Helmet Mounted Displays for the 21st Century: Technology ...

1. INTRODUCTION
Helmet Mounted Displays for the 21st Century:
Technology, Aeromedical & Human Factors Issues
The introduction of Helmet Mounted Sights and Displays
into operational aircraft. and the increased mass of the
helmets due to the incorporation of the additional image
source, optical trains, the combiners/prqjection system as
well as the wiring. optical supports etc, and the subsequent
changes in centre-of-gravity have all conspired to push the
biomechanical safety aspects in the wrong direction. Most
current work is in minimising the mass of these components
and lowering the CofG of the head mounted mass by a
number of clever design fixes, and this is producing some
reductions in helmet or head-mounted mass. The average
current tlight helmets weigh in the region of 1.5kg (3.3lb),
whilst the lightest is in the region of I.lkg (2.5lb). In some
cases these lower masses results in a reduction of impact
protection and. whilst in some cases, this may be acceptable
for operational reasons. reduction in impact safety margins is
not generally or widely acceptable. To complete the head
mounted weight. the mass of an oxygen mask, at some 300g
(0.66lb), must be added. Helmets incorporating displays are
of course heavier and the current avJerage mass, excluding
oxygen mask. is in the region of 1.9kg (4.2lb) and 2.2kg
(4.8lb) with 0, mask Fig I gives an indication of the static
loads on the head. counterbalanced by the posterior neck
muscles.
15.4kg
Fig 1: Head/neck loading biomechanics
With helmet mounted displays, compensation for the
increase in mass due to the display components is often made
by a trimming of the helmet structure and a subsequent loss
of helmet impact safety levels. If the use of this modification
(i.e. helmet ‘trimming’) route is not an optimum w’ay to
engineer lightness into a helmet display. a fundamental
rethink of the helmet design concept maybe, and this is the
approach taken by DERA Farnborough in one of their long
Dr G M Rood and H du Ross
Systems Integration Department, Air Systems Sector
Defence and Evaluation Research Agency, Farnborough
Hampshire GUI4 OLX, UK
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term research programmes (Ref I). The aim was to review
new lightweight, stable designs, with improved centres of
gravity and moments of inertia, which would improve pilot
performance and reduce the risk of neck and spine injury in
fast jets during highly agile manoeuvres and ejection. For
helicopters the mass reduction is intended to improve the
protection in the crash case by reducing the inertia of the
HMD and to improve the impact protection for multiple
impacts.
The approach was to investigate the mechanics of new
materials for the head/helmet interface, integral lightweight
shell structures and an assessment of the human
characteristics of integrated design functions.
2. MECHANICAL & STRUCTURAL ISSUES
Materials for a stiff, two part clam-shell helmet. of sandwich
construction, together with multi-layer high impact
attenuating liners were modelled. constructed and tested, and
have produced data to design closer fitting helmets shells,
Alternative methods of producing form tit liners were used
and evaluated and highly conformal stable liners produced.
Head aggregation modelling techniques, using data from
laser scanning of heads, were analysed to develop size rolls
for future designs.
2.1 Form Fit Liners
In the design for the mechanics of materials for the
head/helmet interface, maximal head coverage and
conformality were considered high priority. Following
initial materials surveys three approaches were studied to
produce head liners; live head mouldings, liners moulded
from replica heads and liners machined from laser scanned
head data. The first method was abandoned early due to
thermal and toxicity effects, the second proved to be an
effective approach but was superseded by the use of laser
scanning techniques and CAD/CAM manufacture of the
form fit liners. This methodology was more attractive and a
cheaper approach in the longer term with the benefit of high
stability and the optical alignment being completed in the
computer following head laser scanning. Due to the
personalised nature of the form fit liners. the optical systems
can be set for the helmet. eliminating heavy adjustment
mechanisms.
2.2 Shell Construction
In the process of determining the helmet design. basic
integral lightweight shell structures and impact attenuating
liners were constructed and impact tested, both physically
and with a Finite Element model initially as representative
Paper presented at the RTO HFM Symposium on “Current Aeromedical Issues in Rotary Wing Operations”,
held in San Diego, USA, 19-21 October 1998, and published in RTO MP-19.

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hemi-spheroid test shapes. Gradual evolution in shape led to
a helmet shaped construction that formed the basis of the
lightweight helmet shell.
