Scale Soaring UK

A couple of weeks ago I broke an aeroplane. I know, hard to believe isn’t it?
The model in question was my 5m Kestrel 19 all moulded model from Pavel Gerasis in the Czech Republic. I have had it for a dozen or more years but it is still a favourite model.
I was flying at my local slope in a 20mph wind and had set up for my landing approach. I was coming in quite quickly with only a little flap due to the wind and was adjusting my height with the airbrakes. At about 5 ft altitude the model encountered what I think was wind shear and it literally dropped out of the sky, hitting the ground hard. As if that wasn’t bad enough it bounced back into the air and continued past me back out over the edge of the hill. The encounter with the ground knocked off the rudder and appeared to have damaged the tailplane. It continued along the slope for a short distance with very little control before impacting the side of the hill.
I collected the pieces, dumped them into the car and retired home where I stuck them in a corner of the workshop in disgust.
Having finished my latest project and with no new build on the bench I have retrieved the wreckage and decided that the Kestrel is repairable.
The fuselage is cracked in front of the fin and the wing joiner system is ripped out and broken. The right wing is creased top and bottom just forward of the flap/aileron location. The tailplane has been totally gutted where it attaches to the fin. All in all its not as bad as I expected so the repair is on.
Brian.

Hi Brian,count your blessings mate it could have been a lot worse,yours is repairable.

We also have those glass walls on our slopes landing strip. Your going along nicely wondering whats for tea in the knowledge that you will be taking home an unbroken model when suddenly Poof, crash, bol--cs another glass wall. Who puts them there? Well if anyone can repair it you can. However having worked more years with glass cloth and resins than I care to remember if I can help in any way just holler.

Thanks for the offer of guidance Terry, it is much appreciated. However I originally came from the world of competitive F3b/F3f/F3j where our everyday machines were "mouldies" and mid airs were frequent. I seemed to spend half my life in those days repairing airframes to the point that I became allergic to epoxy resin despite all me PPE. Thankfully years of building wooden gliders has reduced the symptoms and I can now glass a model without too much difficulty.
I don't know about glass walls but I do know about big holes in the air. This was a new phenomenon to me on this particular slope which has the reputation of having a safe landing zone. When I spoke to one of my mates he said that he had experience the same thing the day before and in similar conditions. This slope has a steep face with a sharp edge and then a gently tapering slope to the rear. We are now theorising that the 20-23mph wind created the conditions for wind shear further back up the hill.
I know that I got off pretty lightly and that the aircraft should repair reasonably well. However I can now practice a different set of skills for a while and also take the opportunity to do a full restoration/renovation on the model.
Brian.

I have now actually made a start on the repair process. I have begun with the tailplane as it sustained the worst damage. I initially thought that I would have to build a new tailplane but after due consideration I decided to repair. If the reair didn’t work out then I still had the option to build new.
I opened up the underside to get rid of all the crushed and ripped surface exposing the balsa spars. As there was a chunk missing out of the middle of the spar I used a couple of thin carbon fibre strips, glued and clamped to the spars to reinforce the centre. The area to the front and rear of the spar were filled with soft balsa block as they will have to support the retaining screws. The blocks were fixed in place with an epoxy/micro balloons mix. This was also used to fill the gap between the carbon reinforcing strips in the centre. I will have to wait till tomorrow to sand this all down to profile.
I have also started on the rear end of the fuselage by sanding back the paint down to the glass and running cyano into the break.
Brian.

Last night I laid up two layers of 25gm glass cloth/epoxy on the rear boom and lower fin of the Kestrel fuselage and a further two layers on the underside of the tailplane at the centre. Today the fuselage bandage was crisp enough to sand back but the tailplane was still very slightly tacky to the touch.
I got stuck into the fuselage with 180 and 240 wet and dry (used wet with a little detergent added to the water) the glass faired into the fuselage quite well and the fin is now really stiff and secure. While I was messing about with wet and dry I sanded back the red paint trim on the nose as I am going to take this opportunity to finish the Kestrel in a different colour scheme (just for a change). It took a while to get rid of all the colour and I find I am back to the white gel coat in places and the seam line is now visible.
Brian.

