Building more crash-worthy composite planes and cockpits

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

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  1. Oct 2, 2019 #41

    SVSUSteve

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    Once they started mandating helmets, post-crash fire was the next to be dealt with. I heard one of the senior Army aviation safety officers say that he can name off the top of his head every person who has died due to a post-crash fire associated with a crash within design limits. in US Army helos since he was commissioned (20+ years earlier). That's how he demonstrated what they achieved.

    It's a **** shame that we haven't put a fraction of the attention into addressing a major killer. Then again, in EAB we are still facing a certain number of people who think that a shoulder harness is overkill.
     
  2. Oct 2, 2019 #42

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    *facial tic*
     
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  3. Oct 2, 2019 #43

    SVSUSteve

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    That's not a problem with composites. That's a person who doesn't have the common sense to use an epoxy that will stand up to hydrocarbons and alcohol.


    I'm aware of literally only enough to count on both hands. Most of those where due to the builder not following the plans OR, less often, the designer specifying the wrong epoxy. The biggest problem with "composite fuel tanks" is that many composite aircraft have wet wings rather than a separate tank. It's a ***** and a half to make a crashworthy wet wing. In fact, short of putting a fuel bladder into the "tank"...I am going to bet it would be a nasty weight penalty to accomplish it.


    Citation please. I've seen just as many cases due to poorly designed or maintained metal tanks.

    The ironically named "Moronca" comes to mind.

    Usually they rupture because they deform. That either unzips the rivet line or it shears the weld apart. An aluminum tank is one big set of stress risers if it's going to fit in a wing BEHIND the main spar (which is the ONLY place other than tip tanks they belong in a modern aircraft without resorting to a racing fuel cell) because of the difficult of eliminating sharp corners in what is basically a big flat box.

    You ever seen the fuel tanks on a Blackhawk or an Apache? I've got a picture around here somewhere of one crash where the airframe was shredded but the tanks were still intact.

    Baffles (which effectively double as internal bracing does marvels for preventing a hydraulic rupture) or put a fuel bladder (more or less a civilian self-sealing fuel tank) and you'll drop the rate of primary tank failure significantly. Of course, that then shifts the problem to fuel lines pulling out (fix: breakaway valves which are available from racing supply stores) as the structure of the wing separates.

    The tank doesn't have to be "unbreakable" or "impenetrable". It just has to be tough enough to not fail before you're not going to survive the crash either. Anything beyond that is just making the local coroner's job a bit easier.

    Been there, done that. It's a matter of not simply buying "Rutan fabric and epoxy" from Aircraft Spruce and expecting it to be better. The aluminum tanks always blew out before or at the same level of force as the composite ones when we tortured tested a few samples I made. Even with the one we wrapped in ballistic nylon and then covered with fiberglass and Rhino Liner. That coating stopped .22 LR rounds point blank but it still leaked from several spots after the aluminum "core" inside ruptured.
     
  4. Oct 17, 2019 #44

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    If you want to survive a crash, the most important composites will be the one strapped to your head.

    Don't want to wear a helmet? Understand that your chances of dying go way, way up.

    There are lots and lots of racing fuel cells that are essentially "crashproof". Any of the ones with liners, especially.

    Filling regular fuel cells with anti slosh foam will also help prevent them from exploding on impact. The foam has a limited lifespan in fuel of maybe 20, 30 years. Run a clear filter and when you start to see bits of foam it's time to change them out.

    I do agree with the others here that crash prevention seems to be a bigger factor than anything else.. loss of thrust on takeoff seems to be the #1 killer in small airplanes. Do what you can to prevent that from happening.
     
  5. Oct 17, 2019 #45

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  6. Oct 17, 2019 #46

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    Having spent a great deal of time in rally cars and other race cars with 5pt belts, I can also say without a doubt that unless you include some sort of 5pt harness with inertia reels for the shoulder belts, that it's unlikely for the pilot to want to wear them on every flight, which makes them useless. Any safety feature must include pilot/passenger comfort in the design or it may as well not exist.

