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One Eighteenth of a Canberra B.2

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Andy Blackburn01/06/2016 15:18:29
385 forum posts
469 photos
1 articles

Preamble...

The very first version of my Canberra B.2 plan dates back to July 2002 (!), the design has been through a number of (for want of a better term) evolutions, each of which had a different construction, airfoil section (S3021, Clark Y), planform twist etc.

By about 2007 (no sense in rushing these things) I’d settled on one of the best-performing sections with good manners that I could find (HQ2.5/10 blending to HQ 2.5/12), but construction wasn’t easy – the HQ airfoil is undercambered with a delicate aft section, the wing was a fairly complex build and was constructed upside down because there's a compound taper on the bottom but the top is flat, the nacelles were built from thick sheet and triangle section balsa like small “kipper” fuselages and were threaded onto the wing after being planed (an operation that, because of the small size of the nacelles, put the tips of one's fingers at serious risk from the blade of a razor plane) and sanded to shape, and the fuselage was made from an octagon of thick sheet like the old Flair Hawker Hunter so it required contest-grade wood and you had to be good at bending thick balsa. Or good with filler.

I sent out 3 or 4 evaluation copies of the plan to various friends and aquaintances to see what they thought, Terry Lidstone of the West Mendips club built one as a T.4 (solid nose), it was covered in Profilm and came out quite light (39 oz) in spite of the rather agricultural construction. Terry’s conclusion was that although there were some constructional difficulties (that upside-down wing again), it flew really well.

However, I was never really happy with the design, so the part-constructed bits have been stuck in a corner of the workshop, staring accusingly at me for the past 7 or 8 years, and I’ve tinkered with the plan over the years with a view to finishing it properly...

The full unexpurgated, oversized story of Scott Edwards' double-size Cranberry is covered elsewhere on this forum (see "One Ninth of a Canberra"), but Scotty's requirement for a flat-bottomed wing was the trigger for what I hope will be this final re-draw of the plan; after a bit of thought it was concluded that Eppler 205 (which requires a generous allowance from Dr Reynolds - or a turbulator - in order to do its best work) should be really good at that enormous wing chord with low drag and lots of speed potential, and that it would still be fine for the (double-sized) tips because they’d still be about 10 inches wide.

However, if we changed the tip section to Clark Y – a magnificent thermalling and general-purpose section – it would be better at low speed (thermalling and landing) and would probably have better manners around the stall as there's about a degree of aerodynamic washout when compared to E205, but we can add a bit (1.5 degrees) of geometric washout to help things along. And since the wing was now flat-bottomed it should be easy to build and could be constructed the right way up.

After a while, of course, I realised that what would work for Scotty's very large Canberra would probably also still work for the original (and more sensibly-sized, some people might say) 43” span model! This was an exciting find and I spent a couple of weeks pulling everything on the drawing apart and re-drafting it; the fuselage is built on a keel, planked, with a disposable jig. Nacelles are now built on the wing and sheeted in situ (thanks to Steve McLaren for this idea), this approach requires more parts but that’s not a problem if the laser-cut parts are used, and is a lot safer on the fingers. Best of all, the Wings are now built the right way up (hooray)! The inner panel are built first on a spar that runs all the way out to the tip, then the wing is tipped-up onto the bottom skin for the outer panels.

All the parts were copied onto separate .DXF files to that they could be laser cut by SLEC who produced 15 sheets of wood in varying thicknesses, printed the plan and sent everything to me for a reasonable sum.

img_1306.jpg

-obviously, I didn't want to say "very reasonable" in case they put their prices up, but there does seem to be quite a lot of wood.

Gratifyingly, I found that all the parts do seem to just slot together with a minimum of fuss; the only time any minor fettling is required is when the balsa is not 100% correct in thickness – some of my 1/8”, for example, was a few tenths of a mm oversize and the 1/4" was about 6.9 mm. However, I’m told this is quite normal.

