Selectable Profile Energy Absorber System for Rotorcraft Troop Seats Stanley P. Desjardins, Dr. (h.c.) Principal Investigator CEO and Chief Engineer Safe, Inc. Tempe, AZ USA
Lance C. Labun, Ph.D. Research Engineer Principal Labun, LLC Tempe, AZ USA
Leda Belden Product Development Engineer Safe, Inc. Tempe, AZ USA
Jin Woodhouse U.S. Army Aviation Development DirectorateAviation Applied Technology Directorate Fort Eustis, VA USA
Abstract A selectable profile energy absorber (SPEA) system was developed for crashworthy helicopter troop seats. This system combines information from an on-seat sensor and the aircraft data bus to tailor the energy absorber force profile not only to the weight of the occupant, but also to the predicted crash severity. The development testing was conducted using a vertical seat orientation on a drop tower under the U.S. Army Aircrew Survivability Technologies Project. This second Transport Rotorcraft Airframe Crash Testbed (TRACT 2) presented the opportunity for Safe, Inc. to independently test the SPEA system on a troop seat in an airframe undergoing a combined vertical and forward crash impact. The SPEA system is designed to use the lowest possible EA force profile considering the available stroking distance. Despite a component failure, the SPEA delivered a peak lumbar force of 572 lbf in a 50th-percentile male ATD and stroked 10 of an available 12 inches. from the constant, or fixed load energy absorbers (FLEAs), selectable constant load EAs (currently referred to as variable load EAs (VLEAs), and fixed profile EAs (FPEAs).
Introduction The U.S. Army has an on-going goal of developing and maturing innovative and advanced crash protection technologies to provide optimum, full-spectrum crash protection for rotorcraft occupants with minimum effect on rotorcraft weight, cost, power, and operational capability. Current crashworthy designs of seating and restraints have been stretched beyond their performance limits, because many of these systems were designed and fielded decades ago for a narrower aircraft performance spectrum and a narrower flying population. The growing flying population currently in the military has expanded the range of anthropometry resulting in an increase in both the physical dimensions and the weight range that the seat energy absorbing system must accommodate. As part of the Aircraft Survivability Technologies (AST) Project conducted by the U.S. Army Aviation Development Directorate-Aviation Applied Technology Directorate (ADD-AATD) to develop a new generation of advanced crashworthiness technologies, a selectable profile energy absorber (SPEA) was developed by Safe, Inc. (Safe). The SPEA was previously referred to as the variable profile EA (VPEA). The name was changed to more accurately reflect the operation of the EA system. This seat energy absorber (EA) technology represents an advance
The stroking force for the fixed, constant-load EA is typically set to limit the deceleration of the occupant to a magnitude just under the spinal injury limit for the 50th-percentile occupant. The result is that this force is higher than desired for the lighter occupants and lower than desired for the heavier occupants. This problem is resolved by the VLEA technology. With this technology, the stroking force (force required to stroke the seat) is adjusted for the particular occupant based on the occupant’s weight. However, the compressive force within the spine is highly dynamic during the crash event (Ref. 1), and investigators in this field have realized that there is an opportunity to provide more protection (either lower peak forces, higher crash velocities, or reduced stroking distance) by exploiting the characteristics of this dynamic response. Applying a varying load as the seat strokes can be accomplished with either a FPEA which provides a single profile for all occupants and crash scenarios (analogous to the fixed constant-load EA) or by variable/selectable profiles. Each selectable profile is designed to provide maximum protection for a subset of the occupant weight range. Under the AST project, the load-stroke profiles were developed through iterative dynamic tests of a seat occupied by Anthropomorphic Test Devices (ATDs) and supported by dynamic computational modeling. In the SPEA system, various combinations of these profiles can be automatically
Presented at the AHS 71st Annual Forum, Virginia Beach, Virginia, May 5–7, 2015. Copyright © 2015 by the American Helicopter Society International, Inc. All rights reserved. 1
selected to make efficient use of the available seat stroking distance. The testing part of this technology development program was conducted with a troop seat equipped with EAs containing candidate profiles. Tests were conducted in three separate sets, with each set using new EAs which incorporated knowledge gained from the preceding tests. The laboratory testing showed that the technology could limit lumbar loads measured in ATDs to values well below tolerable levels in vertically directed crash pulses representative of crashes exceeding the current requirements.