    Inertia-reel harnesses do not meet FIA or SFI regulations for race cars so I'm not sure if any are made, but maybe there are some for aviation?

    3pt belts are pretty effective but not anywhere near as good as 5/6pt ones.

    As Topaz wrote, that PDF book about small airplane crashworthiness is fantastic and is the reason why I only got 4 hours of sleep last night.
     
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  7. Oct 18, 2019 #47

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    You want the ones that meet the military crashworthiness standards. I think MIL-STD-180 and MIL-R-8236 are the applicable regs by the way. I believe the standard is still the MA-16. I think one of them is about 1.75 lbs if I remember correctly.

    Keep in mind that the AGATE guide (the manual you refer to) also cites artificially low human survivability thresholds for deceleration. It's due to badly flawed "well, no one survived so this must be above the limit of human survival" circular reasoning ignoring that the fact that people survive far higher decelerations (especially longitudinally) in auto racing nearly every weekend. The limitation is more often set by the seat and/or aircraft than by the victim.
     
  8. Oct 18, 2019 #48

    SVSUSteve

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    It's really kind of misleading to say "the most important" thing is this, that, or the other. It's like arguing that the wings are more important to stable flight than the tail. It's a system.

    If you don't have a survivable space (the cockpit doesn't crush you), the helmet isn't going to save you.

    If you don't have a crashworthy fuel system but the cockpit is strong enough, the helmet may keep you alive but just long enough to die from burns or toxic gases.

    If you have a crashworthy fuel system and a strong cockpit, but you're seat isn't bolted down securely enough or if you're restraints fail, then you're going to still smash your face into the instrument panel or your chest. Bouncing your chin off your chest is a major risk factor for aortic or cardiac rupture. It's also a reliable way to give yourself a fracture of the basilar skull (the bottom part where your spine and jaw attach). That's actually how Dale Earnhardt Sr died.
     
  9. Oct 18, 2019 #49

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    That is true.

    I survived a high-G crash (not sure on the math here... but 70MPH to 0 in about 3 feet in a rally car and hit the harness hard enough to turn my chest and shoulders purple. I had a WRC style helmet and a HANS device and my neck wasn't even sore after.

    Wearing a helmet and hans all the time in an airplane would severely limit visibility and comfort, but you'd be way safer.
     
  10. Oct 18, 2019 #50

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    Just wear the HANS during takeoffs and landings. That’s what I plan to do. Of course, I am designing my aircraft to have a place to stash my helmet and HANS. It might be less of an option if you have a really cramped cockpit.

    I’ve worn the newer ones and I didn’t think it limited my ability to turn my head to the side as much as people like to claim. Usually those folks are conflating the HANS and the seats in some race cars or have experienced a HANS with the straps going to the helmet set way too tight. You probably should not try to wear one in a dogfight but if one of us winds up in that situation things have gone horribly wrong.

    Plus, with ADS-B In and ADS-B Out being mandated, your odds of having no warning of a traffic conflict are going to be massively reduced. It’s pretty much the closest thing you’ll see to the FAA admitting that unaided “see and avoid” is at best a concept with massive drawbacks in actual practice. At worst, it’s an approach that gives a false sense of security to those who believe in it as a front line defense rather than the method of last resort that it should be. Now if we could just get everyone to make radio calls around uncontrolled airports.
     
    Last edited: Oct 18, 2019
  11. Nov 7, 2019 #51

    Scheny

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    The Cirrus homepage is only stating 26G seats, but it doesn't say that the cabin is also rated for 26G.

    As for the Diamond (DA40, DA42,...) I know that the cabin is rated for 26G and you only find pictures of either slight injures or unsurvivable ones.

