Crashworthiness

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MaverickSawyer

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If you want something that is nearly ideal, my suggestion would be one of the expanded metal foams that were developed for automotive bumpers. I've got a piece of it around here somewhere that was provided as a test sample and it's not appreciably heavier than polystyrene. It also will crush down to a much greater degree without any substantial rebound.
Good point. Take some serious lessons from the automotive industry and how they do impact attenuation. They've spent a LOT of money and man-hours solving many of the same problems you would otherwise have to figure out by yourself.

It's one reason why it's so important to think of the cockpit as a system rather than viewing the components in isolation. The example I normally use to explain it to aviation folks is that you can't design the wings and tail independently because a change in one will dictate a change in another.
That's kinda why I'm planning on designing the cockpit and cabin first when I do get to actually doing serious engineering work on my concept. Plus, that way I have a complete structure that I can weigh and figure out just how much wing I need to build.
 

SVSUSteve

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MaverickSawyer said:
That's kinda why I'm planning on designing the cockpit and cabin first when I do get to actually doing serious engineering work on my concept. Plus, that way I have a complete structure that I can weigh and figure out just how much wing I need to build.
That's exactly what I have done down to figuring out a decent estimate of the electronics etc BEFORE futzing around with the wings, tail and all that.
 

SVSUSteve

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But that's like 6-9G's for the average guy? A loop could break your spine?
Not likely because of the load path , rate of onset (the part of human G tolerance that most people overlook), shape of the impulse, duration, etc. This sort of misunderstanding is why it is best to approach these things by looking at the loads at the anatomical structure you're interested in. How much of a loop's G load is in line with the spine in a way that would produce a compressive force that great? Probably not very much...

There's also a factor that one must consider in a live person when looking at data derived from cadaver experiments. The pre-impact tensing of the musculature surrounding the spine is going to take up some of the load and increase the tolerance a bit. Given that we can't readily experiment on live people in this way, it's hard to quantify the actual effect of it. By the way, the old wive's tale about why drunk drivers get hurt less frequently because they don't tense up is false. It does stand to reason to use the cadaver data for two reasons: 1) it's the best data we can get since the Nazis ruined the use of prisoners as experimental subjects 2) it probably reasonably approximates the behavior of the anatomy especially in cases where the person had no warning and time to tense up (such as a rear passenger in CFIT crashes).

The other point is that...in light aircraft, a considerable number of folks flying them aren't as fit as your average aerobatic pilot and often there are folks flying either as pilots or passengers who have age-related issues that reduce their load tolerance. Thinking of a design for a 30 or 40 year old Dutch glider pilot who is reasonably fit could easily predispose him to serious injury ten to twenty years down the line as he ages even if his BMI and weight stay relatively stable. So while you or I might not get a spinal fracture NOW if subjected to such a load, it's rather short sighted to not be proactive.

In this case and in the case of most aircraft since it's a minority of pilots who actually do aggressive aerobatics, basically design the seat so it triggers at a lower threshold without having to worry as much about the effects of repetitive loading during "normal use". It might be more complicated if one were designing a competition grade aerobatic aircraft but I haven't given that much thought since that's not my area of interest.

As a side note: The only aerobatic induced spine injury I am aware that didn't involve ground contact or some underlying issue like osteoporosis or osteopetrosis was a guy back in the 1980s who ruptured some of the ligaments in his thoracic spine when he attempted a maneuver without his shoulder harnesses fastened (or they failed...I've heard about the case from three different folks and one says they weren't fastened, the other that they failed and the pilot himself says he doesn't recall which it was).

Basically a flailing injury that caused the ligaments to fail under excessive tensile load (which is one of their two common failure modes) and also produce what is referred to as an avulsion fracture (or in some circles colloquially a "chip fracture"; this is the other ligamentous failure mode we see with some regularity in crash victims) where the tensile load rips the ligament away from its attachment and takes a piece of bone with it. Given that he survived and landed the aircraft, I'm guessing it's safe to presume the G load was probably between six and nine G.