Since my last post on the 13th August I have not been idle.
Still working with the tailplane I was able to wet and dry sand down the glass fibre patch on the underside so that it faired in with the existing glass surface. I then marked out the retaining bolt positions (top and bottom) and drilled through from each side. I was able to insert from the underside a couple of thin brass tube guides for the bolts. I was then able to countersink from the top surface a couple of larger diameter holes to receive the bolt heads. I must have been either clever or lucky because when I trial fitted the tailplane to the fin everything fitted perfectly.
However when I trial fitted the pushrod to the elevator horn I discovered that nothing worked. I traced this back to the elevator servo which is mounted in the bottom of the fin. The servo was completely dead. I don’t know whether this might be a contributory factor to the model not responding after it took itself back into the air before the eventual crash. Whatever, a new servo is now on order.
In the meantime however the tailplane was primed and then given two coats of Appliance Gloss White from a rattle can which has left a very reasonable gloss finish.
While I was doing painty things with the tailplane I decided to restore the rudder which was undamaged. The existing red paint was removed with 240 grit wet and dry and the whole surface was taken back to the original white gel coat. It too was primed and then given two coats of Appliance Gloss White from a rattle can. This morning I amused myself by replicating the final white/blue stripy markings using vinyl strips of the appropriate shade of blue.
I have also been working on the wings but that will be in my next post.
Brian.

While glue and paint has been drying on other components I have started work on the wings.
Firstly I wanted to know if the spars had suffered any damage. I supported each wing up off the bench at the root and tip before applying larger and larger weights at mid span. I was surprised that they were able to take the weight of a large car battery; taking up a nice even curve while the centre of the span still remained a couple of centimetres off the bench.
Bearing in mind that the right wing has some surface damage to the upper and lower skins near the trailing edge I also carried out a twisting test. I anchored the root by a carbon stub joiner and the steel incidence peg in a vice fixed to my bench. I then twisted the wing tip by hand and was surprised to find that both wings were still very stiff. More importantly the upper and lower skins round the damaged area were not deflecting. RESULT!

In the crash the retainer pins had been wrenched out of the wing roots leaving jagged holes in the root face. I tidied up the holes and then created a couple of temporary covers from ply and brown packing tape. I mixed up a small pot of epoxy and micro balloons and then emptied the mix into the leading edge “D” boxes before fixing the temporary covers in place with tape. The wings were then set up on end for the epoxy mix to flow back down to the root. The next day I removed the temporary covers and was rewarded with two nicely solid and flat wing roots. All that remained was to mark up the location of the retaining pegs, drill new holes and fit the new pegs in place.

When the crash occurred the wing joiner broke and the joiner tube was ripped out of the fuselage. So also was the anti-crush dowel at the front of the wing root while the brass incidence peg tube at the rear of the wing mounting was twisted and crushed.
A new 16mm alloy main joiner tube has been fitted and a new brass incidence stub tube has been relocated in each wing root rather than one longer length spanning the fuselage. I have refitted the front anti-crush dowel and have installed a new carbon fibre rod as far back in the wing root as possible to act as a rear anti-crush support.
Brian.

All looking very good Brian

This is a model that I have had since 2010 Peter and it has become a favourite, especially in heavy weather. Doing a repair and resto like this is a little bit like sorting out an old friend. I am thoroughly looking forward to flying it in it's new incarnation.
Brian,.

You're doing a lovely job of it, Brian.

I have now had a close look at the rear of the wing that has surface damaged just in front of the flap and aileron. There is an internal balsa sub spar about 10mm forward of the hinge lines for the control surfaces. This is reinforced with a layer of carbon fibre bandage connecting the upper and lower surfaces. The area between this sub spar and the leading edges of the flying surfaces is hollow to allow the surfaces to rotate into the wing when moving up and down.

This sub spar appears to be totally intact and most of the surface deformation lies in the surfaces above and to the rear of the sub spar and is therefore essentially cosmetic. If I tried to reinforce the inside of this hollow area it is likely that the smooth movement of the flaps and ailerons would be affected so I have left it alone. I have however drilled a series of 1mm holes through the carbon reinforcing along the mid-point of the sub spar and dribbled thin cyano into the holes in order to “harden up” the balsa behind (just in case).
I will have to live with the slight buckling of the upper surface because I will know that it is there. Most folks will never notice it.

While dealing with the wings I also rubbed back the red paintwork on each wing tip back to the original white gel coat. It has now been primed and awaits a new coat of paint.
Brian.

There was a bit more progress today with the Kestrel. The fuselage was masked off, cleaned, primed and then sprayed with Halfords rattle can of Appliance Gloss White using one mist coat followed by two good flow coats. I managed to do this outside in the good weather we are having which allowed it to dry quite quickly. I then carefully moved it into the workshop and hung it from the ceiling where it will dry properly and harden before I start to mask it off for the colour. I also dropped in to see my local model shop owner who is going to cut the vinyl markings for me.
Brian.