    BR, Andreas
     
  12. Nov 7, 2019 #52

    Riggerrob

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    Good point!
    I survived a forced landing in a Beechcraft King Air, bu do not remember it. I remember looking at my altimeter rapidly descending through 500 feet. My vision greyed out and the next thing I remember was sitting in the ambulance. I remember everything that happened after the crash: ambulance ride, hospital, etc.
    A week later, the ambulance attendant told me that he walked me from the wreckage to the ambulance.
    That sort of amnesia was probably good for my long term mental health.

    My fear of airplane crashes has been replaced by an allergy to lawyers.
     
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  13. Nov 8, 2019 #53

    Winginitt

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    Agree with most of what you said, but personally I would never build or use a tank that has anything in it or in its construction that could ever begin to disolve. Fuel is an ever evolving mixture of variations. What works well today may be replaced tommorrow with something that isn't compatible with a composite tank or foam. As I have stated previously, I think the best option for survivable plane crashes is designing for lower speed controllability and lower speed landing.

    If you design for crash survivability after the crash you are trying to overcome an event that has been allowed to happen. If you design for being able to control an airplane at very slow speeds and lessen or prevent wing drop......you may prevent the event from ever happening. Without trying to drift, I would like to make a point and hope that others will just consider that point as its intended. In the past I have talked about the ability of the Slepcev Storch to perform a flat turn at very slow speed. The designer flatly states that this is one attribute of his design. So it is possible to design an airplane that has almost no tendency to drop a wing when flown or turned very slowly. Any airplane that does not drop one of its wings during very slow flight is a positive toward never having an event occur. I don't applaud doing things that way for no reason or to show off.....but its nice to know that if you screw up or have an engine out, you MAY maintain control until almost all of the forward energy is disippated and you are as near to the ground as you can get. If that is over a bunch of trees, the slow speed is still your best option.

    That doesn't mean that someone should not take steps to make the fuselage more survivable too, but there are limits to what you can do. A builder may build and modify based on calculation and theory, but they really have no good way to test the results. With aerodynamic upgrades and changes, all modifications can actually be tested for effectiveness without damage to the airplane.
    Most people like to fly their airplane for enjoyment. Being saddled with a helmet and a Hans device is not gonna be a desired option for most people. Where do you stow it and how do you remove it while flying ? If you wear it all the time it cuts down on your ability to see and hear other planes. What about headphones and your helmet? I think motorcycles are a good comparison.
    While the helmet does help a lot in a crash, it makes it much more difficult to be aware of your surroundings and maybe avoid the crash because you heard or saw something that prevented the crash. Avoidance is always preferable to just surviving............


    Note: I respect the other opinions stated here, but I feel safety would best be served by concentrating some of the effort into aerodynamic areas.Look at the main thrust of the automotive industry and you will see lots more effort directed toward avoiding accidents with early warning and self stopping cars.;)
     
    Last edited: Nov 8, 2019
  14. Nov 9, 2019 #54

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    I do not think the car safety argument is relevant.

    The car industry has spent since the 70's researching and developing secondary safety. The big efforts have made cars a great deal safer in a accident.

    Cars did not ignore primary safety at the same time. Nor should we. They should never be exclusive.
     
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  15. Nov 9, 2019 #55

    Winginitt

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    I didn't say they should be exclusive,in fact I said :
    Auto manufacturers have learned that removing the energy contained in the vehicle and in the passenger prior to any impact is just as important as protection after an impact. They have technology in hand to help slow or stop a vehicle prior to impact. That doesn't mean they disregard any safety measure they can incorporate via construction design. It doesn't mean that you can forget about seat belts, safety glass, or fuel pump shut off. Whatever a builder can incorporate into the build to make himself safer, should be done in concert with whatever he can do to slow the aircraft prior to touchdown. I think a four point quick release harness, lightweight foam under a seat, possibly a dampener, and even a manually triggered air bag might all be worth considering when building. As far as actually designing a fuselage thats going to dissipate energy upon arrival at the crash scene, I think thats beyond the capability of homebuilders. When the auto industry spends millions designing vehicles with crushable features, they still have to test them. Thats something a homebuilder can't do. Even after putting their best crushable design into production, they still cannot insure survivability. They have recognized that by employing available technology they can avoid crash events and rely on the mechanical safety devices only after all other options were insufficient. Thats all that I'm trying to point out here. A builder should do all that he can afford and incorporate mechanically......but he should also take the time to incorporate things that may prevent the incident before it happens. If your wing doesn't drop......how much of an increase in safety have you attained ? If your wings can be kept level and your speed slows to 30 mph (or less), you have a much better chance of survival than an airplane that stalls at 45 and drops a wing....no matter what safety devices you incorporated. Its wrong to ignore those considerations.
     
