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1/4 Scale ASK21**

Posted: 19 Mar 2017, 17:18
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.)

Re: 1/4 Scale ASK21

Posted: 20 Mar 2017, 17:36
by roo Hawkins
HI peter I am looking forward to this build. I am glad to be of help. ROO :D :D

Re: 1/4 Scale ASK21

Posted: 20 Mar 2017, 18:33
by Peter Balcombe
Canopy sections cut
Canopy sections cut
Fuz template example
Fuz template example
1st side rough profile
1st side rough profile
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.

Re: 1/4 Scale ASK21

Posted: 20 Apr 2017, 15:09
by Peter Balcombe
Fuz core_fin
Fuz core_fin
Fuz core_wheel fairings
Fuz core_wheel fairings
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.

Re: 1/4 Scale ASK21

Posted: 26 Apr 2017, 16:17
by Peter Balcombe
Fuselage + wheel/fin fairings
Fuselage + wheel/fin fairings
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.

Re: 1/4 Scale ASK21

Posted: 26 Apr 2017, 16:52
by roo Hawkins
great job peter. ROO :D :D

Re: 1/4 Scale ASK21

Posted: 27 Apr 2017, 16:09
by Peter Balcombe
Wing space lines cut
Wing space lines cut
Wing spaces pieces
Wing spaces pieces
Wing centre dry fit
Wing centre dry fit
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?

Re: 1/4 Scale ASK21

Posted: 18 May 2017, 09:07
by Peter Balcombe
Foam plug less canopy blocks
Foam plug less canopy blocks
Foam plug with canopy blocks
Foam plug with canopy blocks
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.

Re: 1/4 Scale ASK21

Posted: 27 May 2017, 16:47
by Peter Balcombe
Glassed fuselage plug
Glassed fuselage plug
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.

Re: 1/4 Scale ASK21

Posted: 28 May 2017, 02:40
by Tom Pack
Holy crap that's a thick airfoil at the root......what section is it and what is the plan for the rest of the wing?

Re: 1/4 Scale ASK21

Posted: 28 May 2017, 07:46
by Peter Balcombe
Many thanks for your tips James.
The LR system you mention appears to be very similar to the Easy Composites pattern coat primer/high gloss system.
I already have some of the primer, so will press ahead with that on this model.

Tom,
The wing section will be as per the full size, so FX S02-196 / FX 60-126 sections at the root /tip. The intention is to achieve the slower scale-like flying speeds.
Yes, the wing root depth is approx. 3", so plenty of room for the joiner system ;)
The wing will be of built-up construction with a commercial top surface spoiler assembly. I haven't designed the wings/tail surfaces yet, but will do this using the DevWing CAD tool in due course, getting Cliff Evans to laser cut the parts for me.
I will probably finish the model as an ATC Air Cadets Vanguard TX.1 example.

Re: 1/4 Scale ASK21

Posted: 28 May 2017, 08:27
by roo Hawkins
hi peter i am glad you are getting on with it well . your work so far looks realy good .I am glad to have helped. ROO :D :D

Re: 1/4 Scale ASK21

Posted: 28 Aug 2017, 19:58
by Peter Balcombe
Root TE side view
Root TE side view
Root TE plan view
Root TE plan view
Root TE filled
Root TE filled
This project Is still progressing, albeit very slowly, in short bursts between other projects.
I am still slowly getting the fuselage plug profile smooth after having added yet more filler all over the plug and then sanded back with 120 grit.
It now looks a lot better, so I am currently looking at getting a better wing root profile edge, particularly where the thin TE curves back to the fuselage.

After trying unsuccessfully to get a good edge profile with filler, I decided to bite the bullet and cut off the TE section and insert a pre-cut 1/16" epoxy board shape which I could fill to, and then sand back to the epoxy glass edge as shown in the photos below.
A further improvement will be to add an epoxy glass sheet root rib which has been matched up to the correct length FX S02-196 section.
I can then finally fair the fuselage into this rib to get what should be a very good match to the built-up wing at the wing joint.

Re: 1/4 Scale ASK21

Posted: 29 Aug 2017, 23:53
by Jolly Roger
P1040233.JPG
P1040242s.jpg
P1040306s.jpg
Hi Peter,

I've been following your build with interest.

Here's some photos of our local K21 this weekend, hopefully inspiring. Keep up the good work!

Rog

Re: 1/4 Scale ASK21

Posted: 30 Aug 2017, 10:26
by roo Hawkins
hi peter good to see you have started again .I use a thin ply at the root ends . keep the posts coming

Re: 1/4 Scale ASK21

Posted: 30 Aug 2017, 17:40
by Peter Balcombe
Fuz root plates in position
Fuz root plates in position
Root
Root
Thanks for the encouragement Roger & Roo.
Roger, I remember my flights in a K21 at Sutton Bank so 17 or more years ago :)

Two fuselage root plates now added and faired in. Just needs some stopper to finish off that area.
I have another pic of the root rib, but it just will not load!!

Re: 1/4 Scale ASK21

Posted: 31 Aug 2017, 22:19
by Peter Balcombe
Just checking the wing/tailplane mounting relationship in order to correctly form the plug tailplane seat at the top of the fin.
Measurements taken from the Martin Simons 3-view drawing indicate that the wing incidence should be set at +2 to +2.25 degrees with respect to the tailplane.

