Small aircraft crashworthiness

[pmccarthy]

With LINE-X the plane would be perfectly intact after a crash and the occupants would be strawberry jam.

 

 

[Dafydd Llewellyn]

LINE-X spray makes anything unbreakable. http://wimp.com/sprayunbreakable/

My next paint job.

Well, if it forms a strong film that shrinks and thus pre-stresses to concrete block in compression (and the porous surface of the block is ideal for that), then it takes advantage of the compressive strength of the block, and overcomes the very low tensile strength of unreinforced concrete. However, that's the reverse of what we have in s typical fuselage structure; the shell is quite strong in tension, but weak in compression, because it buckles. So you might find that a coating of that nature rather less effective than that video would lead one to expect, on the impact resistance of an aircraft fuselage. Of course, it would make the pieces easier to find . . .

 

 

[Dafydd Llewellyn]

Some of you may find the attached of interest. The third image is a photoshop drawing, but the real thing looks very much like that. It has a faired-in cargo pannier underneath the crash cage, which can (provided it's not loaded with hard objects) provide some eight inches of crush capability. The pilot's footwell also contributes to the crush capability; the rudder pedals are designed to move rearwards as it crushes, carrying the pilot's feet with them. This fuselage has the major features identified in the thread. I don't see how to achieve more than this without the design getting silly.

 

 

 

 

 

[facthunter]

There is a practical limit to building a structure that will cover higher velocity events, with an aircraft. In flight fire is a serious event, requiring quick response and getting out after an invert or damaged structure. The seat design including restraints and the cell around the occupants is very important. The front mounted motor helps as shield to some extent when impacting less than very solid objects.

 

Primary safety involves things like visibility handling, control/ stability, performance. and most importantly the pilot's skills and experience and attitude to safety matters. Nev

 

 

[turboplanner]

That looks very promising Davydd, would reduce the chances of the engine/instrument panel rotating in on the occupants if it hit in a shallow dive, or his a stump while sliding along on its nose, and also reduces the chance of the rear fueselage folding up and crushing the occupants from the rear.

 

A similar profile in a low wing would make a huge improvement in protection from upside down crush, and deformation of the canopy preventing occupants getting out in a fire.

 

A horizontal bar connecting the front and rear of the roll cage could be used as high as possible, consistent with getting into the aircraft standing on the wing without being obstructed by the top of the roll cage.

 

I've attached Formula 500 details because all up weight with a driver and fuel is about 400 kg and it spends its time between about 100 and 190 km/hr, so the roll cage has roughly a similar task to an RA aircraft coming in at those speeds, and the design has evolved over 40 fatality free years. Ingress from the ground is roughly the same as ingress from a low wing, but the driver does have to bend to get in and out.

 

The splay shown on the F500 diagrammes came from older, very narrow cars where you squeezed yourself between the chassis members. These days they are parallel or wider at the bottom.

 

In terms of weight, the heavier bars, Chrome Moly 4130 steel tube 32 mm OD x 1.6 mm wall weighs 1.19 kg/metre.While I have weighed complete frames, the figures are archived on paper at the bottom of big piles. The gusseting is based on successive crashes, and allows some parallelogramming and distortion in the crash which provides some progressive crumple.

 

 

 

 

 

[Dafydd Llewellyn]

That looks very promising Davydd, would reduce the chances of the engine/instrument panel rotating in on the occupants if it hit in a shallow dive, or his a stump while sliding along on its nose, and also reduces the chance of the rear fueselage folding up and crushing the occupants from the rear.A similar profile in a low wing would make a huge improvement in protection from upside down crush, and deformation of the canopy preventing occupants getting out in a fire.

 

A horizontal bar connecting the front and rear of the roll cage could be used as high as possible, consistent with getting into the aircraft standing on the wing without being obstructed by the top of the roll cage.

 

I've attached Formula 500 details because all up weight with a driver and fuel is about 400 kg and it spends its time between about 100 and 190 km/hr, so the roll cage has roughly a similar task to an RA aircraft coming in at those speeds, and the design has evolved over 40 fatality free years. Ingress from the ground is roughly the same as ingress from a low wing, but the driver does have to bend to get in and out.

