One Eighteenth of a Canberra B.2

[Andy Blackburn]

I couldn't possibly comment on anything pertaining to any elections which might or might not be in progress...

However, I hesitate to clutter up this thread, but on the subject of the Mk18 eyeball, if I might make a mild observation - that A4A nose profile looks as though it might be a tiny bit high at the tip? And perhaps just a tiny bit short? I could be wrong, and if so, please accept my sincere apologies...

Edited By Andy Blackburn on 04/08/2016 22:15:31

[John H. Rood]

Andy, ha! And honest I don't want to spam up your thread any more, but I'd just add that your observation is precisely why I was thinking of this trial n' error eyeball shaping method as the way to go for my little project. Getting things in 3-D will tell the tale!

I utilized a side profile comparison published by Tommy Thomason here: **LINK**, and I carefully drew up an appropriate centerline, ref points, etc. so I could at least have a reasonably good start-point. But we will see! 

Meanwhile, if you're not completely bored yet, sift through my image stash here at your leisure! **LINK**

Enough yapping from me... back to you and your excellent CANBERRA!

Edited By John_Rood on 04/08/2016 23:20:09

[Andy Blackburn]

PM sent.

BTW - Tailhook Topics - really, really good resource...

[Andy Blackburn]

Well, I've kinda finished the nose transparency - or at least, it's as close as it's going to get! Here's the general view:

and a detail view showing the flat clear vision panel, offset to the right (to clear the bombsight, apparently):

I just need to carefully slice this off (it's only tacked on) extend the base a bit because it'll be recessed into the nose sheeting to a depth of about 1/8" and pack it off to Steve at Vortex.

Edited By Andy Blackburn on 13/08/2016 18:28:18

[Andy Blackburn]

So, with the canopy and nose transparency formers packed off to Steve, I can start to think about finishing & covering; I'm going to use tissue/sanding sealer/dope because film is just too fragile, and I think this approach might be less work (although more waiting around) than glass+epoxy. I thought about glass+acrylic varnish but I've put a line through that for the following reasons:

1. Acrylic varnish shrinks about as much as full-strength dope.
2. It's water-based so, unlike dope or epoxy, it raises the wood grain which requires a lot of primer.
3. To get the same dead-smooth finish as epoxy, it seems to be a bit heavier.

There are four types of tissue available to me at the moment; Peck-Polymers and Sams tissue come in multiple colours but have no wet-strength, so pulling them around compound curves (fuselage, nacelles, wingtips, etc.) isn't going to work. I have a lot of Japanese tissue in various colours, and a few sheets of white "modern" lightweight modelspan, which is very similar to the old stuff but has a slight grain - a bit like the Silkspan that you used to get in Sterling rubber-powered scale kits, but without the wrinkles.

I don't want a starved-horse appearance to the structure so the questions are, which of the two candidates shrinks more, and is that shrinkage more with or across the grain? (Yawn - do stay with me, I think it's worth it)

So I made a couple of 100mm square frames from 1/16" square balsa, all stripped from the same sheet of wood, and carefully covered with tissue to exactly the same tension, as near as I could get it:

then loaded them with the same amount of water:

- and look what's happened when they dried:

- they've both shrunk, but the Jap tissue has shrunk a lot more. That might explain why my indoor scale models keep warping!

After a couple of coats of 50:50 non-shrinking dope/thinners and left overnight, they've both stabilised a bit but the Jap tissue has still shrunk a lot more:

- it's quite a lot of shrink:

I spent some time trying to come up with a new name for the relative units of shrinkage, such as Relative Uniform Normalised Tension (RUNT) units, but then unfortunately realised that they were 100mm squares so a 1mm shrinkage is 1%. Drat.

The numbers are:

New Modelspan

Shrinkage with the grain: 0.3% [note 1]

Shrinkage across the grain: 0.6%

Jap Tissue

Shrinkage with the grain: 0.3%

Shrinkage across the grain: 2.5% (!!!)

 

That was a bit of a surprise.

 

Notes

1. I can hear certain parties complaining now about the accuracy of measurement ( "he can't possibly visually measure to that level of accuracy..." ) ; but you can if you use a straight edge and then find a bit of card or wood that will just fit in the gap, and then measure the card with a micrometer...

Edited By Andy Blackburn on 17/08/2016 07:55:01

[Matt Jones]

Andy, could you please ask Steve if he can do another canopy and nose section please?

[Andy Blackburn]

PM sent.

[John F]

Nice testing there Andy.

The tightness of the tissues on the frames do look different and when working with such small dimensions the difference could be massive.

