1/4 Scale Charlesworth ASK13

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Peter Balcombe
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Location: Clevedon, North Somerset, U.K.

1/4 Scale Charlesworth ASK13

Post by Peter Balcombe » 16 Aug 2019, 10:01

This plan has been around for a while & apparently quite a few built over the years.
As with most other Cliff Charlesworth designs, the plans are available through Cliff Evans (Laser Cut Sailplanes) who has digitised the plans and makes a short kit available.
However, Cliff has recently been made aware that some of the parts don't fit as well as they might, so this build has been requested to investigate and make improvements where necessary.
Also, this build will have the alternative open cockpit (Cabroilet) arrangement.

The supplied plan sheets are big, so you need a bit of space to spread them out ;)
Initial inspection showed that the wings use the Cliff Charlesworth preferred built-up main spar assembly approach (so I will alter the build here to use my preferred build up from lower spar approach), plus the fuselage longerons are balsa (which I will change to spruce & balsa to add more strength).
Also, I felt that a few more build notes would be helpful, especially for a less experienced builder.

Anyway ....
Starting with the fuselage, I first made up a simple fuselage build jig from softwood (bottom rail with vertical end pieces glued vertical at overall fuselage length + 2mm separation). This was marked with centre lines along the bottom strip & up the inside of the end pieces, plus former locations. (I made the end pieces 220mm high to allow a straight edge to be placed along the top, just above the highest former to give an upper sighting centreline.

Next job was to draw a reference line on the plan, just below the keel at F2, with the formers at 90 degrees to this.
The idea is now to fix pedestals at each former position to position the keel strips at the correct height relative to the reference line.
(Note that Cliff's plan has a 'Datum' line marked in the canopy area, but I needed one below the keel).

Next, Cliff arranged for me to receive some keel pieces cut from 1/4" ply, which could then be epoxied together to make up a full length keel, complete with wheel pant surround.
With the keel sat centrally on the support blocks the front & rear formers were glued onto the keel and tacked to the jig end pieces (each spaced off with a couple of 1mm ply strips to allow them to be released later).

All this is hopefully clarified in the pic below.
Attachments
Keel in build jig base.jpg
Keel in jig

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Peter Balcombe
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Re: 1/4 Scale Charlesworth ASK13

Post by Peter Balcombe » 18 Aug 2019, 12:06

Using some 9mm Sq. stripwood moulding from the local DIY store, I then made a series of 'goalposts' which could be screwed to the sides of my jig base, giving verticals at each former position, but with a crossbar above the lower longeron position to allow access to all longerons for insertion.
Next, a straight upper centreline reference strip was fitted to the tops of the jig end pieces & vertical centrelines also marked onto each former (temporary balsa strips being fitted to the open topped formers for this purpose).

With the goalposts in position, the formers were glued to the keel and clamped to the goalposts, ensuring they were were vertical in both axes, using the top reference line to sight along/drop a vertical line down (plumbob!)

Note that formers 5, 6 & 7, together with 2off 5A & the root ribs all slot together to make an interlocked sub-assembly before this is sat onto the keel.
(This is where I found the first issue, as a tab slot was missing on the root ribs & part of a tab uncut on a former - however these were soon rectified).

The longerons can now be fitted at waist, upper & lower positions, noting that the upper longeron needs to be spliced at 3 points to achieve the shape shown on the plan side view.
I chose to deviate from the plan (6mm Sq. Balsa) & use an inner 1/4 x 1/8" Spruce strip + outer Balsa strip of the same size. The aim here was to give a bit more strength and make the front end curves a bit easier. (The waist slot on Former 4 needed raising a few mm & the tops of formers 3 & 4 was also found to be out of line.

Note that all of the issues I am finding have been fed back into the CAD drawing to allow Cliff to update the laser cutting files for future parts supply.
Attachments
Formers on jig2.jpg
Formers on jig
Jig Former 0_2.jpg
Nose former
Jig tail end2.jpg
Rear former
Top view longerons2.jpg
Rear fuz_longerons2.jpg

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Peter Balcombe
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Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale Charlesworth ASK13

Post by Peter Balcombe » 19 Aug 2019, 19:37

With all longerons in position, I realised that I hadn't fitted the upper keel piece between the front two formers!
However, there was no slot pre-cut in the front former (Former 0), so the fuselage had to be released from the jig to get at this.
The slot was duly added & the keel piece glued into place. Drawings have been updated.

I was thinking it necessary to refit the fuselage into the jig as before, but upon reflection realised that if secured to the building jig frame upside down, it would still hold everything straight whilst all of the lower & side sheeting was fitted.
In the inverted state, the rear half of the fuselage lies flat on the board, so it is easy enough to clamp this straight, whilst the nose/tail formers are clamped to the front/rear verticals.