 

The splay shown on the F500 diagrammes came from older, very narrow cars where you squeezed yourself between the chassis members. These days they are parallel or wider at the bottom.

 

In terms of weight, the heavier bars, Chrome Moly 4130 steel tube 32 mm OD x 1.6 mm wall weighs 1.19 kg/metre.While I have weighed complete frames, the figures are archived on paper at the bottom of big piles. The gusseting is based on successive crashes, and allows some parallelogramming and distortion in the crash which provides some progressive crumple.

That crash cage of mine is actually the aircraft primary structure; it's essentially a 25G cage but it also carries the undercarriage, the engine mount, the wing and lift-strut, and the rear fuselage. The skin of it is largely removable for servicing; and since it has "stroker" seats, the control circuitry runs outside the steel frame, between it and the non-structural skin. The total weight of the yellow frame in those photos is 56 lbs. It's "fail-safe" to some extent; the analysis covers a number of single-element failures. The aircraft has two seats in tandem.

The detail of the frame is not evident in those photos, but it can certainly deal with the FAR 23.561 inverted case (3 G inverted). The analysis showed that a "spider" is needed in the windscreen aperture, to minimise local bending stresses at a number of cluster joints; however it has to be made from non-magnetic material , and so is not in the photos.

 

 

[facthunter]

Yes. the high load parts are in a compact area and a space frame is ideal. I would like to see more use of a high temp metal skin between the engine bay and the cabin too extending back and underfloor. Behind the rear seat could be a different form of construction .Nev

 

 

[Oscar]

What you are seeing there is a hybrid: a 'space-frame' occupant cage that connects the mainspar, u/c and engine mounts thus keeping all the highly-stressed components flying along in harmony, and an essentially monocoque rear fuselage cone that provides light-weight stiffness to the empennage. A 'glass outer skin provides aerodynamics and by being easily removable (since it is not providing structural integrity), allows remarkable access to the control runs etc.

 

In the flesh, this is a very deep-chested aircraft - a bit like a Staffordshire Terrier by comparison to the Whippets of the small aircraft world. The 'Stroker' seat requires some of this depth, but there is also depth from an integrated pannier that contains usable load items in an entirely safe 'container' (that has low impact on c/g variation). It is the only aircraft that I've ever seen that one can easily remove a (large) outer panel and access the control runs for inspection, adjustment and maintenance.

 

 

[Dafydd Llewellyn]

Yes. the high load parts are in a compact area and a space frame is ideal. I would like to see more use of a high temp metal skin between the engine bay and the cabin too extending back and underfloor. Behind the rear seat could be a different form of construction .Nev

Those photos are of the plugs for the fuselage moulds, so naturally no proper firewall in there at that stage. Yes, there will be a conventional stainless-steel firewall, extending to the lower skin, with fibrefrax backing. Attached a shot of the rear fuselage, for your interest - the sheet metal bit weighs 30 lbs. There's a vapour barrier between the pannier and the cabin.

I'm not trying to sell you the aeroplane; I'm trying to illustrate how some of the secondary safety feature identified in this thread can be incorporated into the design of a small aeroplane. It's designed for certification under CS-VLA, so of course it has all the safety features demanded by that standard.

 

 

 

[facthunter]

We are talking about design concepts That's fine as far as it goes. Nev

 

 

[rgmwa]

What software are you using to analyse it, Dafydd?

 

rgmwa

 

 

[Dafydd Llewellyn]

What software are you using to analyse it, Dafydd?rgmwa

I used Microsafe, which is an old DOS - based FEA program. I did the analysis almost a decade ago.

 

 

[Dafydd Llewellyn]

We are talking about design concepts That's fine as far as it goes. Nev

Ha - I assume your concern is that in the event of an engine fire, the fuselage skin aft of the firewall needs to be fire-resistant? Let's talk about that, because it has both primary and secondary safety aspects:

Firstly, there is a requirement in FAR 23 and CS-VLA that the fuselage skin directly behind any cowl outlet (by which is usually understood, any area of skin likely to be affected by flames coming from the cowl outlet) for a distance of 600 mm aft of the outlet, must be at least as fire-resistant as normal aluminium alloy aircraft skin (FAR 23.1193(d) or CS VLA 1193(d)). That is normally addressed by adding a piece of metal skin in that area outside the GRP skin. (There is no such requirement in most of the recreational aircraft standards.)