When you're working on such small sections I would have thought that, for accuracy, a simpler shrinkage measurement would suffice. Cutting a piece exactly 100mm square then applying the experiment without the frame would remove any external influences.

That way you can remove any doubt whether the shrinkage measured is down to movement in one direction or a combination of the two directions. it also looks that although the Jap tissue shrunk more it could be a much higher % if the tightness was equal.

[Andy Blackburn]

> The tightness of the tissues on the frames do look different and when working with such small dimensions the difference could be massive.

Well, you say this, but the way it was done was:

1. Lay the frames on the bench carefully
2. Tape the tissue to the bench on one side
3. Attach some tape to the other side, tug carefully and attach to the bench
4. Repeat for the other frame, using the same amount of "tug"

So at the very least, the tension is similar.

[Scott Edwards 2]

I can understand that on a lightweight open structure, excessive shrinkage is a real problem. For covering solid sheet though, like a fuselage, is it still a problem ?

Question 2: I've found that compared to dope, using acrylic varnish on tissue makes it much more 'stretchy' and easier to get round gentle curves. Is there a better technique with dope & tissue that makes it as stretchy as acrylic ?

And finally .... your sanding tool idea for planking !! I've made a couple, and damn they work !!! I put some sandpaper on the outside too though for grip, I found that a bare cardboard surface on the outside was insecure in the palm, and having a sheet on the outside too makes a huge difference.

[Andy Blackburn]

> I can understand that on a lightweight open structure, excessive shrinkage is a real problem. For covering solid sheet though, like a fuselage, is it still a problem ?

It can be, particularly if the sheet is thin or the wood is soft; a lot of film-covered ARTFs have this problem if you squint a bit and look along the fuselage.

> Question 2: I've found that compared to dope, using acrylic varnish on tissue makes it much more 'stretchy' and easier to get round gentle curves.

Well, that depends - if you're putting it on dry with dope or acrylic varnish then the water in the varnish will expand the tissue and make it go around compound curves. However, if you do the following then I prefer dope;

1. Apply two or three coats of thinned non-shrinking dope or (preferably) sanding sealer, sanding after each coat.
2. Dampen the tissue and lay it on wet, smooth everything out, stretch around compound curves.
3. Flood thinner through the tissue to attach it to the surface whilst the tissue is still wet.
4. Let everything dry.
5. Two or three coats of thinned non-shrinking dope or (preferably) sanding sealer again, sanding after each coat.

Jap tissue needs less dope to fill the weave but it has its own issues.

[Andy Blackburn]

Things are still happening on this build, but one of the perils of having a new motorbike in the summer is that one tends to ride it...

I've got the transparencies back from Steve at Vortex, this isn't a particularly clear picture but they don't look too bad at all:

The plastic is quite thick (just over 0.9mm at the base, so I suspect it was probably 1+mm before vac-forming) but it looks vaguely promising, going to try fitting it tomorrow.

The rest of the structure has got to that point where you apply filler...

...the stuff gets everywhere and it it stops raining later this evening I can start to sand it off. I can also have another go at putting my fingers through the wing skin, I seem to be good at that.

Stuff to do before covering is:

1. Fit the nose transparency, needs a small rebate in the planking at F1.
2. Fit the cockpit, cut out the cockpit opening and agonize about how much cockpit detail to leave out.
3. Fit the hatch; I have some 6mm x 2mm magnets from Maplins
4. Er... that's it, I think.

I'm sticking with the "non-shrinking dope & lightweight Modelspan" covering process, and I think I'm going to have a go at brush-painting Humbrol enamels, if I can get some decals of the right size I'll use them otherwise it might have to be multiple discs of Solartrim in the time-honoured manner.

[Andy Blackburn]

We're officially on holiday at the moment but I've managed to get a bit of modelling time. I've been agonising about the nose transparency because although the plastic is about 1/32" thick and the moulding is quite sturdy, it is in rather a vulnerable position. After some lengthy consideration I thought the best thing to do was to just accept that it might sustain damage and just get used to the idea that it might occasionally need replacement.

So I've changed the plan again and am going to glue it to the front of F1 which might not be particularly secure, but if it comes off there's a reasonable chance of a clean break which will make replacement much easier.

Steve has provided a couple of sets of mouldings, so I've used the slightly imperfect set on the basis that if I make a mess of it, it doesn't matter quite so much; separating the nose transparency required a sturdy knife and careful use of a razor-saw:

... and after a few patient minutes with some wet and dry, used wet, it's just about the right size:

That's a view from the bottom, the clear vision panel is offset to starboard (to clear the bombsight, apparently).

