Building more crash-worthy composite planes and cockpits

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

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  1. Sep 29, 2019 #1

    RSD

    RSD

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    Thought that I would start a new thread about how to build more crashworthy cockpits to try to bring this information together in one thread as it is currently split across threads about foam, threads about cloth and a few others.

    BoKu you mentioned that hybrid fabric that you used to be able to get from Soller - did it look like this -
    [​IMG]
     
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  2. Sep 29, 2019 #2

    Vigilant1

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    First a note: We're just talking about composite construction here, right? You've put this thread in that section (thanks), but on some devices/screens, the title of a thread is a LOT more prominent that the text showing the sub-forum.

    Would you mind if we take a step back and establish your objectives with regard to crashworthiness, or what you'd like to accomplish?

    I'll offer this (as a non expert), but you'll want to write your own for this thread: "What are the factors involved in the design and construction of a composite cockpit structure that can reasonably be expected to protect occupants from severe/lethal injury in an impact with forward velocity of 60 MPH and a vertical velocity of 20 fps at the most common impact attitudes. (A good case can be made for going to higher speeds than this, I'm just putting this mark on the wall). The cockpit structure should prevent the incursion of external objects (e.g. trees, rocks, etc ) and other parts of the plane (engine, wing spar, etc) from entering the occupant area during these impacts. The structure should also provide suitable attachment points for individual restraints.
    I imagine you'd want to cover the design and building of the "tub" itself, and also means to reduce occupant G loading (e.g. crush zones inside and outside the "tub"). For the tub, it would be interesting to know, at these forces, if "corrugated" beams/panels (with a CF web connecting the inner and outer skins) offer advantages over the more usual foam core. I think we might find that the foam/epoxy interface becomes a failure point at some extreme loading.

    Billski touched on the subject of a composite "tub" that can withstand 26Gs a little bit in this recent post.
    Anyway, my 2 cents.
     
  3. Sep 29, 2019 #3

    TFF

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    There use to be a member here who made middle weight formula race cars. He went off to build the best crash cockpit. There have been others too. You can search HBA.
     
  4. Sep 29, 2019 #4

    Hephaestus

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    So I'm tagging along for selfish reasons.

    I've been contemplating my cockpit rear former, and seatback lately.

    Intention is to have the ballistic chute mounted behind the seat. Only other good place would be between the firewall and canopy (instrument bay/header tank area) not sure I love that idea either.

    Integrating rollover protection with the canopy surround (where canopy mates to fuselage) seems to make the best sense.
     
  5. Sep 29, 2019 #5

    Vigilant1

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    A related issue is egress if the plane is upside down. It can be tough to work that one out in the case of a low-wing plane with a conventional canopy. Even with a breaker/chipper tool and all appendages operational, it might be a struggle. Designs with a separate windscreen and gullwing canopies with removable hinge pins provide a good way to get out (as long as the inverted plane tips one way or the other and nothing jams) and the centerline beam gives good structural support for the windscreen frame, which can also be rollover protection.
     
  6. Sep 29, 2019 #6

    Hephaestus

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    I'm going to worry about surviving the impact first. I'm ok waiting for help as long as there's not a fire.

    That said, I've never had an aircraft that didn't have a survival axe mounted within reach.
     
  7. Sep 30, 2019 #7

    Winginitt

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    Obviously a good place to start is seat and restraint design. If one doesn't survive the impact, all other improvement is moot........

    My second thought is not only surviving impact but still being physically able to respond and act. That would also encompass seats,restraints, and whatever it takes to maintain some sort of protected area.........

    My third thought is how to insure there is no after impact fire.......

    After that we can worry about actually being able to get out of the airplane.

    I think the first consideration should be selecting an airplane that has good slow speed flying characteristics and a slow stall. I don't think airplanes with 60+ knot stall speeds give a pilot much chance of survival.
     
    Last edited: Sep 30, 2019
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  8. Sep 30, 2019 #8

    RSD

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    I realised after I had posted that I should have put the word Composite in the thread title somewhere - if you are on good terms with a mod feel free to make an edit.

    Not a problem.

    Firstly I'm talking more about increasing survivability for the survivable crashes - for what is a survivable crash I would say those where the pilot has some control over the airplane until just before impact or other problem causing a crash. We aren't talking about the really high speed at vertical angle type crashes or ones where a wing has come off - I think we all agree that they are unsurvivable unless it is your really lucky day.

    Some examples of the sorts of crashes I'm talking about for this thread -
    • one of main landing gear legs collapses on landing
    • overrunning the end of the runway and hitting a ditch and flipping (bad landing or aborted takeoff)
    • engine out forced landing onto a roadway but hit unseen overhead powerlines crossing the road while on final resulting in a hard but not high speed impact with the road at (pick an angle)
    • engine out forced landing into a ploughed farmers field
    • doing circuits at the local busy GA training airport and you are at 30 feet on final when someone else tries to land on top of you
    • engine out forced landing just after takeoff and due to lack of height you have no choice but to hit a building
    We could probably whittle that list down to - crashes occuring during takeoffs or final/landings (planned - or forced but with a degree of control).

    I'm open to people's suggestions but given the takeoff / approach / landing speeds of most of the sorts of aircraft that homebuilders build how does 80 knots (roughly 90 MPH) and a sink rate of 30 fps sound? Probably with injuries we need to consider not just the blunt force trauma type of injuries but also ones like spinal injuries.

