Crashworthiness

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autoreply

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I've witnessed a couple of crashes/crashlandings and most of them were mainly vertical forces. On the other hand, most of them were gliders.
Two spun on final and another stalled into the ground after a cable-break, both with a near-horizontal attitude and excessive vertical speed. In all 3 cases there were casualties, as there were in another incident, involving a frontal crash into a mountain.

Of the motor aircraft I saw crashing, most either topped/winged-over or dug their nose in the ground after a bad landing or hitting a ditch.

I seriously doubt you can make an aircraft crash-resistant (or at least survivable) for any crash with significant vertical speed, they don't even manage to make a car safe at our stall-speeds.

As Jim said below as well, most crashes ofcourse happen at a considerable speed, but you usually have plenty time to get rid of that by simply sliding to a rest.

For a completely different question: why do some pilots use helmets? Heavier=> higher load on your neck and a higher risk of injuring it right?
 
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JimCovington

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Wouldn't most crashes with significant forward motion (i.e. nose down) be high speed and thus higher to design for?
Remember that forward motion generally won't stop as quickly (unless you've found the only concrete wall in the field) but vertical motion generally will. That means that even if a larger portion of your crash velocity is forward, you may still have higher G forces in the vertical axis.

Of course, you could also catch a wingtip and tumble- you'll really got to consider a LOT of factors.

FWIW, one of NASA's crash tests drops the airplane vertically (no forward motion) at a nose-down angle of 30 degrees. They also have a "swing rig" that provides forward motion and tries to make a flat impact with both forward and vertical velocity.
 

GESchwarz

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You have all made good points. The seat suspension is only for the vertical component of a crash. The horizontal component is a bit easier to deal with. I'm going with a welded steel tube fuselage and a crush zone of weaker aluminum structure of about 6" thickness in the belly...that's where my radiator and ducting are. The steel cockpit cage underside will have a slight resemblance to a tobogan.

My favorite WWII fighter is the P-47 in part because it offered gobbs of protection for the pilot.
 

Davefl42

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How do they make the AG birds survivable? I've seen numerous pictures of crop dusters crashed with injury but not fatal?
 

bmcj

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I think a large percentage of ag-bird accidents are due to clipping something (tree, fence, ground) during level flight. This means that a healthy cage around the pilot is probably the best protection. For the ag accidents that follow the typical GA type accident (i.e. - stall/spin), I suspect the fatality rate is comparable.

A wire cutting blade on the ag plane canopy is also essential for those unseen power/telephone wires.
 

harrisonaero

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I don’t have a lot of time to devote to online discussions, but because I do get some good info out of these forums I’ll contribute as well.

1. It’s not the impact that kills you or breaks your back, it’s the rebound.
2. You need energy attenuation, not just deceleration. Typically this requires more than just foam.
3. For the homebuilder, a very practical method of accomplishing attenuation is the “crushed can” type of aluminum seat bottom that wrinkles and absorbs the impact. I suggest starting with at least a 6” stroke.
4. Don’t just consider the seat structure, also consider HIC (head injury criterion). Many accidents are survivable as long as the the occupants don’t experience head/chest impact.
5. Don’t even consider flying in a light plane that doesn’t have at least a 3-point restraint.
6. If you’re serious about designing your own crashworthy seat then go beyond the forums and do your homework. Don’t be afraid of using our money- the FAA has an entire facility devoted to crashworthiness and in particular seats… it’s called the FAA Civil Aerospace Medical Institute (CAMI) laboratory.
7. Don’t just consider the seat, consider the entire aircraft’s ability to attenuate energy. In order of safety: tube-and-fabric, aluminum, wood, composite.
8. Consider engine and wing placement: low wing tractor being the best, high wing pusher being the worst.
9. One test is worth a thousand expert opinions.
 

GESchwarz

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Harrison,

I appreciate your breaking radio silence. Could you expound on the rebound, the first subject on your list. I don't understand why the rebound would be more dangerous than the initial impact. Are you talking about all of the banging around within the cockpit, i.e. trauma caused by striking things within the cockpit?
 

JimCovington

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I think he's referring to the rebound that some are trying to avoid with foam.

Imagine this scenario in a springy seat:
1) Flat impact occurs.
2) Your spine continues its trajectory down.
3) The seat springs compress due to the force from your body's mass.
4) The springs reach their compressive limit and begin to return.
5) Your spine is still headed down while the springy seat is headed up. This is the point at which you have the most force on the spine, and this is usually where it snaps.
 

autoreply

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4. Don’t just consider the seat structure, also consider HIC (head injury criterion). Many accidents are survivable as long as the the occupants don’t experience head/chest impact.
Why then use a helmet? That increases the load on your neck considerably, only to break it earlier.

