Perceived discomfort for tri-axial helicopters vibrations
Laurianne DELCOR(1) , Étienne PARIZET(2) , Julien CAILLET(3) , Julie GANIVET-OUZENEAU(4)(1) Laboratoire Vibrations Acoustique (LVA) & Airbus, France, laurianne.delcor@airbus.com(2) Laboratoire Vibrations Acoustique (LVA) , France, etienne.parizet@insa-lyon.fr(3) Airbus, France, julien.caillet@airbus.com(4) Airbus, France, julie.ganivet-ouzeneau@airbus.com

Abstract
Vibrations are an important contributor to perceived discomfort in helicopters cabins. A test campaign wasconducted in order to understand how discomfort varies for helicopter tri-axial excitations. Six experiments
were conducted, which involved 30 participants. Subjects were seated on a six degree of freedom vibrationtable. They had to evaluate discomfort of several stimuli. The ﬁrst three experiments investigated the inﬂuenceof vibrations due to the blade passage frequency of the main rotor, to the tail boom modal behaviour and to
a combination of these two sources. The impact of a phase shifted vibrations and of amplitude-modulated
excitations were studied in a fourth and ﬁfth experiments. Finally, the last experiment focused on real signals
measured on several helicopters of Airbus ﬂeet. Results showed that 1) vertical vibrations represent the major
contributor to discomfort in helicopters; 2) for amplitude-modulated excitations, discomfort only depends on
the amplitude of the modulation, not on the modulation frequency; 3) out of phase stimuli are perceived in thesame way than phased stimuli; 4) helicopter frequency content above 30 Hz does not increase discomfort; 5)
ISO2631-1 comfort index is a good predictor of discomfort evaluations.
Keywords: Perception, vibrations, helicopters

1INTRODUCTION

Helicopter manufacturers are closely interested in ride comfort because it has become an important marketingargument. They need to have a full understanding of comfort parameters and to be able to estimate it fromin-ﬂight measurements, in order to offer a good product to their customers.
Ride comfort is partly governed by vibration-related comfort. The ISO2631-1 standard [1] makes it possible toevaluate the discomfort produced by any transportation means based on vibration measurements at the points ofcontact between the person and the machine. However, this standard has been the subject of studies that haveshown that it does not accurately estimate discomfort from all types of sources, and deserves to be slightly
adapted for each application [2].
This is why this study aims to verify the suitability of ISO2631-1 to helicopter vibrations. It will focus on raw
signals measured in helicopters but also on signals reproducing the main characteristics of vibrations measured
in helicopters . This paper will only present subjective evaluation of discomfort due to the excitation related tothe blade passage frequency, which represents the highest excitation in the measured spectra.

2METHODS

2.1 Participants
Thirty people participated in the experiment. This group consisted of 7 women and 23 men, aged between
22 and 55 (average=40, std=9.8 years). They were recruited from Airbus and were engineers in vibration oracoustics. Participants had to sign a waiver beforehand attesting to their good health (no recent fractures orsurgery, no retinal problems...).
Before exposing participants to vibration, the RMS value of the weighted acceleration normalized to an 8-4289

hour reference period of all vibration signals in the session was computed using the ISO 2631-1 method. The
obtained value was 0.12 m.s-2, which is below the 0.5 m.s-2 threshold value (for which a remedical action
should be taken), and far below the daily exposure limit value of 1.15 m.s-2, as deﬁned by the regulation [3].
2.2 Test equipments
The experiment used a vibration simulator (Airbus Cube in Toulouse). This bench is a six degrees-of-freedom
simulator, but only translational vibrations were used in this study.
The test means allows a 50 mm displacement on X and Y axes, and a 100mm displacement on vertical axis.
Nominal frequency bandwidth goes from 0 to 250 Hz for all axes.
On the top platform, whose dimensions are approximately 150 cm × 150 cm, a standard helicopter seat wasﬁxed. Subjects participated in the experiment one by one.
The vibrations played by the Cube for each participant were recorded by three accelerometers. The ﬁrst one
was a conventional triaxial accelerometer ﬁxed to the ground at the level of the seat legs. The other two weretriaxial pad accelerometers attached to the seat and the backrest. The sampling frequency was 204 Hz and all
signals submitted to each participant were recorded.
2.3 Stimuli
In order to understand the inﬂuence of different vibratory excitations on perceived discomfort, real signals
and synthesized stimuli were played.
The real signals were derived from ﬂight measurements performed in several helicopters of the Airbus ﬂeet.
Vibrations were measured at the ﬂoor during stabilized forward ﬂight phases.
Eight signals from eight different helicopters were selected to be played to participants. In four of these signals
the high frequency content (above 30 Hz) had a high level. A low-pass ﬁltering was applied (cut-off frequency: 30 Hz) which created four modiﬁed signals. The objective was to know if comfort analysis can be limited
to the low-frequency range or should include upper frequencies (mainly due to harmonics of the main rotor
excitation). All together, 12 "real" signals were presented to the participants.
The synthesized stimuli aimed to represent typical vibrations of helicopters. Different sources of vibration
have been studied but this paper will be limited to the main rotor blade passage.
The amplitudes of the measured vibrations vary from 0.05 to 0.5g and are generally higher on the vertical axis.
The blade passage frequency varies according to the number of blades and the type of machine from 17 to 30
Hz.
In that case, the frequency was ﬁxed to 17 Hz and the inﬂuence of level in the three directions was studied,using a full experimental design. Factors were the vibration level in each direction. Each factor had 3 levels,
which lead to an overall number of 27 stimuli. In the vertical direction, levels were 0.1, 0.18 and 0.32 g. In
the two horizontal directions, levels were 0.057, 0.1 and 0.18 g. In all cases, the increase between two levels
was 5 dB.
A previous study showed that for single frequency vertical vibrations discomfort depends on the vibratory velocity. Hence, by choosing a low frequency vibration, higher frequencies discomfort can be inferred. Amplitudelevels varying in 5dB increment allowed to have a signiﬁcant variation in discomfort (differential threshold =1.5 dB [4]) and to cover a wide range of amplitudes with only three levels.
All signals (real and synthesized ones) had a duration of 5 second. They were presented one after the otherwith a 5-second pause between two stimuli.

