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1/4 Scale ASK21

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Peter Balcombe
Posts: 1140
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

1/4 Scale ASK21

Post by Peter Balcombe »

Whilst not expected to be a fast build, my plan is to attempt to produce a scratch built 1/4 scale model of an ASK21 with GRP fuselage and probably built-up wings & tailplane.
The plan Is to build a glassed fuselage plug from blue foam along the lines of Roo's examples, then make a mould from this in order to be able to produce one or more glass fuselages, coupled with built up flying surfaces designed using DevWing/DevCad.
Even at 1/4 scale the model will have a span of 4.25m and a fuselage length of 2m, so requiring a car transport length of just over 2m.

The first step in building the fuselage plug was to source some blue foam (Styrofoam) as used by Roo, but my local SIG depot has stopped stocking any extruded foam (possibly because extruded foam uses CFCs in its manufacture).
However, Roo managed to put me onto someone selling used Styrofoam wall insulation blocks from a local building renovation, so I bought a few blocks. These had unusable areas caused by glue impregnation/removal, so the 1.2m x 0.6m x 75mm blocks had to be glued together using Evostick (solvent free) to form an overall block from which the fuselage core could be cut.

The fuselage core drawing was fairly easily generated from an ASK21 3 view in my Martin Simons Sailplanes book, scaled up to 25% full size.
The side view and plan view drawing s were printed out on several A4 sheets, taped together to form the overall drawings.
The side & plan view outline were then scribed into the foam, highlighted with a biro & then cut with a bandsaw to give a foam box of the correct outline.
The foam core photo also shows the canopy and wing root outlines, plus pairs of red 4mm holes carefully made in each area as registration holes. The red paint was applied to 4mm rods pushed right through the foam core in order to clearly mark the holes all the way through, so that they could be easily picked up again after the core had been sanded to shape.
In fact the canopy sections were hot wire cut from the foam core using simple thick card templates before the squared off block was shaped. The canopy blocks were then stuck back into place using Evostick (but the cut lines contine to show where they need to be removed later.)
Attachments
Foam block with outline
Foam block with outline
Foam core cut
Foam core cut
Canopy blank
Canopy blank
Plan on foam block
Plan on foam block

roo Hawkins
Posts: 831
Joined: 18 Mar 2015, 20:12
Location: Northamptonshire

Re: 1/4 Scale ASK21

Post by roo Hawkins »

HI peter I am looking forward to this build. I am glad to be of help. ROO :D :D

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Peter Balcombe
Posts: 1140
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

With the canopy blocks hot wire cut & then stuck back into place, card templates were made for each of the 6 fuselage cross sections provided on the Martin Simons 3 view.
These templates were then used to profile a small band at each location, before the intermediate sections of foam were roughed out using a pad saw and coarse rasp.
Now that the basic shape has been arrived at, the profile has to be converted into the required smooth flowing curves using a Surform plane.
This work is currently still ongoing, but I seem to be slowly 'getting there'.

You may notice that the fuselage plug currently has no fin, wing root, wheel fairings etc.
These have been initially left off to give me more chance of getting the flowing fuselage lines.
Only once these have been achieved, will I then start adding the various lumps & bumps, fairing these in locally prior to overall fuselage glassing.
Attachments
Canopy sections cut
Canopy sections cut
Fuz template example
Fuz template example
1st side rough profile
1st side rough profile

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Peter Balcombe
Posts: 1140
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

Work has restarted after a gap of a few weeks, but the basic fuselage has now been smoothed and its now time to start adding the lump & bumps.

First up is the fin (after making up a smaller hot wire cutter) which has been initially stuck on with Copydex, then the joint reinforced with White glue prior to fairing into the rear fuselage core.
Also added are the main and rear wheel fairings. these have also been cut from blue foam and initially stuck on with Copydex prior to fairing in.
Attachments
Fuz core_fin
Fuz core_fin
Fuz core_wheel fairings
Fuz core_wheel fairings

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Peter Balcombe
Posts: 1140
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

Wheel & fin fairings now in place and fairing in process nearly complete.
Next job is to cut a wing centre section blank & cut the corresponding fuselage aperture.
Attachments
Fuselage + wheel/fin fairings
Fuselage + wheel/fin fairings

roo Hawkins
Posts: 831
Joined: 18 Mar 2015, 20:12
Location: Northamptonshire

Re: 1/4 Scale ASK21

Post by roo Hawkins »

great job peter. ROO :D :D

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Peter Balcombe
Posts: 1140
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

Today's task was to make up some wing centre section templates and hot wire cut a 200mm long section to sit in the fuselage.
I then bit the bullet and cut a big lump of the fuselage out to include the wing shape, cutting the wing shape from this as a 2nd step, leaving the required space to insert the longer centre wing section as shown in the photos below.

I initially left the centre section somewhat over length, to allow it protrude 13mm each side of the fuselage to accommodate a significant LE fuselage fillet, but now think it better to reduce the fillet length to around 6mm so that the centre section is less prominent, particularly at the TE which will ultimately be curved.
What do you think Roo?
Attachments
Wing space lines cut
Wing space lines cut
Wing spaces pieces
Wing spaces pieces
Wing centre dry fit
Wing centre dry fit

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Peter Balcombe
Posts: 1140
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

Wing stubs fitted, canopies removed and now just about ready for glassing.
The current plan is to glass the canopy blanks separately from the main fuselage.
A canopy land will be glassed around each fuselage side canopy recess, which when flatted back will allow the canopies to be replaced (with release tape used between them and fuselage) before the complete fuselage is given a skim coat of filler before rub back/filling to achieve the final plug.
The canopies will then be removed again prior to mould making.
Attachments
Foam plug less canopy blocks
Foam plug less canopy blocks
Foam plug with canopy blocks
Foam plug with canopy blocks

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Peter Balcombe
Posts: 1140
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

Fuselage now treated to lengths of carbon tow along top and sides of the rear fuselage to stiffen this up prior to glassing.
Glassing used a single layer of 290gsm plain weave cloth in several pieces to cover all blue foam except that I only covered approx. 1" width on the insides of the canopy cutouts (albeit with 200gsm cloth).
On my next fuselage plug, I will use a couple of lighter layers (200gsm) as Roo does for his fuselages, rather than a single layer of thicker material. I had a few areas where the heavier material didn't conform/stick well to the foam - resulting in having to cut these 'bubbles' out later and fill.
The glassed fuselage has now been given a skim coat of a polyester type, 2 part filler to fill the glass weave.
More filling will be necessary to achieve a nice smooth plug shape ;) ;)

The plan is to temporarily refit the glassed canopy blanks after applying a release tape just inside the coaming perimeter. The complete plug will then be finally filled & smoothed, ready for a pattern coat primer which should be able to be buffed up to a gloss finish.
Attachments
Glassed fuselage plug
Glassed fuselage plug

James Brand
Posts: 52
Joined: 14 Sep 2015, 20:37
Location: South Devon

Re: 1/4 Scale ASK21

Post by James Brand »

for plug painting use LR plq02 LR PLQ04, the PLQ04 is a solid high build primer that can be polished but........ I use it as a fairing compound with a guide coat of black then lacquer with the PLQ02, dry sand from P800 to 1500 then wet to 2000 then polish with 3M products..... finally marbocote mould seal and a few cotes of fastcote or fastcote lp..............you end up with a ceramic like super gloss finish......

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