Crashworthiness

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by GESchwarz, Jan 30, 2009.

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  1. Jan 30, 2009 #1

    GESchwarz

    GESchwarz

    GESchwarz

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    I'd like to start a discussion on GA Crashworthiness. Although there is plenty of information out there on how to protect the occupants of a vehicle, particularly automobiles, I see less application in airplanes.

    Of particular interest to me right now is how protecting my back against a high vertical G crash. I have seen a military heilocopter seat that had a cushion that was about 9" deep. I thought that might do the trick. The upholstery foam would have to have just the right spring rate to hold firm at 1 G, yet be able to have sufficient compression all the way up to 9 Gs or so to protect the spine.

    Does anyone know what type of foam fits this requirement?

    My tandem seat/pilot in front design has the pilots rear 4.5" above the spar. That's more than a lot of designs out there, but I'd like more compression distance. I'm thinking that I could mount my seat on a swing arm that would cause the seat pan to travel in an arc that is forward and downward, ahead of the spar, allowing my rear a greater stopping distance in a high vertical G crash. The top of the seat would travel a vertical path. The seat belts will be 5-Point so that I don't "torpedo", or is it "submarine"?
     
    Last edited: Jan 31, 2009
  2. Jan 30, 2009 #2

    BBerson

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    Most military helicopters have a crash absorbing seat structure. The seat has an energy absorbing device to absorb the deceleration. The foam cushion is not designed to absorb crash energy. In fact a foam cushion can be worse than no foam because the pilot impacts the bottom of the seat just as the helicopter is bouncing back up.

    What you want is a material that can absorb the energy. Some of the seats have a "torshok", a device that deforms steel wires as it strokes. Otherwise, some type of crushable metal might work.
    BB

    torshok:http://www.stormingmedia.us/48/4802/A480240.html
     
    Last edited: Jan 30, 2009
  3. Jan 30, 2009 #3

    addaon

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    For vertical impacts, I can provide a small data point. I had a paraglider "incident" while wearing a harness with 7.5" of foam protection under my butt (I can look up the foam type if you're interested). A 1200 fpm decent onto packed dry dirt terrain, with no other protection and with landing almost entirely in the direction of my spinal column on the foam (no feet taking impact) lead to a single slightly cracked vertebra.

    (Dana discusses paraglider airbags next; while these are awesome, especially the pure ram air ones, I was on a classic foam harness.)
     
  4. Jan 30, 2009 #4

    Dana

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    Paraglider harnesses oftan have an "airbag", a nylon fabric bag filled with soft foam rubber (just enough to hold it open). It's not totally sealed, in the event of an impact the air escapes at such a rate as to control the decelaration.

    Others dispense with the foam, which is bulky (an issue when hiking up the hill) and use a ram air inflated airbag with a valve flap. This serves the same purpose, but doesn't give protection in the first few seconds after launch.
    [​IMG]

    -Dana

    New Yorkers like to boast that if you can survive in New York, you can survive anywhere. But if you can survive anywhere, why live in New York?
     
  5. Jan 30, 2009 #5

    Topaz

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    Yeah, I was reading through this and thinking the same thing as your first paragraph.

    What you need is any device or crushable object that provides a relatively even decelleration over a larger distance than a simple seat cushion could provide. Look up the maximum G-tolerance of the human body in that direction (I don't know it offhand, but it should be available from multiple sources). After that, it's just like designing landing gear - you have a 'design' vertical impact velocity, and you need to find the decelleration distance required to dissipate that velocity while keeping below the G-tolerance limit. Then some mechanism to smoothly apply a relatively even force over that distance so that there are no 'spikes' in the loading.

    I've seen deformable metal structures used for this (airliner seats are a prime example - the legs bend and collapse predictably), crushable foam or metal honeycomb (also commonly used for this purpose on planetary lander gear legs - the Apollo LEM was a good example), or actual shock absorber/dampers on a pivoting structure. Anything that will smoothly decellerate the load.

