Small aircraft crashworthiness

[Dafydd Llewellyn]

The purpose of this Thread is to bring to people's attention, and invite discussion on, some aspects of aircraft design that affect survival and injury in an emergency landing situation, in small GA and recreational aircraft.

 

The history of awareness of this aspect of aircraft design starts with the introduction of seat belts, then the addition of upper body restraint, and has now progressed to dynamic testing of cockpit assemblies using instrumented dummies to measure forces in the lumbar spine, and to check for head impact.

 

Formal design requirements for these matters are not uniformly applied across the full spectrum of aircraft classes; dynamic testing has not so far filtered down to the "watered-down" design standards for recreational aircraft; and of course no formal standards are applied to experimental aircraft for this.

 

The rapid growth of General Aviation in the 1950s and 60s mainly involved aircraft of American origin, certificated to U.S. Civil Air Regulations Part 3 (CAR3). This standard demanded, in essence, a seat belt and associated anchorages capable of resisting a 9G retardation of a 170 lb (77 Kg) occupant. It did not address the ability of the cockpit structure to resist deformation.

 

The CAR 3 requirement was carried over into FAR 23.561, when that superseded CAR 3 in about 1965; however CAR 3 aircraft continued to be manufactured under "grandfather" clauses, well into the 1980s.

 

FAR 23 introduced a requirement for upper body restraint in 1988, and made it retroactive (FAR 23.2) to aircraft manufactured after December 12, 1986. (Australia had mandated upper body restraint for front seat occupants in the 1960s).

 

Consideration of heavy objects above and behind the occupants was added - not sure when, possibly FAR 23 amd 44 or thereabouts.

 

The FAA introduced dynamic seat testing in 1988, by the introduction of FAR 23.562; this requires a form of "barrier crash test" , but is mainly aimed at ensuring that the compressive force in the lumbar spine does not exceed 1500 pounds under the prescribed impact conditions, which for the principal case require an impact at 60 degrees from below the aircraft longitudinal axis. This requirement has not as yet been applied to the design standards for recreational aircraft.

 

 

[Dafydd Llewellyn]

The general reaction of manufacturers to new technology in regards aircraft safety, is "we don't mind doing it if everybody else has to do it" - except that each increase that requires special testing, adds another barrier to new entrants. The dynamic seat testing issue has been a watershed because of this; in effect, the applicant had to provide several test specimens, and the only accredited test facility was at one time located in the U.S.A.. Each test cost something like $US 25,000 to run, and destroyed the specimen in the process. There are at least two tests, so if the applicant's design managed to pass, compliance with FAR 23.562 might cost as little as $100, ooo - but the problem with dynamic testing is that it's not merely a case of firing the specimen at a " brick wall" - the test must achieve a reasonable approximation to a defined acceleration / time history (referred to as "pulse shape") - and that is as much a result of the crush behaviour of the specimen as of anything else - that requires trial and error, so in practice the cost is likely to be several times that.

 

This sort of thing has forced some manufacturers to turn their back on "mainstream" aviation, and instead focus on marketing experimental kits, or on recreational aircraft whose design standards do not include things like dynamic testing or overturn safety, or fatigue life aspects. That market demands short build time, good looks (which often involve "styling" features that are counter-productive to safety) , and performance. Generally, safety takes a back seat, because the average consumer neither understands what is missing, nor wishes to do so.

 

We have now had fifteen years of experience with experimental amateur built aircraft in Australia; whilst that's not really very long, given the build time of most kits in the hands of the average consumer (if there is such an animal!) - we are starting to get a glimmer of an understanding of what their advantages and disadvantages are.

 

How important are safety considerations to you?

 

 

[turboplanner]

We were recently discussing the inherent weakness in an open top cockpit.

 

I went looking for details of an Indianapolis crash into the wall, which resulted in a major redesign of that class of car to try to make major wall crashes survivable.

