Teaching Series

Teaching Series
Windblast Testing
Dept of Human Engineering, IAM, IAF
Introduction
How is Windblast Testing done?
Ejection from an aircraft will subject an aircrew
to airflow, the velocity of which can vary from 0
knots on one extreme (ejection on ground at 0 knots
aircraft speed) to 600 knots at other extreme; the
highest velocity at which the modern fighter could
be flying at the time of ejection. This sudden
exposure to ambient air flow once canopy is
jettisoned / removed is known as windblast.
Exposure to such high velocity wind can cause
petechial haemorrhages, sub-conjunctival
haemorrhages, chest injury and flailing of head and
extremities. Apart from the spinal injuries due to
high catapult force, the windblast injuries can also
cripple an aircrew and affect post ejection survival.
Restraint systems provide some protection against
flailing of limbs. The helmet / visor and oxygen mask
provide protection to the head and face. The life
preserver unit (aircrew jacket) protects the chest.
Windblast testing hence becomes an integral part
of the battery of certification tests for helmet, visor,
oxygen mask and Life Preservation Unit (LPU).
Wind blast testing is carried out in specially
designed test facilities that are different from the
standard wind tunnel. The wind tunnel provides
laminar flow in which aerodynamic properties of
an aerofoil are studied. A wind blast test facility on
the contrary provides sudden high speed wind blast
to assess its human effects. As of date a wind tunnel
facility is available at the National Aerospace
Laboratory (NAL) Bangalore but there is no Wind
blast test facility in India. The facilities at the
specialised labs in France and USA are used for
wind blast testing and certification of the indigenous
flying clothing under development.
Testing Facility
The typical set up of a Wind blast test facility
is shown in figure 1. The main components of the
system are as follows:High wind speed generator
The windblast facility produces a high velocity
Fig 1: A Windblast Test Facility
(Reproduced from http://www/dtbtest.com/eLibrary/wind-blast-testing.pdf-United States)
Ind J Aerospace Med 55(2), 2011
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Windblast Testing : Dept of Human Engineering
air flow field (100-700 knots) similar to the
aerodynamic environment during ejection. This is
accomplished with a controlled expansion of high
pressure compressed air (11 Mega Pascal) which
exits through nozzles. The nozzle assembly is
arranged to provide uniform airflow. Thus, the
broadened windblast airstream more accurately
simulates the ejection aerodynamic conditions
Anthropomorphic test dummy (ATD)
Hybrid II/ III anthropomorphic dummies are
used to simulate the ejectee. This Mannequin should
have a biofidelic instrumented neck. The neck is
instrumented to record the forces on exposure to
wind blast. The test Mannequin is instrumented to
measure the biodynamic neck loads and
acceleration experienced by each part of the body.
The dummies are available in 3 sizes viz. 95th
percentile, 50th percentile and 5th percentile US
anthropometry. The selection of size of the dummy
does not really affect the test and usually 50th
percentile dummy is used. The dummy is suitably
clothed with correct sizes of various items of flying
clothing viz. helmet (with visors), oxygen mask, LPU
and Anti G suit.
Ejection seat
The clothed dummy is placed in a ejection seat
placed directly in front of the jet nozzle. The seat is
mounted on suitable stand so that the included angle
as in the aircraft. The certification of wind blast
testing and certification can be done for specific
ejection seat for the following reasons:(i) The pitch of each seat in aircraft may vary
(e.g. Martin Baker Mk 10 seat has pitch of
25°.
(ii) If a visor flies of on exposure to wind blast,
the timing of that event with ejection seat
sequence (that is specific to each seat) is
assessed to see if the flown off visor would
46
affect the ejection sequence in any way.
Data recording equipment
The dummy is instrumented with 3 axis head
accelerometers, 2 pressure sensors for the eyes,
bending moment and tensile force transducers in
the neck (C1 & C7 vertebra). High speed video
cameras (500 - 1000 frames/s) are installed to
record the wind blast and its effects, which enables
to analyse the specific events and assists in failure
analysis in time domain.
Objectives of Windblast testing
The wind blast test assesses the following
objectives:(a) To demonstrate that the Aircrew helmet and
visor does not fly away from the head during
ejection
(b) To demonstrate the structural integrity of the
flying clothing
(c) To demonstrate pilot’s facial protection
(d) To measure the helmet induced forces and
movements at occipito-atlantic joint, in support
of neck injury hazard assessment.
Standards to perform a Windblast Test
Wind blast testing of helmet and visor assembly
is carried out in accordance with Mil standards
87174 A and Mil-V-25951/1 (AS). The testing is
recommended to be carried out max windblast
velocity of 600 ± 60 KEAS. The time of exposure
to the maximum velocity should be 300ms ± 50ms,
and the rise time to reach peak velocity should be
within 125 ± 20ms. The total windblast duration
should be at least 3.0s. The testing is recommended
to be carried out in Head on position. Additional
testing is also recommended with head yawed to
left and right and in head pitch up condition.
