Engineering for occupant safety

[old man emu]

Whys is it that after 120 years, are we still building airframes like T-model Fords? We are still making the basic box kite design that Hargraves used in his experimental work. In the past 60 years, the automobile industry has expended probably billions of dollars to engineer occupant safety into their vehicles. Why hasn't this knowledge been applied to aircraft, especially those at the lower end of the MTOW spectrum?

 

Have a look at pictures of aircraft which have impacted solid objects while having a degree of forward motion. That cuts out those impacts resulting from a flat spin where the whole of the fuselage impacts the ground at the same time. In every one of those pictures you will see that, for front-mounted engined aircraft, there is massive distortion of the front of the aircraft, extending back into the passenger space. This is a sure indicator that the airframe around the engine was very rigid.

 

The rigidity of the front of the aircraft caused the Impulse of the impact to be transmitted through the airframe without diminishing. (Impulse in physics means the change in momentum over a period of time. The longer the time, the smaller the Impulse.) It is the magnitude of the impulse that determines the severity of damage to the airframe and occupants.

 

The research done by the automotive industry has resulted in the change in the design of chassis from the rigid ladder type to the less rigid unibody (monocoque). Of further value has been the application of data obtained from crash tests to design impact absorption (crush zones) which apply the impact forces to deforming the structure, making noise and heat. The data is also used to design construction that diverts the direction of the impulse away from the passenger cage, or prevents the uncrushable engine and gearbox from entering that cage.

 

I know that weight is the enemy of aircraft. Perhaps a solution would be to use the geodetic structure that Barnes-Wallis first applied to the Wellesley and later the Wellington. Barnes-Wallis hit upon a revolutionary structural idea – rather than building an aircraft structure on the principle of a beam, which supports an external aerodynamic skin, he developed a new type of structure which had the structural members formed within the aerodynamic shape itself. This required the structural members to follow the curved outer shape of the fuselage and wings. https://www.barneswallisfoundation.co.uk/life-and-work/geodetic-aircraft-design/ Basically what he did was divide the rectangles that are found in a ladder frame into a lot of smaller triangles.

 

Geodetic :the shortest possible line between two points on a sphere or other curved surface. A geodetic (or geodesic) airframe makes use of a space frame formed from a spirally crossing basket-weave of load-bearing members. By having the geodesic curves form two helices at right angles to one another, the members became mutually supporting in a manner that the torsional load on each cancels out that on the other. In addition to being comparatively light and strong, the fact that the geodetic structure was all in the outer part of the airframe meant that the centre was a large empty space, ready to take payload or fuel.

 

The structure was good for war planes as it could take a lot of damage without breaking.

http://ww2today.com/wp-content/uploads/2012/09/Wellington-bomber-fire-damage-595x301.jpg  http://www.ww2aircraft.net/forum/attachments/aircraft-pictures/171269d1309098874t-battle-damaged-aircraft-ww2-wellington5.jpg

 

 

 

[onetrack]

Go for it, OME! Produce your best design, and upstage Bex! I've often thought along the same lines. But for RA aircraft, the improved cabin design must also stay cheap and affordable.

I have also often wondered why RA aircraft designers haven't reverted to the "fuselage in halves" design of the Mosquito, utilising modern lightweight materials, rather than wood or plywood.

[Geoff_H]

Have a look at the amazing survival rate of the Mooney.  Tail is monocoque but cabin is steel frame with aluminium covering.  This is design for minimum weight and maximum survival.  

[kgwilson]

In a motor vehicle weight isn't a major problem. Most aircraft engines hang off the airframe at the front so it has to be both strong and light enough so designing it so the engine and mount break off and go under a strengthened firewall/fuselage as many car engines are designed to do in a crash is likely to introduce weight problems. Then you could design an ultra strong safety cage like they have in rally cars but I suspect the same problems would arise as well as ease of entry/exit. Some aircraft do have a stronger passenger frame than others.