 

Both F1 and Indy car cockpits are open top. F1 technology is too expensive for us, but from memory the Indycar monococque structure got wide walls, maybe 120 mm box sections to improve the structure.

 

In searching, I came across this story. While only indirectly relevant to the subject, it does give us in insight into the classification of fatality type, and some of the things to design for to prevent it. A reasonably frequent cause of death in flying is sudden deceleration (paragraph 4), where the aircraft can appear to be completely undamaged, yet the occupants are dead.

 

http://blog.parathyroid.com/race-car-deaths-medical-causes-racing-deaths/

 

I'll keep looking because I'm sure the body structure changes on the Indy Cars were listed somewhere.

 

 

[SDQDI]

I look at it a little like the difference between cars and motor bikes, the bigger GA planes that I could use to take the whole family here and there with are the ones I would like to know had a fairly rigourous test phase. But our smaller kit built ones and to a certain extent small factory built I see more as being similar to a motor bike and personally while still having safety in mind I would not expect them to be put through an expensive test to prove it. Obviously if that was a requirement it would kill the smaller operators.

 

 

[Chird65]

I look at it a little like the difference between cars and motor bikes, the bigger GA planes that I could use to take the whole family here and there with are the ones I would like to know had a fairly rigorous test phase. But our smaller kit built ones and to a certain extent small factory built I see more as being similar to a motor bike and personally while still having safety in mind I would not expect them to be put through an expensive test to prove it. Obviously if that was a requirement it would kill the smaller operators.

I agree, this is my pastime and although I looked into the survivability rate of crashes in the KR2 range of aircraft, it was not the only criteria. Things such as ease of build, support group, performance etc. were the other criteria. It is interesting that the general comment after looking at photos of crashed KR2' is "wow they were lucky to walk away from that". The plane looked bad but the occupants were in reasonable shape. Is this because of good design? I don't think so as it is low wing and bubble canopy. The basic structure around occupants is 5/8 wood and ply. The last thing may be that the planes are light and the stall speed is low (mind you KR2 only just stall in the 40 - 50 knot range)

 

Interestingly, in my study of the KR2 the issue of Pilot currency in type, was the largest contributing factor. Unlike the Bike/Car comparison other road users were not my biggest worry.

 

 

[Old Koreelah]

Very useful info. Civilised countries investigate what went wrong in order to prevent a recurrence. Recent moves to investigate all recreational aircraft crashes might cost us more, but could yield safety dividends.

 

It cost the motor industry billions to improve safety in modern cars. Over the last three decades our Rescue Squad has seen the results: far fewer crashes and greatly reduced rates of death and injury.

 

Mass production of modular items like airbags has brought the price down. Surely quite a lot of those safety ideas can be adapted to small aircraft design.

 

There is scope for home builders to improve safety considerably. Perhaps this forum could do more to share design ideas.

 

I have incorporated a dozen or so safety innovation into my little aircraft. Some I have tested, others I hope will help in case of a prang. I'm happy to share these ideas.

 

 

[Head in the clouds]

The purpose of this Thread is to bring to people's attention, and invite discussion on, some aspects of aircraft design that affect survival and injury in an emergency landing situation, in small GA and recreational aircraft .....

Thanks Dafydd, an excellent subject for discussion.

 

Rather close to home for me too. Where small aircraft are concerned I had always held similar thoughts to those of SDSQI and Chird. From my earliest sport flying days and then into the first part of my commercial career, which was mustering in small helicopters, it seemed that those who crashed generally died or were severely maimed, so the message was clear - don't crash. Or if a crash becomes inevitable then have a well pre-planned procedure which hopefully keeps the aircraft under control throughout the event and dissipates the energy away from yourself. Not often likely to happen, of course, but better to have a plan than not, especially when your working days are spent low and slow over frequently unfriendly terrain.