 

Secondly, the cowl outlets in my aircraft are pieces of 0.016 inch wall stainless steel tube - i.e. they are in a regulatory sense, parts of the firewall. What you see in the photos is the chute in the fuselage skin plug to cater for them. The stainless steel tubes finish about 3/4 of an inch clear of the skin.

 

Thirdly, an engine cowl is supposed to be as fire-resistant as aluminium alloy (23.1193©); again, that's not carried over into most recreational aircraft standards. Containing an engine fire in an aluminium-alloy cowl is rather akin to boiling water in a paper bag; it depends upon the cooling supplied by the external airflow to prevent the cowl from melting. This makes the use of GRP cowlings an interesting question; in the past, people simply used a "fire-retardant" resin and added 15% of Antimony Trioxide to the resin, and let it go at that. But any GRP resin is an organic material, and it is not truly fire-resistant to the extent that aluminium alloy can be when it is cooled on the non-fire side; and GRP is a much less effective conductor of heat, so cooling the outside is not particularly effective.

 

A single-engine aircraft requires a "fire life" of the cowling of 15 minutes (look up the definition of "Fire resistant" in FAR Part 1). That is likely to require something additional to a bare GRP cowling; there are various approaches - one could glue Fibrefrax blanket to the inside - if one had a truly fireproof glue - but the insulation must not be able to absorb spilled fuel or oil, so that won't answer. Intumescent paint sounds good, except that it's likely to fall off when it chars, rather than remaining in place to protect the cowl.

 

I've not seen anything that I'd consider a truly fire-resistant engine cowl on any recreational aircraft. I have some ideas on this, but not yet tested. At least, the cowl outlets on my aircraft are visible to the pilot.

 

 

[turboplanner]

We built a Fibreglass Reinforced Plastic monococque truck mounted fuel tank for Shell using Monsanto fire retardant resin. Shell had bought it based on evidence of a truck crash with a similar tanker in Europe where the tank contained the petrol during the fire which just burned where it was boiling out of a fractured section which had taken the impact. The tank had contained the fire for the duration until firefighters put it out.

 

The objective was to produce an alternative to aluminium tankers which had a maintenance cost when cracking occurred, however Shell got cold feet and switched the tanker to heating oil operations.

 

It would be interesting to see where fire retarding resin is today in terms of strength, fire performance and cost vs standard laminating resin.

 

 

[Dafydd Llewellyn]

How thick was the layup? In a massive (by comparison with a typical aircraft cowl) layup, the temperature of the laminate can be controlled for a while, by the use of hydrated aluminium oxide powder in the resin - around 50% by weight. The latent heat of vaporisation of the water is quite effective - while it lasts.

 

Also, you do not have a tornado of air inside the tanker, as you do inside an engine cowl, so there won't be sufficient oxygen to get a really hot flame inside. So not really a valid comparison.

 

Most readily-available fire-retardant or heat-resistant resins I've come across are getting pretty hot and bothered at around 300 F. "Fire-resistant" requires 1100F. You may be able to get better than this, but it won't be available in small quantities. Derakane 510A plus 15% W/W of Antimony Trioxide just managed to meet the FAR 23 requirement, but it was largely char by the end of the test, with barely any structural strength remaining.

 

 

[turboplanner]

It's a long time ago but maybe about 3 1/2 ozs x 4, or about 8 mm thick.

 

Yes, the oxygen was excluded from inside the tank, but the flames were working over the outside of the tank.

 

I recall reading something about the NASA re-entry skin design where someone discovered it didn't have to be fireproof, just achieve a slow detrition rate (there is a specific name for it), and that's what they did before they went to ceramic tiles.

