Lightweight but safe aircraft seats?

Discussion in 'Workshop Tips and Secrets / Tools' started by cluttonfred, Mar 26, 2016.

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  1. Mar 26, 2016 #1

    cluttonfred

    cluttonfred

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    As always, aircraft design is full compromises, especially when it comes to performance-robbing weight vs. safety features.

    When it comes to aircraft seats, I have seen everything from simple canvas slings in a vintage Cub to massive leather covered armchairs in fancy kit planes. Then again, seats are not just about looks and comfort. If the worst happens and you crash your plane, the design of the seat and whether or not it allows some give to reduce deceleration without bottoming out can make the difference between a sore bum and broken back.

    The classic Tony Bingelis articles on DIY seats for homebuilt aircraft are available here -- Cockpit and Cabin Interior -- and at least one manufacturer is offering slick, lightweight (only 6 lb each!) seats here -- http://www.blackmaxbrakes.com/comfortlite-seat. Can we do better?

    In thinking about a lightweight T-18 project, I had in mind a nylon canvas over aluminum tube design a bit like a custom-made lawn chair, but I do wonder about safety and was thinking I'd have to build a mock up and test it to see how it might fare in a crash.

    Of course, crash safety goes far beyond just seat design, but let's stick to that topic for now. Does anyone have any designs/photos/tips to share on building lightweight but safe aircraft seats? SVSUSteve, I am hoping that you and others with specific technical knowledge of crash safety might have some thoughts on this subject.

    Cheers,

    Matthew
     
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  2. Mar 26, 2016 #2

    PTAirco

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    I think I mentioned this before but Tony Bingelis often useful advice goes awry with his ideas of seat construction. Using old lawnchairs. Now maybe in his days lawnchairs were made in the USA and of sterner stuff, but what he proposes gives me the chills. I'd say you have a 2.0 G ultimate seat there before it gives way and you're sitting on your jammed up elevator controls. I have lots of childhood memories of lawnchairs breaking under 1G in the backyard in summers gone by. Please pay more attention to the thing that supports the single biggest weight (you) in your small airplane. Sometimes this is more than the engine and would you seriously propose using a lawnchair to make an engine mount?
     
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  3. Mar 26, 2016 #3

    TFF

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    I would think that with a T-18 like an RV that your but is sitting on the wing structure cross through. No place for a sling, just a pad.
     
  4. Mar 26, 2016 #4

    ekimneirbo

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    I assume you will have a harness type of restraint rather than just a lap belt. The seat would need to just absorb any downward impact. Some type of thick foam block placed under the seat and inside a plastic liner would be light and absorb/transition some/most of the impact. If you could find some of the stuff like they use for packing large crates (maybe 3" thick). A thin sheet of aluminum with some bead rolls raised on it and placed on top of the foam should spread the impact some. You could
    try dropping a kenlite block on it from different heights to see how much effect it has. If the somewhat fragile block survives, then at least you know its helping. If you feel really brave, you could use a watermellon instead of a kenlite block.:)
     
  5. Mar 26, 2016 #5

    Swampyankee

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    Until fairly recently, the culture of aviation safety was "don't crash!" The trouble with that logic is that crashes happen, and GA crashes happen at a high rate and GA crashes have lower survival rates, largely because of design features. You are the most irreplaceable, irrepairable, easily damaged item in the plane. Yeah, you won't screw up, put maybe the vendor who sold you the bolts that hold your prop on did.
     
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  6. Mar 26, 2016 #6

    Topaz

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    Yeah, a lot of designs do it that way. And there are a lot more crushed vertabrae in the world as a result. Bolting the seat pan to the spar is not even an "adequate" solution, let alone a "good one", regardless of the level of padding used.

    Crashworthiness is a subject I've been researching very strongly the last couple of weeks, for my own project. While I make no claim to even a tiny fraction of SVSUSteve's knowledge on this subject, I'm really starting to see some of his points in an entirely different light. Current regulations regarding light aircraft crashworthiness are almost worthless in terms of how much protection they actually provide the occupants of aircraft. Even a casual examination shows that a lot of people have died in accidents that their body could withstand, but the airplane folded up around them because the regulatory standards don't ask enough of the manufacturers.

    Just as an example, FAR 23 and EASA CS 22 require that an airframe protect occupants against significant injury or death up to 9 g's in a forward (longitudinal) impact. But the human body, properly restrained, can withstand up to 45 g in such an impact, without significant injury. I've heard from reliable sources that the actual human tolerance is probably double that, but nobody was willing to be a volunteer in tests beyond 45 g or so. So the body can withstand at least 45 g, but our airplanes are only required to protect us up to 9 g. That's insane.

    Any discussion of seat "safety" has to take into account the rest of the system of features that together protect the occupant, and the types of crashes and resulting loads that are developed. Where you attach the seatbelts (to the seat? to the airframe? both?) affects their ability to protect the pilot, and the amount they stretch under load is really remarkable, requiring a close examination of the "flail envelope" through which the pilot's head and body can move, even when restrained by a five-point harness.