 

About 5-6 years ago I started the design of a new plane that is rather different from the majority of the LSA types that predominate these days. The design is driven by the need for quick folding wings, retractable gear and forward side-by-side seating for best visibility. The design has loose parallels to a folding Drifter (love Drifters!) but with speed and comfort. True to my form I gave little or no consideration to crashworthiness, I just accepted that it was a design to be flown conservatively in terms of having large smooth places to put down in case of power failure, as I do as far as possible in any case. Compared to a Drifter this one would need quite large paddocks though, and that would mean flying higher on most flights which to a significant degree would negate the value of the terrific visibility provided by front seating. And so the first dilemma presented itself.

 

All of the components of that plane were made about three years ago and the assembly of 60-70% of it completed by about 12 months ago. During the assembly I posted an open log on a US forum site and it attracted a lot of interest and comment. Most of that comment had to do with the folding aspects and the rear engine installation. There was also a lot of discussion about the less than optimal aerodynamics at the wing centre-section, tapering aft fuselage, cooling airflow and airflow to the prop. Some people predicted dire consequences for the stall characteristics due to the double divergence on the top of the wing from the combination of its own curvature and the fuselage taper, others opined that it would have little or no adverse effect as the tailbooms acted as effective spanwise flow fences.

 

The point is - in hundreds of posts absolutely no-one at all mentioned the very poor crashworthiness of the design from the points of view of frontal impact or rollover. Any significant frontal impact would result in the engine and its mounting structure - which carries the shoulder harness - to come forward and the first point of resistance would be the backs of the crews' heads. No problem though - just don't crash ...

 

From some time beforehand, and during the build, there was a character on this other site who was generally considered to be a ghoul and also a bit of a pest. He was/is at Uni studying everything to do with crash trauma and apparently has the world's largest private database of crash information. He is often consulted by the NTSB and similar, and he visits as many crash sites as possible adding to his collection of data. His fervour in the matters of crashworthiness bordered on obsession and he drove many of us to despair but as the numbers of fatalities grew exponentially over a few years there, and in Australia too, I started to become affected by his insistent 'preaching'. The final straw for me came when I lost more friends apparently needlessly as their aircraft folded up on them in relatively minor impacts.

 

This influence caused me sit back and have a long and hard think about what I really want and need in an aircraft, and how I could achieve as many of the original design goals with an airframe that would provide the occupants with the very best possible protection in event that the worst happened. Our Nemesis on the other site didn't just preach about the need for crashworthiness, he generously and helpfully spelled out all the design features that would contribute to occupant protection in a crash. In high wing aircraft the majority of them are common sense and most of them add little or nothing to cost or weight, for the most part they are just a matter of re-configuring minor aspects of the layout. I also benefitted greatly from talks with Bill Whitney about his experiences with the crash-cell developed for his Boomerang design. As Dafydd has mentioned, the structural aspects of maintaining the integrity of the cabin space in low wing/canopy designs is very much more difficult and likely to add considerable weight or lose a lot of space to achieve successfully.

 

It might be that I'm just getting older and more conservative now, but resulting from this I've now had a very frustrating turnaround in the middle of my previous project. Consequently I am now about 80% through CAD modelling a completely different design that I hope to begin to build in perhaps six months, and which will probably provide more fun, definitely more utility, and certainly vastly less risk of injury or death if things go wrong. It's a STOL design and I've moved away from sheetmetal for this one and gone back to a welded CRMO steel and fabric fuselage. Properly designed they arguably provide the best crash cage at any given cost and weight.

 

 

[Dafydd Llewellyn]

Well, I've been thinking about this since before I looked at those RV accidents; and along the way some additional experience has led me to the following; see what you think of it:

 

Firstly, in regard to energy-absorbing seats:

 

These are a proven life-saver then the aircraft hits the ground flat, rather than nose-down. A normal fixed undercarriage (to FAR 23 standards - a lot of Exp Cat ones are way under this) is designed to a limit descent velocity around 10 feet per second; it will collapse at anything more than about 12 feet per second. A seat pan that has six inches of controlled collapse at close to but not over 1500 pounds, can increase the survivable descent rate (i.e. keep the load in the lumbar spine below 1500 pounds) to over 25 feet per second. The first attempt at this of which I am aware was the Chipmunk Safety Seat devised by, if my memory serves me, Vulcan & Sirrahlie, (Australian Aeronautical Research Laboratories) which used four blocks of polyurethane foam between to squares of plywood. You have to get a specific grade of foam, and the long-term durability is questionable, because such a seat may have to last decades before it is needed.