    It all sounds more complicated than it is - a several-inch-thick block of higher-density plastic foam fixed under a seat structure designed to fail a little ways below the vertical human G-tolerance load will go a long ways towards protecting your back in most real-world cases. Vertical impact loads are inherently unpredictable, so doing a highly-detailed analysis for a specific velocity case is a bit pointless. Better to just set a 'cutoff velocity' below which the structure doesn't come into play (won't be distorted by 'normal' hard landings) and then do the best you can with the available space for impacts more energetic than that.
     
  6. Jan 30, 2009 #6

    GESchwarz

    GESchwarz

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    Thanks for your valuable inputs. I see that I have some R&D ahead of me. It seems that any resistive method of extending the decelleration distance would be benificial, provided that the extension (compression) continues to the upper reaches of the spine G limit.
     
    Last edited: Jan 30, 2009
  7. Jan 31, 2009 #7

    Mad MAC

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    I think that energy adsorbing foam is probadly the most practical to improve the crash worthiness of homebuilts given the effort required made the seat base stroke somehow. Here is a interesting report of engery adsorbing foam as applied to gliders.
    http://www.streckenflug.at/shop/images/dynafoam_ostiv.pdf

    There is a good article on cockpit design here. http://www.ostiv.fai.org/CkptRoeg.pdf

    Robinson helicopters are a good example of crushable seat bases, made from sheet ally.

    As always there is various FAA documents (but those that refer to dynamic seats tend to point to SAE satndards that cost arm and a leg).
    For the installed seat belt geometry there is the AC21.34.
    Static strength of seats is covered by TSO-39a (it has a copy of the NAS809 standard in the back).
    AC25.562-1b appendix 3 covers some of what changes you can make to a seat without changing the dynamic performance (although most of it is of little value for homebuilts).
     
  8. Jan 31, 2009 #8

    Dana

    Dana

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    I was starting to type "how many airplane crashes actually involve a straight down impact anyway?", thinking the nose down crash is more likely. However, then it occurred to me that the only ailrplane crash I've witnessed involved just that (pancake after engine out on takeoff and subsequent stall) and something better than the Kolb sling seat would have prevented the back injuries the pilot suffered. That and my own crash in an old Quicksilver weightshift (broke my tailbone as well as the leg injuries, I was very lucky it wasn't worse) illustrate that such protection is a good idea. Something to think about for my new design.

    Still, the steep impact, and the common engine behind pilot design that seems the only option for my design, seems more of a dangerous issue.

    -Dana

    Dullard: someone who can open an encyclopedia or dictionary and only read what they'd planned to.
     
  9. Jan 31, 2009 #9

    Midniteoyl

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    Seems to me that most crashes are pancakes... the pilot usually is still trying to get the nose up when it impacts.
     
  10. Jan 31, 2009 #10

    GESchwarz

    GESchwarz

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    Excellent input everyone. Mad MAC, those reports are just what I've been looking for.

    The following crashworthiness and safety features are what I am designing into my plane. Can you think of any others?


    • Cockpit roll caged constructed of welded 4130 steel tube. This is the type of occupant protection provided in automotive racing. Because this structure is stronger than all that surrounds it, the surrounding structures will absorb a greater percentage of the energy as they breakaway in a violent crash.

    • Long-stroke, shock absorbing seats to reduce risk of back injury.

    • Long-stroke, trailing link landing gear of the type used on the Navy’s FA-18. This gear is designed for hard landings. More importantly, once you contact the ground, the shock struts prevent the aircraft from rebounding back into the air; the plane is on the ground to stay. Many fatalities have occurred after the aircraft has returned to the air at low speed in an uncontrolled manner.

    • Tricycle landing gear configuration.

    • Large trim-adjustable horizontal stabilizer for maximum authority for forward CG shift.

    • Five-Point Seatbelts.