The impact tests, to the IJK standard for helmet test, BS
6658, demonstrated excellent impact attenuating properties.
The manufacturing development and Impact Assessment was
carried out by the Structural Materials Centre at DERA
Farnborough and a ‘cardinal point‘ specification (i.e. the
maximum of technical freedom) given as to the technical
direction needed. Double Diaphragm Forming techniques
were used to successfully produce the fibre reinforced skins,
carbon tibre inners and glass tibre outer. and significant
development made in the press-forming of high performance
cellular materials for use a energy absorbing liners. A
sandwich construction helmet shell had reduced mass
compared to conventional single skin GRP or ABS helmets,
but maintained the high stiffness necessary to eliminate the
mechanical decoupling of optical systems used in more
flexible helmets. Until the final designs, it is difficult to
provide comparative masses, as it is possible to design the
helmet either for maximum impact attenuation or minimum
mass for acceptable impact limits, The aim, however, is to
produce a helmet mass of initially 1.6kg (3.52lb) which
includes optics and oxygen mask - i.e. a basic helmet mass of
1 to 1.2kg - with high levels of impact protection This. with
a better understanding of the materials technology, should
allow this I .6kg mass to be reduced in time to around 1.4kg.
As a result of enhanced stiffness, and the utilisation of high
performance materials, the shell demonstrated impact
performance and energy management greatly superior to
current conventional helmet shell designs.
2.3 Impact testing
The combination of tibre reinforced plastic skins and a
structural foam core. in this design, results in the stiffness
providing improved load spreading capability, which. in
turn, leads to greater energy absorption per unit shell
deflection, as the impact is spread over a greater area of
surface foam.
Fig 2 shows the force-deflection response results of helmet
shells impacted onto the flat test anvil. The results
demonstrate excellent energy management during the first
impact event, with the plateau ‘g’ level being controlled to a
level approaching 250 ‘g’, implying a good choice of foam
yield stress for this level of load spread.
The results for the second impact event show reduced ‘g’
levels when compared to the first test. The force at which
crushing onset occurs is lower due to shell damage sustained
during the initial impact. This reduces the load spreading
capability and hence the load required to crush the foam. A
second region of crush occurs in these impacts implying that
another layer of foam material has started to deform.
0 5 10 15 20 25
Deflection (mm)
Fig 2: BS6658 Impact Test Using Flat Anvil
Fig 3 shows the force - detlection response for shell impacts,
using the Type A hemispherical anvil tests, and show a much
decreased crushing load when compared to the flat anvil
tests. This is due to a smaller area of foam being crushed
because of the concentrated loading. Conventional shell
design generally causes the hemispherical impact test to be
the most severe, and therefore selection of foam yield stress
is based on the impact scenario. The foam selected must
therefore be higher density than that required for the flat test
due to reduced load spread. Thus the sandwich construction
allows the foam to be selected based on the flat test. in the
knowledge that the hemispherical test will automatically be
passed.
0
0 4 8 12 16
Deflection (mm)
Fig 3:BS6658 Impact Test Using Hemi Anvil
The optimum mass solution for helmet design, based on the
pass levels quoted in BS6658 Type A, is therefore not to
achieve the lowest possible ‘g’ level. but to allow the
deceleration to rise in a controlled manner, until stable
crushing of the energy absorption layers occurs at a level
approaching 300 ‘g’. This design philosophy serves to
maximise energy absorption per unit shell deflection. giving
the lightest possible shell design to pass the test standard.
However. by a fuller understanding of this design process it
would be possible to either maximise protection for a defined
helmet mass or to minimise the mass for a defined protection
value. Any solution in between is; of course. also
achievable.
A comparison of the transmitted impact levels for
conventional flying helmets and the lightweight helmet are
shown in Figs 4 to 7. For this design of helmet, the
philosophy was to provide a helmet lighter than conventional
helmets that would provide similar impact performance. In
this case the helmet was around 40% lighter than current
flying helmets.