 

It is possible to design an energy-absorbing seat that uses a form of energy-absorbing device whose crush characteristics are not affected by how fast it happens. If the design standard allowed a simple force-deflection curve analysis from a "static" crush test - the area under the force-deflection curve represents the energy - then energy-absorbing seats would become vastly less costly. Not as good as a FAR 23.562 device, but much, much better than nothing. My first attempt at this was a seat base essentially comprising four empty beer cans between two pieces of plywood, with a hole drilled through the walls of the cans, just under the top, to prevent a high "starting" load. It was even less durable than the polyurethane blocks, but the raw materials are not a cost issue, so one could replace it every year, I suppose. This sort of thing needs to be installed in a "bin" that can carry side loads; and one of the problems is getting it to always collapse without tilting. Now that "memory" foams such as Temperfoam are more readily available, they offer a better answer. Gliders use this approach, but it needs more crush distance than is commonly available in a glider, to be really effective.

 

Obviously, sitting on the mainspar of a low-wing aircraft is NOT the way to address this problem.

 

Secondly, a non-deforming cabin structure:

 

Large, stabilised box-sections members either side are an obvious way when frontal impact is the principal issue, as in cars; but increasing the fuselage width of, say, an RV 6 by 240 mm is not likely to be attractive to consumers. It can be done with smaller members, but the added weight is also an issue. However, a non-deforming cockpit also needs something ahead of it to do the crushing - and that's a major problem in most aircraft. High-wing aircraft can achieve an adequate cabin structure provided they use hefty windscreen pillars, but even they can't cope with a direct head-on impact.

 

So the answer is to prevent the aircraft from impacting on its nose. The Seabird Seeker showed me how that can be achieved; what it needs is for the stalling characteristic to be changed from the "classical" form, where the nose drops uncontrollably until the aircraft regains flying speed, to a "minimum steady flight speed" situation, where the stick hits the back stop before the wing stops flying. If the pilot tries to stretch the glide, the aircraft will develop a high sink rate, but it won't dive into the ground. A high sink rate is also lethal - unless you have effective energy-absorbing seats. If you combine these two strategies, the "RV type" accident could be made far less lethal. The "ground aversion" response of pulling back as hard as possible will result in the aircraft arriving "flat" and hopefully sliding along on its belly, so the won't be a massive nose-on impact.

 

Thirdly, overturn protection / escape:

 

The simplest way to achieve that, is to choose a high-wing layout.

 

 

[Bruce Tuncks]

One clue to how people think about safety is to see how many wear helmets when they fly. Not many... but I reckon a couple of bicycle helmets stored behind the seats on my Jabiru would be a good idea if you had a power failure at altitude and had time to put them on..

 

I also like the idea of doing a modification to the seats if it were possible, and indeed anything that is affordable and achievable.

 

 

[turboplanner]

Jab is an interesting design - a lot of stories about how tough they are, usually after a forced landing which clearly shows the aircraft didn't hit anything anyway.

 

FRP weight for weight is stronger than steel and FRP gives a very good body with polyurethane foam so FRP sandwich panels can be made on compound shapes and can be very strong - I've seen a hand laminated FRP sandwich panel refrigerated shipping container slide off a truck and reduce a Holden to half width without suffering any damage other than surface scratches.

 

However there's one report from a guy who had to do a forced landing, ran out of room, so did a ground loop and sideswiped a fence, breaking his arm.