In a crash scenario, 6-9 G decelerations (measured at the vehicle floor) kills people all the time due to inadequate restraints, being out of position during impact or the collapse of structure. This is why duration and rate of onset are so important. The relatively gradual application of G load in aerobatics versus....say....falling out of a window onto a concrete driveway largely explains why the former seldom harms anyone outright but the latter can inflict serious or lethal injury.
 

BJC

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FYI, g load in a Pitts, which has a too-upright seating position, typically is around 4.5 for a loop. When I was serious about aerobatics in the Pitts, my highest g maneuver was a pull roll push push humpty, entered from redline. The pull was 7.5 and the final push out was -6.5. I don't fly that hard anymore.

Shoulder straps are normally loosened for aerobatics in the Pitts. Tight shoulder straps hurt the shoulders and cause a bend rather than a linear stretch of the spine, and even tight straps do not prevent sideways movement.

Probably the maneuver that is most damaging to the airplane is a snap roll with a high entry speed.


BJC
 

Hot Wings

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The other option would be to have something that is going to breakup and can be forced out of the way as it compresses beyond its maximal attenuation. That would allow- theoretically- for a greater attenuation in a given stroke distance because it would maximize the functional stroke distance.
How about Portland* cement? No, not joking. To be more specific foamed cement. It can be made in densities less than 8 pounds/ft3 with a compressive strength an order of magnitude above most foams so it might actually weigh less. When it is compressed it crumbles to dust in a very progressive and constant manner. It's one quality that might have to be fixed, or worked around is that it doesn't tolerate vibration well.



*Portland is a speculation. All of my exp[experience is with type G.
 

GESchwarz

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Only testing will provide the answer. The material must resist deformation throughout the stroke in a ductile manner. Would a brittle material like a porous concrete behave that way?

I chose welded steel tube for my cockpit cage over riveted aluminum or composites, because steel resists even when deformed, whereas aluminum tears like paper and can crack like glass. Composites shatter in a cloud of shards and dust.
 

Hot Wings

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Would a brittle material like a porous concrete behave that way?

.
Agreed that testing would be needed, but the samples I remember testing failed in a surprisingly "ductile" way, with zero rebound. Making foamed cement is very easy. All you need is cement, water, some dish detergent, and a blender. Some additives to consider: cotton flox and other fibers such as glass or polyester, micro balloons (or phenolic spheres) and fly ash.
 

MaverickSawyer

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Agreed that testing would be needed, but the samples I remember testing failed in a surprisingly "ductile" way, with zero rebound. Making foamed cement is very easy. All you need is cement, water, some dish detergent, and a blender. Some additives to consider: cotton flox and other fibers such as glass or polyester, micro balloons (or phenolic spheres) and fly ash.
Would using some sort of polymer adhesive like an epoxy help with the vibration issue? Or would that make it too springy and strong?
 

Hot Wings

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Would using some sort of polymer adhesive like an epoxy help << >>
Maybe. It's hard to describe foamed cement. The closest material that I can think of that you might have any hands on experience with is the green foam used by florists. I suppose saying it's sensitive to vibration is not completely accurate. It can be abraded easily but if protected from surface impact of the vibrating surface it's in contact with, say by a urethane skin, it might survive well.
 

SVSUSteve

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Making foamed cement is very easy. All you need is cement, water, some dish detergent, and a blender. Some additives to consider: cotton flox and other fibers such as glass or polyester, micro balloons (or phenolic spheres) and fly ash.
I know we have this desire to come up with our own ideas but why reinvent the wheel when there are off the shelf options that are known quantities? There's a point where being "experimental" becomes counterproductive and/or potentially more hazardous.

My concern with a foamed concrete- beyond weight and behavior under a load- is how it will behave during a service life. The other issue is the reliability of one batch or recipe to the next.

Testing it to the capability of your average homebuilder isn't likely to give you the same degree of information you can get by simply going with something commercially available

It can be made in densities less than 8 pounds/ft3 with a compressive strength an order of magnitude above most foams so it might actually weigh less.
Versus using Rohacell or foamed metal which is a lot lighter (in some forms) and already have behavior profiles that match what we're trying to do.
 

