Crashworthiness

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JimCovington

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Mar 30, 2009
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217
Location
Burlington, VT
Using a simplistic theory of deceleration=>constant is the only way to go for these types of calculations for amateurs. If I were in the business of crash protection, I wouldn't use the simplification. A collapsing metal structure will probably peak early and then yield more easily as it collapses; a bed of foam beads will peak near the end as the beads compress.

Here's why the ratio works. (PS - I tried other numbers, and they were within rounding error in every case. Let me know a specific condition you find that's not.)

Use the formula a = (v^2-u^2)/2s for both acceleration and deceleration. Simplify it to a = v^2/2s for both conditions, as you are either starting or stopping at rest (the G forces will be opposite signs, of course.)

v is the same in both conditions - the speed you reach after a freefall is the same as the speed you must decelerate from.

Use the following variables:
G= G
a = rate of deceleration
ff=distance in freefall
st= distance to stop

Solve for v:

v = sqrt(2 G ff)

v =sqrt (2 a st)

Substitute one v for another (they're the same value)

sqrt (2 a st) = sqrt (2 G ff)

square both sides (remember to check for the correct sign at the end)

2 a st = 2 G ff

solve for a

a = G (ff/st)

And there ya go....deceleration (in Gs) is your freefall distance divided by your stopping distance.

/In honor of this post, I've changed my avatar. No, that kayak's not flying and yes, it's about 6" above the water after falling 15'. I'm about .05 seconds away from a compressed spine.
//And no, I didn't say "Hold my beer and watch this" before I did it.
 

mz-

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Nov 26, 2009
Messages
144
Location
Finland
This pdf is a nice report primarily on the effects of manuvering loads on human circulatory system. Nice website. I was able to find evidence of articles on crash testing but was unable to actually access the articles themselves.
Yeah, a large portion of the reports are not scanned in yet. There is a download pdf link on the report's search result page if it is scanned in. Otherwise there's a request link.
 

GESchwarz

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Oct 23, 2007
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1,179
Location
Ventura County, California, USofA.
Jim,

I had to sleep on that ratio. At first it didn't seem that it could be that simple. But by this morning it made perfect sense. It's actually simpler than your explanation, but I don't have time this morning to explain it...I'm having a hard time putting it into words or a formula, but it's just the basic fraction reduction method I always use and it's based on the fact the average velocities of both acel and decel are the same, so they cancel. See what I mean...I know what I'm doing but I can't yet explain it. I'll get back on this. gotta go.

By the way Jim, it's good to have you on board. I'll want you to be with me when I'm doing my foam block tests.
 

autoreply

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Joined
Jul 7, 2009
Messages
10,738
Location
Rotterdam, Netherlands
Jim,

I had to sleep on that ratio. At first it didn't seem that it could be that simple. But by this morning it made perfect sense. It's actually simpler than your explanation, but I don't have time this morning to explain it...I'm having a hard time putting it into words or a formula, but it's just the basic fraction reduction method I always use and it's based on the fact the average velocities of both acel and decel are the same, so they cancel. See what I mean...I know what I'm doing but I can't yet explain it. I'll get back on this. gotta go.
V squared divided by 2 A gives the distance travelled (in correct units)

Thus V^2/(2*a)=s or a=V^2/(2*s)
 

GESchwarz

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Oct 23, 2007
Messages
1,179
Location
Ventura County, California, USofA.
Here are some excerpts from MIL-S-58095A, Crash Resistant Seats…

Para 3.5.1

…the system shall be designed to avoid brittle failure.

Para 3.11.4

…The seat cushions shall be designed for comfort and durability and not as a device to absorb crash energy.

Para 3.12.2

…not less than 12 inches of vertical stroking distance shall be provided.
 

Mad MAC

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Joined
Dec 9, 2004
Messages
579
Location
Hamilton New Zealand
Found this the Aircrash Survival Design Guide Vol 4, Aircraft Seats, Restraints,Litters, & Cockpit/Cabin Delethalization
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA218437&Location=U2&doc=GetTRDoc.pdf

This has a number of interesting things i haven't seem any were else including head, arm & leg strike arcs, peddle spacings to prevent jammed feet.