modeling and dynamic seat testing were used in developing the technology. The SPEA system design and performance were described in the program final report (Ref. 5) and a summary was presented in a previous American Helicopter Society (AHS) paper (Ref. 6). In adjustable constant force EA systems (i.e., VLEAs), the EA force must be low enough to assure that the initial peak in the lumbar force does not exceed human tolerance. The profiles developed in the SPEA program lowered the rate of onset (i.e., how quickly the load reaches its peak). One consequence of the lower onset rate is that less dynamic overshoot occurs. Once this dynamic peak has passed, the EA force applied to the seat can be safely increased. These considerations are the basis of the profile EA concept. In this type of EA, the force is increased at a slower rate than the constant force EA, but then surpasses the force that can be safely applied by the constant force EA. Thus, ultimately, more energy is absorbed within an identical stroke distance. One potential benefit is to provide the same safety for the occupants as a conservative constant force EA, but to achieve that same protection level in a shorter stroking distance.
This paper reports the next step in the technology demonstration wherein, an SPEA equipped test seat was installed on the CH-46 aircraft during the second Transport Rotorcraft Airframe Crash Testbed (TRACT 2) full-scale crash test (Ref. 2) conducted by the NASA Langley Research Center at NASA Landing and Impact Research (LandIR) Facility, Hampton, Virginia on October 1, 2014. This paper presents an independent evaluation of the SPEA by Safe.
Selectable Profile Energy Absorber Technology The SPEA technology concept was developed to provide optimized crash protection across the entire occupant size/weight range and across an expanded range of crash impact velocities. The seat EA system is designed for future aircraft, which will be equipped with a predictive crash sensing system. The SPEA will use information from that sensing system to select an optimal EA setting for the predicted crash. Such an Active Crash Protection System (ACPS) is currently in development, also under the AST Project (Ref. 3) by ADD-AATD. In accomplishing the goal of this program, an energy absorbing system was developed that maximizes the use of the available stroking distance between the seat pan and the floor. The system takes as input the weight of the occupant to select an initial EA setting designed for a specified occupant weight and adjusts the seat EA to that setting soon after the occupant is seated. This initial profile would be selected on the basis of a ‘nominal’ or specified crash pulse, for example a test pulse in MIL-STD-85510 (Ref. 4). If during flight, the ACPS signals that a crash will occur, then the SPEA system uses the predicted vertical velocity of the crash together with the occupant’s weight to revise the EA setting. Thus, the EA force profile is tailored to both the occupant and to the crash. For a lower-than-nominal velocity crash, the deceleration force applied to the occupant is reduced from the initial setting to minimize trauma to the occupant. For a higherthan-nominal velocity crash, the force is increased to the level that will impose the least achievable trauma to the occupant that is consistent with the available stroking distance (seat will not hit the floor to cause a force spike in the spine). A series of energy absorbing load-stroke profiles were developed that could be used separately, or in combinations, to absorb the maximum amount of energy possible, while not exceeding lumbar tolerance limits. The system selects a load-stroke profile to provide the energy absorption needed to decelerate the occupant using nearly all of the available stroking distance for all crashes, thus minimizing trauma. To meet program goals, computational
The SPEA system absorbs impact energy by deforming up to five selectable EA components. Each EA component is fabricated to deliver a specific force profile as the seat strokes. One EA component is permanently engaged (base EA) and four are selectable. By selecting different combinations of these four selectable EAs and the base EA, 16 different force-stroke profiles can be created (Figure 1). These profiles were developed by conducting dynamic tests using a 5th-percentile female ATD, a 50th-percentile male ATD, and a 95th-percentile male ATD. In an iterative series of seat drop tests using a seat crash impact pulse with a higher velocity change than specified in MIL-S-85510 (Ref. 4), the load-stroke profiles were optimized to absorb a maximum amount of crash impact energy within 12 inches of seat stroke, while limiting lumbar forces to magnitudes below the Injury Assessment Reference Values (IARVs) (Table 1) throughout the seat stroking duration (Figure 2). These three load-stroke profiles were then used to create the family of profiles for the five component EAs. The series of sixteen (16) EA load-stroke profiles were generated by selecting different combinations of these five component EAs, such that adding the force developed by deforming each selected component EA generates the desired composite profile. The series of profiles covers the range of occupants from 5th-percentile female to 95th-percentile male for a range of crash impact velocities (vertical) from 30.2 to 54.7 feet per second (fps). This range of crash velocities represents a spread from 50 to 164 percent of the crash impact energy corresponding to the 42.7 fps requirement in MIL-S-85510.