The Canopy looks OK as well:

And the overall effect is, I think, pretty good:

The only problem now is that having cut out a dirty great hole for the cockpit, it needs to be filled with something . Representations have been made to the usual quarters and with a bit of luck I won't have to make my own pilot and ejector seat (which I am given to understand is a Martin-Baker Mk 1C).

Edited By Andy Blackburn on 07/09/2016 19:57:28

[Scott Edwards 2]

Looking damned fine ! Steve McVortex knocks out damned good stuff doesn't he

Idea ... Could the nose transparency be made detachable and held on by magnets ? It could pop off in a crunch and be easier to replace ? It could also be used to gain access to nose weight or even a switch ?

[Andy Blackburn]

That's not a bad idea, Scotty. Might need to locate it with a couple of short 1/8" pegs...

[Andy Blackburn]

The cockpit furniture should be arriving from Real Model Pilots around the first week in October, so after a couple of coats of 50% thinned sanding sealer and a fair bit of sanding it's on with the covering and I suddenly realised that whilst I'm familiar with smaller open-framework models covered with Jap tissue, I've not covered anything this size with Modelspan on a solid surface for about 20 years.

However, it all seems to be going quite well; it's surprising what you remember, and it's really quite therapeutic as well as being considerably quicker than expected, for example:

1. Tissue cut to shape with scissors and darted at the root;

2. Tacked down by flooding thinners through;

3. More thinners, and some water at the tip where it needs to go around a compound curve;

4. More thinners flooded through, and it's done:

- that's only about 5 minutes for that one panel from start to finish, which seems to be fairly typical

Parts have been mainly covered dry so far, the fuselage and nacelles will be covered wet. OK, there's some waiting around for the thinners to evaporate and a bit more time spent sanding off the tissue at the edges and/or wrapping it over the LE or TE, but it's easily less than 10 minutes per side of each component. I'm planning on three or four coats of thinned non-shrinking dope.

[Martian]

Lovely looking build Andy, I'm tempted to use tissue to cover my dossier when complete can you tell me where you bought the tissue please

[Andy Blackburn]

I got a roll of lightweight modelspan from Mantua Models in Windsor, can't remember how much it was. Normally I'd try SAMs Models because I seem to order from there regularly but they're asking £2.30 for what looks like one 20" x 30" sheet, which is a bit over the top. Mike Woodhouse sells the stuff for £0.75 a sheet.

[Martian]

Thank you Andy

[Andy Blackburn]

Actually, I can't believe that SAMs are selling lightweight modelspan at £2.30 a sheet (although that's what's implied on the website), it might be £2.30 for a pack of "n" sheets...

[Andy Blackburn]

Things are still happening here, but life and work keeps getting in the way; wings, tail and control surfaces have been covered, so it's on to the fuselage; doing the more fiddly bits first:

Tearing the edges of the tissue is done to make blending the edges easier, I read about it in one of Peter Miller's books. It works as advertised because once the tissue is dampened down and thinners flooded through, you have to look very hard to see the join and after it's been doped and sanded, the join disappears:

 

Edited By Andy Blackburn on 02/10/2016 16:58:56

[Andy Blackburn]

Here's the last bit of tissue (not counting patches which I wouldn't admit to anyway) :

And the tea ceremony to mark the Laying of the Last Piece of Tissue :

Doping and sanding will now follow.

[Scott Edwards 2]

Well, the tea looks like its been passed by the Devil himself, and you're two biscuits short of a 'proper' tea break. But - your tissue covering technique is immaculate

[Andy Blackburn]

Well, there were only three ginger nuts left in the packet - the astute observer may detect the odd couple of crumbs from the (missing) first biscuit.

I'm on the third coat of thinned dope, and I fear that more coats will be required. However, the stuff doesn't sand well because it always seems to dry not-quite hard - which I assume is a function of the amount of plasticizer - so the sandpaper always clogs; perhaps ordinary shrinking dope would be more sand-able, but I daren't use it because the shrinkage would be pretty severe. Getting a bit worn down with all this doping and sanding, and I keep denting the balsa with the edge of the sanding block! And SWMBO is starting to make pointed remarks about the smell

I'm thinking that the best thing to do for the final coat is probably to spray a coat of grey primer from a rattle can and then sand that carefully as a surface for paint. Thoughts or advice would be much appreciated...

[onetenor]

Wet'n'dry used wet with a drop of washing up liquid in the water will; stop the clogging