Does this sound right?

(Both wing and tailplane sections are as per full size)

Re: 1/4 Scale ASK21

Posted: 31 Aug 2017, 22:57
by Jolly Roger
To some extent it's personal preference.

In practice, a larger angle requires a more forward CofG and results in a stronger stabilising moment in the longitudinal plane. The sweet spot for most glass sailplanes is around 1.5 to 2 degrees, but I have read of 0 degrees for more aerobatic types.

Hope this helps.

Rog

Re: 1/4 Scale ASK21

Posted: 03 Sep 2017, 13:59
by jimbo
Good work Peter. Watching with interest, hope the mould comes out well.

Re: 1/4 Scale ASK21

Posted: 08 Sep 2017, 19:15
by Peter Balcombe
Many thanks all.
A tiny bit more progress - the top of the fin has been cut to shape.
The front step is as per full size as it allows the LE to curve up to the LE of the tailplane which projects forward of the top of the fin.
The rear part Is cut to the tailplane underside profile.
Thus the tailplane will have a small block fitted to its front underside & it then bolts directly onto the top of the fin.

Re: 1/4 Scale ASK21

Posted: 08 Sep 2017, 21:00
by Peter Balcombe
Fin Tailplane seating small.jpg
This the photo that I was trying to add.
It was taken on an iPad held in portrait mode, so image needed to be rotated 90 to have fin vertical.
Image appears on its side in preview but will maybe come out ok when you click on it!

Re: 1/4 Scale ASK21

Posted: 06 Mar 2018, 09:59
by Peter Balcombe
Canopy area blanks
Canopy area blanks
Current plug progress
Current plug progress
A little bit more progress over the past couple of months, but a bit of one step forward & two back in places :(

The plug is now nice & smooth and I have added a front wheel coaming, plus made smaller canopy blanks in order to avoid having a very deep valley between the 2 canopies in the mould, which would no doubt cause problems when I tried to lay up this area.

After studying more photos of the full size, I noticed that the wing root fillets were too big, so that has been sorted.
I have also noticed after finding a good rear view shot that the rudder horn fairings should be much more prominent, so that is the next job.
The main wheel fairing also looks wrong as this should in reality, reflect a bolt-on fairing below the normal lines of the lower fuselage rather than being fully faired in as I have at present. (I did have a problem relating the 3 view profiles to the actual 3D shape in this area).

So... a bit more work before applying the pattern coat & buffing up to a finished plug :roll:

Re: 1/4 Scale ASK21

Posted: 06 Mar 2018, 18:58
by GeeW
Would you like some close up shots with measurements of the rudder cable fairings? (or any other bits of a K21 come to that) As I have one in the workshop this weekend.
Regards

Gordon

Re: 1/4 Scale ASK21

Posted: 06 Mar 2018, 20:26
by Peter Balcombe
29839C9C-F547-4FC7-AE80-541176150849.jpeg
EAC4A618-81F4-4BEB-970A-058E29D3AE23.jpeg
1712D7A7-F6E2-4F51-824A-A9DBD76B5409.jpeg
Thanks Gordon,
I think I have the rudder cable fairings sorted now, as I was able to scale the rear opening from a photo I found on the web.
I fitted some more prominent fairings today, which I hope are much more representative of the full size.

However, I would appreciate a side shot of both the mainwheel and tailwheel fairing areas, plus the nosewheel if you could manage it. I found the tyre sizes in a manual on the web, so think I have scaled the wheels ok.
I think that the mainwheel is a sprung rear swinging arm type, so if you do have the mainwheel fairings off at any time, a view of this would also be interesting in case I think about using other than a hard mounted wheel. I assume the mainwheel doesn’t move all that much, to avoid grounding the fairing on a hard landing.

Some photos on the web seem to show the mainwheel fairing & others seem to show what I assume is the attachment moulding.
The photo below seems to show the fairing in place, wheel at lowest extent.

A further query;
The picture below shows a fin mounted pitot (KDP photo) whereas I have also seen a vertical installation on the top of the fuselage, just aft of the wing (3rd pic). Is the upper fuselage installation also likely to be a pitot, or is it more likely to be a radio antenna?

Re: 1/4 Scale ASK21

Posted: 06 Mar 2018, 21:19
by GeeW
I will take some images for you of wheel fairings.
Main wheel fairing is normally attached with half a dozen screws, but is often seen removed.
In winter it is removed as it fills up with mud and on a hard landing tears off and is then promptly run over! In the late summer it fills up with dried grass which has an annoying habit of catching fire if the brake is used in a energetic manner! That is also why the Air Cadets don't fit their main wheel fairings either.

The factory standard T/E probe is the one fitted half way down the fuselage. It is an optional extra to have it fitted on the fin. The pitot lives in the hole at the very nose of the glider. For aerobatics including inverted a 100mm extension tube is fitted to the pitot as the ASI doesn't work inverted, which can become an issue if things go in anyway awry.

The radio antennae is inside the fin so never seen on nearly all non-metal gliders.

If this weekend goes to plan I will be re-installing the whole undercarriage assembly so can get images of that as well. The undercarriage frame is pivoted at the front edge on a trailing link with big rubber blocks above to take the (sub-optimal landing) shocks.
The amount the wheel hangs down varies for each glider as its maximum extension is dictated by how tightly you do up 2 pieces of string....I kid you not. As you rightly observe there is not much travel, about 75 to 100 mm max.

