Crashworthiness

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GESchwarz

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I don't quite understand what the contention is all about. G=(V1-V0)/t, and no wonder-foam can improve on, or get around that. Long stroke suspension beats short stroke suspension every time. Total reliance on nothing but a seat pad (small "t"), no matter how good, offers far less protection than an a well designed, long stroke (large "t") seat assembly.

If under given impact parameters a Confor Foam pad, or any other pad for that matter, were able to limit your spine to a load of 15 Gs for instance, if you double the pad thickness, then you spine would only experience 7.5 G's.

Any arguement against stroker seats should be centered on the elastomeric characteristics of the system, not on the stroker seat concept itself.

I'm not arguing the merits of, or criticizing, the Oregon Aero products at all.
 
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JimCovington

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I don't quite understand what the contention is all about. G=(V1-V0)/t, and no wonder foam can improve on, or get around that. Long stroke suspension beats short stroke suspension every time. Total reliance on nothing but a seat pad (small "t"), no matter how good, offers far less protection than an a well designed, long stroke (large "t") seat assembly.

If under given impact parameters a Confor Foam pad, or any other pad for that matter, were able to limit your spine to a load of 15 Gs for instance, if you double the pad thickness, the you spine would only experience 7.5 G's.

Any arguement against stroker seats should be centered on the elastomeric characteristics of the system, not on the stroker seat concept itself.

I'm not arguing the merits of, or criticizing, the Oregon Aero products at all.
I think we're arguing both sides of the same coin. V0 is zero in all of these crashes - the question OA poses is: Is there a secondary return acceleration as the seat rebounds while the spine is still headed south? Their foam doesn't contribute to any return "bounce" and your seat frame shouldn't, either.

OA doesn't suggest their pads are a cure-all. But they have their foot in the GA door because you replace a seat cushion in a GA aircraft a lot easier than you can replace a seat. Therefore, they sell a lot of replacement cushions.

As for the experimental side - price out a stack of their foam, and compare that against one of their whole seat assemblies. Aside from the rhetorical question "What's your spine worth?" most of us just don't have the wallets for the whole seat. So once again, they sell more foam than seats to the EXP/AB crowd.
 

GESchwarz

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Regarding the issue of rebound, the foam cannot rebound until after the peak load is reached. It's really nothing more than a type of spring. The foam cannot impart a greater load than what is imposed on it by the highest impact force, therefore rebound in and of itself is not a source of danger. The foams I plan to use as part of the stroker system can be of any rigid type which typically do not rebound much at all. They are one-use and inexpensive. The stuff I tested the other day is an inexpensive rigid insulating foam that rebounded to about 80% of its original height. That indicates a very desirable rebound characteristic. The material is very cohesive, exhibiting no tendency to fragment when compressed to 17% of its original volume.

This taffy-like constant force foam may feel good as a seat cushion but I don’t think that’s what I would want in my stroker system. It would be like hitting concrete. You want the benefit of the full stroke, and the constant force foam defeats that. When I think of "constant-force", I think of tar. Tar is a constant force medium. Constant force materials are great for conforming to shape over time, but they simply are not used in suspension systems, anywhere.

Because the constant force foam does form to the pressure imparted by your buttox and thighs, it is indeed an excellent way to distribute the load to your body thereby preventing damage to the local bone structure of your pelvis. So, it'd guess it's much more of a pelvis-saver than it is a spine-saver, because it doesn't limit Gs the way a conventional linear-force spring does.
 
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vortilon

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The Boeing PT-17 is classic example of poor and good seat design. The PT-17 is probably one of the most crashed airplanes in history. One because they built so many of them and they were used as primary trainers and that alone was responsible for many crashes but after the war they were the main stay in crop dusting and we all know how inherently dangerous that is. The early Stearman was built with wood seats that took many a pilots life and injured even more. The dusters learned early on to replace the wood seats with the later aluminum seats.

One of my good friends Al Stix just crashed his PT-17 at Creve Coure and lived to tell about it. The aircraft was pretty well demolished after the engine quit and hitting some trees. I believe the Stearman is the most crash-worthy aircraft ever built.

Two Men Walk Away From Small Plane Crash - KTVI
 

wsimpso1

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Were all of this dynamic stuff so straightforward and simple, we would have resolved a lot of things a long time ago... Trouble is that the system responds to the crash pulse in a complicated series of events.

Using a simplified model with a single degree of freedom will not adequately model the system. The crash acceleration pulse at the seat starts low, comes up to peak acceleration, and then goes back down, all pretty quickly. Once you know that, you have seat structure (a spring-mass-damper train), then you have the cushion (another spring-mass-damper train), then the crash dummy's butt, pelvis, back, torso, and limbs(another spring-mass-damper train). Each responds as the acceleration of the element below it builds up, and each response takes some small but finite time.

The spec is performance based - it specifies the input pulse based upon measured pulses from crash tests, and then uses a function of the forces developed in the lower spine of the crash dummy to determine likelihood of serious spinal injuries.

