Building more crash-worthy composite planes and cockpits

Discussion in 'Composites' started by RSD, Sep 29, 2019.

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  1. Oct 1, 2019 #21

    Riggerrob

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    “ ..... It is no use the part that the restraints are attached to moving in a different direction than the tub that is holding the person, and likewise seat attachment points also need to be considered like this. For belts the consideration must be for a six point belt not just a four (or less) point belt. ....”

    The primary function of a lap belt is to keep your buttocks in the seat during negative Gs. Lap belts should be worn low and tight across your pelvis bones.
    Shoulder straps help prevent you from head-butting the instrument panel.
    Air bags are a secondary method of preventing you from bed-butting the instrument panel. I prefer the newer air bags installed on lap belts of some Airbuses and the newest Cessnas. They inflate away from your waist, filling the space between your face and the panel.
    A fifth or sixth (crotch) strap prevents lap belts from rising above your pelvis .... and prevent you from “submerging” under the instrument panel.

    If you worry about rapid egress, install quick-disconnect door/canopy hinges that can be released from both inside and outside the cockpit. Apply military-style labels near outside handles to simplify the work of fire-fighters trying to drag you from the wreckage.
    As for r canopy-breakers ... crash axes are nice but require strong arms and room to swing them. An alternative is the one-handed glass-breakers carried by first-responders.

    My expertise is limited to sewing a few seat-belts (FAA TSO C-22) and repairing hundreds of parachute harnesses.
    I have survived one forced landing in a Beechcraft King Air, but hope not to repeat that miserable process. Finally, I have lost far too many skydiving friends and colleagues when jump planes crashed.
     
  2. Oct 1, 2019 #22

    RSD

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    I guess that could be reworded to be

    Surviving the crash is heavily dependant on
    • impact speed and using crushable structures to decelerate impacts on the pilot
    • keeping the structural shape intact around the pilot
    • being properly restrained
    6000 lbs breaking force for seat belts seems about right - about 27 G's on a 100 kg / 220 lbs pilot

    - Wear a helmet - indeed
    - stop objects penetrating the cockpit - yep
    - keep the fuel in the fuel tank(s) - and no vapour spaces
    - crush structure under you and in front of you
    - don't impale yourself on the control stick sounds like a good argument for side sticks
     
  3. Oct 1, 2019 #23

    SVSUSteve

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    Actually, it had more to do with what the military was willing to design for. Even Stapp admitted in one of his papers that the limits of human tolerance to deceleration are far higher than people want to accept (>100 G). There’s a common sarcastic comment among those of us in the crash survivability research community that goes “We have 80-100 G people strapped to 26 G seats bolted into at best 10-15 G planes. Obviously there’s room for continued improvement”.

    Look at Hugh DeHaven’s survival from long falls paper (Google it). It’s an eye opener for most people in terms of what people can actually withstand. The comment by one of the victims is a masterpiece of understatement.
     
  4. Oct 1, 2019 #24

    SVSUSteve

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    Honestly 4130 tube fuselages fare better in real world crashes than room temperature cured composites. People forget (or choose to ignore) the wide gulf between the composite technology of INDY or F1 cars and what’s getting put into general aviation aircraft especially by homebuilders.

    Take a look at the pictures of the Cirrus that ran off the runway at Chapel Hill striking a tree at relatively low speed. Everything forward of the leading edge broke up killing one occupant and nearly doing the same to another. That’s the state of the art (in a practical sense) for GA composite aircraft.
     
  5. Oct 1, 2019 #25

    SVSUSteve

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    It’s not only the going under the instrument panel that’s the problem. The lap belt rides up and compresses the abdomen. Often you see catastrophic liver injuries (pulverized basically) or it raises the intraabdominal pressure so much that it blows out one of the sides of the diaphragm and shoves the stomach and/or intestines up into the chest cavity. Oddly enough that (called a traumatic diaphragmatic hernia) is not invariably lethal. There are a couple of cases in the medical literature where it was not immediately diagnosed.
     
  6. Oct 1, 2019 #26

    Vigilant1

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    That's state of the art for composite GA aircraft. But is it the limit of what can practically be accomplished (cockpit integrity wise) by a homebuilder using composites? That seems unlikely.
     
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  7. Oct 1, 2019 #27

    SVSUSteve

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    I would argue that a layperson probably cancel out any weight advantage offered by composites before you could achieve a comparable level of protection. Trust me... I would have much preferred to be able to layout a composite cockpit structure for my design instead of having to learn to weld. That said, there’s a lot of composite technology built in to help attenuate energy, etc.

    I don’t want to discourage advances but I just have to try to temper the urge to think it is going to be a revolutionary rather than an evolutionary sort of progress.
     
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  8. Oct 1, 2019 #28

    Hephaestus

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    Pretty sure Cirrus has big molds and autoclaves ;)

    We've seen some pretty impressive results from some hard impacts in a variety of homebuilt fiberglass ships too.

    Not to point a finger, but I'm glad Colin Hales little incident in Japan was survived, but **** - that's what I want to avoid personally.
    PAY-Colin-Hales.jpg
     
  9. Oct 1, 2019 #29

    RSD

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    Indeed. I'm hoping to find out more about this in a class that I am taking that starts next week, but my understanding is that a lot can be gained just by having the room climate controlled during layup and curing.

    Just took a look at that accident - the tree struck the left wing rather than the fuselage but the fuselage still broke up really badly - would have to really question how crashworthy the design is.
     