Regards

Gordon

Re: 1/4 Scale ASK21

Posted: 06 Mar 2018, 21:53
by GeeW
Just looked it up. Incidence is +2 degrees on full size.
Dihederal is 3.6 degrees measured from the inboard top wing skins.

See page 20 for drawing of undercarriage
See page 27 for T/E probe location
See page 29 and 30 for rigging and incidence information


Knew there had to be a copy on-line of the Maintenance Manual somewhere!

http://www.aeroclubrieti.it/w/wp-conten ... I-IVWO.pdf

Re: 1/4 Scale ASK21

Posted: 06 Mar 2018, 22:39
by Peter Balcombe
Thanks again Gordon.
If you do get some pictures of the mainwheel fairing, it would also be useful for a fore/aft view as well to show the profile of the fairing itself, relative to the main fuselage (including a scaling object if possible)
The photos show the fairing as a distinct “add-on” to the underside, particularly around the sides, and it is that change in profile that I want to try to recreate if possible.
My current plug has a smooth profile from fuselage to wheel aperture which is very different from real life!

It has just occurred to me that if the fairing comes off as one piece, then it might be possible to take a photo of the fairing sitting on the ground or flat to a wall, looking directly in line with its centre (either top or bottom) and scaled if at all possible using an object of known length. (Top and bottom views would be even better).
I should then be able to see both top & bottom apertures in the same image & can scale these from the photo using CAD.

It may actually be worth thinking of cutting off my existing fairing, re-profiling the lower fuselage to ignore the fairing, then making a fairing add-on to fit in place to complete the plug, much like the canopy blanks.
Peter

Re: 1/4 Scale ASK21

Posted: 07 Mar 2018, 20:49
by GeeW
Peter
Don't cut off anything just yet! The fuselage does not follow a fair line under the fairing. There is a bit of a lip moulded into the belly of the fuselage to accommodate the main wheel fairing.
The fairing is on the workbench so no problem to grab some scaled images of that whilst I'm waving the camera about. The fairing is a strange shape as it is symmetrical on the fuselage join and is asymmetric at the bottom to allow space for the brake caliper.

Gordon

Re: 1/4 Scale ASK21

Posted: 07 Mar 2018, 21:31
by Peter Balcombe
Thanks again Gordon.
I found a copy of the manual on the web when looking for the wheel sizes.
I had previously guessed that the incidence would be about 2 degrees, so that is good although the tailplane seat could be tweaked if necessary.
The wing dihedral is a bit more than I expected but that is easily dialled into the wing design design program when I finalise the joiner arrangement.

Whilst waving the camera about, could you please also point it at the tow release.
Although I think I will install the aerotow release in the nose pitot aperture, it would be good to see how prominent the real release is.
Peter

Re: 1/4 Scale ASK21

Posted: 10 Mar 2018, 09:20
by Peter Balcombe
Gordon,
PM sent with contact details in case you wish to send full resolution pics.

Re: 1/4 Scale ASK21

Posted: 10 Mar 2018, 19:01
by GeeW
I was just about to ask..will send over later this evening as I am on cooking duties this evening.

Gordon

Re: 1/4 Scale ASK21

Posted: 10 Mar 2018, 21:36
by GeeW
Emails sent. I hope they are useful?
Just as a nod to anyone else following this thread I have ready access at the moment to several types of glider.

ASK-21 (both early and late types)
ASW-19b
Standard Cirrus (choice of 2)
DG-1000T
LS-4b
LS-4 (surprisingly different from the 4b, different fuselage fin and altogether) (choice of 2)
LS8-18 (or 15 or 15 with winglets)
Grob Twin II
Grob Astir CS
Glasflugel Libelle H201b
Shempp Hirth Nimbus 2 (choice of 2)
Slingsby Kestrel 20 (A 19 with stretched wings rather than the later MkII which is a different aeroplane)
Grob 109b
LAK 12

Re: 1/4 Scale ASK21

Posted: 10 Mar 2018, 22:33
by Peter Balcombe
Lovely, many thanks Gordon :)
Peter

Re: 1/4 Scale ASK21

Posted: 17 Mar 2018, 19:08
by GeeW
Two more emails should be awaiting your pleasure in your inbox. :D

Regards

Gordon

Re: 1/4 Scale ASK21

Posted: 02 Apr 2018, 14:59
by Peter Balcombe
Many thanks for all the pics and recent dimensions Gordon.
I now have a good scaled 3 view drawing of the wheel fairing, so will break out the foam or balsa shortly to make a more realistic 1/4 scale fairing.
Peter

Re: 1/4 Scale ASK21

Posted: 08 Apr 2018, 19:03
by Peter Balcombe
C8ED2AED-1897-4EFE-AF7E-2A819DC3E481.jpeg
60D97284-543A-4E9B-BEAA-647B85D67ADE.jpeg
Old main wheel fairing removed and underside of fuselage made good.
New fairing made and just glued in place (scaled from full size photos & dimensions kindly provided by Gordon).
Hopefully this looks more like a real fairing now (not that you probably see this view of a full size glider that often!!)
Fairing still needs to be faired in.

