The Evolution Of Energy Absorption Systems For Crashworthy Helicopter Seats 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 . S. P. Desjardins Safe, Inc
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.
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
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
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)
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
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.
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.)
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 CRASH LOAD ATTENUATOR CONCEPTS
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.
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
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: 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
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
OBJECTIVES, Cont’d • To Provide all occupants With Comparable Protection Regardless of Weight, 5th Percentile Female to 95th Percentile Male
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
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.
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.
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.