Small aircraft crashworthiness

[Head in the clouds]

..... Also, you need adjustable rudder pedals, rather than a seat on adjustable seat rails......

 

..... 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....

Agreed, the seat structure has to be fixed so as to design the front of it to be sufficiently durable to not deform in the worst crash event. I wasn't aware of the danger to the ankles of the welded L shaped pedals so that's certainly a consideration for incorporating a different kind of pedal structure.

 

In respect of the need for adjustable pedals, and for the adjustment to be easily made, inflight without risk of compromising control if possible, and independent for each seat of course, I have come to the conclusion that the adjustment method employed in some gliders is probably the best and lightest, see picture below. I'd be very interested in other methods that folk may know about, and systems where the geometry remains the same regardless of pedal adjustment are clearly preferable. Some systems merely move the foot-bar back and forth and that means that the pedals arc downwards when adjusted for long legs and arc upwards for short legs. Brake cylinder attachment/adjustment is another important consideration too.

 

 

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

Yes, though I'm doing mine a little differently, using an 'under-structure' to carry the gear legs, and having the front part of that raking up to the firewall, that rake is then extended further forward as part of the engine mounting.

 

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.

Thanks for the numbers and other info, interesting point about the body ... what do you think about foam, if it's used for the energy absorption? There seems to be two distinctly separate schools of thought, those who suggest the foam should collapse as it does its job i.e. does not return to original shape after overload, and those who see a benefit in rubberised foams which very quickly return to their original shape. The main discussion seems to be centred around the likelihood that a crash that involves the preferred 'skipping' arrival involves several separate impacts and if the first one flattens the foam there's nothing left, and slack belts, for the ensuing ones.

 

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.

I think so too, and the point you made about the door pillars is very relevant. A couple of notable events in the US have seen the far-too-flimsy front pillars of one particular design buckle, following which the entire airframe just collapsed around the occupants. I'll see if I can find the images. I think the front pillar(s) (I'm using four plus bracing) can also benefit by being resolved to the aft fuselage by more than just a single inter-spar member, I'm planning on using a small truss (possibly triangular) at the door head, to resist buckling in that region - still toying with that one at this stage.

 

Other matters for consideration regarding crashworthiness - the energy absorption value of the landing gear. In combination with your excellent Seabird Seeker concept of keeping the crash attitude more horizontal than vertical, then a lot can be gained from long travel and progressive suspension, something which my new design has, albeit that the plan was for STOL reasons rather than unintended sudden arrivals -

 

Here's a sneak preview -

 

 

I'm out of time again, thoughts about flail arcs and distances will have to wait for next time - do you have any numbers/references on that Dafydd? I've based mine on Bill Whitney's testing for the Boomerang.

 

.

 

 

[metalman]

I go on a supercub forum in the states occasionally, one of the problems with the cubs in a stack is they have a single diagonal tube from the froward wing attach to the opposite rear attach, when the wing hits the rear wing rear collapses into the cockpit and does bad stuff to the occupants ,the easy fix is to weld another diagonal the opposite way . It's a pretty popular mod to the old airframes and I think a lot of the new build and copies do it from the start. They would have to be one of the most copied aircraft ever ,and going by the extreme places they fly them I guess pretty good in a crash, there's an alaskan saying that " your not a real bush pilot till you've put a cub into the tree tops" ,,,,,not sure I'd like to join that club though.

 

 

[Dafydd Llewellyn]

Just to throw this one in, empty aluminium drink cans are awesome "crumplers", so good some years back a University competition for a bumper bar design for a crash test actually won over all the scientifically designed and built at great cost crumple designs. They bought a number of drinks and merely spiked a hole in them to drain them then taped them together. Apparently the tab opening gave way at 'X' pressure allowing the can to crumple after offering an initial air cushioning and worked a treat. Might be worth looking at to put under your seats, weighs nothing.

I tried this with four beer cans on end; once they started to buckle, they crumpled at just about the ideal force; however they needed about double that force before the crumpling commenced, and that would provide precisely the lethal spinal load that the ARL work set out to eliminate. Drilling a 10 mm hole transversely across the can at one end largely eliminates the high initial load. There's considerable scope for experimentation, but do NOT put anything under the seat unless you have first measured the force at which it compresses; it the total force for the seat as a whole goes over 700 Kg, it will not provide protection. Ideally it should start to compress a little below that figure, and then maintain close to 700 Kg until it's fully collapsed. If the crumple force of individual elements - say of one beer can amongst a group of four - starts out high and then falls away, there's a risk that the seat pan may collapse on one side rather than uniformly - and that will do you spine no good at all.

