Now one near a school! Ballarat

[motzartmerv]

Glad they are ok. Should be commended for getting out of the soup, hitting a powerline is always on the cards when doing PSL for real.

 

 

[Head in the clouds]

...shows just how strong the aircraft are

Thanks for the pics, rivet spacing looks fine, in my (very humble) opinion the spar is exceptionally lightly built though, so now I'm not surprised at the multiple failures. I'm no engineer though, so I'm not saying it doesn't meet any particular loading requirements or anything like that, obviously they're not designed for landing wingtip first. All that said I've seen lots of examples of aircraft that have landed/crashed tip first and even cartwheeled, and never seen a wing failure anything at all like that...

 

 

[facthunter]

I'm not expert either but they are deep sectioned and light metal rather good at collapsing as it has there. It impacted the nose too. Some of them have the ribs a bit too far apart and the surface oilcans. I'd say it has impacted the wingtip heavily and concertinered?. . I think the spar relies on the skin a fair bit. Nev

 

 

[fly_tornado]

Not a lot too that spar! Tornado uses 48 size rivets to attach the ribs to the spars

 

 

[Kyle Communications]

The rivets in the spar are pretty big...maybe the pics distort it a bit. The spar is not that light when you pic it up. The main web is not 20thou it is probably 1 or 1.5mm thick

 

 

[fly_tornado]

grip range of the SD48BS is 10-12mm

 

 

[Gribble]

Im no aeronautical engineer, but im sure the wings arent designed to be landed on. The crumpling wing might have just absorbed enough energy slowly to prevent any injury to the occupants.

 

AND, ill leave my speculation there, good too see only the aircraft got hurt (and a powerline, some grass and probably some pride).

 

 

[Head in the clouds]

The main web is not 20thou it is probably 1 or 1.5mm thick

It's difficult to tell much from photos but I've built a fair bunch of spars and sheetmetal planes and I'd lay money that web's no more than .032" but more likely .025". And that would be reasonable with closer rib spacing or even web stiffeners but IMHO the amount of material they've removed with the lightening holes has done the spar no favours at all. And I don't credit the hole flanging with providing any anti-buckling benefit either, beyond maybe 2-5%, having performed a few destructive tests with and without them.

 

 

[facthunter]

Regardles , they work Ok in service and no plane is designed to look good after rolling around after hitting a wire in flight. The deformation did absorb the energy and helped the occupants. I'm not inferring the wing was purposely designed to do that. It contributed nothing to the accident. Wires spring out of nowhere when you are outlanding and that is the cause of this unfortunate outcome. fortunately the occupants are not injured and will be going back west soon. One can imagine the disappointment at the damage to a new aircraft and the end of what would have been a happy visit to Avalon. We should see if someone can't contact them and give them some aeroplane people company, (the right kind) and support.

 

Did you talk to them Ding? Nev

 

 

[Kyle Communications]

Too late for me to measure my spar...I believe there is something like 1000 flying there hasn't been any wing failures except for corrosion issues on the wing strut attachment on the underside of the wing in Denmark and bending one back with multigrips after it was bent and stored outside in sub zero conditions. There was a recent failure around a year or so ago when a wing came off in flight in Italy but that was due to someone not attaching the wing properly. The current XL and S have been uprated to 560kg by the factory at +6 and -3 G and I would think they had to do destructive testing for this upgrade. I think the way it is made also has to do with the strength. Each rib has a wing skin tensioner attached between each rib and the sheet on top so all the ribs are locked together and also tied to the skins. These tensioners are not used where the tanks are but I actually made some from scrap from the tank covers and fitted them to mine on the top side of the wing. I couldn't rivet those strips in the 2 tank bays as the rivets would have punctured the tanks but I glued them each one of the strips and also the ones in the rest of the wing both top and bottom sit on the top of the ribs and overlap and they are riveted by the rivet going through the skin then the 2 tensioner strips and into the rib itself

 

 

 

 

 

[Head in the clouds]

I couldn't agree more FH. The wing 'crumple zone' may, by chance, have saved their lives and I'm delighted they're OK, and also very sorry for their loss. And they did the right thing to get out of bad weather and were very unfortunate to hit the wire, and they're hard to see at the best of times.