Re: 1/4 Scale ASK21

Posted: 10 Apr 2018, 17:33
by Peter Balcombe
CB5A47B8-DF3B-45B6-BE70-CFA29B4C8EB3.jpeg
53B56254-8AB4-4CC3-9CAE-B12A5B0697EC.jpeg
Main wheel fairing blended into the fuselage a bit better now.
A final check to fill the last few blemishes, then on with the pattern coat I think.

Re: 1/4 Scale ASK21

Posted: 27 Apr 2018, 17:31
by Peter Balcombe
7B97B0B5-30FA-4E24-AF90-9F4B57397C02.jpeg
After a couple of coats of pattern coat primer, initial sanding with 120 grit, then working through the wet & dry grades (used wet) to 800 grit, followed by a polishing paste, I think I have a fuselage plug about ready for the mould making process.
A photo below shows the fuselage complete with its 2 small canopy aperture blanks which will be hot glued in position after a few more coats of wax polish, then the joints sealed with wax.

Next job is to prepare the parting board.

Re: 1/4 Scale ASK21

Posted: 15 May 2018, 16:21
by Peter Balcombe
Mould box
Mould box
Plug in position
Plug in position
With whole plug prepared with 6 coats of beeswax, it’s time to prepare for making the 2 part mould.
After measuring the fuselage width and getting some 3mm parting board material, I made up a box (screwed only, so easier to dismantle once 1st side of mould made) which allowed the split line to be on the upper parting board level when the fuselage is sat with one wing root on the bottom of the box. The fin is also supported to be at the correct height.
Previously positioned marks on nose, rudder post and top of fin show the splitting line.
Once the fuselage outline is cut out of the parting board (and edges also supported at correct height), the fuselage can be sat into place, the gap filled with wax (carefully cut back to give a flat surface into fuselage) prior to adding mould registration features.
The photos below show the mould box before & after fuselage plug is sat in place.

Re: 1/4 Scale ASK21

Posted: 23 May 2018, 20:20
by Peter Balcombe
B3AF43F9-4277-4BB5-9CFB-5B8E51447D11.jpeg
First side of mould now laid up.
A bit of a learning curve, but hopefully it will come out ok in due course.
Need to get more resin for 2nd side as it used a lot more than I expected ;)

Re: 1/4 Scale ASK21

Posted: 24 May 2018, 07:04
by Simon WS
Looking good Peter. Was making a mould something you fancied trying or are planning on making more than one for your mates?!

Re: 1/4 Scale ASK21

Posted: 24 May 2018, 09:12
by Peter Balcombe
Thanks Simon,
Although initially thinking of going down Roo’s one-off route, I somehow decided that I would have a go at making a mould as a project!
This was partly because at the time, I wanted to try the process to make another fuselage off of an unused DG100 fuselage I have & partly because I thought it might be useful to be able to make a spare if needed.
I haven’t seen 1/4 scale ASK21 models about, so thought it a good subject.

There is a looong way to go yet though ;)

Re: 1/4 Scale ASK21

Posted: 24 May 2018, 16:46
by Peter Balcombe
Mark,
What is a ‘heavy’ moulding?
As far as I know, I am making the mould as a kit manufacturer would - layers as recommended by John Hall.
There is a gel coat plus the equivalent of 1 layer of 300gsm + 2 layers of 450gsm on the fuselage part of the mould, with twice that on the flanges.

Re: 1/4 Scale ASK21

Posted: 24 May 2018, 20:53
by Mark Erwood
Normal industry practice is to make moulds at least twice the lay-up weight of the mouldings that you expect to produce. Seems to give a good life to moulds.

Re: 1/4 Scale ASK21

Posted: 26 May 2018, 14:50
by Peter Balcombe
D036E060-0864-4B14-A7C1-872544E13772.jpeg
Mould layup is probably triple the expected fuselage content, so seems to fit the bill in that respect.
However, lessons learnt for next time:
1. Take more care to ensure parting board gap fill is perfectly level with parting board at all points.
2. Fit fence to rear of fin (for open rear of fin).
3. Limit mould to within about 3” of plug to reduce layup materials needed and weight.
4. Add lots of registration features along intended mould half bolt line.

1st side now carefully removed from parting board (together with plug) and turned over once the overall mould outline had been slimmed down to within 3” of the plug. Mould half bolting hole line marked approx 30mm from plug outline & new registration countersinks added every 12”or so, as the original features were nearly all removed when the mould flange was slimmed down. Bolts will be fitted every 6” or so.
Vertical fence fitted at rear of fin for 2nd side (1st side mould rear will be cut off later).
Nearly ready for making a start on the 2nd side of the mould when I have restocked with resin ;)

Re: 1/4 Scale ASK21

Posted: 26 May 2018, 20:17
by Mark Erwood
Hi Peter,

Sounds as though your mould will be more than adequate. The last mould that I made I trimmed the flanges to about 1". For registration I drilled a 1/8" hole every few inches in the first flange and superglued a 1/8" snap head rivet in each hole. it worked a treat. Good luck with the rest of the build.

Mark

Re: 1/4 Scale ASK21

Posted: 03 Jun 2018, 12:39
by Peter Balcombe
0236641C-9004-4AD4-A083-FCBC3AD7ADC9.jpeg
Completed the mould layup yesterday & have just split the mould.
After a bit of a struggle getting the 2nd side off, the plug came out reasonably intact.
A bit of remedial work is needed where the plug stuck/ or where there are small voids, but I hopefully now have the basis of a useable mould.