Once this has been done, a 1/16" Balsa sheet was fitted on the central flat area ahead of the wheel (drawing is confusing as it says 3/32" Balsa), the wheel pant fitted and the 1/4" Balsa support strips fitted (in two 1/8" layers each side of the keel aft of the wheel. Once dry, the edges of these pieces are bevelled to align with the formers, ready to accept the lowest fuselage sheeting layers.
Note that the rear former has vertical sides, so the rearmost fuselage skin will have to twist nearly 45 degrees over the rearmost inter-former space.
Also note that the top of the rearmost former is approx. 1.25" wide, so the rearmost section of lower skin support widens over the rearmost inter-former space.
Attachments
Basic struc on back2.jpg
Inverted Fuz structure
Former clamp2.jpg
Former clamp
Nose on back2.jpg
Inverted nose centre skin
Lower rear Fuz skin prep.jpg
Rear lower skin support

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Peter Balcombe
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Re: 1/4 Scale Charlesworth ASK13

Post by Peter Balcombe » 21 Aug 2019, 19:00

I mentioned earlier that a 1/16" skin had been applied to the bottom of the forward formers & this has been followed up with a further 1/8" layer as indicated on the plan.
Once the edges of the resultant 3/16" Balsa plate have been sanded back & bevelled to suit to next skin band, further 1/16" Balsa skin bands can be applied, working towards the cockpit coaming. Note that the 1/8" balsa need to be added before the adjacent skin bands are fitted in order to make a stiff enough angled joint (i.e. not as I did it :oops: ).
Note that the drawing shows the landing skid attached to the bottom of the fuselage using two hardwood blocks which appear to mount directly to the bottom Balsa surface (effectively 3/16" thick), rather than tied directly to the keel or formers. (I'm not sure if Cliff Charlesworth intended one of the bottom layers to be ply rather than balsa, but no material type is mentioned as far as I can see.
I'm not sure how robust the keel fixing will be in resisting any unplanned sideways forces during landing, so I may reinforce these fixing points later.

The forward lower fuselage has now been skinned as shown below using 36" long sheet, plus additional balsa skin supports added at the rear of these areas to support the rear skin joints.
An additional balsa skin support has also been added at Former 12 as the last section will be done separately. I have also added a skin support directly below the waist longeron as the upper& lower sections will have to be skinned separately due to the joint angle at Former 12.
Next job is to add the rear fuselage skins.
Attachments
Front lower skins done.jpg
Front lower skin side.jpg
Skin support1.jpg
Rear skin support.jpg

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Peter Balcombe
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Re: 1/4 Scale Charlesworth ASK13

Post by Peter Balcombe » 22 Aug 2019, 17:00

Rear fuselage skins now added, plus the more complex pieces at the very rear.
The plan calls for a 1/16" ply tail skid support to be rebated into the bottom skin, but I decided to use this as a template to cut out the shape from a piece of 1/16" balsa sheet before glueing the ply into the hole and then glueing both to the fuselage, tapering the front into the rest.

Meanwhile, with all but the upper waist band of fuselage skin in place, I clipped the open cockpit canopy base into place on the upper fuselage longeron line & started to populate that, plus sort out the alternative canopy fore & aft fixings.
A new forward upper former was made (based on the front canopy frame former), but extended downwards to be able to attach to the top fuselage longeron. The upper keel piece was replaced with one which extended past F1 to tie into the new forward canopy upper former.
The open cockpit structure has 2 dowels which slide into holes drilled in fuselage Former 5B at the rear, plus a central latch pin at the front.

Also added, are the other curved former top sections which will be skinned as a turtle deck later.
Note that I found that the top of former 5B stuck up above the turtle deck line, so took 4mm off the bottom to improve matters.
Attachments
Rear skin.jpg
Tail skid base.jpg
Open canopy structure.jpg
Top forward fuz.jpg

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Peter Balcombe
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Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale Charlesworth ASK13

Post by Peter Balcombe » 28 Aug 2019, 15:40

More progress with the fuselage skinning, with the front end now done plus rear sides done, leaving the top & area around the rear of the wing root to complete.
I will probably leave the wing root areas & above until the wings have been built and joiner assembly inserted.
Meanwhile, the canopy frame structure has had the last few bits added, so is now ready for skinning.
Attachments
Fuz front skinned.JPG
Fuz rear skinned.jpg
Wing Root skin.jpg

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Peter Balcombe
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Joined: 18 Mar 2015, 10:13
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Re: 1/4 Scale Charlesworth ASK13

Post by Peter Balcombe » 02 Sep 2019, 20:52

The open cockpit frame assembly is now more or less complete apart from fitting the windscreen base supports & a final bit of shaping.
There will now be a short holiday break before the build continues 8-)
Attachments
Open Canopy top.jpg
Open Cockpit side.jpg

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