Crashworthiness

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Joined
Jan 25, 2003
Messages
430
Location
Shirley airport MA
here is the video of it, the velocity at impact seems rather low to me

he appears to completely stall it but is so low that he couldnt have picked up much speed before it hit

if everybody takes a guess you can average it and use the "wisdom of the crowd" to come very close

I say 50 mph

 
Last edited by a moderator:

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,748
Location
Rotterdam, Netherlands
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.

It seems to stand still in the air. Wingspan is around 45'. I guess the "still" altitude is 20 m (65ft).
Using: v^2/(2G)=a we can obtain impact speed, assuming it's a free fall. A = 10m/s^2, a is 20.
(vertical) impact velocity thus would be 20 m/s, or 72 km/h or 45 mph.

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.
I doubt that. There've been extensive crashtests in gliders and the frontal zone is specifically designed to absord that energy (by destructive bending too). There is by the way no "seat pan", the fuselage is one continuous structure (shell)

Here's a good read (in German though)
Sicherheitscockpit - Technik
Sicherheit
 

BBerson

Well-Known Member
HBA Supporter
Joined
Dec 16, 2007
Messages
13,093
Location
Port Townsend WA
Looks like he "ruddered" the turn into a half turn spin. Classic return to runway accident.

As for crash absortion, how does fiberglass absorb energy without yielding? and getting warmer as metal does?
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,101
Location
Orange County, California
GLASS doesn't yield. The mechanical crushing of the epoxy matrix and separating the reenforcement fibers from it absorbs quite a bit. Remember, the structure is a mechanical *composite* of two materials. It's not like a glass vase. In compression, there's lots of opportunity for energy absorption, but generaly people do strength tests in tension, where composites simply fracture without a traditional yield.
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,748
Location
Rotterdam, Netherlands
With the addition that Kevlar itself can absord an impressive amount of energy (it's only used the last 20 years or so and afaik not on this glider). Modern gliders often use a 50/50 kevlar/carbon weave to keep strength and safety on par, without the shattering of carbon (or glass)
 

GESchwarz

Well-Known Member
Joined
Oct 23, 2007
Messages
1,179
Location
Ventura County, California, USofA.
A vertical speed of 45 mph at impact is equivalent to a freefall from 68 feet. The average g force is determined by the stopping distance. Here’s what it works out to:

36" = 24g's
12” = 69 g’s
6” = 137 g’s
3” = 273 g’s
 

Rienk

Well-Known Member
Joined
Oct 11, 2008
Messages
1,365
Location
Santa Maria, CA (SMX)
A vertical speed of 45 mph at impact is equivalent to a freefall from 68 feet. The average g force is determined by the stopping distance. Here’s what it works out to:

36" = 24g's
12” = 69 g’s
6” = 137 g’s
3” = 273 g’s
One of the topics at the ESA workshop earlier this month was crash protection. One process being tested was having a composite seat pan suspended on nylon webbing, which supposedly will stretch/fail with a certain load, absorbing significant energy without rebound.
A solution mentioned which I would like to try is used on helicopters; using an aluminum tube designed to turn "inside out" upon a crash. Apparently, this works really well, though I don't know anything about the sizing of members. It seems that a such a system set at 45 degrees would help in both vertical and/or horizontal energy absorption.
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,748
Location
Rotterdam, Netherlands
The guys from the Tiger (you know, the wannabe-Apache) told me their crew's weight is limited by those seats. If I recall the explanation correctly it's a multilayer honeycomb structure under the seats that absords enough energy to be able to walk after even the words landing.

@ GESchwarz, with such "low" energies, the amount of G's is usually less important as the impact energy. 20G's will rupture your organs, but if speeds are low enough to make it very short duration one can sustain much higher G's.
 

GESchwarz

Well-Known Member
Joined
Oct 23, 2007
Messages
1,179
Location
Ventura County, California, USofA.
The guys from the Tiger (you know, the wannabe-Apache) told me their crew's weight is limited by those seats. If I recall the explanation correctly it's a multilayer honeycomb structure under the seats that absords enough energy to be able to walk after even the words landing.

@ GESchwarz, with such "low" energies, the amount of G's is usually less important as the impact energy. 20G's will rupture your organs, but if speeds are low enough to make it very short duration one can sustain much higher G's.
True...

A 5 mph impact with a stopping distance of .25" will yield 41 g's, with a transition time of .0057 seconds. This will hurt.

A 50 mph impact with a stopping distance of 2.1' will yield the same 41 g's, but the transition time is now up to .057 seconds, an order of magnitude longer. This can kill.

I wold guess that the multi layers are design to progressively crush at higher loads...each layer is yields at a different load. I would imagine that honeycomb yields in a somewhat brittle manner. Rigid foam on the other hand is more squishy, not so brittle, therefore it compresses progressively over a range of loads.
 

