Quantcast

Crashworthiness of structures, was "General question

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

orion

R.I.P.
Joined
Mar 2, 2003
Messages
5,800
Location
Western Washington
re: Crashworthiness of structures, was "General question

Given that this post had drifted off the subject of the original thread, I thought it deserved a new heading so that the two questions posed might get a more specific answer.

Regarding the wood structure, assuming that the plane was built of quality materials and that proper adhesives and protection were employed, it's likely that even 60 year old wooden airframe should be sufficient for the job at hand. But in order to determine its suitability, a close inspection will have to be performed by someone who is well versed in wood construction and restoration.

About ten years ago I saw a 40+ year old Barracuda parted out because the grandchildren of the original owner did not want to take the risk of passing on an old, wooden structure that they had no way of evaluating. After they parted out all the components they could, they decided to let a few local builders in on the job of airframe disassembly, through the use of a few sledge hammers. Interestingly enough, the plane was in such good condition that quite a few of the blows simply bounced back, nearly taking out the person swinging the hammer. Smaller, thinner structures eventually did break but the spar never did and neither did much of the fuselage. The beefy core structure had to finally get disassembled with a chainsaw.

As far as the crash protection issue is concerned, the best structure for protection of the occupants is one that will be able to absorb a lot of energy. It's not necessarily the "strongest". In this comparison it may actually be the composite that is better in a crash since the composite can take quite bit of energy and dissipate it through deflection and spring-back. I've seen post crash Glasairs for instance that took quite a beating but with little injury to the occupants.

Aluminum on the other hand may be strong but will lose its integrity the moment the reinforced monocoque shell loses integrity and buckles. After that point there is actually little there to absorb the energy, the result being that the aluminum shell simply collapses in on the occupants.

But this is not an easy comparison since it must be assured that all design conditions are equal, which I think in the case of the Cirrus and the Bonanza they aren't necessarily so.

And the one thing that is not equal between the two is interior space. The Cirrus has much more occupant volume and is markedly more comfortable - a pretty important criteria for those that fly long legs.
 

D Hillberg

Well-Known Member
Joined
Nov 23, 2010
Messages
1,310
Location
very low low low earth orbit
re: Crashworthiness of structures, was "General question

With wood its conditional, No rot,No bugs, The rest can be repaired, Advivserary Circulars A/Cs and other referance materals on the FAA web site. Glues have advanced from the 1900s, and the Bonanza is a fine plane.
 

orion

R.I.P.
Joined
Mar 2, 2003
Messages
5,800
Location
Western Washington
re: Crashworthiness of structures, was "General question

I haven't seen a post crash Eze personally so I don't know. I did fly in an Eze some years back and while the structure looked fine for flight, I don't think I'd feel all that comfortable in one in a hard landing - not a lot of structure in the cockpit's flat sides. But that's just a cursory glance opinion.
 

D Hillberg

Well-Known Member
Joined
Nov 23, 2010
Messages
1,310
Location
very low low low earth orbit
re: Crashworthiness of structures, was "General question

Seen a lot of (long ez & velosity) experimental and other ( Helicopter,Fixed wing crashes), With the rear mount the engine will pass through you in a violent crash,With the engine in the front it becomes a battering ram, Composets are strong but in an impact like in an typical aircraft crash you have multiple impacts and all the composits after the first impact are compramized and have little protection during the run out, Where an aluminum structure deforms and remains with the continued forces till distruction absorbing energy, The test is take a coke can and stomp on it with differant perssures,Now take a plastic spoon and bend it till it breaks. The can will absorbe the energy and the spoon will store that energy till it PoPs. Used to help NTSB and others pick up wrecks, You learn alot about how things are built.
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,762
Location
Rotterdam, Netherlands
re: Crashworthiness of structures, was "General question

Seen a lot of (long ez & velosity) experimental and other ( Helicopter,Fixed wing crashes), With the rear mount the engine will pass through you in a violent crash
Yepz, but what's violent? If the crash is so violent you can't survive it anyway, why bother with a stronger and heavier rear mount?
Composets are strong but in an impact like in an typical aircraft crash you have multiple impacts and all the composits after the first impact are compramized and have little protection during the run out
Yeah, that's the opinion usually expressed by pilots. It's also false, composites can be far superior in crash survivability, if properly designed.
[video=youtube;AtrzvwayniM]http://www.youtube.com/watch?v=AtrzvwayniM[/video]
The speed measured when his car clipped the barrier was 300.13 km/h (186.49 mph), at a 75-degree angle, subjecting Kubica to an average deceleration of 28 g. After data from the onboard accident data recorder had been analysed it was found that he had been subjected to a peak G-force of 75 G"
So this 185 mph frontal crash should've resulted in the total destruction of the car during the 2nd hit wouldn't it?
There's a reason virtually all motorsports use composites, exactly for that reason; crash survivability.