Andy Blackburn01/06/2016 15:29:56
385 forum posts
469 photos
1 articles

Wings

The inner panels are built first:

img_0169.jpg

There's a fair bit of careful infill needed, and the spar strength has to be tapered off towards the dihedral break:

img_1319.jpg

The wing joiner is 5 mm carbon rod running in carbon tube, the incidence pegs are 3/32" wire. I should have taken more care with the alignment because I had to make a small compensation when installing the incidence tubes in the fuselage.

img_1321.jpg

Outer panels are made by sliding the bottom outer wing skin into place and then tipping the wing up so that the spar is flat on the wing skin.

img_1326.jpg

Ready for the top skins:

img_1330.jpg

 

Edited By Andy Blackburn on 01/06/2016 15:30:16

Andy Blackburn01/06/2016 15:48:47
385 forum posts
469 photos
1 articles

Nacelles

Nacelles are now built directly onto the wing, the only thing you have to be careful of is keeping the vertical keels dead straight:

img_0222.jpg

After a bit of experimentation, I discovered that it's possible to pre-set a compound curve in a piece of light, straight-grained 3/32" balsa but running both sides under a hot tap for a minute, leaving it for 10 minutes and then just bend it with the fingers, scraping the inside with a fingernail to induce curves in both directions. When it's dry, you're left with this:

img_1386.jpg

Sheeting the nacelles is then pretty straightforward, but there's a lot of waiting for wood to dry:

img_1410.jpg

I'll detail it on the plan including some cutting patterns but basically the process is

  1. Pin some scrap bits of 3/8" square onto the wing surface 3/32" from the formers so that the sheet fits between, and sand a the sheet for a reasonable fit.
  2. Tack-glue it in place with medium cyano.
  3. Take off the bits of 3/8" square and properly glue it to the wing surface, I don't like kicker (awful stuff!) but in this case it's very useful.
  4. When it's set, carefully dampen the skin, leave it a few minutes, apply some medium cyano and then carefully roll it onto the formers, one or two at a time, cutting a dart at the front as necessary (see picture) and some V-shaped cut-outs to allow the excess wood to bend.

Here's the almost-finished nacelle:

img_1419.jpg

- hasn't turned out badly, needs planing, sanding and the jet-pipe...

img_1420.jpg

..and of course the starter fairing, made from rings of balsa mounted on a 1/8" dowel, spun in a drill and sanded:

img_1414.jpg

Mark Kettle 101/06/2016 15:50:55
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1873 forum posts
1259 photos

Subscribed Andy yes.

Andy Blackburn01/06/2016 15:51:25
385 forum posts
469 photos
1 articles

Tail Surfaces

The tail is just 1/4" sheet, cut into some odd shapes to try and avoid warps, with some strategic spruce reinforcement:

img_1364.jpg

img_1367.jpg

Andy Meade01/06/2016 16:01:18
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2163 forum posts
562 photos

Beautiful work, and I want one. Who can I speak to about getting a plan and a set of parts please Andy?

Andy Blackburn01/06/2016 16:30:26
385 forum posts
469 photos
1 articles

Fuselage

Ah, the interesting bit...

One of the changes to the original plan was to add a traditional battery box. I still make them 1 1/4" square to fit old-style NiCd batteries...

img_1370.jpg

Glueing the keel pieces showed up a minor difference between the plan and parts - unusually for me, I didn't fuss about it (paper will expand and contract with atmospheric conditions) and just went with the parts.

img_1371.jpg

However, here's where there was a minor hiccup:

img_1374.jpg

When the laser cuts wood, it's a very precise cut but the cut itself is V-shaped, and on thick sheet it can be 0.75-0.8 mm wide at the top of the V; this means then when keel pieces are trued-up for gluing together, there can be a gap of up to 0.5 or 0.6 mm, as shown here. I've added some thin sheet to adjust the fit of the keels, but it looks as though there might have to be some adjustment of the parts patterns to allow for this.

The other issues were:

  1. I hadn't really though in sufficient detail about where the servos would be mounted;
  2. It turned out that the snakes were mostly in the wrong place, and the holes were too small anyway, and
  3. The side keel pieces were both cut from 1/4" sheet and the wood was slightly bent - normally this wouldn't be a problem (wood does this all the time) but they were bending in such a way that the fuselage was twisting during assembly.

