the ISO 2631-1 standard. However, this study has shown that the vibration values on the HP, especially at low frequency and in the z-axis, where the most vibration is generated, have been greatly reduced.
While strut-mounted solutions such as those employing roller-screw inerters have shown promise in attenuating structural vibrations in helicopters [Reference Tamer, Masarati, Zilletti and Bottasso36], these systems often require major structural modifications. In contrast, the QZS-based seat cushion developed in this study can be easily integrated into existing pilot seats without modifying airframe components, offering a practical and modular solution for vibration mitigation.
In the future, it is expected that QZS isolators will be widely used in almost all systems with users generating low frequency vibrations. Since it is currently designed for the Sikorsky helicopter UH-60 model, it can be used in all UH-60 aircraft. In the future, it can be used in all helicopters with similar features. It is expected to be beneficial for the users of mechanical systems that produce vibration. In time, as a result of the surveys conducted on HPs using seat cushions with QZS isolators, it is predicted that the isolator will provide more reduction in vibration levels and more improvements will be made in terms of comfort.
Funding statement
This work was funded by the Erciyes University Scientific Research Projects Coordination Unit (Bilimsel Araştırma Projeleri, Erciyes Üniversitesi), under the project number FYL2024-13632. The funding source had no role in the study design, data collection, analysis, and interpretation, writing of the manuscript, or the decision to submit this article for publication.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/aer.2025.10057.