Re: 1/4 Scale ASK21

Posted: 03 Jun 2018, 17:40
by Peter Balcombe
Mark,
Good tip with ‘just in case’ scenario.
I have a number of registration points included in the mould halves (added when original ones were going to be cut off!).
Will clean various stuck plug areas off tomorrow & then attack the other damaged areas.

Re: 1/4 Scale ASK21

Posted: 03 Jun 2018, 19:41
by Peter Balcombe
Mark,
I now understand what you mean by a ‘heavy’ mould :)

Re: 1/4 Scale ASK21

Posted: 04 Jun 2018, 06:45
by roo Hawkins
Well done Peter. Will have to do a mould some time myself.

Re: 1/4 Scale ASK21

Posted: 04 Jun 2018, 10:09
by Peter Balcombe
I see what you mean Mark.
Although looking reasonable from a distance, my moulds look as if they need a good deal of attention upon close inspection.
I think I I’ll get John Hall to have a good look & advise when he calls by in a week or so.

Re: 1/4 Scale ASK21

Posted: 10 Jun 2018, 10:49
by Pete Marsden
Fuselage mould.jpg
This is a great effort! Having made epoxy one-offs for years I know just about everything that can go wrong! (and in my case still does!)
Just for info I enclose a pickie of a fuselage mould I recently made.
Without wishing to tread on toes the following may be of use:
I finish the pattern with two pack paint polished to a mirror, 7 coats of release wax, and then take an epoxy mould from this.
Never mix epoxy lay-ups with polyester ones. i.e. if you finish the pattern in epoxy or two-pack, don't try and take a polyester mould from it. It will invariably stick solid.
Make the mould symetrical: i.e: - one coat of gel (very thinly applied), an epoxy/microfibre fillet along all tight corners, one layer of 100gsm glass To prevent the weave showing through), 2 to 3 layers of 200gsm, one layer of 100gsm and another coat of gel. This should prevent the mould warping with age.
Put a fence around the whole lot - this too gives rigidity.
Use locating pegs - this can be done simply by drilling dimples into the first half of the mould, re-waxing and then make the second half.
Regarding the dividing plate: - I don't get too hung up about the initial fit of this (I use 9 mm MDF with a lower fence to hold this the correct height above the workbench to allow the plug to fit centrally). I taper the hole in which the plug fits and with everything firmly held, I apply a fillet of filler into this notch. Once set I remove the plug and sand this fillet flush with the surface of the dividing plate. The plug should now fit perfectly.
Hope this helps a bit.

Re: 1/4 Scale ASK21

Posted: 14 Jun 2018, 20:42
by Stuart Ward
Peter
Thanks for the explanation
But for someone that has only built woodies and would like to try to build a glass fuse, most of the paragraph above was gobbledygook. Maybe someone could start a new thread with step by step instructions on making moulds. Even small mould like control horn fairings.
Stu

Re: 1/4 Scale ASK21

Posted: 26 Jul 2018, 10:40
by Peter Balcombe
Mould_side.JPG
Mould_top.JPG
Not able to find space to take the ASK21 mould to the Shrivenham aerotow, so a couple of pics to show current progress.
The excess mould flange material has been removed and the inner mould surface voids filled. Just needs a final surface polish now prior to making the first part.

I have decided to make an epoxy part (rather than polyester resin) as with a slow hardener this will give me a lot more time to get the various glass cloth & roving layers in properly before the resin gels.
The downside is that I will need to make both sides and then join them in one hit - using glass overlaps from each side to form the central joining bond. The upside is that I hopefully get a complete part in one go.

However, before all that, I will be having a practice on some much smaller moulds that John Hall has kindly lent me.

Re: 1/4 Scale ASK21

Posted: 01 Aug 2018, 06:29
by Andrew Ray
Superb work there Peter. I have always liked the K21, lovely lines for a 2 seat training glider.

Re: 1/4 Scale ASK21

Posted: 14 Aug 2018, 20:08
by Peter Balcombe
5ED15AEB-E002-4291-8D99-5312E53A4C2E.jpeg
C3102C5B-53E2-4F4B-8FA8-4FE66A77F130.jpeg
Epoxy glass fuselage laid up last Saturday in a marathon 6hr stint.
After discussion with John Hall it had been decided to lay up each half with matching join line overlaps using slow cure epoxy resin. The 2 halves were put together whilst the resin was still wet & the overlaps smoothed down onto the other side to form the complete part in one sitting.
After the normal cure period, the part was post-cured for 8hrs or so at 30+. I finally managed to release it from the mould this afternoon after a bit of a struggle (as anticipated due to the less than perfect nature of the mould surfaces)
However, it is now out and should be useable after tidying up some surface blemishes.

I still have ply wing root & tail fin top to epoxy into place as these were left out at the lay up stage to reduce work.
An idea of the result is shown below.

Re: 1/4 Scale ASK21

Posted: 15 Aug 2018, 08:36
by VinceC
Excellent work Peter. When does production start?

Re: 1/4 Scale ASK21

Posted: 15 Aug 2018, 08:54
by Peter Balcombe
Thanks guys.
Still a long way to go to get a flying model, but at least I now have something to attach things to!

The mould needs some repairs & surface polishing before I can even think about re-use, and probably isn’t good enough for serious ‘production’ use - at least with my fibreglassing skill level. We will see where we go once the design is proven.