 

In the 30s the fabric provided a fair bit of the strength when wrapped tightly around the frame & this often caused the aircraft to crash when the tension got released by gunfire or fire. Barnes Wallis's geodetic frame provided all the strength and chunks of it could be blown away and the rest still continued to provide sufficient strength for the aircraft to continue flight. I don't know why they never continued with the design. Maybe it was a bit complicated to build and/or repair

[Geoff_H]

Some of the very early experimental aircraft used Kevlar in their construction.  A flat spin with the pilot walking away was regarded as fantastic safety.  Problem is that it is tough not strong.  Some cloths use carbon fibre and Kevlar interwoven to give strength and toughness. I designed and started to build a glass aircraft with a layer of Kevlar.  It was far too heavy and I abandoned the project at fuselage stage.  

[Geoff_H]

The Mooney cabin is a safety cage design

[Old Koreelah]

The Jabiru airframe seems to tick most of the safety boxes; it has an exceptional record of aircrew survivability.

Heavy engine and its mounting frame in front, fiberglass from there back, with a big strong wing structure around the cabin.
Fiberglass may not be as light and strong as carbon, but is better at absorbing impacts progressively.

[Blueadventures]

11 minutes ago, Old Koreelah said:

The Jabiru airframe seems to tick most of the safety boxes; it has an exceptional record of aircrew survivability.

Heavy engine and its mounting frame in front, fiberglass from there back, with a big strong wing structure around the cabin.
Fiberglass may not be as light and strong as carbon, but is better at absorbing impacts progressively.

Absolutely agree, proven performance. Jabiru is a winner hands down every time. I recon they should offer a closer to stol performance wing in their range.

Edited April 4, 2021 by Blueadventures

[skippydiesel]

Yah! I am with those thinking composite.

 

In my mind (or whats left of it) compost constructed  is the use of different materials to create the whole  ie metal, wood, fabric and of course plastics.

 

Again in my mind using the best material for the job/application and not being wedded to one/two is what we should be aiming for - so why not a geodetic metal cockpit safety  frame with plastic skin ?  Why not a flexiable laminated wooden spar covered in water proof epoxy ?  Whats wrong with using fabric - fantastic stuff used in the appropriate places. I have never believe safety must involve/result increased weight,  that is   just an excuse by lazy designers.

[Geoff_H]

If you are looking at composite aircraft and are serious about the amount of energy that your frame will absorb before you start absorbing the energy, (not so good), but want a strong light structure you would use an interwoven carbon fibre and Kevlar cloth. It costs around five to ten times that of straight glass but in Izod impact tests absorbs over twice the energy of straight glass.  You will be paying for increased crash absorption.  How many of us would pay for that option?

[old man emu]

Whoa there Neddy! Before we wander off into appropriate materials - which is an appropriate path to take - let's get the skeleton done first, then think about its skin.

 

I've has second thoughts about a geodesic structure. I think that it might be too inflexible for the goal of reducing the magnitude of the Impulse. What we want is for the structure to crumple, but not transfer the Impulse to the cabin area. Perhaps that means putting some sort of cockpit pod inside the fuselage. 

 

This is a plan for an Old Timer RC plane that we can use to illustrate some points.

It's clear that the fuselage is basically a girder construction with the cockpit. The problem is that the cockpit is located in a space between the girders, which might allow everything forward of it to intrude into that space.

[Geoff_H]

I must disagree that structure strength is more important than energy absorption of the structure upon impact. A totally rigid structure will not impart an infinite force that will break the structure.  Impact absorption is more important as this reduces the energy available to cause very high stresses that will break the structure.  The triangular steel frames work by the high impact absorption on deformation of the steel.  The real performance indicator of a good design in a crash is the energy to deformation and in an aircraft the weight must also be a parameter. When considering theses parameters carbon fibre with Kevlar wins.  Triangular steel frame design is in the same class but heavier.  

[kgwilson]

Here is an example of the front taking the impact. This J230 went in hard a mile or so from YSGR in 2016. Both passenger & pilot walked away & 1 had a cut on the head the other a sore leg. The momentum swung the airframe 180 degrees & the entire front end including the engine disappeared underneath & left them still strapped in looking at a bunch of cows.

[kgwilson]

After the 180deg turn the fuselage slipped back off the engine etc. You can see that the engine was running pretty well on impact.

