The Impossible Repair

It’s that time of the year when enforced hibernation makes many of us think about repairing some of the previous season’s damage.  Having had more than my fair share of dramas over the years, I have come to realise that more often than not, what seemed like a total right off at the time is with care, repairable.   Every disaster is different and not all moulded models are the same, but here is what I did to rebuild a wing tip.

The model is an Electra F5J 3.7 mtr thermal soarer.  It’s  construction is the usual hollow moulded epoxy/ carbon/foam/glass sandwich, and is of a fairly light weight.  The tip in question was almost totally destroyed in the arrival when my radio link failed, which was traced to a faulty Lipo  The model rolled inverted and quickly disappeared behind trees about ¾ mile from my flying field. I was fortunate that apart from the fuselage tail boom being broken, this was the only significant damage.

To start I cut away as much of the remaining damage as was needed to enable me to decide if a repair could be done. This photo shows my first attempt to piece the bottom surface together with scraps of thin carbon. This did not really work.

As the main spar and the aileron spar had to be replaced I decided the best repair would be a ‘traditional’ balsa rebuild, but with carbon reinforcement. I started by making a paper template from the other good undamaged tip. The template idea was also dropped as the work progressed.

As the repair progressed, the missing moulded surfaces were replaced with balsa sheet and I decided to make the tip from foam. I thought this was the only way I could reproduce all the curves. I cut a rough shape from blue foam and epoxied it, with carbon pins, to the rebuilt tip.

This shows the underside as I start to sand the repaired area, and foam tip, to shape.

Once the repair had been sanded to an overall smooth finish I wanted to complete the repair without adding any more weight than was unavoidable. I thought that to try to replicate the models colour and finish was a waste of effort and decided to use Textreem spread tow carbon. This is the material used in many of the lightest thermal soarer’s.  

I coated the surface of the repair with wing skinning epoxy and laid the cloth on to it. The epoxy soaked through the cloth and I removed as much as I could by much blotting with kitchen roll paper. Once I had the epoxy evenly spread through the cloth and it all looked ‘dry’ I covered the completed area with  a thickish polythene cut from a rubble sack. I then carefully pressed this down all over onto the carbon with fingers and the edge of a credit card. Once again the epoxy that remained came through the cloth to the surface.

To finish off I covered the job with as many magazines and weight as I could balance on the work bench and left it overnight to cure.  The picture shows the underside after sanding and trimming.

I covered the underside first because the tip needed to be strong enough to allow the top surface to be sanded to its final shape. Once this was done the same process was followed which can be seen in these last two photos.

The model had this repair plus a smaller one on the other tip and to the fuselage boom. The total gain in weight is approx. 40g which I thought was pretty good. The repaired tip is approx 25g heavier than the other, but as far as I could see it did not show in flight.

If you are in a similar situation, I hope these few words will help you make a start on that ‘impossible’ repair.

Peter Mitchell