    • Energy absorbing fuselage sub structure utilizing a low-wing configuration with extra deep spar, and a thick double-shear panel cockpit floor. Part of the engine coolant system has been located there which aids as a barrier and absorber of energy.

    • Anti-Stall/Spin features include vortex generating leading edge stabilizer gloves to prevent stalling of the horizontal stabilizer. Forwarded mounted twin vertical tails to prevent blanking by stabilizer.

    • Angle of Attack indicator to warn of approaching stall and to aid monitoring for adequate lift.

    • Heavy elevator control to provide feedback to pilot. Use of bobweight.

    • Harry Riblett’s GA37A318 airfoil and “Hershey Bar” wing planform for predictable, soft stalls.

    • Large Fowler type flaps to achieve low stall speed.

    • Mazda rotary engine, which has no mechanical failure modes that would cause an engine shutdown. Only the seals can fail, in which case only a minor reduction in power results.

    • Redundant fuel and ignition systems.

    • “Padded Cell” cockpit designed to eliminate sources of injury to the occupants in a crash.

    • Redundant quick release canopy mechanism.

    • Fuel tanks located away from the cockpit, behind the main wing spar, outboard of the wing center section.

    • Halon fire extinguisher inside cockpit.
     
  11. Jan 31, 2009 #11

    Topaz

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    A few things come to mind:
    1. Several of these items are going to be pretty heavy. Is the cockpit crash-cage also the actual structure for the forward fuselage? If it's separate, you're carrying around a lot of extra weight. Better to beef up the forward fuselage structure a bit, rather than make something totally redundant.
    2. Unless you're planning on rough-field operations, pilot training is a better solution for safe landings than a heavy trailing-link gear. If you plan to make hard, carrier-like landings, then that's another matter.
    3. "Energy absorbing fuselage sub structure utilizing a low-wing configuration with extra deep spar, and a thick double-shear panel cockpit floor. Part of the engine coolant system has been located there which aids as a barrier and absorber of energy." - Again, potentially heavy, and I'm not sure I'd want scalding-hot coolant spraying around the cockpit after a crash, either. Ouch!
    4. Wankle-type rotary engines do, indeed, have fewer mechanical failure modes, but the overall reliability is also a function of how well the engine has been developed and tested for aircraft use. Even though there's fewer things to go wrong, the rest of the systems on the motor might not be as well adapted to the aircraft environment, and might make the overall reliability lower than a dedicated aircraft engine. My personal thinking is that, if you want an 'ultimately reliable' aircraft engine, go with a certified, purpose-designed aircraft engine. They've got literally decades behind them, working out the bugs. A rotary won't have that development period accomplished.
    5. Twin vertical tails aren't going to help you much. Modern jet fighters use twin tails because they're capable of operating at very high angles of attack and twin tails help prevent the fuselage from blanketing the tails. You won't run into that. Twin tails have been discussed ad nauseum here. A search should turn up much of that discussion.
    6. Mounting the fuel tanks outboard of the center section will mean more cockpit management tasks in-flight, as you work to keep the fuel load balanced between the tanks. Failure to do so will result in a tendancy to bank in the direction of the more-full tank. Distracting the pilot with any additional task incrementally increases the chance of pilot error, so you'll need to balance the 'once-in-a-blue-moon' risk of inboard tanks against the 'every time you fly' risk of having them outboard.
    7. Make sure you have a very good ventilation system in the cockpit if you're going to include a halon extinguisher. Halon is an asphyxiant, and you'll be just as dead if you pass out after using it and lose control of the aircraft.
    Probably the best book I've seen on safety in aircraft design is David Thurston's Design for Safety. A very good reference. JAR-22 (the certification standards for gliders) also contains quite a bit of good information, since gliders tend to have a lot less structure around the pilot for crashworthiness. It may be somewhat redundant to the OSTIV report, however. Still might be worth a look.


