1. 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 .

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

3. APPROACH Review Present 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

4. NEED FOR CRASHWORTHY SEATS
Established in the Late 1950’s and Early 1960’s 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

5. NEED FOR CRASHWORTHY SEATS, Cont’d
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)

6. DECELERATION – TIME RELATIONSHIPS, Z DIRECTION

7. DECELERATION – TIME RELATIONSHIPS

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

9. 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 = sec.
GL = 14.5 G
k = 14.5/48 = 0.30
Then from the above equation :
S = in.

10. 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 in.)

11. 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

12. FIXED LOAD ENERGY ABSORBERS (FLEA)

13. DYNAMIC OVERSHOOT

14. 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.

15. UH-60 BLACK HAWK ARMORED CREWSEAT, INVERSION TUBE E/A

16. EH101 FOLDABLE TROOP SEAT, WIRE BENDER E/A

17. BELL 230/430 PILOT SEAT, CRUSHABLE COMPOSITE COLUMN E/A

18. FRENCH/GERMAN TIGER ARMORED CREWSEAT, METAL CUTTER E/A

19. A129 ITALIAN ARMORED CREWSEAT, TUBE AND DIE E/A

20. BELL 230/305 MEDICAL ATTENDANT SEAT,STRAP BENDER E/A

21. V-22 OSPREY TROOP SEAT, TUBE AND DIE E/A

22. VARIABLE LOAD ENERGY ABSORBERS
Fixed Load System is Designed for the 50th Percentile Occupant
Effective Weight of the Lightly Clad 50th Percentile Occupant is 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

23. VARIABLE LOAD ENERGY ABSORBERS, Cont’d
Assuming the Same 60 lb Movable Seat Weight, the Total Effective Weight Range that the Load Limiting System Must Decelerate are:
5th- percentile: lb
95th -percentile: 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

24. VARIABLE LOAD E/A ADJUSTMENT RANGE

25. V-22 OSPREY ARMORED CREWSEAT, WIRE BENDER VLEA

26. UH-1Y ARMORED CREWSEAT, INVERSION TUBE VLEA

27. FIXED PROFILE ENERGY ABSORBERS (FPEA)

28. BELL 230/260 PILOT SEAT, STRAP BENDER, FPEA

29. LOAD-STROKE PROFILE VS CONSTANT LOAD, MILITARY REQUIREMENTS

30. UH-1Y TROOP SEAT, WIRE BENDER, FPEA

31. 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

32. OBJECTIVES, Cont’d
To Provide all occupants With Comparable Protection Regardless of Weight, 5th Percentile Female to 95th Percentile Male

33. CONCLUSIONS
The Following Concepts Suggested in the Late1960’s and Early 1970’s 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

34. CONCLUSIONS, Cont’d 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.

35. CONCLUSIONS, Cont’d
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.

36. COMPARISON OF FIXED PROFILE SHAPES

37. CONCLUSIONS, Cont’d
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.