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Test Seat During the SPEA development effort, a test-bed troop seat was designed and fabricated to perform the iterative testing required to develop the profiles. This seat, shown in Figure 3, incorporates a combination floor-ceiling mounting scheme. The EA system consists of two crushable aluminum tubes having the same force-stroke profile. The two EA tubes add to create the total EA force. The profiles are created by varying the outer diameter of the tubes along their lengths. The profiled tubes are located between pairs of lined rollers that rotate on fixed steel axles. The inertial loading of the occupant and seat mass draws the EA tubes through the rollers, crushing the tubes and allowing the stroking part of the seat to move relative to the seat frame. A portion of the occupant and seat kinetic energy is absorbed by the tube-crushing process, reducing, or limiting, the loads transmitted to the occupant.
Figure 1. EA Force-Stroke Profiles.
The stroking part of the seat rides on guide tubes using three custom acetal bearings located between each of the outer seat bucket frame tubes and the two guide tubes to reduce friction and prevent binding during the stroke.
Upper Stationary Cross Member
Figure 2. Measured Lumbar Force Compared to IARVs. Lower Stroking Cross Member
Table 1. Injury Assessment Reference Values (IARVs) – Hybrid III ATD.
Source Safe, Inc. JSSG2010-7
5thpercentile Female, lb 940
50thpercentile Female, lb 1,150
50thpercentile Male, lb 1,400
95thpercentile Male, lb 1,760
1,281
NA
2,065
2,534
The IARVs (axial lumbar load tolerance limits) used for the referenced effort (Ref. 5,6) and presented in the Table 1 above are identified by Source as Safe, Inc. The Joint Services Survival Specification Guide (JSSG-2010-7) (Ref. 7) limits are also shown in Table 1. Safe developed the spinal tolerance limits independently using the actual injury history of an energy absorbing seat model (i.e., UH-60 helicopter crew seat) that had experienced hundreds of crashes. These tolerance limits are more conservative than those generally accepted, but felt to be safer and more appropriate than the JSSG recommended values.
Figure 3. SPEA Test Troop Seat.
Test Following the completion of the SPEA development effort, Safe was provided an opportunity to independently demonstrate the SPEA system as a participant in the TRACT 2 (Ref. 2) full-scale crash test as previously mentioned. For this full-scale system test, the previously tested troop seat was refurbished and fitted with a 3
test-specific profile EA system. The load-stroke profile used on the test seat was the profile that the SPEA algorithm would have selected based on occupant weight and the crash pulse predicted for the test. The only difference was that on the test seat the profile force was generated by two identical tubes rather than a combination of different tube profiles. The test troop seat is shown (Figure 4, with ATD in green flight suit) installed in the test bed airframe. The seat is floor-ceiling mounted with all loads in the Z direction reacted by the ceiling structure and loads in X and Y directions reacted by the floor and ceiling. The seat was occupied by an Aerospace Hybrid III 50th-percentile male ATD, clothed in light-weight flight clothes. The performance of the troop seat with SPEA was evaluated using the instrumentation in the ATD combined with data from the seat interface to the airframe and from sensors on the airframe itself.
longitudinally and laterally (X and Y directions). The accelerometer mounted on the floor fixture of the seat recorded a severe, short duration vertical (Z direction) pulse. This pulse exceeded 120 G. The ceiling accelerometer recorded a far lower vertical acceleration (~30 G peak). The Safe seat test was identified as Experiment 11, and the two accelerometers nearest to Experiment 11 were checked for confirmation of the floor accelerometer (Figures 6 and 7). Accelerometer #28 mounted on the floor and accelerometer #29 mounted nearby on the right sidewall were consistent with the ceiling accelerometer. Although the sidewall sensor showed a second peak of ~40 G, it did not confirm the 120 G recorded by the floor sensor under the experiment. After reviewing the mount of the seat floor sensor, it was concluded that this sensor could have moved or been impacted by elastic motion of the airframe causing the brief, high impulse. The following discussion is based on the vertical input from the ceiling accelerometer.
Figure 4. Troop Seat Mounted in TRACT 2 Airframe. The drop test vehicle was a CH-46E Sea Knight airframe which had been modified with several experiments, including several different subfloor structures, seat designs, restraint designs, and advanced biofidelic ATDs. The pulse was intended to be ‘severe yet survivable.’ The test plan called for a horizontal impact velocity of 35 fps and a vertical impact velocity of 26 fps. The aircraft attitude was planned to be 2° pitch up, 0° yaw, and 0° roll. The impact surface was flat earth consisting of soil classified as silty sand. The impact pulse was measured by accelerometers on the floor below the experiment and in the ceiling above, in addition to the accelerometers listed in the TRACT 2 test plan, that were mounted at various locations on the airframe.
