Scale Soaring UK

Having designed a set of built-up wings for the PT Pik20, a friend gave me a PIK20 fuselage which had previously been "repaired" after having its tail snapped off. Also, the damaged fin had been split with a razor saw around the original join seam in order to clean up the inside of the fuselage join area.

Whoever had attempted the previous repair had used a shiny blue outer surface carbon? tube, but had apparently used an epoxy or similar glue which was still gooey in places & had certainly not bonded very well to the original polyester glass. I was able to twist out the tube in strips & then clean out the goo/harder stuff fairly easily, cleaning off further using an IPA wetted cloth to get back to clean polyester glass.
The additional external glass epoxied directly onto the top of the original smooth gel coat was easy to peel/pick off!
It also looked as if the fin post had been similarly glued as the hard brownish glue could be sliced/pinged off from the original polyester glass.
There was also an equipment mounting tray fitted in a similar way in the cockpit area. This also pulled off in one piece pretty easily as shown below.

Although I didn't get any photos of the original state, I include a few taken after I had removed the joint tube, finpost, tailplane mount, cockpit tray and as much of the joint material as i could to get back to the original polyester glass surfaces.
Apart from the rear fuselage/fin break, the forward end of the fuselage is pretty much undamaged.

My repair plans are currently:
1. Make a new polyester glass rear fuselage internal support tube, using the outer gel coat surface as a mould. This is then slit to remove from the fuselage and the slit widened to allow it to be compressed enough to slide inside the fuselage break area & be attached with polyester resin.
2. Put a lightweight glass cloth layer on the inside of each fin half to stabilise these before rejoining & then carefully refit to the fuselage over the new tube stub.
3. Once the fin has been correctly reassembled, pick off the gel coat around the joint & apply an external glass bandage before filling as necessary to achieve a smooth finish before repainting the whole fuselage once other minor damage had been repaired.

Hopefully, apart from a little extra weight, it should give a firmly bonded repair. We shall see in due course

During a gap in other build activity, I have managed to tape up the rear fuselage to act as a release layer, then apply a few overlapping layers of glass with polyester resin to make up an inner reinforcement tube.
The resultant tube was then carefully slit without much gel coat damage to enable it to be opened out slightly & released from the Fuz.
It came off nicely & the Fuz. cleaned up again.
The slit is now being widened as required to allow the sleeve to be compressed enough to be inserted into the fuselage.

Well, the reinforcing tube needed shortening & the slit widened a fair bit in order to be able to squeeze it together enough to fit into the fuselage (from the front!) & polyester resin bonded into place.
Meanwhile, a layer of 25gsm cloth was applied to the insides of the fin halves to back up the cracks & holes.
The fin halves were then held together using masking tape all around the seam & re-joined using some 1” wide glass tape & polyester resin.
I am now ready to fit the fin back onto the fuselage.

Fin re-attached & a start made in removing gel coat around the break so that an external bandage can be applied over the joint prior to making good with lightweight filler.
The gel coat will probably also be locally removed to make good other major gel coat damaged areas.

Going well Peter, watching with interest as I used to have one of these and it's such a pretty aircraft. I do like a glass fus/foam wing combination too.

Thanks Stephen.
Tail joint now given a couple of layers of external glass cloth prior to filling once the resign has fully cured.
Meanwhile, the forward fuselage major damaged gel coat areas has been picked off to remove the worst & cyano run into the exposed glass layer prior to filling (thankfully none large enough to warrant external glass cloth).
I think I will add a glass layer to the inside of the forward fuselage (wing joiner to nose) just to provide additional stiffness as there are various areas where there is minor gel coat crazing & I am not going to pick it all off!
The small added forward weight is hardly likely to matter as it will be needed anyway to balance the tail repair.

Once the rear fuselage joint has fully cured, I will tidy up the remaining external fin damage by picking off the gel coat in the worst affected areas and filling. The hardwood internal tailplane support block can also be replaced using polyester resin.