Clearly, some redesign and re-drawing of parts would be necessary, and some more parts would have to be cut. Oh dear. That's not what I said at the time, of course, but the sentiment was broadly similar...

Fast-forward several weeks and SLEC had produced and sent another sheet of poplar ply formers, and we could finally get down to the fuselage. Basically, it's built upright on 1/4" keels with some disposable jigs to hold everything straight, it can be almost completely assembled and then hit with cyano to lock everything in place:

img_1426.jpg

This is a stressful photograph - I had to clear the workbench!

Here's a close-up of the incidence peg fixing:

img_1427.jpg

The idea is that when everything is tacked in place, you can remove the incidence pegs and pins and lift it off the bench, add the remaining structure and then sand everything to shape before slotting back into place, adding the snakes, fitting the servos and then planking the top part of the fuselage; once that's done, it should be rigid enough to plank the bottom without anything being pinned down. That's the theory, anyway .

Scott Edwards 202/06/2016 08:15:35
150 forum posts
90 photos
Really pleased you've got this project going again. A new plan from the Blackburn stable is always an event. Following with great interest !
Andy Blackburn14/06/2016 20:43:02
385 forum posts
469 photos
1 articles

Fuselage Part 2

Amazingly, the revised snake routing was nearly correct, only a few minor changes required;

img_1430.jpg

Here's a close-up of the rudder servo mount arrangements:

img_1432.jpg

...and another view showing how it's retained:

img_1435.jpg

We had a small ceremony on Saturday to mark The Laying Of The First Plank:

img_1441.jpg

As you can see, no expense was spared on the accompanying refreshments - genuine Sainsburys organic ginger nuts, those are...

This is a few more planks added (on Sunday, I think); I'm using tapered planks because the nose former tapers quite a lot, parallel planks are less work to prepare and would have to be narrower (maybe 4.5-5 mm as opposed to about 7.5 mm) because of the small nose former diameter, so there would have to be more of them, and some would have to be angled at each end. I don't know which approach is quicker.

img_1442.jpg

I'm using thin/medium cyano to glue the planks to the formers and Deluxe Materials Aliphatic Resin to glue the planks together; I did try Titebond as it's supposed to be the best, but for me it's too thick, difficult to apply and gets everywhere...

This was the state of play at about 6 pm today (Tuesday):

img_1443.jpg

I've just lifted it from the jig and as hoped, it's very rigid with just over half the fuselage still to be planked.

As you may have noticed, I've had a change of plan at the tail end - rather than continuing the planks to the last former, I think it's going to be much easier to in-fill the last fuselage bay with thick (and light) sheet and then plane/sand everything to shape afterwards.

Steve McLaren14/06/2016 21:23:59
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202 forum posts
202 photos

Oooh, Up-town Top Planking! Wasn't that a song?

Dave Hopkin14/06/2016 21:33:03
3626 forum posts
291 photos

FOUR ginger nuts for the first plank laying ceremony............. that's nothing short of outrageous!!!

should only be 3......

Nice looking Canberra though

Scott Edwards 215/06/2016 11:27:08
150 forum posts
90 photos
Damned good planking there. Are they all 1/8 thick ? For the solid infill tail end, at a guess you can hack out some of the inside to save weight ?

Any comedy quips from the Mrs about building a balsa cucumber ?

Edited By Scott Edwards 2 on 15/06/2016 11:27:54

Steve Houghton 115/06/2016 11:58:21
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1850 forum posts
136 photos

Watching with great interest thumbs up

Andy Blackburn15/06/2016 15:44:13
385 forum posts
469 photos
1 articles

> Up-town Top Planking! Wasn't that a song?

I really couldn't say, of course, as it was before my time...

> FOUR ginger nuts for the first plank laying ceremony............. that's nothing short of outrageous!!!

Well, obviously a spare is required in case one drops into the tea during the ceremonial dunking process...

