Saving a Write Off

My old and faithful electric Xplorer 3.5 suffered one more nasty arrival last summer. This one due to a radio fail, not me.

Despite an extensive search before the model was found it spent over a week lost in an uncultivated area of hedgerow with its nose buried up to its wing in soft earth. The damage did look bad and at the time I could not face making the repair so I put the model in a cupboard and forgot about it.

6 months later a wanted request on the forum got me thinking about it again. I thought it would repair quite well. This resulted in me offering it to a couple of guys who had expressed an interest. The pictures below show the damage did nothing to encourage them to take it on, so I decided to do it myself.

This model was my first all moulded electric thermal soarer, and it has always been a joy to fly so I wanted to see it usable again. The wing is f3J construction, before spread tow carbon, and is strong. A previous accident 2 or 3 years ago did damage the centre panel, and at the same time destroy both tip panels, but this latest event did more serious damage to the centre panel.

These pics show some of the damage to the centre panel, it can be seen that the top and under side skins were split and although it can’t be seen here, it was subsequently found that the top carbon spar had been almost completely severed.

So how to repair?

I first thought of removing all the damaged skins and replacing them with balsa covered with glass cloth. But this meant that I would also have to re paint, and anyone who has seen the models I have repaired will know that I don’t like spraying.

In the end I decided to retain the damaged wing skins and repair them by piecing them together. The top surface was repairable by glassing the inner side of the shell, and as I needed to cut a big hole in the underside to do this and gain access for the spar repair this decided things for me.

Here you can see the underside of the panel after I cut it open.

This removed the damaged skin so that I could repair it and gave access for the spar repair.

To repair the spar I first had to remove the balsa webbing. I ground out as much of the balsa in the damaged area as I could safely reach with my Dremel, and then carefully finished the cleaning out with a balsa knife and Permagrit tools.

Here can be seen the hole in the spar webbing and the intact lower carbon spar.

I have repaired a number of models (not all mine) with broken spars. So far as I know, none of them have failed since, so this is how I do it. First I make a length carbon fibre plate to suit the size of the spar. In this case the broken spar is thin, about .8mm thick and approx. 25mm wide. To make the repair I cut a carbon plate 1mm thick and approx. 80mm long. Replacement webbing was made with built up sections of 8mm balsa.

The spar replacement carbon plate is first pushed into place under the broken spar and the replacement sections of balsa webbing then forced into position between the lower good spar and the repair. Plenty of 30min epoxy is used to glue it all in-place with each part of the repair.

The wing panel was carefully lined up with a mark one eyeball and left for 24hrs under weights to hold it all in place.

Once the epoxy cured, I drilled a 1.5mm hole all the way through the wing from top surface to the bottom, through the spar and webbing, either side of the spar break. Into each of these hole’s I cyano a 1.5mm carbon fibre rod. The rods pin the spar repair plate to the original spar and stop any possibility of a glue failure to be caused by the shearing effect of excessive loads in flight or a heavy landing.

The wing skins were pieced together, with glass cloth epoxied onto the inner surface with wing skinning epoxy.

To finish the repair the surface sections must be replaced so that the monocoque strength is retained as far as is possible. I made joining strips by laying up carbon cloth with 1mm balsa, and these were then epoxied into the wing forming a ledge for the repaired pieces to be fixed to.

One small area of the leading edge had disappeared in the crash so I moulded a piece of carbon cloth to the leading edge shape and this was cut to size and fixed in place with cyano.

The panel is now almost ready, it’s not pretty, but it is strong, weighs approx. 25g more and will fly again.

One tip panel suffered a small amount of damage and this was repaired in the same way as the centre panel.

All the equipment in the model, except for the lipo and the nose spinner survived the crash, and will be used again. However the rx was replaced by Multiplex, although they could not actually find a problem with it. I will also replace the 2 Futaba S3150 rudder and elevator servos in the fuselage, with the much smaller MKS- DS6101. Nothing wrong with the 3150’s, but as they are in the fus, under the trailing edge of the wing, their weight means that more weight is required up front to balance the model.

Having done a trial balance on the re-assembled model, I find this now means that I can use a smaller, lighter weight lipo and the overall result is that the total flying weight is only increased by only a small amount despite the extensive repair.

When the weather warms up I can always clean it up and re-spray it :o)

Update 23 Feb 2014

I have flown the model now and it is just like it was before the crash – excellent