    Impact attenuation downward, to reduce spinal injuries, takes more than a little foam. The latter is likely to break down under regular use anyway, as people step on the seat getting in to the airplane. The AGATE crashworthiness design book I linked a few days ago recommends - as a minimum - a 4-6" stroke for the seat, with the energy-absorbing material or structure generating a constant 1200-1300 pounds of force during the stroke. Those are more than "block of foam" levels of loads, and a foam block that could generate sufficient loads over a 4-6" distance before bottoming out would probably be closer to 7-8" thick before use. Better to use a hard seat and controlled deflection of dedicated sub-seat structure to provide the necessary stroke. That's how airline seats do it.

    It's also important to ensure that the seat impact attenuator can handle multiple impacts coming from different directions. A couple of the more-frequent impact scenarios involve one end of the airplane - nose or tail - impacting first, then "slapping" the rest of the airplane into a flat impact with the ground. The secondary "flat" impact can actually be the more dangerous, so the seat, belts, and impact attenuator have to withstand and protect the pilot in the first near-longintudinal impact and still have enough action left to protect him in the secondary "nose/tail-slap" flat impact.

    There is no "simple" solution that can be resolved just using the seat. A comprehensive solution involving a rigid cockpit cell, a forward "crush zone", multi-point restraints, a vertical impact attenuator, and other design points is really necessary if you're going to provide realistic protection to the pilot.
     
  7. Mar 26, 2016 #7

    TFF

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    There is no seat pan on a RV, you sit on a cushion Velcroed to the floor. Seatbelts and backrest is all there is there if you dissect it. Part of the problem if you want compact airplanes, there will not be a place for anything extra because there was no place for it in the first place. Afoot is a regulation to retrofit old helicopters with 23G seats and fuel cells; great you say I want safe. What it does is eliminate one passenger on average weight wise. No R-22 could ever be flown again two person. Cost will make ADS-B out look like pocket change. People who have helicopters like Brantleys and Hillers will just have to turn them into playground equipment because there is no support or money for an STC to be profitable.
     
  8. Mar 26, 2016 #8

    Topaz

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    I understand all that. And yet the fact remains that current regs - and many designs - do not provide even minimal crash protection for the occupants. Steve is right on that score. Retrofitting old designs that had little or no crashworthiness features is likely a poor way to go. As you say, doing so would likely render the aircraft unusable. But that's no excuse to continue being negligent with regards to crashworthiness on new designs. We've got the data, and we've got design methodology that can mitigate these issues. Why wouldn't you incorporate them into a new design? My little project is about as small an airplane as you can get, at least in terms of the fuselage, and yet I've already identified ways to incorporate both a forward crush zone and a full-size vertical impact attenuator under the seat, both at extremely little penalty in terms of weight and volume. There's no excuse for any larger design to do any less.

    To be clear, I'm not talking about a "safety airplane." I understand the consequences of trying to make something "safe", as an absolute. Some realistic provision for mitigating impact dangers ought to be included in any new design, however, to a level appropriate to the aircraft's performance. But what we're doing now is wholly inadequate, and even if I couldn't take longitudinal crash resistance all the way out to 45 g, only going to 9 g is absurd and needlessly dangerous. People are getting killed or crippled that don't have to be. I don't see how any "it's too heavy/expensive" argument can be thrown up in good conscience to counter that.
     
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  9. Mar 26, 2016 #9

    cluttonfred

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    Thanks, all, these are all valid points and I am not dismissing the idea that seat design should be integrated into an overall aircraft safety package, but I did ask for thoughts on making a light but crashworthy seat. Please lets set aside the larger questions of designing for safety overall and try to concentrate on a light but safe seat adaptable to existing designs. It's not a complete solution but it's much better than nothing.

    On the specific example of the Thorp T-18, I do not yet have a set of plans -- I am still debating on pulling the trigger for $350 plus shipping -- but as far as I can tell there is nothing between the occupants rear ends and the floor other than whatever seat you put in. The rear spar is at the bottom of the rear cockpit bulkhead and front spar under their knees. There are no controls passing under the occupants aft of the knees since the elevator pushrod, rudder cables and flap lever all pass through the central tunnel. That's bad in the sense of no crushable structure under their rear ends, but good in that there is nothing already in the way to prevent you from adding something. Here's a useful pic.

    t-18_iso.jpg

    PTAirco shudders at the though of assembling a seat from recycled lawn chairs, and I agree. My concept was to follow the design concept of a lawn chair -- an aluminum tube frame with separate cloth sections for seat and backrest -- to create a light but crashworthy seat. Straps of nylon webbing woven together might work, and according to this site, 2" nylon webbing has an ultimate strength over 5000 lb and stretches by about 2% when wet, which might be a good way to get it good and tight when installing.