 

And a couple of days ago I saw a photo on another site of a Jab which had landed in a paddock with ditches, side view and the engine compartment/nose/prop had flicked down and was/may have been sitting on the ground and had torn out the windscreen pillars in the process, leaving the occupants as the next progressive crumple items.

 

I'm not critical at all of Jabiru with these comments, just pointing out that you can use the strongest construction materials, but sometimes the stresses come in the areas you least expect.

 

 

[Dafydd Llewellyn]

The reference I was trying to remember is (Australian) Dept. of Supply Aeronautical Research Laboratories Structures & Materials Report No. 316, by A. P. Vulcan and S.R. Sarrailhe, July 1967. I have it in hard copy only. It shows what one can do (given a suitable basic design) for a few dollars. The Chipmung seats were designed to accommodate a WW2 style seat-pack parachute, so they had a suitable "bin" under the pilot to accommodate a crushable element. It was normally filled in civil use with two abominably hard Kapok cushions; and the natural frequency of a body resting on those resulted in a lethal 30G "spike" going up the pilot's spine. Instances of the two occupants being found dead in an undamaged aeroplane, sitting in a paddock with the engine still ticking over, were not unknown.

 

 

[Yenn]

In my opinion one of the greatest safety items is a 5 point seat belt. when I started flying many years ago Cessnas and Pipers had a lap belt only. A sudden arrival almost guaranteed your head would hit the panel. My Corby Starlet, which was designed before experimental aircraft has a 4 point harness and relies on the turtle deck to hold the shoulder belts. I would think the shoulders would be restrained, but the front of the fuse would collapse with dire results to your legs and feet. The RV4 I fly seems a lot more crashworthy, but if the tail folded up, the shoulder harness would slack off. I just hope that it happens later rather than sooner. The semi stressed skin with a load of rivets would I hope give a reasonable crumple rate. I have built what I hope will be a crumllable seat base out of riveted .032" sheet aluminium, topped with a temperfoam home made seat.

 

 

[Old Koreelah]

Great post, HITC. I am one of those who followed your design on the other forum without mentioning any safety issues.

 

My love of having the command centre up front probably stems from the Blanic. (They say your first flight has a profound impact.) They still have a safety dividend in terms of excellent visibility, but forward cockpits will have to wait until we can get that heavy engine out from behind our heads. Electric may be the only safe way.

 

 

[Old Koreelah]

Jab is an interesting design ...you can use the strongest construction materials, but sometimes the stresses come in the areas you least expect.

Too true, Turbs. Because of weight considerations, aircraft designers have far less latitude to build in safety than do car designers. Better to use materials that can both carry flight loads and also progressively deform in a prang.

I'd rather crash in a Jabiru than a steel cage. The FRP absorbs a lot of energy, and the panels do you less harm than smacking steel bars, however well-padded.

 

 

[Dafydd Llewellyn]

Great post, HITC. I am one of those who followed your design on the other forum without mentioning any safety issues.My love of having the command centre up front probably stems from the Blanic. (They say your first flight has a profound impact.) They still have a safety dividend in terms of excellent visibility, but forward cockpits will have to wait until we can get that heavy engine out from behind our heads. Electric may be the only safe way.

Erhm - There's a trick or two to mounting a heavy engine on a pylon behind the cockpit; if you do it the simple, obvious way, what you have is a sledgehammer.

One way to avoid that, is what Thurston came up with for the Colonial Skimmer and the Teal - which is to use a couple of struts angled well forward at their base. The pylon is designed to fail first at its rear attachments, whereupon it will do a "pole vault" on the struts, which in effect throw it clear over the top of the cockpit. The way I'm doing it on the Blanik, is to have two attachments at the rear, and one at the front, arranged in an isoceles triangle. The rear attachments are designed to fail first; but they are extremely unlikely to fail simultaneously; so the pylon will pivot forward and sideways around an axis through the remaining rear attachment and the front attachment, which are eyebolts set to facilitate this pivoting - so it will go to one side or the other of the cockpit. The pylon and its attachments are designed to withstand 18 G forward inertia load.