  16. Nov 10, 2019 at 2:45 AM #56

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    You were fortunate of not remembering. The last thing I remember after the mid-air was the ground about 3' in front of the nose or the aircraft coming at me at a very high rate of speed. Don't remember hitting the ground, just waking up with gasoline pouring on me. From then on I remember to much and wish I didn't. Living got very hard, and the mental trauma was worse than the physical even with having to get my heart beating again.
     
  17. Nov 10, 2019 at 3:03 AM #57

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    Absolutely. Crash safety and good low speed characteristics are both important (along with a lot of other things). It would be great to be able to discuss crashworthiness without the need to preface every post with a missive on the value of safe low speed handling, the importance of reducing velocity/kinetic energy, etc. That is all understood.
    If we had a thread on reducing stall speeds, I'm hoping no one would barge in repeatedly to expound on the value of a crashworthy structure. It is true, but not relevant.
     
    Last edited: Nov 10, 2019 at 4:01 AM
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  18. Nov 10, 2019 at 5:28 AM #58

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    For some reason you don't seem to take exception to numerous mentions of using tubing, only to my suggestions that the most effective way to make a cockpit survivable is to find ways to reduce speed for landing and or impact . Thats what he is asking for,suggestions about making the cockpit survivable. Suggesting a way the op might not be considering doesn't mean it isn't something that makes a composite cockpit more survivable. After all, he is designing a new airplane, why not think about anything that aids the structures and the pilots survivability? Often when someone starts out with a new idea, they overlook something obvious and later wish they had considered it. The op flatly stated he was looking at 90 mph landing speeds and high sink rates. I have to ask why he thinks lesser speeds are not obtainable when he also states that the crash conditions he is talking about are ones where he expects the pilot to have a modicum of control while going down. I would hate to think I wouldn't have full control of any airplane at 90 mph, or that I was going to lose control just below that airspeed. Just because an airplane is made from composite materials doesn't mean it can't also be designed to fly at a reduced speed as well as incorporate the latest composite technology. If you have actual information as to how composites can resist impact damage, please feel free to post it and stop telling me that my suggestion isn't relevant to making the cockpit safer. We are at post 57 and I haven't seen any real information from you on how the composite material itself can be done in any manner that increases its integrity or the safety of the pilot while attemping to survive a 90 mph crash. I think the Lancairs have lots of engineering for their composite designs, and if memory serves me they also have one of the highest fatality records when they do go down. Why doesn't Lancair come up with some mode of applying composite technology that makes them more survivable? Speed at impact....thats why. Please stop worrying about my suggestions and put some objective information into the thread about how you think this composite material can be fashioned in some manner other than existing methods that will aid safety. At least I'm trying to provide food for thought rather than worrying that suggestions for a safer cockpit don't fit with your vision of how to accomplish it. If I had a thread on reducing stall speeds and someone offered some suggestions on how to make the cockpit safer while reducing those speeds, I would welcome the insight. I mean, all suggestions are meant to accomplish a common goal.....