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  10. Oct 1, 2019 #30

    Hephaestus

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  11. Oct 1, 2019 #31

    BJC

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    Negative g flight should not be done with a single seat belt.

    A belt for negative g needs to be worn across the top of the legs, further down than the pelvis. A tight crotch strap helps during negative g by working with the lap belt to hold the legs tight without squeezing them together. Typical dual seat belts for aerobatic aircraft cover both the upper legs and the pelvis, but it is the lower belt and crotch strap that restrain under negative g.


    BJC
     
  12. Oct 1, 2019 #32

    Winginitt

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    RSD: I'm designing a new experimental plane that will use carbon fibre extensively in the fuselage and wings so I figure that I might as well design it with improved safety built in.

    Winginitt: There was a fellow who was building an airplane a few years ago that was being built with carbon fiber.
    He had an extensive racing background (Indycar ?). It was a really slick looking airplane and I believe he was incorporating some of his technical experience into making it safer. Somewhere near its completion I think financial problems ended his build. I can picture the carbon fiber airplane but can't recall any details. Might be worth a little searching on the net. It was a really nice airplane.

    RSD: For fire I see that as a two part equation -
    • using incombustible materials wherever possible
    • keeping the fuel in the fuel tanks and avoiding vapour spaces in the tanks through using tanks using similar construction to those found in motor racing.

    Winginitt: My one suggestion here is that you consider using aluminum or stainless tanks . Personally I do not trust
    any type of composite tank. Its not just that they may soften or disolve as fuel mixtures vary, but any minor defect can allow fuel to seep and weaken structure. It only takes a few pin holes and time.
    I always felt that a metal tank encased in Linex would be virtually bulletproof in a crash.



    Linex will burn if subjected to a flame, but if a tank doesn't rupture you probably have no flame, or much less flame and more time to escape.
     
  13. Oct 1, 2019 #33

    Dan Thomas

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    The typical aluminum tanks in GA aircraft wings will burst when the airplane hits the ground hard. The fuel is still moving forward as the airplane decellerates within a few inches, and the pressure surge against the tank walls blows it out and sends fuel everywhere. It peels the skins off the wings. A full tank is vapor-free, but that case makes the biggest mess due to the weight of fuel.

    You'd need a tank that refuses to burst, and it would need to be in an airframe cavity that cannot produce anything that would pierce it. That's a big job.
     
    Last edited: Oct 1, 2019
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  14. Oct 1, 2019 #34

    Dan Thomas

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    Seems to me that composites are much more flammable than aluminum or tubing covered with Poly-Fiber or some other flame-resistant system. There have been Cirrus fatalities due to the igniting of the airframe when the parachute rocket fires. In one famous midair, the parachute worked ok but the airplane was burning as it slowly descended. All occupants perished.
     
  15. Oct 2, 2019 #35

    Winginitt

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    I think that the best way to view the safety of composite vs aluminum for fuel tank use is to start with the idea that an aluminum tank does not have the problem of chemical destruction by fuel to contend with. Problems that occur over time are well documented for various composite fuel tanks. That gives aluminum an edge because it will not cause as many crashes.
    Once impact is imminent, the aluminum tanks have some ability to deform before rupturing.......but rupture is occasionally a result. A composite tank will most likely crack if deformed much. No one currently makes an unbreakable or impenetrable tank of any kind that I'm aware of. I think encapsulating an aluminum tank would go a long way toward maintaining the integrity of a fuel tank. It's an unproven speculation on my part, but testing a small composite tank and a small aluminum tank w/linex would be pretty easy. Just fill em with water and torture test them by dropping them from some height.......and again on rocky terrain.
     
  16. Oct 2, 2019 #36

    toyohabu

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    The US Army based on data from the Vietnam War has made great strides in survivability, the greatest killer identified was post crash fire. Here is a paper published in the early 80's that is a good summary of their findings.
     

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  17. Oct 2, 2019 #37

    Hephaestus

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    On the fuel tank question... Wouldn't a polyethylene tank make more sense?

    We've seen the videos of them being filled and dropped and survive. I don't quite understand the love of anything but a race or custom tank?
     
  18. Oct 2, 2019 #38

    billyvray

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    Jettisonable fuel tanks?
    It complicates things, but would keep you from burning up after surviving a crash.
    I recall Terrance O'Neill designed them into his Magnum Pickup bush aircraft - Mounted right below the cockpit as part of the aircraft (not like an obvious fuel pod I mean). I've love to have plans of that aircraft.
     
  19. Oct 2, 2019 #39

    Vigilant1

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    +1. Crosslinked polyethylene (PEX) is very strong, tough stuff. Standard tank sizes are available and not expensive.

    Breakaway fittings that stop the fuel flow are available, but not easy to find IIRC.
     
  20. Oct 2, 2019 #40

    SVSUSteve

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    I seem to recall that most of the major assembly parts of a Cirrus are room temp cured or simply put in a heated room but nothing like what happens with FIA or INDY approved cars (then again, some parts of an INDY car has 6 mm of carbon and another fabric to reduce the possibility of fragments flying as far). However, it's been quite a while since I did any poking around about Cirrus.

    The other point everyone forgets about homebuilts versus something like a Cirrus is that your average homebuilder is a rank amateur compared to the person designing a Part 23 composite bird.

    I've seen a few in remarkably good shape. Pressurized Lancairs, if they come down under some reasonable degree of control (and they don't catch fire as composite are want to do), hold up decently. I'd not want to bet my *** on one but they are pretty sturdy due to the pressurization.
     

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