1/4 Scale ASK21

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Peter Balcombe
Posts: 437
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

1/4 Scale ASK21

Postby Peter Balcombe » 19 Mar 2017, 17:18

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.)
Foam block small.jpg
Foam block with outline
Foam core cut small.jpg
Foam core cut
Canopy blank small.jpg
Canopy blank
Plan on foam block.jpg
Plan on foam block

roo Hawkins
Posts: 402
Joined: 18 Mar 2015, 20:12
Location: Northamptonshire

Re: 1/4 Scale ASK21

Postby roo Hawkins » 20 Mar 2017, 17:36

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: 437
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Postby Peter Balcombe » 20 Mar 2017, 18:33

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.
Canopies cut small.jpg
Canopy sections cut
Fuz template small.jpg
Fuz template example
1st side profiled2.jpg
1st side rough profile

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Cliff Evans
Posts: 530
Joined: 13 Mar 2015, 10:32
Location: Bristol

Re: 1/4 Scale ASK21

Postby Cliff Evans » 21 Mar 2017, 15:15

As I said Peter, glutton for punishment!

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