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Evaluation of a Helicopter Rescue Basket for Safe
Human Carriage John A. Plaga Air Force Research Laboratory Abstract:
Background: Expeditious aerial
evacuation of civilians with helicopters is limited by the inability to extract
more than one person with a helicopter rescue hoist. The small cabin size also limits the number
of evacuees (5-6) to be carried to safety on each flight, and often results in
long-term separation of families. A 15
person capacity helicopter rescue basket (Precision Lift Heli-Basket HB-2000)
was recently provided to US Army Guard and Air Guard HH-60 helicopter units
through a Congressional plus-up.
Objective: Evaluate the
Heli-Basket during flights while tethered to an HH-60 to determine the
potential of injury to the occupants during all phases of flight (take-off,
transport, and landing). Methods: A large and a small anthropomorphic manikin
were placed in the Heli-Basket along with instrumentation to measure manikin
and basket accelerations, rotations, velocity, and heading. Tests were supported by the 106th Rescue Wing
(RQW), F.S. Gabreski Air National Guard, Purpose: The purpose of
this study and analysis is to determine if the Precision Airlift, Inc.
Heli-Basket HB2000 can be certified for human carriage. This Congressionally mandated piece of
equipment was bought for both the Army and Air National Guard. The US Army Soldier Systems Center (Natick
Laboratories) did extensive flight testing and certified the Heli-Basket only for
cargo carriage. The Air National Guard
(ANG) Rescue Wings have articulated a Homeland Defense requirement to provide
short range “water rescue” and “high rise building” emergency human carriage
capability using the Heli-Basket. Objectives: The following are the objectives of the
study: a. To
determine if the Heli-Basket is safe for human carriage b. To define
the operational limits of the Heli-Basket for human transport Test Overview: The Air
National Guard Air Force Reserve Test Center (AATC) managed the test program
which was conducted at, and supported by, the 106th Rescue Wing (RQW), F.S.
Gabreski ANGS Suffolk CO.,
Figure 1.
Flight Route A total of five sorties were flown at
various flight conditions to evaluate the stability and acceleration safety of
the Heli-Basket. Three phases of flight
were examined for stability and acceleration safety of the Heli-Basket: flights encompassing rectangular circuits,
landings, and takeoffs. The flights
through the circuits (Figure 1) were conducted at air speeds of approximately
20, 40, 60, 80, and 100 knots, with various payload configurations (Figure 2)
including a 103-lb Lightest Occupant In Service (LOIS) manikin and a 218-lb
Large Advanced Dynamic Anthropomorphic Manikin (ADAM) manikin. The manikins were secured in the basket with
lap belts. The takeoffs and landings
were either on a hard surface such as concrete or a soft surface such as
grass. For a description of each test
condition or “cell”, see Table 1 below.
Figure 2. Test
Setup Table 1. Test
Cells
Data Processing: Data
collected during the flights included manikin accelerations and angular rates
using the LOIS-manikin-mounted digital Data Acquisition System
(DAS), Heli-Basket position and velocities using an Embedded GPS/INS (EGI)
system, and standard digital video from a camera mounted in the Heli-Basket,
pointing at the manikins. Each of these
three systems had its own timing system which had to be correlated against each
other. The DAS had a time reference with
respect to the triggering of data collection as well as an IRIG time (days,
hours, minutes, and seconds) with respect to GMT. The EGI system logged the data against day-seconds,
which is the number of seconds that have passed since the beginning of the
current day at the Prime Meridian (GMT).
The video digital time display was synchronized to an atomic clock, but
to local time (GMT – 5). The data on the
DAS were collected at 1000 Hz, and on the EGI at 16 Hz for periods up to 52 minutes. This resulted in large amounts of data from
each sortie for a variety of test conditions.
In order to make the data manageable and to easily identify discrete
test conditions, data were parsed out of the total data set from each of the
five sorties and organized into discrete test numbers and test conditions
(Table 2). Table 2. Text
Matrix
*L+A+B = LOIS + ADAM +
Ballast Takeoffs and Landings: The takeoffs
and landings were treated as discrete events, so the DAS and EGI data were time
shifted so that the event (takeoff or landing) occurred at time = 0. The data from the takeoffs were processed
from 2 seconds prior to the liftoff to 8 seconds after liftoff, and the data
from landings were processed from 6 seconds prior to landing to 4 seconds after
landing. The DAS data were filtered at
200 Hz and reduced to an output rate of 500 Hz, and the EGI data were processed
using no filtering and the native 16 Hz sample rate. These data were analyzed against impact
injury criteria that were developed primarily for ejection seat evaluation and
crash load analysis using AnalyzeTest Version 0.0.18 software that was
developed in-house (AFRL/HEPA). The
injury criteria included the Dynamic Response (DR) model for 5 axes (+/-X, Y,
+/-Z), Multi-axial Dynamic Response Criteria (MDRC), and analysis of a moving
average on the chest accelerations. The
limits for the DR and MDRC correspond to approximately 0.5% probability of
injury, and each of these criteria are shown in Table 3 below along with the
extrema values from all of the tests conducted.