BS6658 Type A Impact - Flat Anvil - Rear of Helmet
0
‘G’ e
%
0 2 4 6 8 10 12 14
Time (ms)
Fig 4: Impact 1
o 0 2 4 6 8 10 12 14
Time (ms)
Fig 5: Impact 2
BS6658 Type A Impact - Hemispherical Anvil - Rear of
Helmet
0
8
2.4 Modelling
0
0 2 4 6 8 10 12 14
Time (ms)
Fig 6: Impact 1
0 2 4 6 8 10 12 14
Time (ms)
Fig 7: Impact 2
Finite Element Analysis (FEA) of impact behaviour was used
in refining the mechanical characteristics of the shell and
liner materials and to reducing the dimensions and masses of
the impact attenuating liners. Further 3D FEA modelling
techniques are being used to model the behaviour of the
skull, brain, fluid and cervical spine region to obtain a better
understanding of the head under impact
2.5 Head Aggregation and Sizing
To identify methods for defining new size rolls for the
helmet, head aggregation and 3D CAD modelling studies
were constructed using the head models of 139 male
Caucasian subjects. Software has been developed to align the
heads to form aggregated head models. The individual head
scans were landmarked on the screen and the positions of
these points were then used to align the whole or sections of
each head in the database. The resulting model is known as
an aggregation model and represents a volume which
encapsulates all of the database members. This aggregation
model can then be interrogated within software. using a quasi
Gaussian map, to provide summary surfaces at particular
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percentile positions, Thus. the resulting information is a
series of 3-D stacked surfaces. that, for a particular
aggregation alignment, describe the percentile sizes of the
population. These surfaces can then be used within a 3-D
design environment to produce conceptual and production
models of future helmet systems. The aggregation criteria
and the percentile shell choice can bc optimised for the
particular application for which the head gear is to be used.
Five head registration criteria were used to produce an
assessment of the minimum and maximum surface envelopes
which enclosed all of the heads in the aggregations.
Currently it is estimated that there may be 2 - 3 sizes for the
front section and up to nine for the rear portion.
3. HUMAN FACTORS ISSUES
A series of human factors tests have demonstrated the
practical acceptability of some of the component parts of the
assembly. Helmet stability. thermal aspects. pressure
breathing, noise attenuation. sizing criteria and airf‘low
management aspects were investigated.
3.1 Stability
A series of stability measurements were conducted
comparing the stability of form tit liners to those of the
current in-service t!K helmets. The stability improved in
both slippage and torsion, and. although there is a great
variation between axes and sub,jects, a maximum
improvement of 15:l was achieved. The factors which
contributed to this variability were estimated to be partially
helmet related - support areas ctc and partially human related
- skin tautness, hairstyles and lengths etc. These
improvements in stability will contribute to the reduction in
overall head-mounted mass by allowing an optical design
that will need a smaller exit pupil. Currently the liners have
been perceived by the test sub.jects as very stable.
comfortable. vvithout hot spots and preferable to traditional
helmet design. An indication of the stability achieved is
shown in the three axes. compared to conventional flying
helmets is shown in Fig 8.
Mean Relative Stability - 2 Standard helmets 81
Lightweight helmet
0 HI Helmet H3 Helmet n Lightweight
3.2 Thermal aspects
Fig 8:
Rx
Roll
RY
Pitch
Rx
Roll
One obvious concern was the thermal conditions in an
enclosed helmet. Motorcyclists, of course, survive in hot
climates in helmets of this type, but the application to the
airborne environment has greater implications in terms of
potential thermal degredation of aircrew performance. An
experiment. measuring skin temperatures at five areas of the
head, was carried out using subjects exercising sufficiently to
double the natural oxygen uptake, each lasting one hour.
Thermal build up was measured at three ambient
temperatures (20°C, 28’C and 35OC). Fig 9 shows the mean

head temperatures against time for both the form-fit helmet
and a conventional tlying helmet. The bars at the end of the
plots shows the range of final temperatures.
These levels are in an acceptable range. comparable with a
conventional hclmct. indicating that, for these cases.
additional cooling would not be necessary, but, in the real
operational environment, stress levels, physical workload
during combat, solar heating etc., may necessitate cooling.
and these thermal conditions will be further assessed.
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Fig 9: Comparison of Mean Head Temperatures (5
Subjects, 5 Sensors)
In order to provide an acceptable interface between the liner
and the skin. a number of materials wcrc assessed, resulting
in the mechanical characteristics of traditional kid leather.
well known to be acceptable, being chosen. Techniques have
been dcvclopcd to bond the leather to into the high curvature
surfaces such as the female moulding of the human head
lining, creating a highly acceptable surface for skin contact
with little increase in interface pressures with the subjects
tested so far and head contact pressures of less than one-third
of those in existing UK helmets.