The existing wing joiner & incidence pin boxes/tubes will probably have to be replaced as Murphy’s Law is sure to declare that they are in slightly the wrong place for the new built-up wing design, but that will be left until later in order to keep that area more rigid during other work

More pics when I have got back to a tidy fuselage ready to move forward with normal installation activities.

Joint plus other cracked gel coat areas now tidied up and ready to paint once the wing joiner positions have been changed to suit the new built-up wings.

Old fuselage wing joiners were in about the right place, but were 14mm blade type, so had to be removed anyway to change to the new 15mm type
However, this is just as well as the previous joiner had also been fixed in place using epoxy!
Removal just needed a hacksaw blade through the middle of the joiner assembly, followed by a tweak on each half to release cleanly from the polyester fuselage

New 15mm joiner strip is now available, so the previously part built wings have been extracted from their hiding place in the workshop for finishing prior to making up a new scissors type fuselage joiner box - fitted using Polyester resin.

If anybody out there with a PT PIK20 is able to trace around their Tailplane & Rudder assemblies & attach as 1:1 pdfs, it would be greatly appreciated.
Many thanks,
Peter

Whilst waiting for glue to dry on assembled balsa skin pieces for the top of the 2nd wing, I have decided to fabricate a fuselage scisssors joiner assembly from 2 rectangles of 1/8” ply, plus 2 pieces of 1/4 x 1/8” Spruce strip (sawn diagonally across width over the 145mm length to give 6mm to zero wedges).
With the parts glued together around 2 lengths of 15mm joiner tube, I have an assembly giving 2 1/4 degrees angle on each side.
The joiner tubes are a little over length & are only dry fitted, so can be removed to encase the assembly in glass tape/resin prior to fitting in the fuselage. The joiner tubes will be slid/fixed into place from outside the fuselage to allow them to be cut flush with the outside of the fuselage root fairings.
Once everything is in the right place, the joiner assembly will be bonded to the inside of the fuselage with polyester resin/flock
Then for wing attachment, a fully removable steel joiner strip slides into each wing/side of the joiner box, rather than have the joiner permanently fixed into the wing as for Pat’s original suggested approach.
Hardwood blocks will be fitted inside the fuselage to retain rear incidence rod location tubes at the correct dihedral angle.
Wing retention will be via Multiplex Uniloc fixings.

Steady progress Peter, there looks to be a lot of work in those wing ribs

StephenB wrote: ↑06 Apr 2019, 05:39 Steady progress Peter, there looks to be a lot of work in those wing ribs

I let the laser cutter take the strain (courtesy of Cliff Evans - Laser Cut Sailplanes) as all of my built-up wing designs are produced in CAD, so I have cutting files available

Dummy LE & hinge spar planed/sanded flush with the ribs and the top skin applied.
Starting to look a bit more like a wing now.

Now the brake can be fitted from the top and servo hatch/cables fitted prior to adding the bottom skin.

Now with wing skins on and fitted to the fuselage using the joiner box assembly

Having got the wing joiners fitted, attention turns to the tail feathers & having got Cliff Evans to cut some tailplane ribs & spars for me, it’s time to start glueing them together to produce a built-up tailplane.
Basic framework now together, with 3mm Liteply hinge & Aileron LE spars, prior to fitting balsa doublers to give 6mm hinge line thicknesses.
The Aileron LE will be angled for top hinging (tape).
The square ply plate in the centre is to locate & retain the fixing bolt (packed out with balsa to the rib outer profiles before adding skins)
I will probably add the balsa top skin next in order to provide a bit more rigidity prior to fitting the spar doublers.

The top skins were attached as 2 pieces (partly to use up spare pieces of balsa sheet) after fitting the balsa block above the hold-down plate.
Turning the assembly over, the rib building tabs can be removed, fixing hole drilled through (using hole already in the ply plate) and then the 1/16” elevator LE spar doublers fitted behind the spar (which will be angled for down-going clearance), lower fixing plate balsa block added, plus spars sanded flush with the ribs.
Balsa infill also added in centre sections of the elevator to allow the rudder clearance cutout to be made later on.
3/16” Balsa doublers also fitted either side of the central ribs, forward of the hold-down plate, to allow for an anti-rotation peg to be fitted later.
Then, on with the bottom sheeting which is currently clamped up, using temporary spruce strips to ensure a straight TE.