 

My point is simply an interest in the structure. As amateurs we must learn wherever we can, and its not the mass of a web that matters, so much as the support of it and the big zones between the ribs are clearly the weak link here. Web stiffeners, maybe two per rib bay could possibly strengthen that whole wing by say 30-40% for just a few grams weight.

 

Even more interesting to me is that normally if a wing (or any beam) fails it would break in one place and that's it. But when this one first failed, it bent and then that damaged place actually must have become stronger, in its failed condition, than the next unfailed bay which then failed and became stronger than the next etc, u'know? Never seen that except in a truss, so that indicates to me that the web wasn't doing its job.

 

 

[Head in the clouds]

Yes KC, those skin tensioners might be better called skin stiffeners and they do a grand job, main purpose being to stop oil-canning of the top surface skins. So its worth considering that an I beam wing of that span and thickness really shouldn't be bending enough to cause canning in the first place unless the web wasn't doing its job. Those skin stiffeners will, though, add considerably to the overall strength of the wing monocoque, so you did well to add extra in the tank area. However, in that crash via wingtip, the progressive roll-up of the wing was possibly contributed to by the progressive failures of those stiffeners.

 

 

[BRL]

I think bit of over thinking is going on in regards to the structural attributes of the damaged savvy I say three cheers to the savvy and that no one was hurt.

 

 

[Ultralights]

the spar is plenty strong enough. and im sure you will find it surprising to know that the robin aerobatic training aircraft only has 5 ribs along the wings entire length. the strength is in the skins. dont forget, in a braced high wing aircraft, the loads are shared by the lift strut, and by design, the loads are longitudinal along the spar to the wing root from the lift strut join at the spar. so the only 100% bending moment in the spar is from the lift strut join to the wing tip. the wing root mounts are under a compression force.

 

Lightning holes in the structure also add considerable stiffness.

 

i see the staged failure as a good thing, it means the failure at one point hasnt led to total failure as the next point gets overloaded. it looks like the impact/ folding failure has stopped at each rib/spar join and not become a sudden catastrophic failure of the whole wing. only the ever increasing weight during the ground impact has caused the wing to fail progressively at each rib point, and not suddenly at the wing root of lift strut locations.

 

 

[Guest Maj Millard]

Nothing wrong with the Savvy wing, you just can't design an aircraft to withstand every type of crash scenero. They are built light to fly, not hit the ground. The cabin structure is still in one piece, and that's all that mattered in this one. Wings are definitly expendable in crashes like that. Bloody power lines !!...sad loss of a nice aircraft but the owner is around to build another one ........... Anyone know where I can get a check-out in one of those 3-engined Cessnas ??...................................Maj...

 

 

[Head in the clouds]

U/Ls consider your 50% strutter OK? Say 45% of the lift is outboard of the strut due tip losses. That 45% of the lift tries to push that part of the wing up and that half of the wing is unstayed. The inner half is supported at each end and also pushing up. That support difference between the halves is unbalanced, no support at the tip, right? So there is a large bending moment, the forces are not purely longitudinal.

 

Have a look at a Robin web thickness... Also note the extensive use of web stiffeners between each lightening hole.

 

Hole flanges don't provide significant stiffness or any strength, they're there to control stress in the material and thereby reduce buckling tendency. Build a spar with unflanged lightening holes, stand it up on a flat surface holding one end, the other end will want to flick twist one way or the other. Flange the holes and it'll stand straight for whatever that's worth, in use the ribs hold it straight anyway. Put thumb pressure on the hole flange and it'll collapse - try it, I'll lend you my flangeing dies. Hole flanges look good, thats about it...

 

I'm not suggesting there's anything wrong with Savannah wings, they're proven! But folks also keep saying we need to investigate failures and isn't it a good thing to learn where we can?