So if you want to play about with such options, build yourself a test rig and make sure it collapses nice and evenly, with the correct load.

 

 

[metalman]

Dafydd would you have contact details for the guy doing the RV mod, I'd be interested to have a look at it

 

 

[Old Koreelah]

...this thread...shows me that whilst there are a minority of people who do care about " secondary" safety considerations, and some serious thoughts there, the majority of people really aren't interested. This demonstrates yet again what Edsel Ford found out - safety is not a marketable commodity...So we get what we deserve - stylised garbage.

A perhaps harsh judgement Dafydd, but I totally agree that few people look under the skin for safety features.

My little Jodel now has a dozen or so improvements that address most of the safety concerns raised here. They cost me quite a lot of money and several years of rebuilds and testing, but most are invisible to the casual observer.

 

Never having won anything in my life, I regularly enter judging in the innovation category at NatFly. Sadly the judges seem unable to see beyond the fairly ordinary paint job; one year I offered to demonstrate some of the innovations, including my automatic battery isolator. The judges pretty well bruised me aside.

 

After reading this thread I will make more improvements, particularly to the seat and harness.

 

 

[Old Koreelah]

...Is there any recreational aircraft available, for a reasonable price, that has a higher crash worthiness inherently built in, that is readily available?

Jabirus come closest.

 

 

[Dafydd Llewellyn]

Agreed, the seat structure has to be fixed so as to design the front of it to be sufficiently durable to not deform in the worst crash event. I wasn't aware of the danger to the ankles of the welded L shaped pedals so that's certainly a consideration for incorporating a different kind of pedal structure.In respect of the need for adjustable pedals, and for the adjustment to be easily made, inflight without risk of compromising control if possible, and independent for each seat of course, I have come to the conclusion that the adjustment method employed in some gliders is probably the best and lightest, see picture below. I'd be very interested in other methods that folk may know about, and systems where the geometry remains the same regardless of pedal adjustment are clearly preferable. Some systems merely move the foot-bar back and forth and that means that the pedals arc downwards when adjusted for long legs and arc upwards for short legs. Brake cylinder attachment/adjustment is another important consideration too.

 

[ATTACH]32079[/ATTACH]

 

Yes, though I'm doing mine a little differently, using an 'under-structure' to carry the gear legs, and having the front part of that raking up to the firewall, that rake is then extended further forward as part of the engine mounting.

 

Thanks for the numbers and other info, interesting point about the body ... what do you think about foam, if it's used for the energy absorption? There seems to be two distinctly separate schools of thought, those who suggest the foam should collapse as it does its job i.e. does not return to original shape after overload, and those who see a benefit in rubberised foams which very quickly return to their original shape. The main discussion seems to be centred around the likelihood that a crash that involves the preferred 'skipping' arrival involves several separate impacts and if the first one flattens the foam there's nothing left, and slack belts, for the ensuing ones.

 

I think so too, and the point you made about the door pillars is very relevant. A couple of notable events in the US have seen the far-too-flimsy front pillars of one particular design buckle, following which the entire airframe just collapsed around the occupants. I'll see if I can find the images. I think the front pillar(s) (I'm using four plus bracing) can also benefit by being resolved to the aft fuselage by more than just a single inter-spar member, I'm planning on using a small truss (possibly triangular) at the door head, to resist buckling in that region - still toying with that one at this stage.

 

Other matters for consideration regarding crashworthiness - the energy absorption value of the landing gear. In combination with your excellent Seabird Seeker concept of keeping the crash attitude more horizontal than vertical, then a lot can be gained from long travel and progressive suspension, something which my new design has, albeit that the plan was for STOL reasons rather than unintended sudden arrivals -

 

Here's a sneak preview -

 

[ATTACH]32080[/ATTACH]

 

I'm out of time again, thoughts about flail arcs and distances will have to wait for next time - do you have any numbers/references on that Dafydd? I've based mine on Bill Whitney's testing for the Boomerang.

 

.

The best single reference is the USAF Crash Survival Design Guide; it's available on the web.

Re the bevel or chamfer at the lower front corner of the fuselage frame, see the attached photo - it shows the initial impact gouge from the RV-3 at Toowoomba. The Goulburn Sting made an almost identical gouge. The front lower corner needs to be shaped to try to avoid this sort of dig-in behaviour. Both those aircraft "bounced" after the initial impact; the patch of dead grass in the background shows where the RV 3 ended up; but this initial dig is what caused the high initial deceleration.