 

The thing is to forsee these eventualities and design around them.

 

 

[Old Koreelah]

The reference I was trying to remember is (Australian) Dept. of Supply Aeronautical Research Laboratories Structures & Materials Report No. 316, by A. P. Vulcan and S.R. Sarrailhe, July 1967. I have it in hard copy only. It shows what one can do (given a suitable basic design) for a few dollars. The Chipmung seats were designed to accommodate a WW2 style seat-pack parachute, so they had a suitable "bin" under the pilot to accommodate a crushable element. It was normally filled in civil use with two abominably hard Kapok cushions; and the natural frequency of a body resting on those resulted in a lethal 30G "spike" going up the pilot's spine. Instances of the two occupants being found dead in an undamaged aeroplane, sitting in a paddock with the engine still ticking over, were not unknown.

A day or two back I saw (perhaps on another thread) a picture of a full-protection pilot seat. Maybe that's what designers should be starting with. I went a part of the way: my seat is moulded to my butt, has lumber support and sits on a crush zone. There is only about 80mm of vertical seat movement available, but some aircraft have none.

 

 

[pmccarthy]

It is amazing what energy can be absorbed if you have the space. A cage full of people falling down a mineshaft when the rope breaks can be safely arrested using deformable strip pulled through rollers at the shaft bottom, to a predetermined maximum g loading. Of course, it all depends on the space available.

 

 

[Dafydd Llewellyn]

In my opinion one of the greatest safety items is a 5 point seat belt. when I started flying many years ago Cessnas and Pipers had a lap belt only. A sudden arrival almost guaranteed your head would hit the panel. My Corby Starlet, which was designed before experimental aircraft has a 4 point harness and relies on the turtle deck to hold the shoulder belts. I would think the shoulders would be restrained, but the front of the fuse would collapse with dire results to your legs and feet. The RV4 I fly seems a lot more crashworthy, but if the tail folded up, the shoulder harness would slack off. I just hope that it happens later rather than sooner. The semi stressed skin with a load of rivets would I hope give a reasonable crumple rate. I have built what I hope will be a crumllable seat base out of riveted .032" sheet aluminium, topped with a temperfoam home made seat.

Agree completely about the five-point harness; but when one uses an energy-absorbing seat, you find the cockpit has to be designed to cater for it. You can't just buy one off the shelf and slap it in; unless perchance your aircraft has a side-stick.

To get the benefit of a five-point harness in the RV, I would suggest that it would need some pieces of 25 x 25 x 2 mm Grade 250 or 350 square mild steel tube tucked under the canopy sill rails - and then move the shoulder harness anchorage to structure that is connected to them. You need to get the shoulder harness attachments at the right height - see attachment

 

 

 

[M61A1]

A day or two back I saw (perhaps on another thread) a picture of a full-protection pilot seat. Maybe that's what designers should be starting with. I went a part of the way: my seat is moulded to my butt, has lumber support and sits on a crush zone. There is only about 80mm of vertical seat movement available, but some aircraft have none.

You might like the ones used in the ARH Tiger, ballistic armour bottom, back and sides, with sliding wings around your shoulders, on two almost vertical rails with crushable spacers, and around 400mm under the seat (warning label says "do not store anything under seat"), all fitted with 5 point harness with inertia reel. Bit hefty though, I would guess around 50kg per seat.

 

 

[dlegg]

Seems that latest car technology relies to a degree on air bag safety. Can this be adapted to low wing aircraft successfully?

 

I mean incorporated into the construction, rather than just the add-on seatbelt type that is for sale.

 

 

[bexrbetter]

had a lap belt only. .

For those of you unaware, cars banned lap belts many years ago, the forces applied to the base of your spine and your guts doubling over with savage force, besides the obvious extra forces applied to the head impact, were found to be killing more than those not wearing one.

 

Seems that latest car technology relies to a degree on air bag safety. Can this be adapted to low wing aircraft successfully?