Regarding the seat cushions not to absorb energy in the MIL-S-58095A, this may have to do with the stoke inefficiency of the foam and the substantially higher crash load criteria of the military.
 

RacerCFIIDave

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Joined
Feb 8, 2008
Messages
412
Location
Asheville, NC
For for US military documents, MS, AN, MIL-HDBK etc I tend to use https://assist.daps.dla.mil/quicksearch/

There is also these articles by tony Segal. These don't seem to have really been publish outside of the UK.

http://www.streckenflug.at/shop/images/dynafoam_freeflight.pdf
http://www.spina-bac.fr/yankeeromeo/DSI/foam_segal.pdf
Very good stuff.... :ban:

I just finished reading them...and am going to forward on to the technical/safety staff at IRL...in addition to using several of these ideas in my 2 working designs of the moment...:gig:

Dave
 

berridos

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Oct 10, 2009
Messages
906
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madrid
Regarding the article in the previous post it would maybe make sense to make a seat that has three sandwich layers and increasing progressively the thickness of the fibre layers inbetween the honeycomb panels. I think the deformation/crushing of the honeycomb could be more progressive and controlled.
 

cluttonfred

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Feb 13, 2010
Messages
7,125
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World traveler
Reviving an earlier thread with a random thought...

I have been thinking about inexpensive ways to provide energy absorbing materials and structure in two key areas, under seats and in front of the cockpit, to allow for both frontal and downward energy dissipation in a crash.

It seems relatively easy to design a seat to hinge or slide on rails to allow downward movement, or to provide a crush space in front of the cockpit which is deliberately weaker than the cockpit structure...but what to put in that space to provide lightweight, predictable energy absorption? My idea is...beer cans! (Or soda cans, but beer cans sounds more entertaining.)

Empty aluminum cans could be filled with different materials (expandable polyurethane foam, loose polystyrene pellets, shredded cork, trying a few different materials for weight, practicality and strength) and tested to determine their crush strength.

It would be then be relatively simple to arange the cans to provide incremental resistance by staggering the height of the cans under the seat panel, for example, and varying either the number of cans (the seat rests on two, with four more an inch lower, and eight more two inches lower, etc.) or the material (the seat rests on two weak ones, with two stronger ones an inch lower, and two even stronger ones below that, etc.).

Besides, the research will allow me to drink beer for the benefit of science and safety. ;-)

Thoughts?

Cheers,

Matthew
 

GESchwarz

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Oct 23, 2007
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Location
Ventura County, California, USofA.
Matthew,

Certainly there are many ways to accomplish the same goal, but without some degree of actual testing and measurement of g force you won't really know how much protection you'll be getting. One other feature you will want to include is some degree of dampened rebound; you don't want to be stuck in the bottom of a cockpit as you may have difficulty lifting yourself out. An adequate design will have 12 or more inches of downward travel. Clearly the plane must be designed around such a volume of space.

Rienk,

That's an interesting link and really demonstrates how some people have more money than common sense. The crush zone of all vehicles is in the very front. If the pilot is in the very front...he gets to meet Elvis, and every other aviator that made the same mistake of flying and crashing in a design where he was in the crush zone.
 
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Topaz

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Jul 29, 2005
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Orange County, California
Rare photos of glider cockpit coming apart at the moment of impact

Shoreham - Sunday - Talk Photography
That was difficult to see. I'm very glad this pilot will live to tell the tale. I lost a friend last year in a virtually identical crash, in his case from a low-altitude stall-spin. Watched him dive in just like this. Then I had to help hold him down - he was in so much shock he didn't know his leg was a pile of meat and he had internal injuries. Took the instrument panel right in the chest.

****. Be careful out there, people.
 

GESchwarz

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Oct 23, 2007
Messages
1,179
Location
Ventura County, California, USofA.
Excellent contribution George, thank you.

Would anybody venture to guess the velocity at impact? With than I can do an approximate calc on g force in the third image where his rear meets the earth.

The brittle failure contributed very little to the rate of deceleration of the pilot and all points aft. The peak loading was saved for the seat pan and main gear structure, where the main crush area terminates, and additional structure begins.
 
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