The Evolution Of Energy Absorption Systems For Crashworthy Helicopter Seats

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The Evolution Of Energy Absorption Systems For
Crashworthy Helicopter Seats
S. P. Desjardins
Safe, Inc

• The Fourth Aircraft Fire and Cabin Safety
  Conference
• Lisbon, Portugal
• November 15-18, 2004
• Originally Presented at the
• AHS 59th Annual Forum and Technology Display
• May 6 8, 2003
• Phoenix, Arizona
• .

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OBJECTIVE AND PURPOSE

• Objective Trace Development of Energy Absorbing
  Systems Early 1960s to Present.
• Purpose Assess the Current State-of-the-Art,
  Identify Any Areas of Concern, and Recommend
  Future Efforts.

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APPROACH

• Review
• Early Work and Concepts
• Process and Rationale That Lead to the Different
  Approaches
• Approaches Used by the Different Suppliers
• Present
• Advanced Concept
• Concerns About Current Requirements
• Conclusions

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NEED FOR CRASHWORTHY SEATS

• Established in the Late 1950s and Early 1960s
  by AvCIR
• Survivable Crash Environment was Determined
• Concluded that a Properly Restrained Occupant
  Could Survive the Resultant Loading in the X and
  Y Directions, but not in the Z

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NEED FOR CRASHWORTHY SEATS, Contd

• Loading in the Z Direction Exceeded Human
  Tolerance and Needed to be Limited
• Approach Support the Occupant in a Seat that
  would Stroke when the Load Reached the Tolerance
  Limit (Limit Load)

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DECELERATION TIME RELATIONSHIPS, Z DIRECTION
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DECELERATION TIME RELATIONSHIPS
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IDEALIZED RELATIONSHIP

• Where
• S stroke or deformation, in.
• G gravitational constant (32.2 ft/sec2 or 386.4
  in. /sec2)
• tm time to Gm, sec.
• Gm Maximum deceleration, G
• GL Limit-load deceleration, G
• k constant GL/Gm

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SEAT STROKE CALCULATION

• As an example, consider a triangular pulse
  representing a change in velocity of 42 ft/ per
  sec. with
• Gm 48 G
• Tm 0.027 sec.
• GL 14.5 G
• k 14.5/48 0.30
• Then from the above equation
• S 11.02 in.

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AIRFRAME STROKE CALCULATION

• Where
• S Stroke or distance traveled, ft.
• V0 Initial velocity, ft/sec.
• Vf Final velocity, ft/sec.
• g 32.2 ft/sec.2
• G Average deceleration of airframe, 14.5 G
• S 1.89 ft. (or 22.67 in.)

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CRASH LOAD ATTENUATOR CONCEPTS

• Crushable Column
• Rolling Torus
• Inversion Tube
• Cutting or Slitting
• Tube and Die
• Rolling/Flattening a Tube
• Strap, Rod, or Wire Bender
• Wire-Through-Platen

• Deformable Links
• Elongation of Tube, Strap, or Cable
• Tube Flaring
• Housed Coiled Cable
• Bar-Through-Die
• Hydraulic
• Pneumatic

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FIXED LOAD ENERGY ABSORBERS (FLEA)
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DYNAMIC OVERSHOOT
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FIXED LOAD DESIGN CRITERIA

• Human tolerance is a function of time-under-load.
• It was determined through analysis and test that
  to retain a tolerable time-under-load
  environment, the limit load, LL, should be set at
  14.5 G.