 

The only way to judge structures is to deliberately fail them or learn when they fail in use. This is a great opportunity to gain understanding, why stick the head in the sand folks...?

 

 

[Guest Maj Millard]

HITC, In 1974 I was in a Cessna 182 crash. We left one wing 30 Ft behind the fuselage wreckage, the other wing was still attached, but bent in two. Two dead, two survived. Weak wings not built strong enough ??....no we hit the bloody ground hard, something had to give ! .same difference with this Savvy...................................Maj...

 

 

[cooperplace]

HITC, In 1974 I was in a Cessna 182 crash. We left one wing 30 Ft behind the fuselage wreckage, the other wing was still attached, but bent in two. Two dead, two survived. Weak wings not built strong enough ??....no we hit the bloody ground hard, something had to give ! .same difference with this Savvy...................................Maj...

main thing is they're OK; bummer about the trimotor Cessna, it was certainly one of a kind............

 

 

[Jabiru7252]

The idiots say this was a Cessna, sure looks like a Savannah to me. Then again, I am getting old.

 

Apparently it clipped power lines. No mention as to why they did what they did.

 

http://www.adelaidenow.com.au/news/national/light-plane-crashes-at-primary-school-injuring-pilot-and-passenger/story-fndo1svg-1226586890487

 

 

[Head in the clouds]

HITC, In 1974 I was in a Cessna 182 crash. We left one wing 30 Ft behind the fuselage wreckage, the other wing was still attached, but bent in two. Two dead, two survived. Weak wings not built strong enough ??....no we hit the bloody ground hard, something had to give ! .same difference with this Savvy...................................Maj...

No, sorry Maj, you absolutely and completely miss the point. Your wing broke in two, right? And that's what I said you'd expect from any beam structure failure. This wing broke in seven, but it's also a beam structure. Trusses break into multiple failures, not beams... So, much as I'm not knocking the Sav wing, the point is that it could easily be made much stronger without much weight penalty because the caps don't look like they have the web support they need. I do this for a living mate...

 

 

[fly_tornado]

already on it.

 

 

[Old Koreelah]

HITC, In 1974 I was in a Cessna 182 crash. We left one wing 30 Ft behind the fuselage wreckage, the other wing was still attached, but bent in two. Two dead, two survived. Weak wings not built strong enough ??....no we hit the bloody ground hard, something had to give ! .same difference with this Savvy...................................Maj...

In this case the wing is part of the "crush zone." The energy used to destroy the wing greatly reduces the G-forces imposed on the people aboard.

 

 

[turboplanner]

A lot of engineering speculation here since the aircraft momentum into a very compressed object - a cable - would have resulted in a massive, narrowly targeting force given it was hit at full flying speed. In other words regardless of one aircraft design vs another - you take a hit like this and land uninjured on the ground, and you can be thankful for whatever engineering method was used.

 

Expected results possibly could be calculated if the angles of the aircraft and cable etc were known.

 

 

[facthunter]

Without video etc it won't be done. Nothing coming from that accident should cause an investigation into the airframe . HITC has a special interset in it and would point out lots of areas of possible improvement . I inspected a written off technam and found out quite a few things about it that I wouldn't have done any other way. Some good , some bad.

 

The more I get involved with the type of planes used in the U/L movement, I have had to come to grips with the sometimes "crudeness" of the construction and the simplification and lack of sophistication and the numerous places where things could have been done better. However when you run the calculator over the materials and sections and loads, they make the requirements. There's excess structure in some places and just enough in others . It's a balance of cost weight complexity serviceability and such." Affordable flying". I would say so. Nev

 

 

[Old Koreelah]

You're on the money, Nev.

 

Just before an arrow was released, a medieval longbow was said to be 90% broken. Perhaps the same could be said of aircraft, where excess weight is inefficiency. The more strength we build into our aircraft the less efficient they become. Airliners are built to take expected loads; in a crash they get converted into confetti. If a section survives intact it was too heavily built.