    As for other things to think about, some things come to mind along the lines of keeping you out of a crash in the first place:
    1. Cockpit visibility. It's easier to hit what you can't see.
    2. Clearly marked, standardized, intuitive-function cockpit controls. Right down to the throttle and mixture controls, flap controls, etc. There are standardized designs and colors for just about everything. Designing intuitively-functioning controls is an art unto itself.
    3. Ample, easy access for all inspection plates and maintenance hatches. You'll be more likely to use them if they aren't a PITA to use. There's a hatch on our Callair A-9 towplanes at my soaring club that takes 24 screws to remove(!).
    4. In the same vein, stupidly-easy and ample visual access to all the stuff you'll want to inspect during preflight. You do preflight for a really good reason. Why make it hard on yourself?
    5. Pay careful attention during construction for wire- and cable-chaffing dangers. Vibration and a sharp corner will saw through even steel cable eventually. Electrical wire is even more fragile. This has brought down airliners.
    6. Good anti-collision lighting and a good transponder. Don't skimp. This is keeping you away from the other hard bits in the sky.
    7. The best L/D and minimum sink you can manage while still meeting your cruise performance goals. Every motor can fail. When it does, you're suddenly piloting my kind of airplane, like it or not, and you want the best possible one you can have. The only safe spot to put 'er down might be a few miles away, and you won't make it if you have the glide ratio of a brick. L/D and sink rate will also help with range and climb rate, respectively, so they're also giving you a plus in the bargain.
    Those come to mind immediately. More may follow.
     
    Last edited: Jan 31, 2009
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  12. Jan 31, 2009 #12

    GESchwarz

    GESchwarz

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    The crash cage is the structure just like the Mooney.
    The long travel trailing ling landing gear is designed to absorb 3G's, that's all, but the main thing is the shock absorber for NO REBOUND.
    Yes, the cockpit floor will be a little heavy, a sandwich of aluminum sheet and plywood. I don't expect a rupture where I'd get hit by hot water spray. The whole idea of the steel cage is to keep outside what is outside from coming inside.
    Rotary fuel and ignition systems by Tracy Crook have stood the test of time in the air.
    I'll look into the fuel management issue. I just don't want to burn if I don't have to.
    Do you have a better idea for a fire extinguisher? I would think that anything that would extinguish a fire may also extinguish me too. I was betting the I could get the fire out first, then pop open the canopy or whatever feels right at the time. Is there a Standard Operating Procedure for extinguishing inflight or post crash fire short of getting the heck out of there?
     
  13. Jan 31, 2009 #13

    Topaz

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    Land. Quickly. ;)

    A chemical powder extinguisher wouldn't be an asphyxiant, but I'm not sure I'd want to breath that stuff, either.

    Just provide some good ventilation and halon should be fine. They're going to be getting increasingly hard to find, however, so you might want to choose something that isn't going to be banned completely soon.

    You'll want good ventilation for sunny days, anyway. It's amazing how much like a greenhouse a cockpit canopy can be. As long as whatever ventilation source you have provides a smooth and significant flow of air through the cockpit, you should be okay. Popping a door is always an option, although popping a canopy usually leads to it departing the aircraft. It's hard to pilot an airplane with a two- or three-digit speed blast of air in your face and no goggles, so generally canopies aren't opened unless you plan on leaving the airplane Right Now.
     
  14. Jan 31, 2009 #14

    Norman

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    A few years ago a friend of mine told me about a plane crash he was in. After everything stopped moving he noticed that the windshield was several inches closer to his face. When he got out he could see that the seat was leaning forward. He was surprised that a home built had an effective energy absorbing seat support so he looked under the seat. This drawing shows what he described to me. It's just a length of C-channel attached to the structure at such an angle that in a crash the webbing will bend and absorb some energy. I have it at 90 degrees but it could be mounted at some other angle to absorb more vertical stress. Say you hit the ground at 45 degrees and 13 Gs. 13 at 45 breaks out to 90 vertical and 90 horizontal. With the webbing vertical most of the displacement during the bend is going to be horizontal so this won't be much of a back saver (and in fact my friend hurt his back)
     