 

 

 

[Old Koreelah]

I tried this with four beer cans on end; once they started to buckle, they crumpled at just about the ideal force; however they needed about double that force before the crumpling commenced, and that would provide precisely the lethal spinal load that the ARL work set out to eliminate... Ideally it should start to compress a little below that figure, and then maintain close to 700 Kg until it's fully collapsed...

Progressive absorption of impact energy is the key. Years ago this bloke, after doggedly working on his invention, finally got traction and the product is now on the market.

http://www.abc.net.au/tv/newinventors/txt/s2006698.htm

 

 

[Dafydd Llewellyn]

Dafydd would you have contact details for the guy doing the RV mod, I'd be interested to have a look at it

Sorry, I don't; it was Graham Moodie, in Brisbane; he was doing this about ten years ago. I suspect he passed away a few years ago. Look up some RV6 owners in the Brisbane area, there was quite an active group there, so most of them will know more than I do, and there will be a number of aircraft that have this mod.

 

 

[Dafydd Llewellyn]

Daffyd, you make a very valid point in that in considering an aircraft purchase it must be designed from the outset with safety in mind. There aren't any homebuilts or LSA varieties that I have seen that really can claim this. New certified "safe" aircraft are simply priced too high for us recreational pilots. I.E new 172 is close to $500k, and just how robust and safe is it really? Granted, a lot safer than a RV, Zenith, Tecnam etc, but a high impact accident I think is a hard thing to survive regardless of aircraft design.Clearly, operating a Drifter or Thruster carries a higher risk than a 172, protection wise that is. But the pilot must be aware of this and operate them accordingly.

Is there any recreational aircraft available, for a reasonable price, that has a higher crash worthiness inherently built in, that is readily available?

No, I don't; I do know of one or two that are abominably bad in this regard. The attitude of the manufacturers is that incorporating these features will add to the cost and reduce the payload, which means the product is less likely to sell, in a very competetive market. "It's no use building the World's safest aeroplane if nobody buys it".

However, I think you can see that most of these features need not add much to the design cost or weight, they mainly need a different philosophical approach in the initial design stages.

 

 

[Guest asmol]

Dafydd, the worlds safest aircraft is one that stays in a hanger.

 

Dafydd, i have never meet you, I dont know you, I only know of you by my readings here but I have strong suspicions you are motivated beyond normal interests in these discussions.

 

You 'seem' to be positioning yourself as some sort of expert on all things aviation and seem to be subliminally planting seeds that some plane designs are dangerous and others are not. Please declare your interests up front with the manufacturers you represent, this would be the fairest way for your contributions to appear unbiased.

 

BTW i dont fly and dont have a plane.

 

 

[Head in the clouds]

Sorry, I don't; it was Graham Moodie, in Brisbane; he was doing this about ten years ago. I suspect he passed away a few years ago. Look up some RV6 owners in the Brisbane area, there was quite an active group there, so most of them will know more than I do, and there will be a number of aircraft that have this mod.

Ooohahhh, Graeme Mudie (note spelling) would be rather sad to hear of his demise. I'm pretty sure he's alive and well still, I had a chat with him not all that long ago, and used to rent space in his hangar, so I know him and his lovely RV6 very well. I can't find his contact details MM, but I'm sure you'll find him easily through the GCSFC. I'm sure he'd love to chat, he's a very affable fella, formerly headed up the the SAAA chapter called SAOG (Sport Aircraft Owners Group) near Brisbane. I seem to recall you're visiting up here at Christmas, would be a good time to visit Heck Field at Jacobs Well and see Graeme's RV perhaps? It's the nicest one I've had a look at, he builds some lovely one-off replica historical sports cars too.

 

Dafydd, the worlds safest aircraft is one that stays in a hanger.Dafydd, i have never meet you, I dont know you, I only know of you by my readings here but I have strong suspicions you are motivated beyond normal interests in these discussions.

 

You 'seem' to be positioning yourself as some sort of expert on all things aviation and seem to be subliminally planting seeds that some plane designs are dangerous and others are not. Please declare your interests up front with the manufacturers you represent, this would be the fairest way for your contributions to appear unbiased.

 

BTW i dont fly and dont have a plane.