Once I have added the internal ply wing roots/tailplane mount and checked that my planned wheels will fit, I can progress the built-up wing and tailplane designs ready for part cutting.

Re: 1/4 Scale ASK21

Posted: 15 Aug 2018, 21:24
by roo Hawkins
Well done Peter. A lot of hard work I think I will keep with my lost foam method. But it is some great work you have done look forward to seeing how the rest of the model comes together. .Roo :D

Re: 1/4 Scale ASK21

Posted: 25 Aug 2018, 21:13
by Mike Forsyth
Hi Peter, I have been watching this build and was wondering what weight of cloth you used in your epoxy glass fuselage, and how many layers? and did you use a gel coat or was this just layed up just using epoxy as the first layer?

Re: 1/4 Scale ASK21

Posted: 25 Aug 2018, 21:50
by Peter Balcombe
Hi Mike,
I used an epoxy compatible polyester gel coat (Easy Composites GC50), then a 100gsm layer from nose to wing root, plus 2 layers of 300gsm (one twill? & one plain weave) using 5 main pattern shapes per side.
In addition, I used 300gsm blisters in the wing root, main wheel & tailwheel fairings, plus strips/rovings around all of the canopy flanges.
The layup was as recommended by John Hall who has made all the fuselages for the Pat Teakle range over the years.
1/4” ply wing roots have now been epoxied into the insides of the wing roots & glassed over.

My only issues were where the mould surface wasn’t perfect (primarily around the wing root/canopy flanges) & my layup also had a couple of voids behind the gel coat in similar areas.
However, as I didn’t have a pigment in the gel coat (GC50 is clear only) then it isn’t really a problem as I have to paint the part in any case, so this will also cover the small amount of filling needed.
Peter

Re: 1/4 Scale ASK21

Posted: 27 Aug 2018, 10:15
by Mike Forsyth
Thanks for the information, it will come in very handy. One last question what was the weight of the fuselage? I always wanted to try making a glass fibre fuselage, but I think I will try on a smaller model first, just to test the water so too speak, I don't think I'm brave enough to do a 1/4 scale model, Even though you have a few imperfections you have done a remarkable job on your very first glass fibre fuselage I can only hope I'm that lucky.

Re: 1/4 Scale ASK21

Posted: 27 Aug 2018, 11:05
by Peter Balcombe
Mike,
Bare fuselage weight with ply root blanks fitted is 1.7Kg for a fairly bulbous 2m long fuselage. (Wing is 70mm thick at root).
Not sure if this is heavy or not, although I gather that epoxy resin fuselages are usually lighter than polyester resin ones, but this obviously depends on the quantity of cloth & resin used in each case.

Re: 1/4 Scale ASK21

Posted: 27 Aug 2018, 11:43
by Simon WS
Fantastic achievement Peter.

Simon

Re: 1/4 Scale ASK21

Posted: 27 Aug 2018, 13:11
by Geoff Pearce
Well done Peter, will you be challenging Roo for fastest build? :lol:

Re: 1/4 Scale ASK21

Posted: 27 Aug 2018, 13:57
by Mike Forsyth
Thanks, Peter. 1.7kg for the fuselage is not too bad, I will be watching this with special interest. You have done a Fantastic job 😃

Re: 1/4 Scale ASK21

Posted: 27 Aug 2018, 16:34
by Peter Balcombe
Geoff,
I don’t think Roo has anything to worry about 🙂
It’s taken 18 months so far to just get a fuselage.
However, wing & tail designs are done, so maybe have something to maiden by next Spring.

Re: 1/4 Scale ASK21

Posted: 13 Sep 2018, 22:24
by Peter Balcombe
Tailplane internals
Tailplane internals
With fuselage available, I have designed a tailplane arrangement using DevWing, transferring the output to DevCad to work up the detailed cutting templates and annotated build plan.
Cliff Evans kindly cut some parts for me, so the tailplane has started taking shape as shown below.
The plan is to embed a thin wing servo in the rear of the tailplane, this driving the elevator via a short pushrod to a ball joint on the bottom of the central elevator half joiner. The linkage sits in a slot cut in the top of the fin.
The tailplane is attached using a single bolt and anti rotation pin.

Re: 1/4 Scale ASK21

Posted: 14 Sep 2018, 06:43
by Simon WS
Looks very neat Peter
Simon

Re: 1/4 Scale ASK21

Posted: 16 Oct 2018, 18:14
by Peter Balcombe
Tailplane & Elevator underside
Tailplane & Elevator underside
Tailplane & Elevator top
Tailplane & Elevator top
After a bit of a delay (to remake the elevator in order to avoid added filler weight), the tailplane top sheeting has been added and the elevator dry hinged to check servo operation.
Everything looks good, so the lower building tabs have now been removed and lower spars fitted ready for application of the lower sheeting.
Note that tip blocks have still to be added to give the final shape.

I will investigate fitting of a servo hatch to allow access, plus fitting the tailplane to the top of the fin.
Rudder construction has also started, so I hope to be able to check the overall fit of the tail feathers soon.