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2.4 Procedure
Participants assessed the discomfort produced by vibration by direct scaling. They had to give a numericalvalue corresponding to their discomfort, using a continuous scale numbered 0 to 50, labelled from "no vibration"to "extremely uncomfortable". Between these extremes, ﬁve semantic labels helped participants to give theiranswer : "5 - a little uncomfortable", "15 - fairly uncomfortable", "25 - medium", "35 - uncomfortable" and"45 - very uncomfortable". This scale is based on the one provided in the ISO16832 standard [6] for loudness
evaluation. Participants gave their evaluations aloud, during the 5-second pause between two signals, and theexperimenter stored them in a computer.
Sample signals were played to participants at the beginning of each experiment. This allowed them to become
familiar with the stimuli and the discomfort rating method. Sample signals were taken from the stimuli sets ofthe experiments. They were chosen to represent the full range of stimuli for each experiment.
The experiments were presented in a semi random order to the participants. Within each experiment, thesignals were played in a random order. Unfortunately, due to practical limitation of the bench and to time, only
two random draws for all 30 participants were made.
2.5 Analysis methods
Repeated measures variance analyses (ANOVA) were used to process the data.
For visualization, perceived discomfort ratings given by participants are normalized. This is done for each
assessment, by removing the average of all ratings of the participant, and adding the average of all ratings ofall participants.

3RESULTS

Six participants had to be removed from the dataset due to a malfunction of the vibrating bench during anday of testing.
The average of the participants’ normalized evaluations of the real helicopter signals varies from 16 to 29
points of discomfort. This corresponds to discomfort between the "a little uncomfortable" and "uncomfortable"labels. In comparison, the average of the ISO2631-1 comfort indexes computed from vibrations measurementson the participants varies from 0.2 to 0.7 m.s−2 , i.e. from "no uncomfortable" to "uncomfortable" dependingon the standard. All discomfort evaluations given by the participants are therefore included in the discomfort
estimates proposed by the standard.
Figure 1 shows the details of the averages of the normalized evaluations for each signal as a function of theassociated values of ISO2631-1 comfort index. It shows that the perceptive evaluations and comfort indexes ofthe standard follow an increasing law and are well correlated (corr=0.93).
Figure 1 shows that there is little difference between perceived discomfort assessments of unﬁltered signals(circles) and ﬁltered signals (triangle). The effect of frequency content ﬁltering greater than 30 Hz for thesesignals is studied with a repeated measurement ANOVA. Two factors are introduced: the ﬁltering or not of highfrequencies (two levels), and the type of machine (four levels according to the four helicopters). The results ofthe ANOVA show that only the type of machine signiﬁcantly impacts discomfort (p=6.5e -16).
For stimuli representing the blade passage frequency, the normalized averages of discomfort assessments range
from 23 to 40 discomfort points. This means that participants found these stimuli "a little uncomfortable" to"uncomfortable". The associated comfort index of ISO2631-1 range from 0.4 to 1.6 m.s−2 , i.e. from "slightly

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Mean of norm. disc. evaluations

50

extemely uncomf.

45

very uncomf.

HC1

HC3 f

HC5HC7HC8

HC6 f

HC2HC4HC6HC3HC7 f

HC4 f

40
uncomfortable3530

medium2520

a littre uncomf.

1510

very few uncomf.