GESchwarz

Well-Known Member
Joined
Oct 23, 2007
Messages
1,179
Location
Ventura County, California, USofA.
I doubt that. There've been extensive crashtests in gliders and the frontal zone is specifically designed to absord that energy (by destructive bending too). There is by the way no "seat pan", the fuselage is one continuous structure (shell)
I've been thinking about that, and why the failure of the airframe stopped crushing at the seat. The explanation is simple...The weight of the pilot was driving most of that destruction. All subsequent conversion of energy (destruction) continued to occur unseen within the body of the pilot. The energy of the airframe had been expended sufficiently to not collapse the forward fuselage any further.
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,748
Location
Rotterdam, Netherlands
I've been thinking about that, and why the failure of the airframe stopped crushing at the seat. The explanation is simple...The weight of the pilot was driving most of that destruction. All subsequent conversion of energy (destruction) continued to occur unseen within the body of the pilot. The energy of the airframe had been expended sufficiently to not collapse the forward fuselage any further.
Given that the gliders mass is around 200 kg (450 lbs) that seems unlikely to me. The wing didn't break (it barely scratched the ground and was intact after impact), so at least the full wing weight (over 250 lbs) was still working during the full impact. Idem for the tail that doesn't seem to be deformed and weights over 100 lbs.

Also note that the aircraft bounced up after the nose crashed into the ground. One can notice the wheel compressed in the last "dive" attitude picture. Than it uncompresses a bit when the aircraft springs up in the air, to collapse after the aircraft falls again, this time in a horizontal position.


Here's a full post on what happened:
http://www.homebuiltairplanes.com/forums/hangar-flying/8294-oh-wow-talk-about-lucky.html#post77284
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,101
Location
Orange County, California
I've been thinking about that, and why the failure of the airframe stopped crushing at the seat. The explanation is simple...The weight of the pilot was driving most of that destruction. All subsequent conversion of energy (destruction) continued to occur unseen within the body of the pilot. The energy of the airframe had been expended sufficiently to not collapse the forward fuselage any further.
No.
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,748
Location
Rotterdam, Netherlands
I love it. Certainly none of us know what we're talking about - We have no data.
We do, and that's what I'm basing my opinion on.
http://www.homebuiltairplanes.com/forums/hangar-flying/8294-oh-wow-talk-about-lucky.html#post77344

If we want any real progress in active safety, we should get rid of all the old-wives tales and look at what actually works. All those pilots, ranting about composites and that they're unsafe are stopping progress in safety. More bluntly said, they're preventing more pilots to survive a screw-up.

Just look at your most valuable asset, your brains. Try finding a aluminum or steel crash helmet. Than have a look at the best ones (in terms of crashworthiness). All composite, most Kevlar. And just assume those designers do have an idea what they're talking about and they're in a very competitive market...
 

topspeed100

Banned
Joined
May 4, 2009
Messages
4,063
Location
Oulu/Finland
A few years ago a friend of mine told me about a plane crash he was in. After everything stopped moving he noticed that the windshield was several inches closer to his face. When he got out he could see that the seat was leaning forward. He was surprised that a home built had an effective energy absorbing seat support so he looked under the seat. This drawing shows what he described to me. It's just a length of C-channel attached to the structure at such an angle that in a crash the webbing will bend and absorb some energy. I have it at 90 degrees but it could be mounted at some other angle to absorb more vertical stress. Say you hit the ground at 45 degrees and 13 Gs. 13 at 45 breaks out to 90 vertical and 90 horizontal. With the webbing vertical most of the displacement during the bend is going to be horizontal so this won't be much of a back saver (and in fact my friend hurt his back)
I wondered the Air France 358 crash in Toronto were the captains seat came loose at the impact; Air France Flight 358 - Wikipedia, the free encyclopedia

There is a lot of moment arm in a high seat that is bolted only from the bottom. Is there a way to secure the seat from higher ?

Flight 358 http://www.youtube.com/watch?feature=endscreen&v=5DcRipBqhEM&NR=1
 
Last edited:

GESchwarz

Well-Known Member
Joined
Oct 23, 2007
Messages
1,179
Location
Ventura County, California, USofA.
A great contribution Synergy.

Most GA planes, Homebuilt included, have little if any design provisions for crash survivability. That is one of the reasons I'm building my own design. When I get some time I'll attach some pictures of my seat and cockpit assembly.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
6,700
Location
Saline Michigan
Great video from the test series that NASA did. I remember the Scientific American articles on this, plus some TV shows on this. Takeaways:

Cockpit has to be sturdy enough to hold shape fairly well - failing here means direct damage to the human;
Seat has to protect against vertical crash pulse - the seat and (perhaps the floor structure) are the only things that can do this and we lose ALL of our vertical velocity over a very short travel;

Harness system has to keep you in place - failure here means secondary collisions and direct damage to the human;
Nose down contact is deadly - Yeah on a hard surface some nose down was tolerated, but flat to slightly nose up was much better, and on dirt, well, nose down was only a little better than a barrier crash.

A couple of other observation that I feel a need to point out:

Nose down contact was needed to produce large longitudinal accelerations, and they really crunched up the cockpit. If you built up the cockpit enough to largely hold shape in a serious nose down crash, the airplane may well become unflyable for weight, and the decels will still be huge and thus tough to make survivable;

If the pilot can get the nose above the local terrain, and the sink rate relative to the local terrain low, current airplanes will protect you pretty well. Improvements in crashworthiness that we are talking about here are largely in the form of enlarging the survivable envelope, but since the energy goes up with the square of the velocity, well, you are still going to have to get the vertical velocity and total velocity down before contact if you intend to live through the event. That comes back to pilot capability all over again.

Billski
 
2
Top