> Damned good planking there. Are they all 1/8 thick ? For the solid infill tail end, at a guess you can hack out some of the inside to save weight ?

All 1/8" apart from the small bits on the centreline - bits of soft 3/16" squashed into place. I'm hoping that the tail end will be light enough that hacking bits out won't be worth the bother.

> Any comedy quips from the Mrs about building a balsa cucumber ?

She hasn't seen it yet. I think she saves her best Nora Batty voice for my efforts

Edited By Andy Blackburn on 15/06/2016 15:46:12

Mike Chantler19/06/2016 22:58:32
10 forum posts

following smile d

Andy Blackburn26/06/2016 16:25:07
385 forum posts
469 photos
1 articles

 

Been a busy couple of weeks so maybe not quite as much progress as hoped, but I've been fettling the tail surfaces and other bits. The stabiliser alignment was pretty close (stab tip to bench measurement 118 mm one side and 119 mm the other) so only required a little bit of adjustment, the fin and rudder wasn't quite vertical so that'll need looking at later:

img_1447.jpg

I think that doesn't look too bad. Shall we have a little treat and put it all together? ...

img_1455.jpg

Not bad.

This is the tail bay in-fill being pinned carefully before glueing:

img_1457.jpg

And this is it with the stabiliser off. This just requires a bit of care with a coarse permagrit block and several try/fit attempts to get the right shape, it takes a while but I can't really see an easier way of doing it.

img_1462.jpg

The tail end is now also relatively stiff, so it's probably back to planking tomorrow.

Edited By Andy Blackburn on 26/06/2016 16:26:18

Phil Cooke26/06/2016 17:54:10
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1897 forum posts
1268 photos

very accurate planking Andy - I bet you were dying to give all those little proud edges a gentle rub down! That will look like a work of art once its all rounded and smoothed down!

Scott Edwards 230/06/2016 10:27:21
150 forum posts
90 photos
Having been lucky enough to see this in the flesh, the quality and accuracy of Andy's work shows me how much I still have to learn. This is going to be a beautiful Madel, and a classic PSS design.
Andy Blackburn04/07/2016 15:56:42
385 forum posts
469 photos
1 articles

I assure you that Mr Edwards is exaggerating - there are loads of bits where it's not quite right, possibly because the planks might be a little too narrow (7.5 mm) and are difficult to get to run straight against existing planks without persuasion from masking tape.

And yes, I have already trimmed off some of the proud edges - just couldn't stop myself...

Should have an update later on today.

Andy Blackburn04/07/2016 22:02:41
385 forum posts
469 photos
1 articles

This was a few days ago - planking coming along well, but slowing down because each of planks is getting to be a different (and odd) shape:

img_1469.jpg

I always seem to need masking tape to persuade adjacent planks to fit together properly, otherwise they're so flexible that they tend to fit where they're pinned but not in-between the formers; that probably means that the grade of wood I'm using (very light, about 6 lb/cu ft) is probably a bit too light, and/or the planks are a bit narrow - the 1/8" sheet was initially stripped to 7.5 mm, 8.5 mm would have been fine, possibly even 9mm.

This is the jig that I used to taper the planks the right amount for most of them:

img_1467.jpg

...and this is the tail area that's been in-filled with 3/8" sheet; you can just about see the different pieces of wood that were used to do the job. Also, the balsa cement has blushed which means that I'm doing the right thing in building when the weather is unsuitable for flying

img_1471.jpg

A quick look inside the tail area where it's filled-in with 3/8" sheet - you could perhaps dremel some of that out, but it might not be worth the bother.

img_1472.jpg

- and (drum roll, please) the last plank has been laid:

img_1475.jpg

..just to show it's not smoke and mirrors, here's the top:

img_1474.jpg

Next job is to add soft block around the fin & stabiliser, tail cone and sand to shape - after leaving it for at least a couple of days to make sure the aliphatic resin has hardened properly. Then I suppose I'd better have a go at the formers for the canopy and bomb-aimer's transparency.

Edited By Andy Blackburn on 04/07/2016 22:05:54

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