 

 

[Dafydd Llewellyn]

Yep, ablation is the term you want - the trick is to form a strongly-adhering layer of char, which acts as an insulator. The use of hydrated aluminium oxide works that way, as far as I can see. The term "fire-retardant" in respect of resins systems seems to mean that they do not themselves support combustion, but slowly char. There's very little evidence of real thought in regard to fire resistance, in most small aircraft cowl designs.

 

 

[Oscar]

It's a long time ago but maybe about 3 1/2 ozs x 4, or about 8 mm thick.Yes, the oxygen was excluded from inside the tank, but the flames were working over the outside of the tank.

I recall reading something about the NASA re-entry skin design where someone discovered it didn't have to be fireproof, just achieve a slow detrition rate (there is a specific name for it), and that's what they did before they went to ceramic tiles.

I suggest that the petrol inside was actually acting as a coolant - given the thickness of the matrix - and may well have done so for a while until enough of it escaped (burned-off), had the fire brigade not been there - then BOOM!. A metal tank would likely have reached BOOM rather more quickly, so one up for the 'glass, I believe. Most vehicle fuel tanks nowadays are (as far as I am aware) some breed of cross-linked PE, so let's not disparage a 'glass, or plastic, fuel tank. I don't believe there have been any crash-and-burn accidents for Jabirus (essentially, always 'glass tanks in one form or another), whereas there have been quite a number of complete annihilation fire results with metal -tanked aircraft (e.g. the RV6 at Gatton recently),which I suggest results from stress rupture of the tank structure allowing fuel to leak at a fast rate and once ignited, it's going to burn until exhausted / smothered.

 

This image of the Cootes' tanker fire at Mona Vale shows how extreme a ruptured steel petrol tank spreads fire: http://resources0.news.com.au/images/2013/10/09/1226735/408924-3f57f8a6-307d-11e3-a489-bd783c3f5e8c.jpg

 

I built several alloy fuel tanks for small racing cars (before these had to be built by a certified builder), with the requisite foam fill, and they are a bugger to build effectively to avoid cracking at the seams and the restraint points (there are design tricks, and they are definitely in the category of 'don't try this at home'). I'd personally far prefer a 'glass tank built from aromatic-resistant 'glass than any other medium (it is hard to get, but available).

 

 

[facthunter]

Dafydd, the Citabria which I sold a while ago had a glass cowl . Which appeared much like any other glass. I can't recall much metal sheet below my feet. With the wind blowing as it does when you are flying, the heat is pretty intense if you have an oil or fuel fire, so you have to act very quickly. It's not an area that a ballistic chute is any use either.. This is an area that needs more attention An easy access fuel isolator is required too. Nev

 

 

[Dafydd Llewellyn]

Dafydd, the Citabria which I sold a while ago had a glass cowl . Which appeared much like any other glass. I can't recall much metal sheet below my feet. With the wind blowing as it does when you are flying, the heat is pretty intense if you have an oil or fuel fire, so you have to act very quickly. It's not an area that a ballistic chute is any use either.. This is an area that needs more attention An easy access fuel isolator is required too. Nev

Nev, I agree; and a fuel shut-off valve within the pilot's reach is a requirement for just about any certification standard. The multiple fuel valves one sees in some amateur-built versions are not acceptable for certification.

The Citabria was a CAR 3 aircraft; CAR 3.625(b) contains the same requirement as FAR 23.1193(d).. I recall the Citabria did have metal skin on the fuselage underside, back to the undercarriage (it was openable, as an access panel). I flew a 7GCBA and the 8KCGB (if I have those numbers right) at Tocumwal; and a friend owns a 7GCBC, so I'm tolerably familiar with them. You can't pick a fire-retardant resin by its appearance, normally.

 

Fire is something a lot of people turn a blind eye to, possibly because it's too horrible to contemplate. This is foolishness; with a little thought, the risk of an in-flight fire can be vastly reduced, and the survivability vastly increased. It gets a lot of attention in certification, but only to the level demanded by the relevant standard. One of the things I do not like about the Rotax 912 is that the fuel system is above the engine. That's an unnecessary fire risk, as was proven by the Continental IO-360. The optimum engine layout for fire risk is exhaust on top, fuel system underneath - and the Navajo and Cessna 400 series are set up that way. However, cowl outlets ahead of the windscreen are not unknown, amazingly. Somebody must be insane . . .