Ind J Aerospace Med 55(2), 2011
Windblast Testing : Dept of Human Engineering
Post-test activities
After the test a detailed visual inspection of
the test items is performed without touching
anything. Photographic documentation is done and
all findings are recorded. All the test items from
the manikin are disassembled and thorough visual
inspection is done. The visor is subjected to Photo
Elastic inspection method. A raw data is prepared
using all the sensor data records. The actual
airspeed is calculated and the data sheet is
completed.
Pass - Fail Criteria
(a) General pass - fail criteria. There should be
no structural damage, which can compromise
head and facial protection. This includes
breaking / tearing / cracking or loosening of
any pilot safety related elements.
(b) Visor pass – fail criterion. The visor is
deemed to have failed if any cracks or
significant deformation is observed.
(c) Helmet retention pass - fail Criterion. Any
loss of helmet, including visor and oxygen
mask from the Manikin’s head is deemed as
a test failure.
(d) Pilot facial protection pass - fail criteria. Max
rotation / slipping of helmet assembly with
respect to Manikin’s head should be such that
Ind J Aerospace Med 55(2), 2011
the gap between visor and mask (Nose
Bridge) should not increase by more that ½”
from pre-test position. Facial areas, not
protected by the helmet or mask, should not
be stroked by visor or any helmet component.
The oxygen mask should be displaced and the
mask – hose junction should remain intact.
The Oxygen hose should not flatten / crack.
(e) LPU, Anti G suit, flying overall and gloves
pass – fail criteria. The cloth, pockets and
stole (in sea version of LPU) should not show
any tears. The zip should not open. The gloves
should be retained.
Conclusion
Wind blast testing of flying clothing is essential
step in certification of flying clothing for fighter
aircrew. It is essential to understand the difference
between wind tunnel testing and wind blast testing.
This teaching note brings out the test methodology
and pass – fail criteria. Flying clothing that has
passed this stringent test is expected to be retained
on the aircrew during an ejection and remain
functional even if the ejection were to take place
at high speed. The aeromedical significance of this
highly technical test lies in the surety that the life
support systems would continue to protect the
ejectee throughout the ejection and post ejection
survival.
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Aviation Medicine Quiz
ACCELERATION
1.
The mechanism behind PLL is that the retina has a single blood supply from the central retinal
artery which is an end artery that penetrates the globe at the optic disc and forms multiple branches,
becoming more numerous and smaller in diameter towards the periphery.
(A)
This mechanism is well proven to be true
(B) This explanation is oversimplified and does not explain more uniform pattern of light loss in
some individuals.
2.
3.
4.
48
(C)
This explanation takes into account foveal blood supply.
(i)
Only A is true
(ii)
Both A & C are true
(iii)
All are true but B is more appropriate
(iv)
Only B is true
The common end points of ascertaining G level tolerance in centrifuge research are:
(A)
100 % PLL
(B)
50 % CLL
(C)
Blackout
(D)
GLOC
(i)
All are correct
(ii)
Only A is true
(iii)
Both A and B are correct
(iv)
Both A and C are correct
An equipment on which specific exercises can be performed to improve the effectiveness of
AGSM is known as:
(i)
Strainometer
(ii)
G meter
(iii)
AGSM Myometer
(iv)
Statergometer
Under + Gz , the effects on FRC and Tidal Volume in a subject wearing an AGS which starts
inflating at 2 G, are as follows:
(i)
Both FRC and TV progressively reduce with increasing Gz
(ii)
Both FRC and TV progressively increase with increasing Gz
(iii)
FRC reduces but TV progressively increases with increasing Gz
(iv)
Both FRC and TV rise initially and then progressively fall at increasing Gz
Ind J Aerospace Med 55(2), 2011
5.
6.
7.
8.
9.
The effect of PBG and AGSM on acceleration atelectasis is as follows:
(i)
The level of +Gz at which acceleration atelectasis develops is increased by AGSM but
reduced by PBG
(ii)
There is no effect of AGSM and PBG at the level of +Gz at which acceleration atelectasis
develops
(iii)
The level of +Gz at which acceleration atelectasis develops is increased by PBG but reduced
by AGSM
(iv)
Both AGSM and PBG increase the level of +Gz at which acceleration atelectasis develops
Under +Gz, when closing volume of the lungs is reached, the oxygen tension of the gas trapped in
the non-ventilated alveoli falls within a few seconds, by absorption of oxygen into the blood to equal
that of the mixed venous blood. The blood flowing through these alveoli thereafter constitutes a
right to left shunt. According to Gliaster (1970), the proportion of cardiac output shunted in this
manner at +5 Gz may amount to:
(i)
30%
(ii)
40%
(iii)
50%
(iv)
60%
Chest radiographs in acceleration atelectasis reveal:
(i)
Basal lung collapse
(ii)
Loss of costophrenic and cardiophrenic angles
(iii)
Shadowing at both lung bases
(iv)
All of the above
The effects of negative Gz on the cerebral blood flow include:
(i)
Sudden increase in transmural pressure in cerebral vessels after few seconds of
onset of - Gz
(ii)
Cerebral blood flow increases
(iii)
The increased blood flow can cause engorgement of cerebral vessels leading to loss of
consciousness
(iv)
None of the above
The limit of negative Gz tolerance is set by:
(i)
Discomfort in the head
(ii)
Oedema of soft tissues of the face
(iii)
Loss of consciousness due to cerebral ischemia
(iv)
All of the above
Ind J Aerospace Med 55(2), 2011
49
10.