Re: 1/4 Scale ASK21

Posted: 24 Nov 2018, 19:22
by Peter Balcombe
Tail group check.jpg
Tail group check2.jpg
With rudder built up and hinged onto a Liteply/balsa finpost, I finally found time between other builds to fit the tailplane fixing Tee nut at the top of the fin and also cut out the tailwheel slot in order to check that the rudder & elevator were clear with the tailplane in place.
Happily it all fits, so I can now tidy everything up before moving on to the wings.
The tailplane overhangs the front of the fin a little more than true scale, but that is because my fin is a little less deep than it should be (3-view scaling error :o ) - but hopefully no-one will bother to much when it is all done.

Note that the bit projecting above the tailplane in the photos (looks like a flag) is actually a piece of masking tape attached to the rudder hinge pin wire to hold the top of the rudder in place as the fin-post is only dry fitted. The wire will be replaced with a removeable carbon rod in due course.

Re: 1/4 Scale ASK21

Posted: 17 May 2019, 18:21
by Peter Balcombe
A little more progress today after printing off a wing root rib, complete with datum line in order to transfer the datum line to the fuselage root ribs, together with the main joiner & rear incidence tube centres.

With the wing datum defined, I strapped a length of substantial hardwood strip against the wing root, with one side along the datum line. I also made an 1/8” balsa negative tailplane root section template so that with this sat on the top of the tailplane, I had a flat surface parallel to the tailplane datum.

Thus equipped, I was able to check the longitudinal dihedral & was pleased to find that the wing sits at +2 degrees, or just over, wrt the tailplane 🙂

So, out with the P38 filler to make a smooth tailplane bed at the top of the fin (essentially just filling the 1/8” gap at the very rear of the tailplane. Once that had set & been tidied up around the sides, short lengths of 16swg brass tube have been let into the top of the fin & tailplane for an anti-twist peg.

Re: 1/4 Scale ASK21

Posted: 29 Jun 2020, 18:16
by Peter Balcombe
K21 Outer panel start_small.jpg
K21 Wing structure1_small.jpg
K21 Wing build2_small.jpg
K21 Wing build3_small.jpg
After more than a years delay, I at last have the bench clear to get back on this build.
So..... with a wing parts set kindly cut by Cliff at LCS, let the build recommence.

The wings use top & bottom main spars, bonded full length to a central Ply web to form an 'I' beam.
So after making up the main spars, the lower one was laid over the plan, supported on blocks to achieve the correct height. The outer ply web was then glued to the top of the spar, using a few ribs to correctly position it, before adding the remaining outer section ribs, plus Balsa Dummy LE, Dummy TE, Aileron hinge spar & Aileron LE to produce the complete structure apart from LE & stock Balsa TE.
K21 Outer panel start_small.jpg
The process was then copied for the Inboard section, including the rear spar & web to tie everything together and give a pretty complete structure apart from the upper main spar & lower rear spar.
The brake box will be defined by 1/8" Spruce spars yet to be added. The photos also show the carbon joiner tube which slots through the root ribs/spar web. The main joiner will be a 16mm diameter high carbon steel (machine slide) rod, with 5mm rear incidence rod.

Re: 1/4 Scale ASK21

Posted: 03 Jul 2020, 11:15
by Peter Balcombe
Joiner tube_small.jpg
Brake installed_small.jpg
Joiner tube boxedin_small.jpg
R21 Rib doubler_small.jpg
With all ribs in place, the joiner tube has now been epoxied in position and then fully boxed in with ply sheet & epoxy to make a solid lump tied to the spars.
The rear incidence tube has also been added & boxed in using hard balsa, faced with 1/16" ply.
Airbrake fitted & pushrod routed through to the servo bay.
Lastly, a 1/8" balsa doubler strip has been fitted around the wing taper break rib in preparation for skinning, which I have decided to do as separate Inboard & outboard sections.
The structure now just needs to be sanded over to get the False LE & TE to profile prior to applying the full width upper skins.

Re: 1/4 Scale ASK21

Posted: 05 Jul 2020, 11:24
by Peter Balcombe
Wing top sheet_small.jpg
Wing tops sheet below_small.jpg
Upper sheeting has been applied in 2 sections once rib doubler had been fitted at the taper break.
I now need to find the Brake blade outline in the upper sheeting - if I can find the piece of paper I wrote the measurements on ;)
Aileron servo cable threaded though & now awaits more cable to be able install the Brake servo cable.
Photos below show the current state, noting that a 1.5" TE strip will be fitted once the sheeting is complete.
(The front part of the ailerons is already included in the build so far, complete with angled aileron LE spar).

Re: 1/4 Scale ASK21

Posted: 10 Jul 2020, 10:01
by Peter Balcombe
29D2B662-3E9D-4A8B-A943-0CFF4BC2541E.jpeg
5D875D66-EAE4-4B58-B0DE-E34799D7E99B.jpeg
Underside now sheeted & servo hatches opened up.
1.5” TE stock now being applied to the rear edge, starting with outer section - aligning upper surface to wing profile (excess lower surface to be removed later when profiled to wing section).

Re: 1/4 Scale ASK21

Posted: 10 Jul 2020, 18:44
by Peter Balcombe
DE72EF6C-FD96-4953-92CD-9AA19FE5731A.jpeg
With bottom spar tapered as required this morning, the 2nd wing structure is well on its way - with not a drop of Cyano in sight!
The Ply web sitting on the top of the wing is for the rear spars yet to be fitted. (The web fits from the underside anyway).