Haha - asmol, Dafydd rarely blows his own trumpet, though he can be a little dogmatic on occasions. He's one of the most experienced and best known Reg35 aeronautical engineers in Australia, and without any barrow to push that I'm aware of ...

 

 

[Old Koreelah]

Dafydd, the worlds safest aircraft is one that stays in a hanger.Dafydd, i have never meet you, I dont know you, I only know of you by my readings here but I have strong suspicions you are motivated beyond normal interests in these discussions.

 

You 'seem' to be positioning yourself as some sort of expert on all things aviation and seem to be subliminally planting seeds that some plane designs are dangerous and others are not. Please declare your interests up front with the manufacturers you represent, this would be the fairest way for your contributions to appear unbiased.

 

BTW i dont fly and dont have a plane.

Simple, Asmol. Just spend an hour or two and read thru Dafydd's posts. Then you might figure out who he is and what he's on about.

 

 

[metalman]

Ooohahhh, Graeme Mudie (note spelling) would be rather sad to hear of his demise. I'm pretty sure he's alive and well still, I had a chat with him not all that long ago, and used to rent space in his hangar, so I know him and his lovely RV6 very well. I can't find his contact details MM, but I'm sure you'll find him easily through the GCSFC. I'm sure he'd love to chat, he's a very affable fella, formerly headed up the the SAAA chapter called SAOG (Sport Aircraft Owners Group) near Brisbane. I seem to recall you're visiting up here at Christmas, would be a good time to visit Heck Field at Jacobs Well and see Graeme's RV perhaps? It's the nicest one I've had a look at, he builds some lovely one-off replica historical sports cars too.

 

 

Haha - asmol, Dafydd rarely blows his own trumpet, though he can be a little dogmatic on occasions. He's one of the most experienced and best known Reg35 aeronautical engineers in Australia, and without any barrow to push that I'm aware of ...

Yep, I'm hoping to tie down at hecks while visit family, I'll track him down, thanks

 

 

[Dafydd Llewellyn]

Simple, Asmol. Just spend an hour or two and read thru Dafydd's posts. Then you might figure out who he is and what he's on about.

I'm a retired professional aeronautical engineer - what the industry used to know as a "CAR 35" engineer - i.e. I held a CASA design signatory authority for aircraft modifications and repairs from 1974 to 2011, when CAR 35 was superseded. I have also been involved in the certification flight testing of several Australian aircraft. I've always been in the capacity of a consultant; I have no commercial interest in any aircraft manufacturer or importer. I'm 73, and simply trying to pass on some of what I have learned, on the basis that there is no point in repeating other people's mistakes; you should find some fresh ones for yourselves. My main interest is to see an Australian grass-roots aircraft manufacturing industry get up and going; I was the chairman of the QLD Aircraft Manufacturers Association for the few years it existed. I have been involved in the investigation of a number of aircraft accidents.

 

 

[Dafydd Llewellyn]

Ooohahhh, Graeme Mudie (note spelling) would be rather sad to hear of his demise. I'm pretty sure he's alive and well still, I had a chat with him not all that long ago, and used to rent space in his hangar, so I know him and his lovely RV6 very well. I can't find his contact details MM, but I'm sure you'll find him easily through the GCSFC. I'm sure he'd love to chat, he's a very affable fella, formerly headed up the the SAAA chapter called SAOG (Sport Aircraft Owners Group) near Brisbane. I seem to recall you're visiting up here at Christmas, would be a good time to visit Heck Field at Jacobs Well and see Graeme's RV perhaps? It's the nicest one I've had a look at, he builds some lovely one-off replica historical sports cars too.Ah - very glad to hear that.

[gandalph]

Dafydd, the worlds safest aircraft is one that stays in a hanger.Dafydd, i have never meet you, I dont know you, I only know of you by my readings here but I have strong suspicions you are motivated beyond normal interests in these discussions.

 

You 'seem' to be positioning yourself as some sort of expert on all things aviation and seem to be subliminally planting seeds that some plane designs are dangerous and others are not. Please declare your interests up front with the manufacturers you represent, this would be the fairest way for your contributions to appear unbiased.

 

BTW i dont fly and dont have a plane.

Wow Asmol! Do you get all of your exercise by jumping to conclusions?

 

I've been reading Dafydd's posts for quite a while and I think the "Subliminal" is probably the least accurate description of his posts you could have used. Clear, unambiguous, direct and sometimes blunt to the point of gruffness would be a more accurate description. I've met him on a several occasions over the last 40 years or so and he has always been extremely helpful, informative, polite and friendly. He's the closest thing to a true polymath I've ever met. Is he opinionated? Yes absolutely. Does he have the qualifications and diversity of experience to support his contributions to these forums? You be the judge.