(After a couple of hours with a restraining neck helmet you're broken as well, not very appropriate for flying.)
 

GESchwarz

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That doesn't make sense to me because the spring will not begin to extend until after the spine has stopped heading down. After all, it is the spine that is doing the compression. The spring will not begin to extend until the load on it begins to decrease. That won't happen until after the peak load passes. The spine is the driver all the way up to the peak load, then it becomes the driven. The rebound force applied by the spring after the peak load will always be less than before the peak load by the amount of energy converted to heat in the deflection of the entire system.

Regarding the helmet, you're right, it is a trade off. Go light.
 

JimCovington

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That doesn't make sense to me because the spring will not begin to extend until after the spine has stopped heading down. After all, it is the spine that is doing the compression. The spring will not begin to extend until the load on it begins to decrease. That won't happen until after the peak load passes. The spine is the driver all the way up to the peak load, then it becomes the driven. The rebound force applied by the spring after the peak load will always be less than before the peak load by the amount of energy converted to heat in the deflection of the entire system.

Regarding the helmet, you're right, it is a trade off. Go light.
Gary,

I understand your logic above, but there's a piece that's missing. Unfortunately, I can't remember it exactly.

I think it has something to do with two facts:
1) The spine isn't doing the compression, it's actually your butt. Your spine can move relative to your butt.
2) Your spine is flexible and loses strength when flexed.

The article (I really wish I could remember where I read it!) about Oregon Aero did a much better job of explaining it; it made sense to me. I wish I could remember the explanation. It was similar to (but not exactly the same as) the "second impact" that applies to brain injuries - first your skull hits, then your brain hits your skull. I really wish I could find the article, it's directly relevant to this discussion. I've emailed Oregon Aero about it but haven't received a reply.
 

autoreply

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Regarding the helmet, you're right, it is a trade off. Go light.
I don't get any benefit of a helmet. Protection against breaking glass or so?

Gary,
The article (I really wish I could remember where I read it!) about Oregon Aero did a much better job of explaining it; it made sense to me. I wish I could remember the explanation. It was similar to (but not exactly the same as) the "second impact" that applies to brain injuries - first your skull hits, then your brain hits your skull. I really wish I could find the article, it's directly relevant to this discussion. I've emailed Oregon Aero about it but haven't received a reply.
It's indeed not as much about a "spring", but more about the delay in response of your various bodyparts, with your body working as a variable spring as well. Some organs will collapse above 20G's and your spline might still be crushing down when your butt is lifted up by the backstroke from the spring. In fact your brain example is an excellent one, it's comparable with being hit in a car from behind, even is your head is against the headrest (the spline) you will get a pretty hard secondary hit by the backstroke of the headrest.
 

autoreply

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Rollover protection - especially in a plane like yours.
How does it work? Having been crushed by my own weight once (less than 130 lbs) when I did a bit too much beer I seriously doubt a helmet will protect me from 1800 lbs of airplane or am I missing the point?

(This being the reason I implemented a roll-over bar)
 

Starman

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Concerning helmets, I can relate some statistics from the motorcycle riding segment of the population. Before helmets there were a lot of brain injuries, helmets helped reduce brain injuries but instead caused broken necks. So your choice is between being a vegetable or being dead.
 

autoreply

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Are you sure your head won't contact the ground at all in a rollover?

testingsafety
No, I'm not. Out of the link I can't (or can't find) whether that happened during this occasion, but I know from another incident where a pretty big pilot was fine, ending upside-down. So for the moment I think that extending your head above the fuselage is a matter of bad seatbelt tension or bad aircraft design.

Concerning helmets, I can relate some statistics from the motorcycle riding segment of the population. Before helmets there were a lot of brain injuries, helmets helped reduce brain injuries but instead caused broken necks. So your choice is between being a vegetable or being dead.
Sure, but that's a whole different issue because the aircraft (your crash barrier) replaces the helmet since you don't have anything else to protect you. In an aircraft, your roll-over cage does the same and when ending upside-down, your helmet might stand the aircraft's weight, but your neck definitely doesn't.

I'm still more than interested in the benefits of helmets however (at least they look cool) and of crashes where the helmet was a benefit.
 

plncraze

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Two other names in the study of crashworthiness are Fred Weick and David Thurston.
Fred devotes a few chapters in his autobiography "From the Ground Up" to his interest in safety which includes the Ercoupe and crop dusters. He worked with DeHaven to design a cropduster which crashed and the pilot walked away. Page 269 of the book has recommendations for crashworthiness.
Thurston wrote a book titled "Design for Safety" which devotes almost 200 page to flight safety including configuration. Thurston designed the Teal amphibean and includes in his book the picture and story of one which force landed into water. The info on config design is interesting.
 
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