Are you modelling the tailplane as per a 'b model or a 'd model PIK?
Just asking!

Regards

Gordon

Gordon,
Good question, but I don’t know!
I have taken the outline from a PT kit foam tailplane core.
What’s the difference between a b & d versions?

With the LE, TE & tips in place, we have a complete tailplane/elevator assembly which can be test fitted to the top of the fin before the previously installed internal block is fixed back into place.
Note that this assembly is intended to be a direct replacement for the standard foam cored version.

If the l/e of the tailplane matches with the l/e of the fin then it would be a 'b model. If l/e of tailplane overhangs the l/e of the fin by 100 mm (full size dimension) then it's a 'd. The reason for this was so that they could extent the top of the rudder upwards by a similar amount to give a bit more rudder area.
There are full size 'b fuselages with the 'd tailplane and vice versa so what ever way you do it you can call it scale!

Gordon,
In that case, it seems to be halfway between a b & d version as per the PT original design.
The tailplane overlaps by about 12mm, so around 50mm on full size
The rudder stops below the elevator & the top front of the rudder is slightly scalloped to provide elevator TE clearance.
I wouldn’t be surprised if Pat based his design on a b version & enlarged the tailplane slightly to improve handling.

A view of the original PT PIK20 build instructions versus the fin top part of my CAD plan.

Plan fuz is definitely a 'd model. L/e position relative to edge of canopy frame line gives it away.

Ok, on with the elevator servo & linkage installation.
I made up a simple cardboard template to hold the servo & bellcrank plates in the correct position relative to the rear of the fin, fitted the servo to its plate & the bell crank to its plate & used the jig to make up the pushrods to correct length.
The jig card was then reduced to minimum size to register the plates when these were taped in, then Polyester resin applied to the bottom of the plates before placing the jigged assembly into the fin & aligning the jig with the fin rear edge.
A piece of spruce used to hold the side of the fin straight & a couple of temporary wedges to hold the plates down whilst the resin cured - job done

Rudder now in place and the flap & aileron made for the 2nd wing.
All flying surfaces about ready to be glassed.
MPX wing connector harnesses made up & ready to be bonded into the fuselage wing root.
Time to get on with the canopy.

With a couple of projects now off the workbench, its time to return to the PIK20 to finish the fuselage outfitting & canopy.

So.. looking at the cockpit coaming, it was evident that both sides had been nibbled by a previous owner, so a decision was needed as to whether to build the coaming width back up to the original width, or reduce to a uniform width.
I chose the latter, so after marking a new line, I attacked the coaming with a half round file to arrive at a better shape.
Next job was to fabricate a canopy frame, so I started with a solid 3 part 3mm Liteply base & added a doubler strip around the edges to give a bit more canopy glueing area.

Although the previous owner had fitted a solid canopy frame without a pilot, the plan is to remove the central area of the initial canopy assembly build to make a recessed pan which can then be equipped with instrument binnacle, seat, full length pilot etc.
In order to be able to get at the pilot after glazing the canopy, I plan to make the bottom of the pan removeable, but alternatively I could go the whole hog & make the bottom of the canopy frame completely open, fitting everything into the fuselage itself - watch this space to see how this develops

I am really enjoying this build/rebuild Peter. I had two of these in the distant past and absolutely loved them. One was built as a sport model and the second was built for Scale cross country events (I remember them through a sweaty haze – just ask Chris Bowles) The sport model died when my son tried a square loop and was a little enthusiastic on the elevator. The second I flew for many years and then sold on.
I suspect that you will enjoy yours when you get it finished.
Brian.

Many thanks for the kind words Brian.
I think the Scale Police would be wanting a word with your pilot, but he appears to have gone AWOL

I’m currently waiting for the warm weather to reappear in a couple of days before moving on with the canopy frame in order to avoid putting the heater back in the workshop.
I’ve decided to fret out the inside of the initial frame (after removing the doubler strips ) to leave a narrow frame which will be stiffened up using a sandwich of glass & 1/16” ply top.

