Workshop

Broken bird :o(
  
October 2012.
The cause of the above crash was me not carefully checking the correct operation of the radio in all flight conditions. (Muppet!) To save time when I set up the model I copied the program from my Vikos and the only difference, apart from throws and differential, was that I had to reverse the direction of the elevators. Well, with my 9ZAP and probably other radios, you need to reverse the elevator in all flight conditions. If you don’t and then throw the “speed” switch as the model is hurtling along the slope edge the ensuing gentle dive will not flatten out with back stick!! The moment it smacked into the ground I knew it was elevator reversed. Was I sick? Damn right!!   
With no way to obtain a replacement wing for my crashed Extreme other than to buy a new one for £350 I had pretty well written the Extreme off. Out in the shed one evening and avoiding Eastenders I began to look at the wreckage and pulled the broken and separated drag spar from the inner panel away from the wing and tried to fit it back in the correct place. Some pushing, shoving, tweaking  and judicious pressure and the application of copious amounts of Beli Zell ca saw it pretty well back where it came from. The separated bottom skin forward of the main spar also went back after the application of some tape and more glue.  The separated bottom skin over the main spar was pried and cut away. Some areas of skin were removed from the main panel to improve access. Progress? Hmm..
The inner panel was clamped and heavily weighted to the bench such that the delaminated flap hinge on the wing panel was held in place and the flap deflected as far open as possible. Copious amounts of Beli-Zell ca was wicked into the joint and allowed to cure overnight. The hinge seems fine but there is some swelling apparent where the skin has not gone back fully but still....

The main problem still remained in that the wing panel was in two parts. The fact that the spars were broken would be serious enough to make repairing this wing with any degree of integrity very difficult and unless you are confident in your abilty to effect a strong composite repair I would NOT recommend repairing this kind of damage. However, my Extreme has now flown many times in big winds (60mph+) and it is holding together perfectly. Having said that, I have no intention of winch launching it!!

Apart from the strength and integrity of the repair the big challenge in joining the broken wing panel would be how to get the two separate panels together straight and without a twist! A jig was needed but how to make one? Borrowing another right wing panel would be perfect but nobody who lives locally has an Extreme. After much thought and cogitation I used the other undamaged panel. How?

A flat board was carefully screwed to the workbench and much time was spent with shims of thin ply and card ensuring the board was absolutely level at both ends and all points in between. This was to ensure the board is absolutely flat and the jig being prepared would result in a straight wing when the two parts were reunited.
 
 
Six jig pieces were made up to support the intact left wing. The plan being to have one at the root and one at the tip, one just short of the damaged area on the root panel and one just short of the damaged area on the tip panel. Two more were made to span the “missing” areas between the two panels.  The short vertical strips of wood strenghthen the jig piece and align with the wing trailing edge. The grey cylinders are lead weights holding eveything flat while the glue dries.
 
 

The jig pieces were then firmly screwed to the board at right angles to the front edge and thier positions carefully marked on the board. The “trailing edge” of each jig piece was flush with the straight edge of the board. Above is the intact left wing panel in position on the jig trying it for size and fit.

 

 

Brown packing tape was stuck to the bottom surface of the wing to stop filler gluing the wing to the jig. Permanently!!

 

Car body filler is applied to the top of each jig piece and the wing will be placed on top of this (quickly).

 

The left wing panel is placed on top of the soft filler and weighted down until the filler has gone hard.
Once the filler had set the wing was removed from the jig and the jig pieces were unscrewed and turned around 180 deg before being screwed back in the same position but facing the opposite way to produce a mirror-image of the wing jig. To maintain the straightness of the wing the “leading edge” of the jig pieces were aligned with a long metal straight edge before fixing to the board again.

Result? A wing jig (of sorts)
 
 

The two broken wing halves could now be put in the jig and brought together and with some judicious removal of glass and carbon shards the main and rear spars and remains of the leading edge and trailing edge sheeting could be brought together. At this point it occurred to me that this palaver could work!! I mixed up some one hour epoxy and stirred in some short carbon fibres and tagged the main and drag spars together and weighted the whole thing down in the jig to cure. This epoxy will be ground away again when I get round to making good the top surface of the spars but by then the main part of the carbon tow spars will be spliced in.

The spar on the Extreme consists of a foam core which is glass covered and then has a heavy carbon tow fillet along the top and bottom edges of the spar joining the spar and the wing skin. This is done front and back of the spar so there are a total of four carbon tow spar doubler/fillets.

The Dremel came out again to cut some access slots to the area in front of the spars. A wide tapering “V” was cut from the breaks in the front carbon tow spar doubler outwards to allow new carbon tow to be laminated in place. The “V” shaped taper should spread the load over a larger area. The batch of heavy carbon tow was from a new EBay source and a bit of a faff to wet out but seemed to go on ok in the end. A length of 5mm carbon rod was incorporated in the repair on top of the carbon tow and helped press the carbon in place and squeeze out excess resin. The top front spar was done first and allowed to set in the jig for a couple of days. Then the bottom front spar doubler was done and allowed to harden in the jig. The two rear spar doublers were done in the same way. Four separate operations which proved quite laborious!

There was a thin carbon skin on the top and bottom of the spars and this was ground away and tapered each side of the break and carbon  before more carbon tow was layered along the surface and pressed in place to squeeze out any excess resin. Again the wing was returned to the jig to allow the resin to cure hard. A length of carbon tow was also added to the front of the rear drag spar over the break. This process took several evenings work spread over about three weeks.

Once it had all hardened a fairly brutal bending/twisting test was given and nothing cracked or creaked. Hmm.. Promising.
To cut a long story short I then replaced and made good the wing skins. Several coats of high build filler were applied and then sanded back until it all looked smooth. Top coat on and it looks acceptable. Flies straight and true. :-) It is about 130g heavier (half of which is lead in the opposite wing for lateral balance)  but for slope work I doubt that will make a lot of difference.
 
The repaired furelage was a crude lash-up to try it all out but I had a spare fuselage and fitting that was the next task. 
 
New fuselage fitted out with new 2.4gHz radio. sorted:)
 
 
 
 
Lots more pictures of the repair here. Yawn!!
 
November 2014 update. Still holding together and as strong as ever :)
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