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Monday, April 21, 2008

Air Crash Investigation

Well the loss of my Ventus at the Sungazer raised plenty of questions , most prominent being what went wrong , followed by could I have done anything differently and finally what has been learnt by this accident.

My flight started with the usual checks, including the correct radio program and movement on all the surfaces, I asked Charl when he launched to really push out and slightly nose down(did not wish to have a missfire like Peter). The Ventus went off well and accelerated at quite a dive I let it build speed and then eased in some up, there was very little response , I pulled full up and the glider went into a verticle climb, I adjusted the trim(normally 3 clicks would put it into level flight). It stalled ... my brain working overtime to establish the problem, had the nose weight shifted? something broken? she started downwards gaining speed , only full up brought it back into a verticle climb and a second stall.

This was becoming a roler coaster ride and I wanted off, third stall and dive and it went into the hill with a thud.

I asked the guys who went to collect the pieces to bring everything back and already started trying to understand what had happened, this glider had flown 3 times before , two flights of about 45 mins , had been pushed a little (recorded a speed of 189km on the logger)and flew well, so what happened.

The weights were checked ,I sometimes fly a five cell battery pack , the weight is marked as such, but I then substitute a sinker of a similar weight if I fly with a four cell pack. Both correct weights in and judging by how they got bent still attached at impact, so weight had not shifted.

Paul Carnell made a comment" looks like your elevator may have been jammed".

This is what it felt like but I still had full up and full down movement. When I studied the elevator linkage things fell into place. The silicon keeper which holds the clevis in place was possibly binding on the cutout.

Sure enough when I checked it, it was torn , it was ok before the flight , so why had I not picked up this problem, one explaination is that I move the sticks full travel when testing and it was sticking/binding in the midrange and locking on top or underneath the cut out.The long pushrod linkages taking up the stain before pushing past the sticking point. I like this theory and will put down the loss of this plane to a small piece of silicon. (Sorry about the photo quality but you get the picture)

Now with all the pieces I had a good look at what failed and how, it verifies your construction techniques, I dont build using a computer and theoretical loads. I build and add a safety margin
on experience and gut feel. Here below is a test of the wing inner panel in this photo the weight(me 80kg) this panel only has one side of carbon tows and the sheer web , no skins, it bends but did not break , so with both sides and skins it is well........over engineered by a fair margin.

I feel that I could do well to build these gliders lighter, but still strong so I started with Ventus no 4, this is to be based on the HQ/ 3/12 airfoil much used on scale ships as opposed to the slippery Mh 32 and Sd 7037 used previousely . So far I have saved 700 grams on the fuselage 100 grams by going the balsa foam route on the tailplane, this equates to a saving of 300 grams in the nose, and it looks like I will get a saving of 1000gram on the pair of wings by carefull layup and choice of materials.
Ventus no 4 will fly soon and if it can have an all up weight of 7kgs it will be a winner
Ventus no 3 ZS-VCX weighed in at 8.7kg A.u.w.
It flies next weekend weather permitting, I just have to get back on the horse after the fall.

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