Re: 1/4 Scale ASK21

Posted: 18 Jul 2020, 12:35
by Peter Balcombe
3AFC2C4D-E574-457A-ABE7-13C8ED76DE21.jpeg
With the top skin now on the 2nd wing & the lower building tabs removed, I couldn’t resist putting the joiner tube in the fuselage in order to try an initial assembly.
Tailplane also sat in position to get an impression of the overall size.
Still a lot of work to be done, but slowly getting there :)

Re: 1/4 Scale ASK21

Posted: 18 Jul 2020, 14:20
by Cliff Evans
Looking good Peter.

Re: 1/4 Scale ASK21

Posted: 21 Jul 2020, 13:20
by roo Hawkins
Nice mate

Re: 1/4 Scale ASK21

Posted: 30 Jul 2020, 15:00
by Peter Balcombe
E3E89C5D-3606-424B-99D9-8B9A1995A279.jpeg
With both wings now fully sheeted & fuselage joiner installation complete, attention moves to completing the wings.
After profiling the lower edge of the stock TE section to match the distinct undercamber in this TE area, the aileron has now been separated on the 1st wing.
(This involved remembering to mark the hinge line on the top surface - defined by rear edge of vertical hinge spar) prior to adding the lower skin). The aileron could then be cut off by inserting a scalpel blade through the top skin, down the rear of the hinge spar & out through the bottom skin at the ends & centre of the aileron. Join up the lower surface points & cut through skin & ribs using a razor saw. There is room for error at the bottom due to the gap formed by the angled aileron LE web.
Once removed the aileron LE bevel can be tidied up.

Re: 1/4 Scale ASK21

Posted: 07 Oct 2020, 10:30
by Peter Balcombe
E61E6DDF-136D-4E1F-BEF7-756037CB243F.jpeg
03A4F6EB-E5BE-4A25-8EA3-EC6F17A1E56C.jpeg
With the wings now having had their glass cloth applied & awaiting a final coat of epoxy to seal the cloth weave, attention has passed to preparing plugs from which I can hopefully ask Barry at Sarik to pull the 2 canopies.

When making the fuselage plug a while ago, I made sure that I saved the canopy cutouts, which were glassed to avoid hanger rash etc. The plan was to use these as the basis for the canopy pulling plugs - although at the time, I wasn’t sure how I would go about this!

In order to produce the canopy plugs, I need to extend the canopy shapes by 1.5” or more at front, rear and bottom, and also make the plug capable of withstanding the vacuum pull without distortion.
The final plug will be cast in Herculite plaster in a simple negative mould bath formed around the extended canopy cutouts.
So ... the first job was to cut some foam blocks having the extended canopy dimensions & cut a recess in these into which the canopy cutouts could be fixed. The remaining foam will then be shaped to profile to also include a moulding release draught angle. The pics below show the canopy cutouts in place in the foam blocks.
Hopefully, all this will work out ;)

Re: 1/4 Scale ASK21

Posted: 07 Oct 2020, 19:21
by B Sharp
I have been patiently for this build to continue Peter. That is an interesting method for making the plugs .
Brian. :)

Re: 1/4 Scale ASK21

Posted: 07 Oct 2020, 20:47
by Peter Balcombe
Hi Brian,
It’s probably not necessarily the normal way to do it, but it’s my first time & I already have the canopy opening blanks saved from the fuselage mould making process ;)

Re: 1/4 Scale ASK21

Posted: 09 Oct 2020, 21:54
by Peter Balcombe
130FB78F-1F1D-4E70-B9F9-6D40C72A4C1E.jpeg
I know that canopy plugs are normally made from a hardwood core, but I am going to have a go at something different - given my starting point with two perfectly profiled canopy opening blanks.

I plan to end up with solid herculite (or similar) plaster moulding plugs, but will be trying a different route to get there. It may not work, but if it does, then it should make life a lot easier than carving two large wooden plugs from scratch.
The extended canopy opening blanks have now been initially filled & shaped to conform to the canopy opening profiles. The pic below shows one of them before the front & rear release angles were increased. The resultant block will also be given more generous rounded corners to improve release tapers prior to sealing with glass & epoxy resin.

Re: 1/4 Scale ASK21

Posted: 15 Oct 2020, 20:45
by Peter Balcombe
59B0FEF3-8FD3-4E70-B45F-8F09598EAEC6.jpeg
Back to the wings, which have now been glassed and also received a thin sealing coat of epoxy resin to leave a gloss finish.
Once rubbed back a bit with fine wet&dry used wet, the ailerons will be hinged & servo installation sorted prior to painting - which might now have to wait a bit!

Re: 1/4 Scale ASK21

Posted: 15 Oct 2020, 22:53
by Jolly Roger
Wings look lovely Peter. The camber on the undersurface looks very realistic.

Re: 1/4 Scale ASK21

Posted: 16 Oct 2020, 08:22
by Peter Balcombe
Jolly Roger wrote: 15 Oct 2020, 22:53 Wings look lovely Peter. The camber on the undersurface looks very realistic.
After talking to Noël Rumers before the wings were designed several years ago, I decided to use the full size FX section as Noël reckoned that with a tip chord of >130mm, that would work & allow the model to fly at a scale speed.
We will see if that is so in due course ;)

Re: 1/4 Scale ASK21**

Posted: 16 Oct 2020, 08:32
by B Sharp
All looking very good Peter, but where do the golf clubs fit into this equation?
Brian. :D