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UH-60 BLACK HAWK ARMORED CREWSEAT, INVERSION TUBE
E/A
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EH101 FOLDABLE TROOP SEAT, WIRE BENDER E/A
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BELL 230/430 PILOT SEAT, CRUSHABLE COMPOSITE
COLUMN E/A
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FRENCH/GERMAN TIGER ARMORED CREWSEAT, METAL
CUTTER E/A
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A129 ITALIAN ARMORED CREWSEAT, TUBE AND DIE E/A
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BELL 230/305 MEDICAL ATTENDANT SEAT,STRAP BENDER
E/A
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V-22 OSPREY TROOP SEAT, TUBE AND DIE E/A
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VARIABLE LOAD ENERGY ABSORBERS

• Fixed Load System is Designed for the 50th
  Percentile Occupant
• Effective Weight of the Lightly Clad 50th
  Percentile Occupant is 142.3 lb
• Assuming a 60-lb Movable Seat Weight, the Limit
  Load,LL, the Load at Which the Seat is Designed
  to Stroke is
• LL GL Wteff (14.5) (202.3) 2,933 lb

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VARIABLE LOAD ENERGY ABSORBERS, Contd

• Assuming the Same 60 lb Movable Seat Weight, the
  Total Effective Weight Range that the Load
  Limiting System Must Decelerate are
• 5th- percentile 172.6 lb
• 95th -percentile 235.2 lb
• With a Fixed Load Energy Absorber, the Resultant
  Load Factors for the 95th - and 5th - Percentile
  Aviators are then
• GL95th- 2,933/235.2 12.6 G
• GL5th 2,933/172.6 17.0 G

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VARIABLE LOAD E/A ADJUSTMENT RANGE
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V-22 OSPREY ARMORED CREWSEAT, WIRE BENDER VLEA
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UH-1Y ARMORED CREWSEAT, INVERSION TUBE VLEA
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FIXED PROFILE ENERGY ABSORBERS (FPEA)
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BELL 230/260 PILOT SEAT, STRAP BENDER, FPEA
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LOAD-STROKE PROFILE VS CONSTANT LOAD, MILITARY
REQUIREMENTS
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UH-1Y TROOP SEAT, WIRE BENDER, FPEA
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ADVANCED SYSTEMS

• OBJECTIVES
• To Combine the Advantages of the Fixed Profile
  (FPEA) with those of the Variable Load (VLEA) to
  Produce the Variable Profile Energy Absorber
  (VPEA)
• To Automatically Adjust the Load Level of the
  Profile to Eliminate the Possibility of Human
  Error in Selecting the Load

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OBJECTIVES, Contd

• To Provide all occupants With Comparable
  Protection Regardless of Weight, 5th Percentile
  Female to 95th Percentile Male

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CONCLUSIONS

• The Following Concepts Suggested in the
  Late1960s and Early 1970s for Use in Energy
  Absorbing Crashworthy Seats Have Been Developed,
  Incorporated into Seats and Are Now in Common Use
  Around the World
• Inversion Tube
• Wire Bender
• Strap Bender
• Metal Cutter
• Tube and Die

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CONCLUSIONS, Contd

• The Evolutionary Process Has Produced
• Fixed Load Energy Absorbers (FLEA)
• Variable Load Energy Absorbers (VLEA)
• Fixed Profile Energy Absorbers (FPEA)
• Variable Profile Energy Absorbers (VPEA)
• An Advanced Energy Absorber Concept (AEA)
• Equipped Seats Have Performed Well in Helicopter
  Crashes.

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CONCLUSIONS, Contd

• A problem Likely Exists With Certification
  Requirements for Civil Seats.
• Efforts to Improve Efficiency Have Lead to Use of
  Fixed Profile Energy Absorbers.
• Performance is Sensitive to Occupant Weight and
  Response Characteristics.
• Civil Certification Requires Testing With Only
  One Size of Dummy, the 50th Percentile.
• This Process Can Result in a Seat Tuned to the
  Characteristics of a Specific 50th Percentile
  Dummy with disregard for its Performance with all
  Occupants of Different Sizes or Response
  Characteristics.

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COMPARISON OF FIXED PROFILE SHAPES
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CONCLUSIONS, Contd

• Since Systems are Now Being Developed That Take
  Advantage of the Unique Response Characteristics
  of the Test Dummy,
• All Development and Certification Testing Should
  Include a Range of Dummy Sizes Representative of
  the Entire Spectrum Of Occupant Weights Expected
  to Use the Seat.
• Dummies Should be Developed and Used that More
  Accurately Simulate the Human Response to Rapid
  Loading in the Z Direction.