Prob not, they need to be handled with care and car manufacturers have special end of line fitment stations with highly trained staff fitting them. Very dangerous and not for the average guy in his shed to be playing with.

 

The biggest issue with safety regs, besides the obvious like cleaning up sharp edges, padding areas, getting the seat of the floor etc. is the addition of weight which of course makes the plane more likely to crash, harder to regain control and heavier impact - simplistically speaking.

 

I would leave things alone except for the standard addition of approved 4 point seatbelts and a Ballistic Parachute - there's your airbag.

 

Oh, and most single seat race drivers have a thin seat insert molded to their body shape which helps spread the loads during an accident.

 

 

[Dafydd Llewellyn]

Seems that latest car technology relies to a degree on air bag safety. Can this be adapted to low wing aircraft successfully?I mean incorporated into the construction, rather than just the add-on seatbelt type that is for sale.

Air bags may well be the least-worst solution for intermediate-seat passengers if there are three or more rows of seats, for whom shoulder harness (other than normal 3-point automotive type) is very difficult to provide. However they do not help if the cockpit collapses; and they do not help the spinal impact case. I'd agree with bex about their dangers they present. I don't think they should be the solution of choice for two-seat aircraft.

 

 

[Head in the clouds]

Some well thought out posts here.

 

I agree with Dafydd that the best way to prevent a rear engine coming into the back of your head is to catapult it over the top, still attached to a wishbone or pair of radius arms. Fairly easy to arrange in the engine pod design of a powered motorglider but unfortunately not so easy in my earlier design, and it would then have the additional problem of requiring a separate resolved structure just to carry the harness points. Sometimes when you've painted yourself into a corner you have no other options but to get your feet sticky or wait for it to dry ...

 

To pass on the main points brought up by our US friend who has analysed literally hundreds of crash sites and broken planes -

 

The single greatest life and limb saver of all is a 'stroker' seat. In simple terms it is a seat which is effectively a box without hard items inside it. The top of the seat is designed to break/crumple/deform/distort so that your backside breaks through the 'lid' of the box and has room to travel into the box. The ideal is at least 300mm of travel but as Dafydd mentioned this can be reduced if the box is filled with material or structures that collapse progressively and cause a deceleration that doesn't exceed 30G or so. The front of the box i.e. the part under the knees must not collapse. In the very worst crash this may mean that you get broken legs, injured knees or similar because it is the underside of the thighs which ultimately stop you from moving forward and greeting the panel or firewall.

 

It's important to remember that a 'good strong seat structure' that doesn't deform in a crash has often been the cause of death due to shock from impact to the spine. I don't have the reference to hand but I recall reading many years ago that a person suspended in a sitting position and then suddenly dropped onto concrete from a height of just 3"/75mm, will be paralysed or killed. Someone with medical knowledge may be able to correct or expand on this ... I don't know any details but I also heard a long time ago that there was a fatality in a glider at Benalla (I think) where an apparently reasonably normal landing had been made but the pilot found deceased. I understand a deflated oleo failed to provide suspension for the wheel on landing - again someone else will probably have more details than I can recall.

 

Based on the above, my new design has a large empty space under the seat which will be filled with a block of impact absorbing foam that will also serve as the seat cushioning. The foam will be a hybrid polymer and laminated from various densities. It resembles the foam used for yoga mats. I have a foam specialist working on a prototype for testing, I'll post more about that when we get to the testing stage. I've also arranged my underseat fuselage structure so that the strong members that support the landing gear legs are beneath the bottom fuselage members, effectively providing a double bottom of structure which would be a help in the event of putting down into a paddock which has rocks or tree stumps in it - they are another hazard to be considered as they can't easily be seen from the air when selecting an outlanding and can rip the bottom out of the plane and any part of the crew that happens to be in the way as well.

 

A stroker seat is something fundamental to the design which must be considered from the earliest stages because it means that the volume beneath the seats cannot be used for structure or controls that could injure if the crew comes into violent contact with them.