I've watched a glider pancake in almost vertical (45 degrees nose down) at something like 3000+ fpm sink on a hard surface. Cockpit stayed in shape (though deformed) and so did the heavy engine structure. Head trauma was the cause, not the composite structure which did impressively well.

Don't let the all too often faulty "common knowledge" get in the way of doing the proper engineering and design..
 
Last edited by a moderator:

D Hillberg

Well-Known Member
Joined
Nov 23, 2010
Messages
1,310
Location
very low low low earth orbit
re: Crashworthiness of structures, was "General question

Indy went to an All carbon tub late 1980s early 1990s and the results were a desaster,The all carbon tubs were changed to aluminum/carbon secondary designs, I worked at Gulfstream LGB and the Composet materals were a pain, The loose screw under foot did thousands of dollars in damage,Most Kit plastic planes do Not hold up in a crash,Seen it over & over, and the Aluminums handling of crash forces were better than the plastic body planes.( The Dynamics and peak stresses to the pilots) From the 16 years of recovering wrecks and 37 years in Aviation I see what worked well and what is a pile. I've seen more people walk away from Aluminum wrecks that were mangled and the likewise composet that was fractured in 2 or 3 pieces with all fatal. And when I say "Violent" the structure is waded up and pieces all over . the plastic planes lose. and a plus on the whole mess,The plastic plane requires hazmat fees and disopsal at its end of a life. Tha aluminum plane at its end is recycled and Pays for the booze at its wake.
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,762
Location
Rotterdam, Netherlands
re: Crashworthiness of structures, was "General question

Most Kit plastic planes do Not hold up in a crash
And that's where proper engineering comes in. If you don't design for safety results may vary (but will probably be bad). If you do design for it, composites can be extremely good at it.
I guess (and that's just that, a guess) something like a C172 is quite a bit more tolerant to crashes (for the occupants) compared to something like a Long-EZ, since at the latter your crumple/deflection zone is only a fraction of the thickness of the Cessna's. That's got nothing to do with material though and everything with designing it for crash safety.
 

orion

R.I.P.
Joined
Mar 2, 2003
Messages
5,800
Location
Western Washington
re: Crashworthiness of structures, was "General question

The problem with this discussion is that there are way too many "what ifs" in a typical crash scenario. Even minor variances in a crash can result in sizable variability of results. Essentially we're discussing energy, which is a function of velocity squared. As such, even minor differences in speed, angle of impact, ground condition, etc. can have significant effect on survivability and even with years of accident recovery or investigation, the data is unlikely to reveal a set trend in characteristics. The only thing we can then take a look at is the practical application of the materials.

Using the aforementioned pop can as an example, when empty, the can can be set upright and with a slow application of even force, can support a sizable weight. But load that can up and slightly tap one side and the can will collapse in on itself in a sudden and catastrophic manner.

Then, take a cylinder of equivalent strength made of composite and what you'll see is that due to the somewhat thicker wall the cylinder will not only hold the same amount of weight (as I said, needs to be designed to the same strength - results even better when designed to stiffness), it will not be as vulnerable to minor wall distortions and so when it does collapse, it will do so slower and to not as great a level as evidenced in the sudden collapse of the original pop can.

But I too have seen post crash results and in the cases that I refer to, the damage seemed to be of less significance, with a relatively high level of survivability. But again, unless one compares two equal designs (airplanes designed to equal requirements) impacting exactly the same, any perceptions or trends may be significantly skewed.