It’s coming on really well Brian. Hope you avoided the usual pitfalls with spraying: runs, pinholes, falling off your chair...

We were going to pop in and see you on our way from Edinburgh to Skye on Tuesday but were too tight for time. Sorry we’re not around when you pass ours next week. Hope you get the Kestrel finished in time.

Rog

You're doing a great job of it, Brian.

We're kindred spirits - I'm doing the same thing with an A-14 - although not as speedy as you.

All pitfalls avoided Rog. Sorry you can't drop by. However will get that canopy to you next weekend.
Thanks for the nice comments Max, I just can't bear to right off a good aeroplane.
Brian.

The paint is now dry and I have now been able to reattach the rudder and to hook up the pull-pull linkage between the rudder and the rudder servo which sits just behind the retract system.
I have been able to reconstruct the carrier for the flight battery and hot glue it into place. The receiver has been reinstalled, plugged up and tested. The most difficult aspect of this has been to tidy up the rats nest of wires so that they don’t get in the road of the retractable undercarriage.
The little guy had become a bit dishevelled in the crash and rather detached from his seat and the rest of the cockpit interior. He is now back to normal and happily (or perhaps not) awaiting his next flight. Finally the canopy was cleaned up and re-polished before fitting back into place.
The wings are also nearing completion. The servos have been checked and are performing fine. All wires and plugs have been checked for integrity. Clevises, servo horns and control surface horns have been checked and are operating with the least possible minimum of slop. The last task was to re-seat the hatch covers using clear tape.
The next task will be to do a full assembly and then re-balance the model. That will have to wait till next week cause “I’m going flying this weekend”.
Brian.

Brian

" Going flying this weekend" ?

Are you coming to Buckminster?

AEB

Yes John, I'm packing the car as we speak.
See you all tomorrow.
Brian.

Having now recovered from a whole lot of flying this last weekend this morning was balancing morning. I got the Kestrel fully assembled for the first time and checked that all the controls were working and in the correct direction and volume. In the early stages of the repair I saw that I had at one stage marked a range of CG positions on the wing root and I had noted their positions at the start of the repair. A quick check on my CG calculator indicated the correct position with a static margin of 10 came right in the middle of the marked range. A safe static margin of 15 came at the front of the marked range. I decided to go with the safe option and with that set up on the balancer I was able to calculate that I needed and additional 40gm of weight in the nose. I am really pleased with this as I expected to require a lot more bearing in mind the repairs to the tail end. I will carry out the first flights and may remove that weight for fine trimming.
Tomorrow is looking possible for the test flights – fingers crossed!
Brian.

That looks gorgeous Brian. Love the blue trim.

It’s nice when an accident gives you the opportunity to refurbish a model and make it tidier than ever, fixing all those little scrapes and dents our models inevitably accumulate, and adding in all those tweaks you’ve been meaning to do for years. Fixing rather than flinging is also good for the planet! Brian Sharp = global warrior.

What a brilliant job you have made of the rebuild Brian, looks great

Looks fantastic Brian - was great to catch up on the weekend and a shame you weren’t quite finished on the Kestrel as it is such a pretty glider.

Simon

Very nice Brian.

Particularly impressed with the quality of the rattle can paintwork.

Mike

Thank you lady and gentlemen for all your kind comments.

Last Sunday I was in my T-shirt flying in beautiful thermals at Buckminster and today I am wrapped up like Michelin Man in a 16 to 25mph wind on my local slope. That’s right; I was performing the maiden flights of my Kestrel 19 after its repair/restoration.
It was actually a bit of a non event with the Kestrel penetrating outwards and upwards with ease. I didn’t even have to alter the trims. I was a bit careful to begin with but was soon throwing the model around as my confidence in the strength of the repairs grew. Loops, rolls, stall turns, reversals and inverted passes were all performed with no undue wing distortion. I even carefully attempted a square loop which the model handled with ease. If anything the model was handling rather better than it has for quite a while. Hopefully I can look forward to another ten years of happy flying with the Kestrel.
I have attached three rather dull and grainy airborne shots as proof of flight. Sorry about the quality but they were taken by me, with my phone, in rather blustery conditions.
Brian.

Congratulations, Brian.

Super stuff Brian!

Phill