Favorable results with hard seats and viscoelastic cushions have proven to be real. This is just mechanical dynamics, it can be modeled and if the values are known for the characteristics, the model can reflect what happens.

Stroker seats were implemented because folks thought that spreading out the pulse would reduce severity. But to get to spinal forces with a stroker seat that were low enough, the impressive strokes that you find in production airplane seats were needed. Rebound apparently plays a role as does damping - the test labs have found that when rebound is small and the cushion is heavily overdamped, the spinal forces are acceptable with much smaller strokes than with a stroker seat.

What I will put to all of you from that evidence is that the stroker seats must have been contributing to the body response. And when your model and your data conflict, revisiting the model will be essential.

Billski
 

bmcj

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If you had lots of room and didn't care about weight or complexity, could you put a seat in a swinging cradle that translates part of the downward momentum into a rotational momentum as it lays you flat on your back relative to the G load?

I know, I know... it's an unrealistic and rediculous proposal, but as a mental exercise, would a Rube Goldberg (Jr) setup like this make an impact more survivable?
 

Dana

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The main difference between the elastic foams and a spring is that foam has some damping, like the shock absorber in your car. It can range from almost no damping (think cheap foam couch cushions) to lots of damping (memory foam). The rigid foams, like cheap styrofoam, absorb the force and don't rebound at all... these are commonly used as part of the energy absorbing material in helmets.

Do some of the "stroker" seats use styrofoam as an energy absorbing media? Put the seat on top of a block of foam, say, 12" thick? Core it out so it crushes at the desired rate?

Re Stearmans, yes, one of the stoutest airplanes ever built. I had the pleasure of getting some basic aerobatics instruction in one in the late 1970's. Back then it rented for $50/hour. Getting a bit off topic, but I can't resist attaching this scanned picture that my father sent me a few years ago... that's the instructor (on the left) and I scratching our heads over something...

It was said that if you crashed a Stearman at any speed and an angle less than 30° you would walk away from it. I don't know if it actually had a published Vne, but during the war two instructors decided to try to pull the wings off of one. They trimmed it for level flight at full throttle, then did a full power vertical dive from 10,000'. At 2000' they both pulled back on the stick with all of their strength, and blacked out. When they came to, the airplane was flying straight and level at 1000'.

-Dana

Ask not what you can do for your country, but what your country is doing to you.
 

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JimCovington

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The rigid foams, like cheap styrofoam, absorb the force and don't rebound at all... these are commonly used as part of the energy absorbing material in helmets.

Do some of the "stroker" seats use styrofoam as an energy absorbing media? Put the seat on top of a block of foam, say, 12" thick? Core it out so it crushes at the desired rate?
Ah, THAT foam.

That foam is close to binary - it crushes or doesn't crush. Its yield rate is not proportional to the force applied, so you need to know just how big your impact is before you core it. The memory foam will dampen the Gs on any impact.

Maybe a combination of the two?
 

wsimpso1

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That is a seat with serpentine elements for a rising rate collapse. Exactly what I was talking about. The elastic load behaviour portion is springy, and you get the elastic springy portion back even after doing plastic deformation to it. That is the source of the rebound that is apparently amplifying the crash pulse.

Billski
 

JimCovington

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That is a seat with serpentine elements for a rising rate collapse. Exactly what I was talking about. The elastic load behaviour portion is springy, and you get the elastic springy portion back even after doing plastic deformation to it. That is the source of the rebound that is apparently amplifying the crash pulse.

Billski
That's not the behavior I expect from that seat. Unless those are titanium S-members, I expect those to permanently deform in a crash.

Only the engineer who designed them can say for sure. (Or a blurb from the JAARS web site.)
 

plncraze

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Those seats are in JAAR's #1 production Helio Courier. They have brought the plane to Oshkosh with these seats. The company also does not release information about them for liability reasons. I do not know if they have ever been tested in a real crash.
The back seat was of the Courier interesting because they did not have any room for a stroked seat so they used thick Temperfoam with holes cut through the material to "tune" the rate of collapse.
In the little research that I have done on crash safety the biggest problem is the springiness of the structure. Foam rubber cushions have exactly the wrong response in a crash meaning that they will allow the person in the seat to continue to travel into the seat (down) while the structure has already rebounded from a crash and is springing back (up).
When you look at the NASA studies of aircraft crashes, where they were crashing flood damaged Pipers off their lunar landing structure, the aircraft were deforming more than was expected and the fuselages were going from tall rounded squares to squat ovals and springing back to their original shape. This means the ceilings of the aircraft were coming down and hitting the occupants in the head. The ceiling would go back into its original shape and nobody would have noticed if it was not on film.
What makes Oregon Aero's tests interesting is, according to an article that I will try to find, is that a very rigid test seat was built which would allow testing of the seats without having to tune a structure as well. During tests they were able to reduce loads on the seat occupant using cushions with a very rigid seat structure.
To simplify what seems to be happening is that you want a rigid structure surrounding a seat cushion that will rebound at the correct rate.
Another complication is the seat belt. If anyone has seen the video of the turbine powered Oracle Raven crashing they will see the pilot coming out of the cockpit almost to where his neck is above the turtledeck and going back into the cockpit. This was with an aerobatic harness. The pilot survived to narrate this crash for a tv show. I can't remember the title of the show.
Once again I am grateful for Billski's input in this forum. From the posts that Billski has put here it seems that he is an engineer of springiness. And that is perfect because an airplane is a bunch of springs flying in formation.
 