    (quote from http://blog.covingtonaircraft.com/2015/11/10/lancair-pt6a/)
    Lancair Stall is 71 mph

    Lancairsmakeup just over 3% of the amateur built ( AB) fleet yet have over 10% of the fatal AB accidents.
    Over half the Lancair accidents in this small sample were fatal while the rest of the AB fleet is just slightly above the overall GA fleet fatal accident ratio of 1 fatal mishap for every 5 accidents. Here’s what’s interesting – unlike the typical high-performance aircraft that have fatal weather encounters, the Lancairs are generally in VFR conditions and involve loss of control – i.e a stall/spin.

    So the pilot starts with a controllable engine out but it often develops into a stall/spin condition before reaching the ground. How do you build a composite that will help?





     
    Last edited: Nov 10, 2019 at 5:43 AM
  19. Nov 10, 2019 at 5:57 AM #59

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    Crashworthiness:
    This, or variants of it, is the commonly understood definition of crashworthiness. Improving crashworthiness means reducing the risk of occupant injury once a crash has occured. It is specifically not about reducing the chances of a crash, reducing the speed at which the crash occurs, or any other thing that somebody wants to talk about. Improved pilot training, improved stall and spin resistance, slower stall speeds, better weather forecasting, safer airports, better runway lighting, proper engine maintenance, etc, etc. All are important in addressing every step of the "accident chain," reducing the likelihood and severity of crashes. All are entirely unrelated to crashworthiness.
     
    Last edited: Nov 10, 2019 at 6:35 AM
  20. Nov 10, 2019 at 7:27 PM #60

    Winginitt

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    Basic Principles of Crashworthiness (https://pdfs.semanticscholar.org/225b/4ff4b954467a26708eb596c3cdbcd304b5db.pdf)
    Crashworthiness can be defined as the ability of an aircraft and its internal systems and components to protect occupants from injury in the event of a crash.

    I interpret that to mean anything incorporated into the design of the whole aircraft that results in lessening impact or injury......not just structural or internal items.
    Below is the considered definition of one of the two leading crashworthiness test institutes in the USA.

    Insurance Institute for Highway Safety
    IIHS evaluates a vehicle's crashworthiness with the help of six tests: moderate overlap front, driver-side small overlap front, passenger-side small overlap front, side, roof strength and head restraints & seats. For front crash prevention ratings, the Institute conducts low- and moderate-speed track tests of vehicles with automatic braking systems.

    The Institute awards points based on how much the systems slow the vehicle to avoid hitting the target or lessen the severity of the impact in the two tests. In the case of an unavoidable collision, lowering the striking vehicle's speed reduces the crash energy that vehicle structures and restraint systems have to manage. That reduces the amount of damage to both vehicles involved in the collision and minimizes injuries to people traveling in them.

    Pretty strong evidence that the overall ability of any moving vehicle/airplane to slow down prior to crashing is considered part of their crashworthiness evaluation.

    While you want to focus in on a narrow definition of crashworthiness, it harms no one and may actually help someone.....especially someone like the OP to broaden the view and consider design features which ultimately place a lesser demand on the structure and its pilot. Lessening speed lessens the need for strengthening the structure and the protection of the pilots head from acceleration. It also contributes an awful lot to structure crashwortiness. In a high speed composite airplane, the designer probably isn't going to get his airspeed down below 50mph with those short sleek wings. He might however incorporate some Hoerner wingtips, some spoilers, and maybe some wing fences. Two of the three can work at only low speeds on pilot command.

    The thing to consider is that once the airplane has reached its stall speed it will more than likely drop a wing due to ineffective controls....and then no amount of cockpit design is going to protect the pilot as the plane spins in.

    Now, I feel that just like brakes are part of any vehicles ability to survive a crash (ie:crashworthiness), so is the ability to dissipate speed/velocity with any means of air braking that helps prevent a spin. You are welcome to view it in a narrower vein if you wish. The OP and anyone else reading this thread can determine for themselves if slowing an airplane beyond normal expectations and retaining control longer is part of what makes an airplane crashworthy. Its each persons choice to view it how ever they wish.

    Do you have any information on how to build a survivable cockpit ? Why not post something objective about that ?
     

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