 

Do yourself a favour and google him. It should throw up an article on "Ageing Aircraft - A real Problem or a Diversion?". His credentials are at the bottom of that article.

 

 

 

[paulh]

Dafydd, thankyou for starting and continuing with this thread, to you and all the other contributors, please keep going, it's all very interesting.

 

I have no relevant experience to offer, so have only been reading, but do take safety particularly for any potential passengers quite seriously.

 

Cool looks are nice but seem more connected with image/ego/eye candy factor than they should be

 

So safety is a factor in aircraft selection for me as a low how recreational pilot with no illusions about skill level and reliability of experimental engines

 

Safe predictable handling, glide range, crashworthiness etc all part of the properly designed package

 

Metal space frame type of structure appeals, not keen on composite construction in the garage at home

 

 

[Dafydd Llewellyn]

Dafydd, the worlds safest aircraft is one that stays in a hanger.Dafydd, i have never meet you, I dont know you, I only know of you by my readings here but I have strong suspicions you are motivated beyond normal interests in these discussions.

 

You 'seem' to be positioning yourself as some sort of expert on all things aviation and seem to be subliminally planting seeds that some plane designs are dangerous and others are not. Please declare your interests up front with the manufacturers you represent, this would be the fairest way for your contributions to appear unbiased.

 

BTW i dont fly and dont have a plane.

I should mention - please do not assume, from my posts, that I regard the RV 6 etc as anything other than fairly typical of straightforward metal aircraft design. I use them as examples purely because I happened to have occasion to study a couple of wrecks in some detail, so I have some data from them. The shortcomings I mention in them are almost certain to exist in most other small aircraft of that genre, to a greater or lesser degree. I saw much the same potential in a T 18 that I had occasion to study in detail, tho it had evidently been a case of pilot LOC due to carbon monoxide. My point - to the extent that there is one - is that the secondary safety of small recreational aircraft could be substantially improved, at little penalty; however this is unlikely to happen unless the consumers start voting for it with their cheque books

 

 

[bexrbetter]

You 'seem' to be positioning yourself as some sort of expert on all things aviation .

This is a safety thread and we should all be on the same bus for the duration regardless of personalities and how they might come across.

 

 

[Powerin]

Airbags built into the seatbelts are available for aircraft, although I suspect not for LSA yet. With no other structural modification do you think these would significantly improve the crash worthiness of LSA or small GA aircraft?

 

http://www.amsafe.com/products-services/general-aviation/seatbelt-airbags/

 

 

[SDQDI]

Airbags built into the seatbelts are available for aircraft, although I suspect not for LSA yet. With no other structural modification do you think these would significantly improve the crash worthiness of LSA or small GA aircraft?http://www.amsafe.com/products-services/general-aviation/seatbelt-airbags/

That certainly looks like a workable feature. The biggest trouble with panel or frame mounted airbags is if the harness is not up to scratch the airbag will do more damage than it prevents. In cars airbags NEED seatbelts to be used to be effective if you aren't wearing your seatbelt the airbag could kill you.

This seatbelt design however seems to be a very easy and effective mod that could be put into most of our aircraft without too much hassle or negative effect (depending of course upon weight)

 

 

[facthunter]

Earlier open cockpit aircraft had padding around most of the places you would contact. The joystick was of wood. I think high wing aircraft have the ability to have a much stronger "cell" for the occupant(s) without much extra weight being added. The prospect of inverting an open cockpit plane on the deck has never thrilled me. Nev

 

 

[dazza 38]

Airbags built into the seatbelts are available for aircraft, although I suspect not for LSA yet. With no other structural modification do you think these would significantly improve the crash worthiness of LSA or small GA aircraft?http://www.amsafe.com/products-services/general-aviation/seatbelt-airbags/

Cub crafters have had them available for a year or two.

 

 

[metalman]

Most modern vehicles have a pretensioner system along side the airbags, it pulls you into the seat at the same time the bags go off, how this would be incorporated into an aircraft would be a huge undertaking, the whole thing would have to be designed around the SRS ,it's not really something that could be added easily.

 

The other option could be a system similar to the side curtain airbags ( they're a different type to the front impact bags) but as with anything there's a down side, trying to fight your way out of a cockpit with inflated airbags would be difficult.