    My thought in terms of testing was to bolt a sample seat bottom to a some aluminum sheet on the ground and drop a 200 lb sack of something on the seat from an appropriate height to simulate a crash, film it and examine the seat afterwards. If folks have suggestions on how best to build this kind of seat, or other concepts for light but crashworthy seats, I'd love to hear them.
     
  10. Mar 26, 2016 #10

    Swampyankee

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    When I was working in the aircraft industry, we had to do some testing for the (must have been hideously uncomfortable) seats used for passengers (self-loading cargo seems a better description of the way they were treated) aboard transport helicopters.
     
  11. Mar 26, 2016 #11

    BJC

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    There is enough weight to be saved in the engine, propeller, battery, alternator, starter, exhaust, fuel tank and instrumentation to allow you to install whatever seat arrangement you want.

    Note, however, that the load path for the pilots weight is into the spar carry-through and the sidewalls and control tunnel near the hips. It will not be simple to install an energy-absorbing crush structure in that space. (Not intended to discourage, but curious as to the solution that you eventually develop.)

    You could use a sling, but I would not.


    BJC
     
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  12. Mar 26, 2016 #12

    autoreply

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    Not that hard if you do it right, but that requires the whole structure. My design is less than a meter tall and still has place for about 4" of crushing distance. Total "crush device" weight is 2 lbs, reinforcements under the seat add another.


    As for OT's question; take a seat you like, and put some 1" thick foam in it. If it's then still comfy; put some foil over it, glass with 1-2 layers of glass and let it cure.

    Pull it out, and add some discrete foam stringers around the back and around the bottom (triangulate the heck out of it). Carbon works better. Then sandwich foam layers between single wet layers of glass below the seat.

    It should be proper structural foam (PVC, PMI) that collapses under overload). You should size density and size of that stack to provide the proper collapsing force, likely in the order of 15-20G's.

    Don't forget the Conforfoam. Otherwise you'll break a few bones in a medium landing and your back and butt will hurt after minutes.
     
  13. Mar 27, 2016 #13

    Swampyankee

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    If I'm reading the FARs (23.562) right, 19g seats are required now.
     
  14. Mar 27, 2016 #14

    Topaz

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    Yeah, it's a little schizoid. The dynamic conditions in §23.562 seem mostly to set conditions for testing, yet the static conditions in §23.561(b)(2) says "The structure must be designed to give each occupant every reasonable chance of escaping serious injury when ..." and then gives the usual 9g forward, 3g up, 1.5g sideways, and 6g down (but that last only if you want to invoke emergency exit provisions under §23.807(d)(4).
     
  15. Mar 27, 2016 #15

    Swampyankee

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    Confusing regulations? Who could believe it?

    It doesn't help that the regulations that count are the ones in effect when a design is first certified, not when it's manufactured.
     
  16. Mar 27, 2016 #16

    PW_Plack

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    Has anyone seen, or better yet sat in, one of these seats? That space under the seat within the pan looks deep enough to contain a not-insignificant crash pad.
     
  17. Mar 27, 2016 #17

    Lendo

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    The Vision core sandwich seat, which is also part of the structure (a strengthener between fuselage sides) has vertical composite strips ( .5" foam and one layer of glass each side) running front to back under the seat, which act as crusher plates. height varies with the curvature of the seat. Can be topped with any sort of cushion you want/ need.
    George ( down under)
     
  18. Mar 27, 2016 #18

    BBerson

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    The crashworthy seat should have a vertical energy absorber. It must absorb smoothly over the stroking distance.
    One such active device uses wires pulled through dies. The wires are deformed (stretched, made smaller diameter) much like wires are made in a wire factory pulled through progressive smaller dies.
    Made for Army helicopters. I haven't seen one.
     
  19. Mar 28, 2016 #19

    Peterson

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    Looking into a plane where pilot/co-pilot seats mount on a box spar running through the aircraft. Thinking weight could be saved while impact absorption increased by mounting the seat framework to the spar using something similar to valve-springs. If wanting to use the nylon webbing in the aluminum frame, stitch a 1/2-1" loop just inside where the stitch would tear in a hard impact. Cover with several thin layers of foam with densest on the webbing and softest on top, upholster to taste.

    How much energy it absorbs will greatly depend on materials used, but it should offer better (?) impact absorption without adding much weight.

    Just spitballing, feel free to correct any errors in concept before I actually try something like this and injure my back on a hard landing.
     
  20. Mar 28, 2016 #20

    BBerson

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    Springs are not advised. A spring absorbs the energy then releases while the occupant is still going down and slams into the returning spring.
    Instead of spring use a crush material like thin aluminum shapes. Beer cans are almost ideal but too thin (.004)
    If you made beer cans out .016" should be near ideal (is my guess). Depends on pilot weight, so simple perfection is likely .impossible. Other shapes are shown in crash study that Topaz posted.
     
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