Figure 5. Video Camera View of Seat While Stroking.
Crash Pulse The pulse recorded by the ceiling accelerometer on the SPEA experiment showed a rapid rise to 22 G in the vertical direction (Figure 6) and subsequently peaked at 25 G. The peak horizontal (X direction) pulse exceeded 50 G, as recorded at the floor, but only 18 G, as recorded at the ceiling level.
Test Results Despite a component failure (seat pan attach brackets), the seat functioned as intended. One video view of the experiment (cropped video image, Figure 5) was recorded and all of the data channels, except the pan accelerations in X and Z directions, recorded usable data. The seat was suspended from the ceiling; the floor attachment fixtures allowed vertical movement, but located the seat 4
Figure 6. Vertical Acceleration Pulse near Seat.
Figure 8. EA Forces and Seat Stroke.
Figure 7. Longitudinal Acceleration Pulse near Seat.
Component Failure
Figure 9. Front of Seat Pan Tilted due to Failure of Pan Brackets.
Both the left and right brackets connecting the seat pan to the seat linear bearings failed during the test. The force data for the left EA shows a sharp drop at 0.1065 s from time zero (Figure 8). This failure of the seat bracket can be seen in the video between frames with times of 32.224 s and 32.226 s. The failure allowed the seat pan to pivot about the point where the seat pan support straps are attached to the seat pan frame tube (Figure 9). The forward end of the seat pan (see line of rivet heads in the tube) tilted downward and the rear section of the tube pivoted upward out of sight. The support straps to the mid-point of the longitudinal seat pan frame tube held, and the deceleration forces from the EAs were effectively transmitted to the occupant. The load in the left EA dropped and then recovered before falling off again ahead of the right EA force. The video clearly shows that the left seat pan frame tube rotated about the suspension point with the forward corner moving downward and the rear corner moving upward. Although the right bracket was also found to have failed following the test, there is no indication of the failure in the right EA force data. Because the video view is of the left side of the seat, the failure on the right tube cannot be seen in the video.
This test seat was one of the two test seats used in developing the profiles as part of the ADD-AATD-funded effort to develop the SPEA concept. The seat had been drop tested six times during the developmental effort; however, the seat pan attach bracket failures were unexpected. The failures may have been the result of the prior testing and/or the combined loading experienced in this test. The failures were brittle failures reinforcing crash safety equipment design principles regarding the use of brittle materials regardless of strength. The failed brackets were fabricated from AL-7068 T76511.
Seat Performance – Spinal Protection The EA force profiles developed by the EA components on the test seat were about 20 percent lower than intended along the entire stroking distance (Figure 10), although they provided the expected shape (except for the initial break-out spike).
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Two other lumbar injury criteria were evaluated: acceleration duration in excess of 23 G and the Dynamic Response Index (DRI). The seat pan acceleration data were not usable; consequently, the duration of seat pan acceleration exceeding 23 G could not be determined. As a substitute, the duration of pelvis acceleration exceeding 23 G was found to be 17.5 ms, significantly less than the 25 ms guideline used for seat pan acceleration. The DRI (Ref. 8) was also calculated using pelvis acceleration due to the lack of seat pan acceleration data. The peak DRI value (Figure 12) of 27.5 exceeds the commonly accepted guideline of 18. This guideline is based on ejection seat operational data with DRI values calculated from seat pan acceleration rather than pelvis acceleration. The DRI result, which would suggest a high risk of injury, is inconsistent with the other two parameters, lumbar force and acceleration duration.
Figure 10. EA Force Profile with Seat Stroke. The SPEA system limited the peak lumbar force to 572 lbf (refer to Figure 11), which is well below the FAA criterion of 1,500 lbf (and the IARV for the 50th-percentile ATD presented in Table 1). From the time plot, it can be seen that the first lumbar force peak lags the initial impact pulse peak, as is typical. In seats with constant force EAs, the first lumbar peak is typically the highest, and assuming that this lumbar peak is below the human tolerance determines the EA force that can be applied since the force is constant through the entire stroke. With the SPEA system, the force profile was designed to limit the initial force peak to minimize the dynamic overshoot and further along in the stroke, the profile force rises after the dynamic overshoot has been managed. In this particular case, the second and third lumbar peaks are higher than the initial peak, but they remain well below the lumbar tolerance for this occupant. The rise in lumbar force is due to the EA force profile applying a greater deceleration force as the seat strokes further (Figures 10 and 11). The SPEA system is designed to apply the lowest possible deceleration force compatible with the available stroking distance. In this case, the seat stroked a peak of 9.9 inches with a final stroke position of 8.9 in (Figure 11) while 12 inches of stroke were available. The low value for the maximum lumbar force indicates a very low probability of injury.