3.3 Pressure Breathing
Although only applicable to fixed wing operations at high
altitude. the initial approach and results of pressure breathing
trials are reported. The helmet uses a new form of oro-nasal
seal which can bc used as the moulding around the face was
sufticicntly accurate to maintain a seal against the face. As a
result, simple ori-nasal and ori-maxilo-nasal seals w’ere
designed and versions produced in silicone rubber and
welded PVC film. A pressure breathing trial was conducted
on six subjects and the results show that maximum breathing
pressures of 70 mm Hg can be maintained, although with
rather high inflation pressures, that is. inflation pressures
higher than the test breathing pressure. The oro-maxilo-nasal
seal was sub.jectivcly considered to be the most comfortable.
The USC of generically conformal 3-D seals should allow seal
inflation pressures to be lower and controlled by breathing
pressures. which arc related to ‘g’ levels, and thus eliminate
the need for mask tensioning - another component of mass
on the heltnet.
To completnent the seals, lightweight low profile inspiratory
and expiratory vtalves are being developed. The resulting
combination, with air passages integral with the shell,
eliminate the need for conventional masks. further reducing
mass and bending moment about the neck.
3.4 Noise Attenuation
Initial noise attenuation trials have shown that a system
integrated into the helmet liner provides some attenuation,
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but not yet to the level of current aircrew helmets, but the use
of lightweight integrated ‘shells’ and active noise reduction
systems, or the use of active noise earplugs (Ref 6) or use of
the Communications Ear Plug (CEP) from the IJSA, would
allow acceptable levels of noise attenuation with a suitably
lightweight attenuation system.
3.5 NBC Issues
The use of a full face helmet, with suitable scaling at the
neck, should allow the NBC protection to be an integral part
of the helmet and the current methods of using hoods of the
AR5 type to be totally discarded (to the obvious advantage
and delight of most aircrew). Aspects such as the need to
gain emergency access to parts of the face need to be
assessed, but, in reality. any change in philosophy between
existing NBC systems and these integrated systems is not
expected.
4. HELICOPTER OPERATIONAL APPLICATIONS
Thus Helmet Mounted Displays (f-IMDs), perhaps of this
type, will be integrated into future helicopter systems and
many of these systems, particularly battlefield helicopters
will have the helmet as an integral part of a Visually Coupled
system: that is the sensors, which will provide a picture of
the outside world to the aircrew, will be coupled to the HMD
with the sensors platform moving in harmony with the pilots
head movement. Helicopters of the AH-64 Apache type
have such a system in use both for pilotage (PNVS) and for
targeting (TADS), and much research is underway as to the
optimal FOV. resolution and other technical aspects of the
HMDs for helicopter operations. One of the aspects of using
HMDs in a weapon system is in the use of off-boresight
targeting and the operational benefits accrued. In fixed wing
operations significant advantages can be accrued from off-
boresight targeting using a helmet sight and target
engagement and missile lock-on times reduced by some 60%
compared to boresight targeting through a Head IJp Display
(HUD), with a capability, with a good missile, of firing at up
to 60 degrees of boresight. Whilst helicopters can use the
high yaw capability inherent in helicopter control to provide
a vestige of off-boresight capability, the time taken to
stabilise the yaw movement and line up the target will
inevitably be longer than using a helmet sight. Thus helmet
mounted sights combined with a good missile or gun
capability should allow enhanced weapon engagement and
release capabilities. A combination of helicopter handling
and control. combined with Helmet Sights and/or displays,
should allow a significantly improved battlefield capability.