wsimpso1

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Build something out of metal, it will have a yield strength, and the material behaves elastically as long as you stay below the yield strength. You push on it, it moves a little, you let go, and it comes back. Put load into it that is a little bigger than the yield strength, and it first goes up that elastic deformation until you get to the yield strength, (the elastic limit), and then the deformation becomes bigger (plastic deformation) until the load applied and the load generated by the metal become equal. Release the load, and the deformation follows the elastic behavior back to zero load. This spring back is a natural part of the process.

The elastic range is quite steep, the plastic range pretty flat , and then the elastic spring back is the same steepness as the original elastic range, but of course it now goes to zero load at a different place...

So yeah, that seat will spring back some, even after a crash load squashes it.

Billski
 

GESchwarz

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The only problem I have with a deforming metal structure is the cost. After each test you have to scrap it and make a new one. Foam works about the same and is a lot cheaper.

Rebound can always be controlled with a shock strut. Every length and width of shock strut is available from the off-road retailers. But I'll bet there are foams that have pretty tame rebound characteristics which are easy to verify by running some simple tests.
 

Starman

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In some of the low profile designs (like all of mine) the seat pan is more or less on the floor or the wing spar so there is no room for collapse and therefore no rebound, is that correct?

Another thing is that a semi reclining position spreads the vertical force over a much larger part of your body. When sitting vertically or nearly vertically all the force is directed through the small area of each vertebrae onto the small area of your pelvis bones. When on your back the force goes directly from the whole area of your torso into the seat back with much less of the bone crushing force. It doesn't all concentrate into one spot like when sitting vertically. So I think a semi reclining position is one of the more important aspects of vertical crash survivability and is a requirement for me.

I was thinking of using a solid hard foam, like styrofoam about six inches thick (with a cushion on top of it), but I read here that it has a very limited range of crush force.
 

orion

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But do keep in mind that in a typical crash the majority of loads come from forward motion. As such, your restraint design is probably more critical than that of the actual seat. For instance, shoulder belts attached too low aft of the seat can cause significant injury including collar bone breaks and in a few cases, even shoulder dislocation.

Rarely does the airplane impact the ground in a flat, pancake attitude and motion - that happens really only in a low altitude stall or as a function of a flat spin, or in a helicopter. Virtually all other crash configurations have a very significant forward motion component or, more accurately, an axial deceleration component.
 

Dana

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...and then the elastic spring back is the same steepness as the original elastic range, but of course it now goes to zero load at a different place...

So yeah, that seat will spring back some, even after a crash load squashes it.

Billski
Not if it's designed so the tubing buckles at the end of its stroke (crumple zone).

But do keep in mind that in a typical crash the majority of loads come from forward motion. As such, your restraint design is probably more critical than that of the actual seat. For instance, shoulder belts attached too low aft of the seat can cause significant injury including collar bone breaks and in a few cases, even shoulder dislocation.

Rarely does the airplane impact the ground in a flat, pancake attitude and motion - that happens really only in a low altitude stall or as a function of a flat spin, or in a helicopter. Virtually all other crash configurations have a very significant forward motion component or, more accurately, an axial deceleration component.
Wouldn't most crashes with significant forward motion (i.e. nose down) be high speed and thus higher to design for?

The "pancake" crash (low level stall) is likely to be relatively low speed, compared to a nose down crash where the aircraft picks up speed rapidly.

I've witnessed two crashes (actually the second one I just barely didn't see, but got there immediately afterwads). Both were Kolb MKIII's, power loss on takeoff. The Kolbs have the fabric sling seat. The first one, the prop came apart at about 40'. The pilot didn't get the nose down fast enough, and the plane pancaked in. Hit left wing low, and the pilot had back injuries (his ass hit the ground), while the passenger (whose ass didn't quite hit) was sore but otherwise uninjured. A better seat would have helped.

The second (which I didn't actually see) was an engine failure (water in the fuel, apparently) at about 2-300'. Pilot tried to turn back and stalled/spun, looks like it did about a half turn before hitting the trees and then the ground, in a nose down attitude. Pilot and passenger were both badly injured, broken leg and pelvis on one shattered ankle on the other. Here it was less a matter of seat shock absorption than lack of structure forward of the pilot (common to most pusher aircraft). Going through the trees almost certainly saved their lives, but a stroker seat wouldn't have.

-Dana

Welcome to the Federal Bureau for Reducing Bureaucracy!
 

bmcj

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Orion makes a good point. I think the only times you are likely to see excessive vertical forces in an impact will be due either to a botched low-level aerobatic maneuver (accellerated stall during pull-out) or to last second recovery (or lack of) from a departure from controlled flight (like a pattern spin, in which case you were already in lots of trouble).
 
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