1/3 Scale T21 by Jilles Smits

[Peter Balcombe]

Just started the test build of this large woodie from Jilles Smits.
As for the Bocian built last year, the laser cut short kit has been provided by Cliff Evans, with many ply parts which appear to slot together very nicely.
As workbench space is currently a bit limited, owing to a few jobs still to finish on my current Olympia (VV401) build, I am starting with the tail feathers. Having said that, the tailplane spans 1.2m!!

First shot shows the tailplane and both elevator spar assemblies which comprise ply parts either side of 4mm balsa. The next shot shows the first elevator assembly once the ribs & T/E have been added, with the 2nd elevator partially assembled.

[Jilles]

Those laminated spars from balsa with plywood liners are a weight saving effort.
I learnt this form my early Goevier model built from somebody else's drawings where even the fuse formers are made like that.
It take a bit more building time but is worth the effort on a short nosed design like the T21
every extra gram at the tail is 6 gram more lead in the nose.

[spike spencer]

Nice one, Peter. That is going to be an enormous thermal-sniffer - but, if its performance is Scale, don't be tempted to get too far downwind !!

Ply/Balsa/Ply sandwich is a great material. I have used it several times on models of all sizes. The joy is that you can make any thickness you want and it responds well to all cutting methods (especially the laser). 0.4mm/3mm/0.4mm for small electric models to 1.5mm/6mm/1.5mm or even larger. The easiest way I have found to produce it has been to make up sheets of 6" wide balsa by the usual tape and glue method, then after that glue has cured and the joint sanded to clear any nibs, spread glue over the whole area and pop the lot into a vacuum bag for a couple of hours. This ensures intimate contact across the whole sheet while curing.

I have used both aliphatic and PU glues for these wide sheets, both easy to spread and will cure quite quickly. Neither type has yet failed on me. I keep finding and using old offcuts from my scrap box.

[Jilles]

Your are lucky to get 6" wide balsa sheets these days here in Australia
I was informed by my supplier that because balsa is now all plantation grown they do not wait until the trees are big enough to cut 6" wide sheets.
However with the ply/balsa/ply construction it does not matter if you have a (glued) butt joined in the balsa

[Peter Balcombe]

Rudder framework assembled (3 diagonal ribs still to be fitted).

[Peter Balcombe]

Tailplane centre section assembly built and servos fitted.
Photo shows underside of the assembly with the 2 Turnigy 225MG elevator servos used in place of Jilles' suggested A55H types.
The large circular hole is where the fuselage mating connector goes.
This assembly will now be glued onto the front of the previously built spar assembly prior to adding ribs & false L/E pieces.

[Peter Balcombe]

Tailplane structure now completed.
The parts are going together really quickly.

[Jason Griffiths]

Looking very nice Peter, looks like you are doing a excellent job as always

[Peter Balcombe]

Have started build up the first main spar assembly during a brief lull in other build programmes.
The main spar is built as substantial box spar assembly with continuous ply webbing front and back from root to tip.
The spars themselves are 1/2"x1/4" spruce, doubled from root to mid section where the strut attaches.
The wing/strut attachment fittings are specified as chrome steel which bolt to the wing main & rear spar assemblies.
The lower wing spar face has a noticeable taper change after the strut attachment on the outboard section.
All of the main ribs have a rectangular hole cut in them for the main & rear spar assemblies, allowing each rib to be slid along the completed spars. The void between the outer edge of the spars and the wing surface is filled with balsa, sanded to the surface profile.

Instead of splicing up many 36" lengths of spruce to make up the spar strips, I have used 2.4m lengths of 8mm x 4mm pine strip available from B&Q, bonding these in pairs to make up each (slightly over thickness) spar strips, albeit with a 300mm extension spliced onto the tip end to give the overall spar length. However, a bonus is that the lower spar taper change can be more easily accommodated, so a preformed lower spar strip was glued up.
The downside is that the spar strip is thicker (8mm), so I need to reduce the root end spar doubler to a single 4mm strip to achieve the same overall spar thickness, thus deviating slightly from the plan. However, I should still have at least the same minimum spar thicknesses everywhere.

I have now glued the outer spar strips to a pre-assembled ply rear webbing strip.
The front webbing strip is shown on the photos below alongside the build. This webbing has slots cut in it to locate the rear of each D box sub-rib.
Also shown below are the root and strut attachment point blocks, constructed from strips of 1/2" x 1/4" spruce strip. These will be profiled to the plan shaped before being securely epoxied in place as they transfer wing stresses to the wing attachment fittings.

[Peter Balcombe]

1st spar now fitted with root end doublers and carbon strip outboard of doubler end.
The 2 hardwood blocks have also now been shaped and epoxied into position.
The front webbing assembly will be added tomorrow to complete the box, which just leaves the fuselage & strut attachment bolt holes to be drilled through using the laser cut webbing holes as guides.