    Yes - and how this is to be achieved - especially the engine - there is probably a good argument here for the engine mount to be part of the same structure as the tub so that it is all one piece in an accident rather than an engine mount breaking off a firewall etc.

    Yes - very important. 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.

    I think that is likely. It would be very interesting to do a design study of the weight differences of a fuselage made with foam core techniques vs much the same plane made with a composite skeleton frame with thin composite panels attached.

    From what I understand those Formula 1 tubs are made with walls that are about 60mm / 2 3/8 inches thick - I'm expecting that a fair bit of that thickness might be nomex honeycomb etc.

    A contribution that I would think is worth quite a bit more than 2 cents actually - because to solve a problem you first need to define the problem and you have helped quite a bit with the development of that.
     
  9. Sep 30, 2019 #9

    RSD

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    Can you remember his user name?
     
  10. Sep 30, 2019 #10

    RSD

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    It does, and so does making a canopy that has a separate windshield as that has several advantages.
     
  11. Sep 30, 2019 #11

    RSD

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    A centreline beam with gullwing canopies works for side by side seating but we need to come up with a suitable option for tandem seating as well. But even an upright aircraft can be difficult to get out of - I found getting out of a Long Ez to be a struggle.
     
  12. Sep 30, 2019 #12

    RSD

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    Indeed - I've seen some seat and restraint attachment points that make me think that survival likelihood is low just because of that in some planes.

    and being able to get out of the seat/restraints quickly as well

    I'm kind of surprised that more GA aircraft don't have engine area plumbed fire extinguishers fitted like the types fitted to rally cars - they don't way much and don't cost much but are mandatory in most rally competitions.

    Indeed.

    A lot of current planes no - but with a properly designed composite cockpit...
     
  13. Sep 30, 2019 #13

    Hot Wings

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    RacerCFIIDave?

    Hasn't posted since 2010.
     
  14. Sep 30, 2019 #14

    Winginitt

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    So what you want to concentrate on is ways to make a safer fuselage ? Not that you are not interested in other safety features, but the main thrust of what you are trying to look into is the fuselage construction itself?
     
  15. Sep 30, 2019 #15

    Dennis DeFrange

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    Also need to allow for the brief incoherence that usually follows an impact . I've been in a few automobile accidents where an have experienced this condition more than once . Impacts can be very confusing , even when there are O injuries . Trying to figure what should be done next can be very scrambled . And fire ?
     
  16. Sep 30, 2019 #16

    Dan Thomas

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    Airplanes, by their nature, tend to hit the ground pretty hard. You might build a cockpit that can take the crash, but the sudden stop will still tear your insides apart and kill you.

    One needs to look at the more common accident scenarios. A lot of pilots crash when they lose control in flight; the stall/spin while trying to clear an obstacle or in a base-to-final skidding turn, or the stupid buzz job and pull-up that gets the accelerated stall and crash. The VFR pilot that continues VFR into IMC and loses control or bangs into a bunch of rocks he didn't see. Flying in high winds or icing conditions. None of these need anything more than better training and a better, humbler attitude toward flying. Crashworthy airplanes or ballistic 'chutes aren't going to save those guys.

    Engine failures? Usually due to lousy maintenance or simply running out of fuel. Better maintenance, including sticking to accepted aircraft practices, and simply refusing to push the range limits of your airplane can go a long way to preventing such accidents.

    Landing accidents are also usually competence-related. Too much speed is often the culprit, and we can track that back to some poor training or the pilot's insistence that more speed on final is safer. Or the guy that got his PPL entirely at a controlled airport with an 8000-foot runway decides to go camping at some remote bush strip without any bush or mountain training. Overconfidence and ignorance break a lot of airplanes.

    I agree on the low-wing nosed-over egress problem. It's one thing I never liked about my Jodel. Getting out would be tough indeed. Factory low-wings aren't much better, and later models have to have some means of letting the door detach, including an external release for the rescue guys. I would vastly prefer a high-wing airplane, since it's structure up top is so much stronger and the doors are kept clear of stuff that might keep them closed.

    Besides, birds are all high-wing affairs. Low-wing birds went extinct a long time ago:)
     
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  17. Oct 1, 2019 #17

    Jimstix

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    Surviving the crash is heavily dependant upon acceleration, the structure not collapsing around you, and being properly restrained. Seat belts have a breaking strength of 6000 lbs because Dr. Stapp found that you won't survive much higher loads. Belts stretch and heads hit things. So, figure the loads at at impact at 2 x 6000 lbf for your restraint fittings and bolts, 6000 for your belts, wear a helmet, keep metal or fuel from entering the cockpit, have a crush structure under your butt, and don't impale yourself on the control stick. Other than that, no sweat
     
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  18. Oct 1, 2019 #18

    RSD

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    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.
     
  19. Oct 1, 2019 #19

    RSD

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    Agree - have been in a few car rally accidents and accidents can cause quite a bit of confusion - especially if they involve a high speed rollover.

    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.
     
  20. Oct 1, 2019 #20

    RSD

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    I'm going to wave the off-topic flag at this one - this thread is in the composites forum because it is making a better safer plane - I can't make a better and safer pilot out of composites - and I'm not a flying instructor so I can't make manufacture better and safer pilots at all.
     

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