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Slingsby Kestrel 19

B Sharp

Slingsby Kestrel 19

Post by B Sharp »

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. :shock:
I collected the pieces, dumped them into the car and retired home where I stuck them in a corner of the workshop in disgust. :x :x :x
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. :)
Attachments
The bits!
The bits!
Cracked tail end.
Cracked tail end.
Ex wing joiner.
Ex wing joiner.
The gutted tailplane
The gutted tailplane
005.JPG
Wing damage top.
Wing damage top.
Wing damage bottom.
Wing damage bottom.
User avatar
terry white
Posts: 508
Joined: 18 Mar 2015, 21:08
Location: wareham,dorset.england

Re: Slingsby Kestrel 19

Post by terry white »

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? :x :cry: 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. :|
B Sharp

Re: Slingsby Kestrel 19

Post by B Sharp »

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.
Attachments
The Kestrel as it used to look.
The Kestrel as it used to look.
B Sharp

Re: Slingsby Kestrel 19

Post by B Sharp »

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. :)
Attachments
Damage cut away.
Damage cut away.
Removing the existing glue with a dentists burr.
Removing the existing glue with a dentists burr.
Carbon strips and balsa filling.
Carbon strips and balsa filling.
Carbon reinforcing strips clamped in place.
Carbon reinforcing strips clamped in place.
012.jpg
Balsa infill held with epoxy/microballoons.
Balsa infill held with epoxy/microballoons.
Rear fuselage being prepped.
Rear fuselage being prepped.
B Sharp

Re: Slingsby Kestrel 19

Post by B Sharp »

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. :)
Attachments
The rear fuselage with two layers of wet glass cloth.
The rear fuselage with two layers of wet glass cloth.
Now dry and sanded back with wet and dry.
Now dry and sanded back with wet and dry.
Fuselage nose stripped of it's red colouring.
Fuselage nose stripped of it's red colouring.
The underside of the tailplane - still tacky.
The underside of the tailplane - still tacky.
B Sharp

Re: Slingsby Kestrel 19

Post by B Sharp »

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.
Attachments
Mounting bolt holes lined with brass tube.
Mounting bolt holes lined with brass tube.
Bolt holes countersunk on the top surface.
Bolt holes countersunk on the top surface.
Bolted neatly to the fin.
Bolted neatly to the fin.
Tailplane/fin attachment from the rear.
Tailplane/fin attachment from the rear.
The complete and painted tailplane.
The complete and painted tailplane.
Underside.
Underside.
The rudder being stripped
The rudder being stripped
Back down to gel coat.
Back down to gel coat.
Completed rudder with Vinyl striping.
Completed rudder with Vinyl striping.
B Sharp

Re: Slingsby Kestrel 19

Post by B Sharp »

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! :D :D :D

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. :)
Attachments
The temporary root cover.
The temporary root cover.
Waiting for the epoxy/balloons to set.
Waiting for the epoxy/balloons to set.
The newly filled holes with covers removed.
The newly filled holes with covers removed.
Attachment pin holes drilled.
Attachment pin holes drilled.
Completed repair with pin in place.
Completed repair with pin in place.
Joiner and both front and rear wing root reinforcers.
Joiner and both front and rear wing root reinforcers.
Wing clamped in place while stub incidence tube dry.
Wing clamped in place while stub incidence tube dry.
User avatar
Peter Balcombe
Posts: 1399
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: Slingsby Kestrel 19

Post by Peter Balcombe »

All looking very good Brian :)
B Sharp

Re: Slingsby Kestrel 19

Post by B Sharp »

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,. :)
User avatar
Max Wright
Posts: 198
Joined: 10 Mar 2020, 19:57
Location: Clayton Bay South Australia

Re: Slingsby Kestrel 19

Post by Max Wright »

You're doing a lovely job of it, Brian. 8-)
Max
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