 

 

[Jethro Belle]

Quote: I should add - a fully-triangulated steel space-frame makes an excellent non-deforming structure, and it's not particularly heavy - but when it does collapse, it absorbs little energy. The cabin structure of the Jabiru does absorb significant energy by minor elastic deformation, and to that extent it is arguably superior to an equivalent rigid steel space-frame; ...

Thank you Dafydd, I have read your posts with serious interest and find them reassuring.

 

Does not a steel space-fame absorb significant energy during deformation, unless it is manufactured from high tensile material and/or has 'brittle' welds. Is not a ductile space-frame capable of significant energy absorption, and if it had a sufficiently high yield strength it would provide equivalent rigid body (separation) protection to a lighter non-ductile structure of higher yields strength?

 

I am not current with aircraft CrMo steels, but believe (from Australian Supplier data) the available tubing is normalized grade 4130 having a typical ductility of 12% at >600 MPa. That makes them pretty tough (energy absorbing).

 

Maybe you are saying is that a space frame built as strong as yours will not buckle until it has killed you with G forces!? Do you think a cleverly designed space-frame could start absorbing energy by crumpling (controlled bucking, which I know is unpredictable unless cleverly designed) before it kills you with G forces? Based on images posted here Jabirus seem to absorb the energy by ripping of the front of your protective cocoon, so it would be no worse than that if members were designed to buckle outwards. This buckling would commence once the other energy absorbing structures are fully crushed.

 

 

[facthunter]

Unfortunately Dafydd doesn't frequent our forums now, much to our detriment, Jethro. It's nearly 4 years since that post. Re reading the lot reminded me of former times. Nev

 

 

[bexrbetter]

Jethro, your thoughts concerning spaceframes are pretty well spot on.

 

I've seen plenty of big hits on spaceframes and to say they don't absorb energy is ludicous.

 

 

[Oscar]

Of course a steel triangulated space-frame absorbs energy. If it didn't, every such structure would simply fall apart when subject to load. This is akin to saying that water is wet.

 

It is the rate of rise of the member fail-point (in part) that impacts the human frame. It is no damn use to you if the stresses upon (for example) your spine being transmitted from the space-frame relieve just AFTER your spine has broken.

 

A tubular steel structure CAN be designed to fail - and in a complex structure, CAN be designed to fail progressively - within the envelope of human tolerance. FAA compliant crash-worthy seats are (often) good examples of this.

 

FEA has improved the ability of designers to create progressively deformible structures vastly less costly than the old 'break-it-and-see' regime. However, even with CNC-controlled welding of 4130, it is (in my opinion) likely you would need to do a full heat-treatment of the completed structure to ensure there are no overloaded junctions etc. May I point you to the commentray regarding Ducati motoGp bikes in the era 2007 - 2009: $m ultimate racing machines with a welded space-frame. According to the most successful rider ever for Ducati, these 'varied with every frame' in characteristics.

 

I have built racing cars from tube-frame structure - I DO like it as a medium. BUT, it isn't the answer to the Maiden's Prayer, it is just ONE way of getting there. Of all the high-performance devices now in use, I believe that a space-frame is only used by KTM on their motoGp bikes - everything else has gone to either composites or forged/cnc-machined alloy beam construction.

 

Jabirus are possibly unique in the ultralight market, in being built from low-tech ( ambient-cure resin and glass reinforcement) materials, which by their nature have considerable elasticity and a slow rising-rate of resistance to load. It is no aberration that the fatality and injury rate for Jabirus is remarkable.

 

 

[Jethro Belle]

Jethro, your thoughts concerning spaceframes are pretty well spot on.

 

I've seen plenty of big hits on spaceframes and to say they don't absorb energy is ludicous.