5. HELICOPTER CABIN/COCKPIT NOISE
One of the issues with the helicopter internal environment is
the continuing problem of noise levels. The development of
new materials to provide lighter airframes, the development
of more powerful engines and the increase in lifting/payload
capacity form the use of better blade designs has resulted in
noise levels in the cockpit and cabin either increasing or
remaining essentially constant. Attacking the problem
through the use of active devices has proved effective, both
operationally and from cost effectiveness. Most systems use
the active noise reduction system mounted in the tlying
helmet, although it is possible to provide area noise reduction
in the cabin and this has been accomplished in Cl30
Hercules trials in the UK. A number of trials have been
completed in UK measuring the effectiveness of ANR in
helicopters during operational flying. In terms of the

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reduction of the risk of hearing damage, trials have
consistently shown reductions in noise at the ear by around
6dB(A). Ref 2 and around 3.5 dB(A) for trials on a Sea King
HA%. Ref 3. Both of these trials resulted in the noise levels
at the ear being reduced to levels that are within the
aeromedical guideline limits of 85 dB(A). During all of
these trials sub,jective opinion was sought from the
operational crews as to their opinion of the overall
effectivcncss of ANR. This was accomplished by means of a
questionnaire for both aircrew and for the survival equipment
crews. who are required to service and support the aircrew
equipment. The general aircrew opinion was highly
favourable. and the opinions supported the hypothesis that
fatigue and stress was reduced, as well as improvements in
communications. The effects of stress and fatigue are
difficult to measure objectively. and some reliance needs to
be placed upon aircrew subjective assessment. Speech
intelligibility can, however. be objectively measured in flight
and this has been done in a number of flights in the USA and
Australia under the auspices of TTCP (AER TP2). Both
speech intelligibility direct and other communication
parameters such as Speech Clarity, Subjective impression of
Speech Intelligibility and Attention Demand are improved.
Figure IO shows data from an Australian Army trial on S-
70A-9 Black Hawk (Ref4 ).
Aircrew rated the speech heard with ANR in operation as
significantly clearer (p i 0.05) significantly more intelligible
(pi 0.025) and significantly less demanding of attention
(p

insert using a similar technology to the circumaural shell
ANR technology. The results are shown in Fig 15, Ref 6,
and arc for an earplug that tits the ear in place of a standard
plug (i.e. not a supra-aural device). The results show good
attenuation in the 30dB area across the frequency range
160Hz to above 4kHz. In helicopters. where the majority of
the high noise generating components are below 1 to 2kHz.
these type of active plugs could provide one solution to the
invasion of high noise levels.
Passive + Active Attenuation
Fig 15: Overall acoustic attenuation for the DERA ANR
earplug
A ‘supra aural’ earplug has been reported (Ref 7) which
provides some increased active attenuation in the higher
frequencies (i.c. above IkHz) and the results are shown for
this device Fig 16 (from Ref 7).
Active Attenuation
Frequency [Hz)
Fig 16: Active attentuation of an experimental ANC ear
Plug
6. CONCLUSION
If the mass and balance targets of future Helmet Mounted
Displays systems are to be met. then it is likely that new
approaches to integrated helmet design need to be initiated,
as the current approach with conventional tlying helmets has
obvious limitations. By the use of new materials and
structures technology, lighter weight helmets can be
designed and built that provide improved impact protection
and stability. whilst reducing the risk of neck and spinal
in.jury. Protection against the helicopter noise environment
and improved communications can be accomplished by
active noise reduction systems which are an integral part of
the helmet design and this type of helmet design has the
potential for incorporating fully integrated NBC protection
7. REFERENCES
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1. Transcripts of five DERA Farnborough research papers
presented at ‘The Design and Integration of Helmet
Systems” Framingham MA December 1997.
2. Hancock M, Hazel A. Leeks C
DRA/AS/SID/682/CR96263/1 .O July1996 An Active
Noise Reduction Trial on Sea King AEW2 at RNAS
Culdrose during March/April 1996.
3. Hancock M, Hazel A, Leeks C.
DERA/AS/SIDICR980242/1 .O April 1998 An Active
Noise Reduction Trial on Sea King HAS6 at DERA
Boscombe Down during December 1997.
4. King R B. Saliba A J, Brock J R, James S H Sept 1997
Assessment of a Helmet mounted Active Noise
Reduction System in the Sikorsky S-70A-9 Black
Hawk helicopter DSTO-TR-0574.
5. Simpson C. A Briefing for Program Manager ALSE
Program Executive Office Army Aviation and Troop
Command December 1992 AFDD Moffet Field.
6. S Rogers 1998 Assessment of the Sound Attenuation
Performance of Earplugs with Active Noise
Reduction DERAIASISIDII .O in publication.
7. Buck K and Parmentier G Active Hearing Protectors:
new developments and measurement procedures
French-German Research lnstitutc of St Louis. France:
Paper IO-I in AGARD CP 596; Audio Effectiveness in
Aviation. 1996.