11.
12.
A study was conducted by Burton (1988) to demonstrate effect of response time of anti-G valve
on G tolerance. It showed that:
(i)
A maximal delay of 1 sec was acceptable for providing best G tolerance when G onset rate
was 6 G/s.
(ii)
A maximal delay of 2 sec was acceptable for providing best G tolerance when G onset rate
was 6 G/s.
(iii)
Response time of AGV did not have any effect on G tolerance of high G onset rates
(iv)
None of the above
As per AGARD 322, for physical conditioning for G protection, a limitation of aerobic conditioning
is kept at:
(A)
Maximum of 9 miles of running per week
(B)
Heart rate should stay above 55 beats pm
(C)
Exercise severity should be calculated as 60 – 80 % of target heart rate
(i)
Only B & C are true
(ii)
Only A& C are true
(iii)
All are true
(iv)
All are false
For G duration tolerance following are true:
(A) The G protection provided by the muscle tensing component is a function of Maximum
voluntary contraction and remains constant for initial 30 sec after which it starts to decline.
13.
50
(B)
After 25 – 30 sec of AGSM, about 1 G of a pilot’s high-G tolerance is lost.
(C)
MVC reduces at rate of about 1% per second when AGSM is done to counter 9 G
(D)
The loss of MVC is not obvious to the pilot
(i)
All are correct
(ii)
Only D and B are true
(iii)
Only C and D are correct
(iv)
B, C and D are correct
The largest study to investigate the acceleration levels associated with visual symptoms was
conducted by :
(i)
Whinnery et al
(ii)
Cochran et al
(iii)
Burton et al
(iv)
Madam Alice Stall
Ind J Aerospace Med 55(2), 2011
14.
Following statements are put forward regarding PBG:
(A)
PBG increases G level tolerance by 0.5 to 1 Gz when used with AGSM.
(B) Giving PBG above 30 mmHg pressures by mask mandates use of chest counter pressure
garments to prevent against excessive strain on chest wall due to distension.
(C) The combination of PBG and extended coverage AGS enables most individuals to maintain
clear vision at +9Gz with little or no straining.
15.
16.
17.
(i)
Only B & C are true
(ii)
Only A& C are true
(iii)
Only C is true
(iv)
All are true
TLSS developed by USAF provides the following:
(A)
Full coverage AGS
(B)
PBG
(C)
Capstain suit
(D)
HFRP
(i)
Only B & A are true
(ii)
Only B & C are true
(iii)
A, B & D are true
(iv)
All are false
Following statement about high G training is incorrect:
(i)
The requirement for centrifuge based high-G training was identified after the GLOC surveys
conducted during the 1980s.
(ii)
The high G training enhances the performance of the baroreceptors resulting in slightly
improved G tolerance in combat.
(iii)
The high-G training enhances aircrew anticipation of circumstances that might result in GLOC.
(iv)
The high-G training enhances aircrew awareness of the potential for GLOC
As far as prevention of aircraft accidents are concerned, which is the best method to initiate autorecovery mode following G-LOC?
(i)
Physiologic monitoring of the pilot
(ii)
Monitoring aircraft stick movement and/or pilot’s head position
(iii)
Using aircraft ground avoidance systems
(iv)
None of the above
Ind J Aerospace Med 55(2), 2011
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18.
19.
20.
As per textbook of Ernsting 4th edition, AGSM can provide more than 4 G improvement in G
tolerance if it is performed correctly. When used in combination with standard AGS, it should
enable most aircrew to tolerate +9Gz for at least:
(i)
5 sec
(ii)
10 sec
(iii)
15 sec
(iv)
20 sec
In an electronic anti-G valve, the pressure to the G suit is controlled by:
(i)
Converting the mechanical signals obtained from the weight to electronic signals and
transferring to G-suit pressure transducer.
(ii)
Controlling the voltage difference between the output of an accelerometer and a G-suit
pressure transducer.
(iii)
A Bang-bang servo controlled system linked to the aircraft accelerometers.
(iv)
None of the above
G onset rates faster than 1 G/s (eg 6G/s) result in a relaxed G tolerance which is lower than relaxed
ROR tolerance by approximately:
(i)
0.3 G
(ii)
0.4 G
(iii)
0.5 G
(iv)
0.6 G
For Answers see page 44
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Compiled by Surg Lt Cdr SK Verma
Dept of Acceleration Physiology,
IAM, IAF, Bangalore
Ind J Aerospace Med 55(2), 2011

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