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  15. Feb 1, 2009 #15

    GESchwarz

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    The more inches your seat can travel while exerting force against a resistive member while doing it, the better. The greater the distance, the less the G load on your spine. The key is to absorb only the greatest G spikes, the ones that will damage your back. With that being said, you wouldn't want to waste travel on G loads that are a lot less than what is dangerous. I think this is critical. So if your back is going to start getting hurt at say 8Gs, you wouldn't want to have spent all your travel absorbing 7Gs then when the full 8Gs hit, you've got nothing left for cushion.

    I'm no expert yet, but I will be before I'm through doing the R&D on this seat. And the nice thing about this forum is that we can bring each other up to speed on what we know. I think that calls for a smiley face. :)

    Thanks for the additional safety items to the list. Keep them coming!!!
     
  16. Feb 1, 2009 #16

    Dana

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    Hmmm. Crashworthiness is one thing (and a worthy goal, within limits); "safe" aerodynamics are another.

    The best approach is to fly a reasonably safe airplane and have and maintain the skills to fly it safely. Purpose-built "safety airplanes" (e.g. the Ercoupe) didn't have any better safety record than comparable conventional airplanes. The Cirrus has its wonderful parchute... and a poor safety record.

    The best way to survive a crash is not to crash!

    That said, you raise come good points. Designing the cockpit and forward fuselage structure to absorb impact is a good idea, if it can be done without adding excesive weight... an overweight aircraft is less safe. There are also little things... visualizing how structural members will move in a crash. You don't want to have the pilot enclosed within an invulnerable crash cage only to be impaled by a minor strut that buckles in just the right (or, rather, wrong) direction.

    Is a low wing truly safer? It could absorb energy... but you're more likely to catch a wingtip on a rock or bush, spinning you around. Also tanks in low wings are more likely to rupture in a landing on rough ground than tanks in a high wing.

    Tricycle gear... I'm prejudiced in favor of taildraggers, I know, but there's a good reason why all bush planes are taildraggers. Nose gears tend to be a weak spot, and when they don't fail, they take the firewall with them.

    "Anti-Stall/Spin features include vortex generating leading edge stabilizer gloves to prevent stalling of the horizontal stabilizer. Forwarded mounted twin vertical tails to prevent blanking by stabilizer"... is this really in issue in a well designed airplane?

    "Angle of Attack indicator to warn of approaching stall and to aid monitoring for adequate lift.
    Heavy elevator control to provide feedback to pilot. Use of bobweight."

    Heavy control forces can be fatiguing, resulting in decreased pilot efficiency. Not sure what you bean by "bobweight". AOA indicators and other stall warning devices are fine, but again, no substitute for knowing your airplane.

    "Harry Riblett’s GA37A318 airfoil and “Hershey Bar” wing planform for predictable, soft stalls.
    Large Fowler type flaps to achieve low stall speed."

    Optimizing for low speed performance is fine, but it of course comes at the cost of efficiency at higher speed. Complex flap mechanisms add weight and complesity (and additional failure modes).

    "Mazda rotary engine, which has no mechanical failure modes that would cause an engine shutdown. Only the seals can fail, in which case only a minor reduction in power results.
    Redundant fuel and ignition systems."

    ""No" failure modes? I can think of many failure modes (albeit unlikely) that would cause shutdown... crankshaft failure, gear failure, seizure, seal failure jamming a rotor, bearing failure, redrive failure... ANY mechanical system can (and will!) fail.

    Not meaning to criticize, just point out the limits of "safety" design.

    -Dana

    The man who would be fully employed should procure a ship or a woman, for no two things produce more trouble" - Plautus 254-184 B.C.
     