 

Edited April 4, 2021 by kgwilson

[Geoff_H]

I think that I would rather have my cabin remain enclosed.  It broke at the highest loaded parts, the side window struts.  I am glad all survived.  

[Old Koreelah]

2 hours ago, Blueadventures said:

Absolutely agree, proven performance. Jabiru is a winner hands down every time. I recon they should offer a closer to stol performance wing in their range.

Too plurry right, Bluey!

I’d be in the market for a STOL Jabiru, but I can understand the factory having no interest in developing a totally new model.

[Old Koreelah]

1 hour ago, Geoff_H said:

I think that I would rather have my cabin remain enclosed.  It broke at the highest loaded parts, the side window struts.  I am glad all survived.  

Having the cabin remain intact would be ideal, but having a structure absorb a large part of the impact is probably more important. Other Jabiru crashes have also resulted in the firewall being ripped out- allowing the occupants to clamber out without serious injuries.

[Blueadventures]

Not really a new model just a wing design and testing. Should result in more sales.  When I see Dan in Mackay I’ll politely hit him up. Their aero engineers need a challenge i’d expect otherwise life will be boring:) cheers.

[Old Koreelah]

3 minutes ago, Blueadventures said:

Not really a new model just a wing design and testing. Should result in more sales.  When I see Dan in Mackay I’ll politely hit him up. Their aero engineers need a challenge i’d expect otherwise life will be boring:) cheers.

It would be more of a challenge than just a new wing, which alone would be a major job (I’d be hanging slats on the front edge)

Doing STOL stuff would require far more elevator range than the standard Jabiru design.

Then there is the testing regime to match the standards of their current models. Expensive.

 

Perhaps our best hope is for someone to develop a home-built modification kit.

[Geoff_H]

Is this failure of the window struts common? It seems like the floor is absorbing most of the impact.  But the doors are not sticking closed and locking the pilot and passenger in.  Must be good if a fire develops.  I still would be somewhat worried at such a significant structure failure in an accident, but it seems to work for the better.  Amazing!  It just shows how not everything is as we would imagine.

[Thruster88]

We will never get good crash protection in light aircraft so better to focus on pilot performance. Stall and spin the aircraft it is all over, no amount of  engineering will change that. Land the aircraft under control following engine failure will in about 99% of cases result in minimal injury. 

 

This RV6 with minimal damage was a  double fatality due to minimal energy absorption under the seats similar to most sport aircraft. There is no room to make it any better. The chairs in a modern cessna 182 are so heavy they are difficult to remove for the annual inspection, they are rated for a 26g impact.  

[Geoff_H]

Many years ago a Chipmunk VH FTA spun to the ground and killed both on board.  Not long after an experimental made of Kevlar also spun into the ground.  The pilot got out and walked away.  The Kevlar all stretched and destroyed but the energy was absorbed by the Kevlar.  When I trimmed Kevlar on a layup the Stanley knife had chips in the blade from cutting it.  Very tough stuff that.

[pmccarthy]

This would provide a good base line for design. There are plenty of them flying in the USA, only one I believe in Australia.

[Old Koreelah]

12 hours ago, Geoff_H said:

...an experimental made of Kevlar also spun into the ground.  The pilot got out and walked away.  The Kevlar all stretched and destroyed but the energy was absorbed by the Kevlar.  When I trimmed Kevlar on a layup the Stanley knife had chips in the blade from cutting it.  Very tough stuff that.

Kevlar laminates are great at absorbing impact and then returning to the original shape, but It’s mongrel stuff to work with; you can’t tell by looking whether the layup is saturated with resin. 

It’s almost impossible to cut the raw fabric. Decades ago George from FGI told me the trick they learned when cutting the sails for Australia II: buy boxes of cheap scissors, cut a meter or so until the blade is blunt then get a new one.

I discovered another trick: extend your layup over a temporary boundary and when it’s hardened, cut it with an angle grinder.

[Old Koreelah]

8 hours ago, pmccarthy said:

This would provide a good base line for design. There are plenty of them flying in the USA, only one I believe in Australia.

Great for visibility, but probably the worst possible configuration on a prang.