50

slightly
uncomfortable

not uncomfortableno vibrations

00.1

0.2

0.3

0.4

fairlyuncomfortable

0.5

0.6

0.7

0.8

ISO2631-1 comfort index (m.s -2 )Figure 1. Standardized discomfort ratings compared to ISO2631-1 comfort indexes of "real" signals. The helicopters (HC) from which the signals are derived are distinguished by different coloured markers. Triangularmarkers indicate signals whose frequency content has been ﬁltered beyond 30Hz.
uncomfortable" to "very uncomfortable", which means that the standard overestimates discomfort a little forthese stimuli.
Analysis of variance showed that the three factors, as well as the majority of their interactions were signiﬁcant.
Only the interaction between X and Y axes was not signiﬁcant.
According to the values of the partial eta squared η p2 , the effect of the amplitude variation on Z is the strongest
one (η p2 = 0.83). The effects of amplitude variations on X and Y (η p2 = 0.5 and η p2 = 0.36 respectively) follow.
Finally, factors interactions have a weaker effect force (η p2 = 0.065 for Y − Z, η p2 = 0.062 for X − Z and η p2 =0.058 for X −Y − Z).
These results are shown in Figure 2a which presents the simple effects of the three factors. These same simple
effects are shown in Figure 2b for the comfort indexes of ISO2631-1. This ﬁgure shows that the variations inthe indexes are similar to the variations in perceived discomfort.

4DISCUSSION

These results show that the index as computed by the ISO2631-1 standard provides a good estimation ofperceived discomfort, whether for signals measured in ﬂight or synthesized stimuli.
This index is already used at Airbus, but does not always correspond to the feeling of the ﬂying crew. This
raises the question of the difference in the perception of discomfort in ﬂight and in laboratory conditions,because passengers are submitted to many other stimuli in the real situation.
The results also show that the vibratory frequency content greater than 30 Hz does not inﬂuence the perceptionof discomfort. Differences between perceptive evaluations and ISO indexes may then appear and shows anoverestimation of discomfort by the standard (see black signals ﬁgure 1).
The results of the experiment on stimuli simulating vibrations related to the blade passage frequency tend toshow that the vertical axis has the greatest impact on perceived discomfort. These results are consistent with
ISO2631-1, which penalizes vertical vibrations more than lateral and horizontal vibrations.

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Mean of norm. disc. evaluations

45

extremely
uncomfortable

extemely uncomf.

X axis
Y axisZ axisvery uncomf.

40
35

uncomfortable30
25

medium20
15

a littre uncomf.

105
0

1.5

10.5

12

3

very
uncomfortableuncomf.

fairlyuncomf.
slightly
uncomfortablevery few uncomf.
no vibrations

X axis
Y axisZ axis2

ISO2631-1 comfort index (m.s -2 )50

0

not uncomfortable

12

Factor level (amplitude)

Factor level (amplitude)(a) Perceptive evaluations(b) ISO2631-1 comfort index

3

Figure 2. Simple effects of amplitude variations on each vibration axis
Overall, the comfort indexes of the standard provide a good approximation of helicopter vibratory discomfort.
However, the semantic labels can be slightly adjusted to better match the feelings given by the participants.

5CONCLUSION

This test campaign provided a better understanding of the discomfort associated with helicopter vibrations.
The ISO2631-1 standard provides a good estimation of discomfort, both for signals from measurements andsynthesized signals. The measured effects of the variation in the amplitude of the vibrations according to theaxes X, Y and Z are in accordance with the recommendations of the standard, which is that vibrations on thevertical axis have the greatest impact on discomfort. The standard comfort index could be slightly improved tobest suit the speciﬁcities of helicopters, by adjusting semantic labels for example. It should be noted, however,that this standard does not take into account the effect of beating that can occur for two sinusoidal vibration
sources of close frequency. This phenomenon will be further tested.

REFERENCES[1] ISO 2631-1,Vibrations et chocs mécaniques - Évaluation de l’exposition des individus à des vibrations globales du corps, ISO, 1997.
[2] Yka Marjanen, Validation and improvement of the ISO2631-1 (1997) standard method for evaluating discomfort from whole-body vibration in a multi-axis environment, Loughborough University, 2010.
[3] DIRECTIVE 2002/44/CE, Directive concernant les prescriptions minimales de sécurité et de santé relatives
à l’exposition des travailleurs aux risques dus aux agents physiques (vibrations), Le Parlement Européen etle Conseil de l’union Européenne, 2002.
[4] Grifﬁn M.J., Handbook of human vibration, Human Factors Research Unit, Institute of Sound and Vibration
Research, The University, Southampton, U.K., 1990.
[5] Bellmann M. Perception of Whole-Body Vibrations: From basic experiments to effects of seat and steeringwheel vibrations on the passenger’s comfort inside vehicles. PhD thesis, Universität Oldenburg, 2002.

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[6] ISO16832, Acoustics — Loudness scaling by means of categories, ISO, 2006.

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