Figure 12. Pelvis Acceleration and DRI. The graph of lumbar force plotted against seat stroke (Figure 11) demonstrates how the increased EA force in the latter part of the stroke is reflected by increasing lumbar loads. This behavior is very different from the response observed with a constant force EA, but because the lumbar force is much lower than tolerance even near the end of the stroke, the likelihood of a spinal injury is very low. Force profile EA systems were initially developed using dynamic modeling studies (Ref. 1). Crash simulation software such as Seat Occupant Model-Light Aircraft (SOM-LA) (Ref. 9) were used to predict the occupant response to EAs with a variety of force profiles. One insight from these studies was that for constant force (also known as ‘fixed load’) EAs, the predicted values for lumbar force and DRI correlated well; that is to say, an EA that delivered an acceptable peak lumbar force also delivered an acceptable peak DRI value. However, once the EA force was varied as a function of stroke, the acceptability of the lumbar force and DRI values were increasingly divergent. In some cases, a profile change that caused the peak lumbar force to decrease caused the peak DRI to increase, while a different profile change that caused the lumbar force to increase caused the DRI to decrease. The DRI criterion models the human torso and head as a single mass supported above the
Figure 11. Lumbar Force Inverted & Half of EA Force.
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pelvis by a single spring-damper system with the natural frequency of the human body. DRI was originally developed for analyzing ejection seat outcomes, where the pulse input is generally trapezoidal in shape, and thus, is similar to a fixed force EA. The guideline for non-injurious values of DRI was determined by calculating the DRI experienced by the occupants in operational (real world) ejections and then comparing the calculated DRI with the actual outcome of the event (i.e., injured or not injured). The DRI is the only lumbar criterion based on living human outcomes, but the applicability of the DRI depends on the limitations of the simplified model and the similarity of the EA input pulses to the pulses experienced in ejections. Consequently, the usefulness of the DRI in judging the probability of spinal injury in scenarios other than those produced by ejection seats, such as crashes and mine blasts, is suspect.
reference force and each moment against a reference moment. Four neck load scenarios are created: NTE for tension-extension, NTF for tension-flexion, NCE for compression-extension, and NCF for compression flexion. The criteria are constructed such that each Nij value must be less than 1.0. In all cases, the values measured in the neck were less than 1.0 and thus, were within the criteria (Table 2), indicating a low probability of injury to the neck. Table 2. Neck Injury Criteria. Neck Injury Condition NTE tension-extension NTF tension-flexion NCE compressionextension NCF compression flexion
Seat Performance – Other Potential Injuries Although the primary objective of the SPEA system is to minimize the potential for spinal injury by minimizing the peak lumbar force, other potential human injuries were considered. These include head injury, neck injury, and torso injury.
Value from Data 0.446 0.462 0.263
Injury Assessment Value 1.0 1.0 1.0
0.278
1.0
The chest acceleration was measured using an accelerometer on the standard thorax mount. Because the chest of the ATD was not expected to impact nor be impacted by any structure, the chest deflection sensor was not operational. The automotive criterion states that the resultant acceleration in all three axes may not exceed 60 G for more than 3 ms (Ref. 11). In evaluating the data from the test, the resultant was determined at each time interval in the data file. The extreme value of this resultant was 32.0 G, which is well below the 60 G limit.
From the video, it was evident that the head struck either the hand or the leg. This impact is reflected in the head acceleration data by a sharp spike in the data for the X direction (Figure 13). Consequently, it is valid to calculate a value for the Head Impact Criterion (HIC). The head acceleration data were used to calculate a value for Head Impact Criteria. The value calculated for HIC is 284, well below the Injury Assessment Reference Value of 700 for HIC15.
The test results for the troop seat equipped with the SPEA system indicated that the head, neck, and chest IARVs were not exceeded. As a result, it is unlikely that the occupant would have incurred injuries in these body regions.