And regarding an earlier post, I was associated with a Grumman dealer here some years back. The association included several post crash investigations of the airframes and as the poster above indicated, in all cases the cabin survived nearly intact with all occupants waking away with only minor injuries. Great structure in that aluminum sandwich.
 

fadec

Well-Known Member
Joined
Aug 18, 2011
Messages
105
Location
New Zealand

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,489
Location
Saline Michigan
The biggest problem you have with doing this comparison based upon looking at wrecks is that, for the most part, composite planes land faster. With more kinetic energy, you can darn well expect worse wrecks, even if they were built to similar strength. Therein lies the real lesson. The slower its landing speed, the less kinetic energy it has during an emergency landing, the less likely it is to kill you then - Choose your design landing speed carefully.

F1 and unlimited hydros have found that when they design the cockpit to be sturdy, the survivability goes way up. Autoclaved carbon is standard in both arenas.

Billski
 

D Hillberg

Well-Known Member
Joined
Nov 23, 2010
Messages
1,310
Location
very low low low earth orbit
Too Heavy-Boats & planes have no common area, Composet crash reactions and the storage of energy in a plastic reinforced structure make design problematic, Sikorsky & Bell did tests and the structures built out of aluminum had lower peaks and transfered less energy to the people. The primary structures are still aluminum and placing composets in other areas. Carbon has another problem when it fractures the barbs stab & cut like a surgens scalple, Ever put your hand in a carbon structure with a strand sticking out? It gets bloody very fast.
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
14,313
Location
Port Townsend WA
I agree with Hillberg, metals absorb crash energy by yielding. Wood and fiberglass and carbon do not yield as well as metal(if at all).
That's why cars still have metal crush zones.
In general, you want two separate structures (cockpit and crush zone) to protect the occupant. The cockpit needs to be strong enough to protect the pilot, but yielding of the cockpit is not desired so maybe carbon (protected with kevlar for fracture barbs)would work for the cockpit.
But the crumple zone should be designed for progressive yield. The area ahead of and below the rigid cockpit should be metal to absorb the energy, in my opinion.
BB
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,762
Location
Rotterdam, Netherlands
Too Heavy-Boats & planes have no common area
You did realize we're talking about 800 lbs boats that can survive 160 mph crashes?
Composet crash reactions and the storage of energy in a plastic reinforced structure make design problematic, Sikorsky & Bell did tests and the structures built out of aluminum had lower peaks and transfered less energy to the people.
So they can't design good enough in composites. Your point being?
The primary structures are still aluminum and placing composets in other areas.
Yes, because those materials are proven, the manufacturing is known, the tools already exist, the knowledge is already there and there is no economic driver for them. They sell 40 year old products for big money and don't have to innovate or improve. If you realize that the Formula 1 racing team (just the racing team, not the whole factory) of Ferrari probably has a bigger engineering budget as both of those companies together, you might get a grasp about the drivers behind these kind of decisions. Aviation simply lacks the budgets to advance rapidly, especially because of all the other complications of certification. The first certified carbon aircraft flew 35 years ago and were superior in every single area. Nevertheless it took 25 or so years before composite/carbon aircraft obtained a reasonable market penetration.

That's market economics, aviation principles and so on, but has nothing to do with engineering difficulties (except for getting to know what you're working with when you already have a employees group that not only has the eduction, but also the mindset and blindfolds for metal) nor has it anything to do with material problems. Most pilots still seem to think that "plastic can't possibly be good." Apply my subscript to them and look at the facts...
I agree with Hillberg, metals absorb crash energy by yielding. Wood and fiberglass and carbon do not yield as well as metal(if at all).
That's why cars still have metal crush zones.
Nope. Several of the high-end cars and many of the racing cars use composite crumple zones.

The reason for normal cars having metal ones are pretty simple, a well-designed carbon/composite crumple zone probably costs as much in production as the rest of the car does...
But the crumple zone should be designed for progressive yield. The area ahead of and below the rigid cockpit should be metal to absorb the energy, in my opinion.
BB
Energy absorption is fairly simple and proportional to: deflection*yield strength (assuming linear behavior). Yield usually is preceded by buckling and thus doesn't do that much. Carbon, dependent on the type doesn't do much better or worse as steel or alu.
If we look at energy absorption per unit weight, which is the only relevant parameter for aircraft, carbon (and kevlar) composites typically do quite a bit better. There's plenty of literature and actual measurements on this. Here just a single one of it:

Source
 

Attachments

Top