Hi, that was the time when we could carry a large plane up a big hill, and then run around all day and enjoy it,at least we can still remember it Brian,
Peter, it's nice to see your rebuild, have been following it, it does bring back memories of my own builds of Pat's models, always thought the Pik20 flew the best of Pat's planes, the first Pik20 I saw, was when Mr Baker brought up a green Pik20 to Crook Peak in the early 70's, that flew well also
Chris

Canopy frame sandwich made using layer of 200gsm glass above & below the initial frame, capped with a layer of 1/16" ply.
The fuselage coaming was protected using a layer of glossy tape to act as a release layer & the assembly could be clipped to the fuselage whilst the epoxy cured to get a good shape.
Once cured, the frame was cleaned up (including removing front section) and has now been fitted with front peg & 3 rear magnets.

Having decided to try to fit out the cockpit a bit, I went for a moulded epoxy glass seat - which is a essentially a curved plate.
First off, I scaled & drew out the side profile and then cut a foam blank, keeping the top part for use later.
Then, with pieces of thickish plastic sheet as the release layers, I laid up 3 layers of 300gsm cloth between an outer layer of 200gsm cloth directly onto the plastic sheet protected foam bed - using the upper foam blank under weights to squeeze everything together whilst it cured.
The resultant curved glass sheet was easily released from the foam blanks & cut to size.
The photos below show some of the steps, plus the resultant fit using my old AH Designs 1/4 scale pilot.
All seems to fit ok, so next job will be to make up the instrument panel binnacle base.

Prior to attacking the instrument panel binnacle, I decided that I ought to get the aerotow release & associated servo fitted.
I therefore took an impression of the inside of the nose, transferred this to a female mould & then cast a lead nose weight.
With a hole drilled through the lead and counter-bored at the rear, the tow release was epoxied in position prior to the whole assembly being set into the nose using silicone sealant - all the better to be able to remove if needed at a later date
I installed an old Turnigy metal cased servo on a ply plate & will now make up the pushrod to complete this aspect.

With the canopy frame now painted, the glazing has been attached.
Meanwhile, an instrument panel & binnacle is under construction.

Decided to get the fuselage repainted whilst the good weather lasted
Have also just finished the Instrument panel binnacle which has been fitted with a panel kindly supplied by Cliff Evans. (Yes, the top left instrument is blue on the full size!)
The intention is to install the binnacle using location pins & magnets so that this is removeable in order to access the nose.

Peter Balcombe wrote: ↑26 Apr 2020, 16:34 Decided to get the fuselage repainted whilst the good weather lasted

Well it is a Sunday..."let us spray".
See what I did there?

After a further delay - forgot to spray the rudder
We now have a completed model at last (bar Rx & straps to hold the pilot down!)
Overall weight when ballasted to get CoG 60mm behind the wing LE (front of main spar, it says on the original instructions) the AUW comes out at just under 9lbs 3oz, or 4.1Kg in new money.
This sits very well with the 9.5-10.5lbs flying weight on the original instructions bearing in mind the rear fuselage/fin repair.

Now to find a spare Rx & tie the pilot down before a trip to Crook Peak ....

Lovely job Peter, the PIK 20 is such a good looker - I prefer this era to the more modern ultra streamlined ships.

Nice one Peter, the Pik20 always looks so well proportioned.

M

The rebuilt PIK20 was successfully flown today at Crook Peak - after jacking up the rear of the tailplane by 3mm! - followed by an uneventful landing
First launch attempt resulted in a dive into the gorse - luckily without damage.
Inspection of the tail angle (fuselage previously broken, so should have double checked that before now ) showed what looked like a negative decalage, so the tailplane bolt was loosened & a couple of strips of scrap fibreglass board slipped in at the rear.

I now need to double check the decalage, as I think a little more under the tailplane wouldn’t hurt. I still needed full up trim & a little up stick to fly level. It may be a little nose heavy, but can look at that when I can get it trimmed for level flight.