Concluding Remarks In this full-scale aircraft crash test, the SPEA system delivered the low lumbar force that was intended. The concept shows great promise in being able to reduce spinal injuries while expanding the severity of the survivable crash environment. This improvement is a result of tailoring the EA force-displacement characteristic to maximize the efficiency of the energy absorbing stroke. The increased efficiency is partially accomplished by design of the profiles to minimize initial dynamic overshoot which enables increasing the EA loads over the majority of the seat stroke. When used in its fully selectable configuration, the SPEA can potentially provide comparable protection across the expanded anthropometry of the current seat occupant population, a goal not achieved in any current EA seating systems. The SPEA system can potentially be applied to a range of seat designs either crew seats or troop seats. Additional effort is required to understand why the EA force profile so successfully lowers the peak lumbar force and yet results in higher DRI values.
Figure 13. Head Accelerations. To evaluate the potential for injury in the neck, the criteria from FMVSS 208 (Ref. 10) were used. The data from the neck load cells are evaluated against four criteria by comparing the extreme value in each of four loading conditions to an injury reference value: neck compression force, neck tension force, neck flexion moment, and neck extension moment. Each force is compared against a 7
Author contact: 6
Labun, L.C., Desjardins, S.P., “Variable Profile Energy Absorbers,” Paper E5 AHS-HRC National Technical Specialists’ Meeting on Rotorcraft Structures and Survivability, 29-31 October 2013. 7 Crew Systems – Crash Protection Handbook, Joint Service Specification Guide, Department of Defense, JSSG2010-7, October, 1998.
Stan Desjardins,
[email protected] Lance C. Labun,
[email protected] Leda Belden,
[email protected] Jin Woodhouse, U.S. Army ADD-AATD
Acknowledgments
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Stech, E.L., Payne, P.R., “Dynamic Models of the Human Body,” Aeromedical Research Lab Technical Report 66 157, AD 740439, November 1969.
The Safe authors acknowledge the support of U.S. Army Aviation Applied Technology Directorate for funding the work performed under contract W911W6-10-C-0032. All of the authors thank Mr. Lindley Bark, Naval Air Systems Command 4.6.6.2 and Martin Annett, Structural Dynamics Branch, NASA Langley Research Center, for enabling the inclusion of the Safe seat experiment in the TRACT2 test. Additionally, Mr. Bark and personnel at Naval Air Systems Command 4.6.6.2 provided the instrumented ATD used in the test and determined the supported weight of the seated ATD. Mr. Bark further assisted in post-processing and delivering the test data and video to Safe.
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Laananen, D.H., Bolukbasi, A.O., Coltman, J.W., “Computer Simulation of an Aircraft Seat and Occupant in a Crash Environment,” TR-81415 and TR-82401, Simula, Inc., DOT/FAA/CT-83/33-I & II, Federal Aviation Technical Center, Atlantic City, NJ. March 1983. 10
Croft, A.C., Herring, P., Freeman, M.D., “The Neck Injury Criterion: Future Considerations,” Accident Analysis and Prevention, Vol. 34, 2002, pp 247-255. 11
Approved for public release; distribution unlimited. Review completed by AMRDEC Public Affairs Office 4 March 2015 (PR #1572)
References
1
Labun, L.C., “Development of an Automatic Energy Absorber System for Crashworthy Helicopter Seats,” TR97256, Simula Safety Systems, Inc., Tempe, AZ; Naval Air Warfare Center, Aircraft Division, Patuxent River, MD, 1998. 2
Annett, M., Little, J., “Evaluation of the Transport Rotorcraft Airframe Crash Testbed (TRACT) Full Scale Crash Test,” Paper D1 AHS-HRC National Technical Specialists’ Meeting on Rotorcraft Structures and Survivability, Williamsburg, VA. October, 2013. 3
Bolukbasi, A., Birchette T., Schaub, J., Moody, M., Fisher, R., Powell, L.A., Choi, Y.T., Hu W., Wereley, N.M., Woodhouse, J., “Active Crash Protection System Enhancements,” AHS International Technical Specialists’ Meeting on Rotorcraft Structures and Survivability, Williamsburg, Virginia. October 2013. 4
MIL-STD-85510, “Seats, Helicopter Cabin, Crashworthy, General Specification For,” November 1981. 5
Desjardins, S.P., Labun, L.C., Pinger, A., “Variable Profile Energy Absorber Development,” RDECOM TR 14-D-01, 2013.
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49 Code of Federal Regulations 571.208.