  17. Feb 1, 2009 #17

    wsimpso1

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    To start at the beginning of this thread, Oregon Aero did some research when they were designing a new seat for the Cirrus. They presented this stuff at OSH, Arlington, etc. They found that their visco-elastic foam cushion systems with the most rigid structures they can put together pass the 19-g requirement without a stroker seat. Their cushions pass the tests with the iron seats used for calibration... It is the production Cirrus seat, and they are offering it for sale as well. It is counterintuitive, but they found rigid structures plus their cushions work, and that even stroker seats were rebounding. So now they are recommending that we build the stiffest bases we can for their cushions.

    Design for Safety is a good starting point. Harry Riblett's airfoils and a design optimized around soft stall is a good idea. Having a good structure around the soft pink things is good. Visibility is always a plus. I don't know as low wings are really any better than high wings - they put an awful lot of stiff structure under your but where you could build an energy absorbing cushion before getting to the structure around the people.

    Don't you believe that any engine is without zero power failure modes - you still have to deliver air and fuel and spark and connect to the prop and cool the darn thing, so you still have plenty of ways to fail.

    I suspect that the most dangerous thing about airplanes is the nut behind the stick and an airplane with fatal levels of kinetic energy. Anything that gives your body more kinetic energy than you can generate without a machine or a gravity assist can kill you, and airplanes do both.

    I think that I am saying, keep it a pleasant airplane to fly. If you make it a pain to operate, it will be safe because it does not fly...

    Billski
     
  18. Feb 1, 2009 #18

    GESchwarz

    GESchwarz

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    Thanks for the critical and valuable feedback.

    Sorry for the confusion...the H tail is for a stowage height restriction, not for stal/spin safety.
    That's right, NO mechanical failure modes. The rotary has no mechanical failure modes that result in sudden or total power loss. They are bullet proof up to 10,000 RMP in automotive racing. I'll be loping along at a leisurely 5,000 RMP.
    I want the heavy elevator feedback so I can feel that I'm pulling. The plane is otherwise trimmed by rotation of the whole stabilizer via jack screw, also like the Mooney.
    Regarding the 318 airfoil, I'm thinking of having a cruise setting that will reflex all of the trailing surfaces a few degrees. What can I say, I'm a mechanism maniac. The thick airfoil is also to lend a bit of structural insurance to my wing fold hinge.
     
    Last edited: Feb 1, 2009
  19. Feb 1, 2009 #19

    Mad MAC

    Mad MAC

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    A few more points.
    If you are going to have large flaps then more power is required to reduce the risk of getting on the back side of the power curve at low speed.

    A separate baggage compartment to reduce the number of things to get you in the back of the head (fire extinguishers do carry some risk in this regard) or foul the controls .

    Are fire extinguishers actually of any in use in flight, I can't recall many cases of they use in flight at least in 2 seats. Post crash if there is a fire, one isn't normally sufficient is it?

    The control column should be designed to collapse when struck to prevent head injury & or a spearing though the gut.

    With anything that strokes care should be taken to ensure that when it gets to the end that it doesn't then cause a spike becasue it hit a sudden change in section.

    There is always breakaway valves for the any fluid systems but they do represent additional risk which is not always justified. Some of the single engine turbo props use jet pumps in a recirculation type system. These have multiple tanks but don't required a tank selector valve.

    If the stick force gradients are close to that in FAR 23 you should be about right any higher will make long flights & rough weather tiring which presents another set of risks.

    In the one day soon list (hopefully), are head up displays and the airbag harnesses.
     
  20. Feb 1, 2009 #20

    Norman

    Norman

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    Well metal dosen't dissipate energy very well until the strain is beyond the elastic limit but if you wanted to prevent movement until a certain amount of force built up you could put a fragile link in the structure that would be rigid up to it.s failure load. For instance in the C-channel seat support you could block open the C with a plywood sandwich with a glue joint of known shear strength. The only load on the ply splint would be from the C-channel trying to deform. So when you put some critical load on the C the splint breaks and then the metal part takes up the load.
     

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