 

Dafydd mentioned that another problem with planes is that they lack forward volume that can be used as crumple zones. This applies to front- and rear-engined as well as low- and high-winged designs.

 

Old K mentioned something about this, saying "I'd rather crash in a Jabiru than a steel cage. The FRP absorbs a lot of energy, and the panels do you less harm than smacking steel bars, however well-padded". I couldn't agree more about not wanting to smash into steel bars, but flying in a fibreglass plane may not be the all-consuming solution either. Statistically Jabs have protected the occupants very well but in general composite planes have a nasty habit of shattering and depositing the occupants around the crash-site. A couple of the recent Jab mishaps have shown the front of the cabin broken and it appeared to me that if the crash had happened just a bit faster the outcome might not have been so happy.

 

The point is, as I understand it, that the well designed steel cage statistically offers the best protection from injury but you certainly don't want to be hitting the steel from the inside, so if we don't have crumple zones to absorb the energy, then what else can be done?

 

Our expert in the US pointed out that if you don't have crumple zones then you have to do something about preventing the sudden stop instead. The solution to that, which has been adopted by a number of aircraft manufacturers, is to get rid of the sharp bottom to the firewall. Instead of being a sharp corner the bottom of the firewall can be given a large chamfer (or radius) and that extra space which is generally not used internally because it is behind the pedal arc, can be used for better cooling airflow out of the cowling. The crashworthiness benefit is that the plane won't tend to 'dig in' if it impacts in a nose down attitude and instead it tends to bounce off and dissipate the energy in a series of skips along the ground. Anyone can imagine that the consequences for the occupants will be much less than they would be if it digs in and comes to a sudden stop.

 

That chamfer also has a huge benefit when employed on a low-wing aircraft, in prevent it digging in an flipping over - the bigger the chamfer the better.

 

There's more but I'm out of time so it will have to wait for later. Keep posting your ideas please folks, this subject is important stuff that will save lives.

 

 

[Dafydd Llewellyn]

HITC, you are right on the money. Having set out to design a "collapsing parallelogram" seat base for my pet project (not the Blanik) it immediately became evident that you have to design the cockpit end of the control system around the seat, otherwise the stick is likely to spear the pilot under the chin. Also, you need adjustable rudder pedals, rather than a seat on adjustable seat rails. So things like the Cessna approach of sliding the sear right back in order to get your legs in the door, are right out. You really have to start with the cockpit, and design the rest of the aeroplane around it.

 

The RV study that we did, was started in order to assess the effect of the rudder pedal design on lower leg injury; I had hoped to get some inkling of the maximum deceleration loads that could be safely applied to the feet, because once you save the fellow's spine, the next question is, will he be able to get out of the wreckage or will he be incapacitated by leg injury? In the event, all we learned was that the common form of rudder pedal, , like an inverted L shape in welded steel tube, is NOT the right answer; in the RV accidents, both occupants had both their ankles fractured by the twisting of the rudder pedals.

 

The "bevel" at the bottom of the firewall is vital.

 

The criterion for a stroker seat is 1500 pounds force in the lumbar spine - which means the seat should collapse at not much more than that load. If you do a simple Newtonian physics analysis, assuming the human body to be a rigid mass, you will come up with a required seat stroke of about 300 mm. However, most interestingly, an early experimental seat test done, I think, by Steve Soltis - I have the report in hard-copy format - showed that about half this distance suffices, because the human body has considerable energy-absorbing capability itself.

 

I agree that a properly designed welded steel tube "crash cage" (and I do not mean something made from boiler tube) is the most cost-effective answer for a small-volume production aircraft.

 

 

 

 

[Old Koreelah]

Now you blokes have got me worried about that steel tube mounted just forward of my wedding tackle. I'd rather a side stick, but that's not so easy in a single-seater.

 

My 4-point seat belt is old and getting hard to adjust. Anyone know where I can get a good 5-point harness from?