Hot Wings

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Versus using Rohacell or foamed metal which is a lot lighter (in some forms) and already have behavior profiles that match what we're trying to do.
Granted there are already materials with known quantities and behavioral profiles. You don't know that foamed cement is not lighter for the same performance. My experience tells me that there is a reasonable likelihood that it could be a superior material compared the known foams of plastics or metal. The only way to find out is to test. To dismiss the possibility of using a new material because "why reinvent the wheel when there are off the shelf options that are known quantities?" is short sighted and exemplifies an attitude that is counter to progress.

It may very well turn out to be a dead end but until someone does the work none of us will know. As for process control - making consistent batches of foamed cement is quite easy.
 

bmcj

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I think new materials like foamed cement warrant testing, but it should be done in a controlled environment for the benefit of everyone, not implemented in a build where its only validation is whether you live or die if you were ever unfortunate enough to crash your plane.
 

SVSUSteve

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Hot Wings said:
You don't know that foamed cement is not lighter for the same performance.
True. I freely admit that and it was part of the message I was trying to impart. My point is that unless we have access to a lot more engineering test capability than your average homebuilder, it's going to be difficult to prove that it works to a reasonable degree of certainty. I would try talking to the local university engineering department and seeing if they have an undergraduate student looking for an honors project? If you supply the materials, they might be willing to help both with getting the recipe correct with minimal iterations and with testing the most practical option(s). Especially younger American professors tend to love an excuse to break stuff "in the name of science".

Plus, just like with adhesives, you need to test each batch to make sure it is behaving the way you want it to. That is going to add a considerable amount of material costs and labor expenses (even if you're doing it yourself you still have to factor that in). Contrary to popular belief, crash testing is some times more than just a matter of "hoist it and drop it" especially when it comes to materials that are completely untested in a similar circumstance. Looking at a recipe that would probably have to be put together by the builder, it just seems like a step that most builders are going to skip (remember....most pilots think they are too good to crash).

While you or I or someone else on this forum might be attentive and cautious to make sure the materials absolutely meet spec....this is homebuilding where it's not uncommon for someone to look for ways to cut corners. One of my favorite books is Crichton's The Lost World because of the engineering professor in the book. He makes a point that you can do your calculations perfectly but if you're too smug (and I'm not accusing anyone here of that), you'll get bitten in the ass by someone not following it to the letter.

That's why it's probably best to stick with something that is commercially available. Basically....eliminate a potential issue with construction and reduce the time it takes to build the aircraft. Both of which are going to make a design much more viable especially in the case of something other than one off designs. It might not live up to the fullest extent of the spirit of experimental aviation but it's a lot less work to achieve the same end result: literally saving your ***.
 

GESchwarz

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Another option is what is often found under helicopter seats. A steel tube structure designed to deform under load. We could make a direct copy. One issue is that the attachment points, both to seat and floor will influence the actual geometry. It is easier to design everything concurrently so that you minimize the compromises.
 

SVSUSteve

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Another option is what is often found under helicopter seats. A steel tube structure designed to deform under load. We could make a direct copy. One issue is that the attachment points, both to seat and floor will influence the actual geometry. It is easier to design everything concurrently so that you minimize the compromises.
I still have never seen an attachment design that would be within the manufacturing and optimization capacity of the average homebuilder is the JAARS S-leg. It's the basis for the anchorages on Praetorian. There may be a more practical option but if it's out there, I have not seen it yet. There's a six-pack of beer in it for anyone who can beat it especially in the setting of the floor warping as often happens in crashes.

One of my friends retired a few years back from the Army as a senior A&P on Blackhawks (he was an E-9 out of Fort Rucker). If you mention to him the original version of the stroking seats fitted on them produces a litany of words best described as obscenities. Apparently it took a lot of effort to get the bugs worked out and was initially a huge headache for the maintenance personnel. It's a beautiful system now though and if one could figure out how to make it in a home shop and minimize potential maintenance issues.
 
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