The analysis from the tests indicates
that the accelerations encountered during takeoffs and landings result in a
minimal probability of injury due to the acceleration. Further analysis of the injury results using
a One-way ANOVA were conducted to determine if there were statistically
significant differences between test variables such as the manikin size,
landings on concrete versus grass, effect of total weight, etc (Table 4). Table 3. Injury
Limit Criteria and Test Results
The results of the statistical analysis
of the MDRC and Resultant Chest Acceleration (Table 4) indicate that there is a
greater probability of injury when landing on concrete versus grass (p =
0.0214), and that there is a greater probability of injury when landing with a
heavy load versus a light load (p = 0.0072).
The effect of the . Table 4. Injury
Limit Statistical Results
Table 5.
Heli-Basket Stability Indices
payload weight on the
probability of injury during takeoffs was not statistically significant (p =
0.0624). Analysis of the accelerations
seen on the heavier manikin (Large ADAM) and the small manikin (LOIS) indicates
that a smaller occupant has a slightly greater probability of injury than a
large occupant (p = 0.0498). The
statistical analysis also indicates that there is a greater probability of
injury during the landing than during takeoff (p = 0.0013). However, it should be noted that the injury
analysis for even the worst-case tests were well below the established injury
limits. Analysis of Circuit Flights: Data were also
extracted from each circuit for each test condition. Flying a complete circuit could take up to 18
minutes to complete depending on the speed of the test. The data from both the DAS and EGI were
converted to day-seconds and the DAS data were filtered at 20 Hz and reduced to
40 Hz, and the EGI data were again kept unfiltered and at the native sample
rate of 16 Hz. The trim and stability were
evaluated by examining the pitch and roll average angles and deviation or
oscillation in the Heli-Basket heading (Table 3) and by examining the test
videos. As one would expect, the greater the
velocity, the greater the magnitude of the trim angles. Note that although the roll angles change
with velocity, this is due to the fact that the Heli-Basket yaws during flight,
and as the velocity increases, the basket lags behind the helicopter resulting
in an apparent roll angle. The effects
of velocity, payload, and the flotation devices on the average pitch and roll
angles are illustrated in Figures 3 and 4 below. These figures indicate that as the velocity
increases to 100 knots, the pitch and roll angles increase to -52 and 51
degrees respectively. The figures also
indicate that the addition of the flotation devices to the low weight
conditions (LOIS alone) reduces the yaw during flight, thereby resulting in the
system increasing in pitch but decreasing in roll. Increasing the weight by the addition of the
Large ADAM and 600 lbs of ballast tends to reduce the magnitude of the pitch
and roll trim angle.
Examination of the data collected on the
Heli-Basket during the flights through the rectangular circuits indicates that
the Heli-Basket is extremely stable with a few exceptions (Figure 5). The Heading Oscillation in Table 5 shows the
relationship between the test conditions and the peak-to-peak oscillation angles. In general, the Heli-Basket had minimal
rotational oscillations during flight, with typical rotations limited to +/- 10
degrees in pitch and roll, and +/- 25 degrees in yaw (heading), with low
rotational rates typically less than 15 degrees per second. However, the lightweight configuration (a
single 103-lb manikin) with the flotation system produced greater rotations and
higher rates, especially at 20 knots. In
this configuration, the Heli-Basket headings varied by +/- 140 degrees (Figure
6) with rates up to 41 degrees per second (Figure 7). As the velocity increased to 40 knots, the
yaw oscillations decreased to +/- 50 degrees.
While there is no evidence that these rates can directly result in injury
to humans, the frequency of these oscillations (approximately 0.17 Hz, or 1 cycle
every 6 seconds as determined by FFTs), can result in motion sickness within a
few minutes of flight. In general,
increasing airspeed, payload weight, or removal of the flotation system results
in a much more stable system.
Additional Considerations: Although a
variety of configurations were tested, it is never possible to examine every
variable. The flight testing focused on
the typical flight regimes envisioned for use with the Heli-Basket with
additional examination of high altitude flights that may be encountered during
mountain rescues. The flights were flown
by only three different pilots, in good weather conditions (sunny, low winds,
40-50 °F), which provided a limited variation in the effect of pilot
performance. But even with uncertainties
of pilot performance, the probability of injury due to accelerations was
extremely low. Summary:
Examination of the data
collected during the Heli-Basket testing conducted 10-12 January 2006 indicates
that there is a very low probability of injury to human occupants due to the
acceleration environment. Flights with
low payloads using the flotation system are less stable at low speeds, but
there is no evidence that this will pose additional risk to the occupants other
than possible motion sickness. Increase
in air velocities tended to make the system more stable and increase the
Heli-Basket’s pitch and yaw angles, especially at velocities greater than 60
knots. Although these higher angles
should not change the probability of injury, they may result in distress to
naive civilian rescuees. Higher
altitudes had no discernable effect on the stability of the system. The overall results of the testing of the
Heli-basket system indicate that there is minimal probability of injury to
human occupants. About the Author:
John Plaga is a research
aerospace engineer who has been with the Biomechanics Branch of the Human
Effectiveness Directorate, Air Force Research Laboratory for 17 years. He has been involved in escape system
research since his graduation from The Ohio State University in 1989. His research projects have included flow
stagnation concepts, windblast deflection studies, biomechanics of
helmet-mounted displays, development of ejection seat instrumentation systems,
studies of ejection seat dynamics, investigation of the Russian K-36 ejection
seat, investigation of the implications of women in combat aircraft, and effects
of downwash on pararescuemen. Disclaimer: The findings and conclusions in this
report/presentation have not been formally disseminated by the Air Force and
should not be construed to represent any agency determination or policy. |
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