 

Thanks @bexrbetter for your confirmation. Dafydd seemed to be distinguishing the crash cocoon surrounding the pilot from the rest of the fuselage spaceframe and I was questioning if making this part 'infinitely rigid' wasn't overkill and excess weight (seemed to be what he thought he was doing in his build). If this is was what Dafydd was saying then the cocoon would not absorb energy (and not intrude on the pilot), instead it would tranfer the force remaining after fuselage crumpling to the pilot harness/seat. Yes I know there is no such thing as 'infinitely rigid': just keeping it simple. Maybe so rigid you die first is the correct phrase .

 

Based on your big hits, did the members bend in or out? Did they protrude into the pilot? What is your opinion about how a Jabiru would handle the same 'big hits'? Fibreglass is pretty amazing in failure. If well made it absorbs far more energy than you expect, not just by massive elastic deflection, but by fibres progressively failing. Clever FRP design can achieve significant energy absorption (I recall flywheel energy storage used carbon fibre that was designed upon failure to absorb energy by becoming 'fairy floss' and avoiding large chuncks taking off your head.).

 

CrMo structure appeals to me because the design does not require Laminate FEA and it provides rigid strong tough points of knowable behaviour to anchor things like the harness, seat etc. Although it is considered old tech by some, for one off home-design/build (where quality achieve and control is hard) it is hard to beat for safety (Also when buying an aircraft someone else has built). You can design it to fold up in certain ways, as many have posted, which is where crash survivable design lies IMHO. Well designed (extensive CAE) and developed (A crash testing program) FRP will win out for factory builds IMHO (Actually what has happened and what Oscar said).

 

Of all the high-performance devices now in use, I believe that a space-frame is only used by KTM on their motoGp bikes - everything else has gone to either composites or forged/cnc-machined alloy beam construction.

.

 

Jabirus are possibly unique in the ultralight market, in being built from low-tech ( ambient-cure resin and glass reinforcement) materials, which by their nature have considerable elasticity and a slow rising-rate of resistance to load. It is no aberration that the fatality and injury rate for Jabirus is remarkable.

 

Yes I saw the fatality stats and they are very low! Amazing and a big plus for Jabiru. I wonder if the energy absorbed wrenching off the front firewall is greater than if they made it stronger so it didn't break free

 

Yes this is the common theme in structural materials. In the pursuit of weight saving the strength is increased, but ductility suffers and with it toughness (energy absorption). Plus with thinner sections bucking is more problematic. In the case of FRP especially CFRP it tends to be load oriented lay-ups that give incredible strength, but as soon as that is exceeded: NOTHING. That means massive loads on the occupant and then fracture with 'no' energy absorbed (the area under the elastic deflection curve of CFRP is negligible because its elastic modulus {for low resin layup beyond the ability of most home-builders} is so high, unlike glass fibre). The other problem with oriented lay-ups is they fracture at minor loads parallel to the main fibre orientation when non-design oriented loads are applied (what happens in crashes). Of course very knowledgeable fibre reinforced polymer experts with big budgets and computational grunt (like F1) can design 'toughness' and crash loads into the structure. Hopefully FRP aircraft builders are up there with F1. My exposure to naval architecture indicates early CFRP yacht designers didn't appreciate, or even consider, what happened when you hit a submerged object: Result = multi-million dollar embarrassment dragged to the bottom in a few seconds by the tonnes of lead in the keel

 

even with CNC-controlled welding of 4130, it is (in my opinion) likely you would need to do a full heat-treatment of the completed structure to ensure there are no overloaded junctions etc

 

I understood a weld stress relief was undertaken to relax weld stresses. What do you mean by a full heat treatment? Quench and temper?:yikes:I was a little surprised about the temper condition variability CrMo suppliers in Australia quote online as aircraft grade. Made me wonder a bit about what home-builders were buying and how they were subsequently heat treating. Must research this more.

 

Unfortunately Dafydd doesn't frequent our forums now, much to our detriment, Jethro. It's nearly 4 years since that post. Re reading the lot reminded me of former times.

 

Thank-you for the update Nev. I suspect he was giving more than he got, so gave up and is posting elsewhere. Shame, but his posts are still great guidance for people like me.