Saab J-21 Build (RCME plan)

[Vince Consentino]

I am scratch building from the RCME plan and have a couple of questions. What is the best method to cut small parts; bulkheads and small curved items, from 1/8” balsa Really tough to cut with razor knife or exacto. Maybe a coping saw? Also the plan has some parts I cannot determine where they go This includes W15 and W16. Also F4T, F3T and there are two F3s. Rest of the cutting is going well. Will post some pictures once I start assembling. This is my first model. Thanks

[Colin Leighfield]

Good one Vince, I’m sure this will be a success and the J21 is an interesting plane, piston engine or jet. I remember this free plan, good choice. Sorry I don’t have it in front of me and can’t comment on where parts fit, but when you stare at things for long enough it generally sinks in eventually!

Using a really sharp knife with 1/8” balsa and patience usually works, sometimes you can cut oversize and get to shape by sanding. Fretsaws are good but a you need a fine blade. A jewellers saw has been recommended by fellow forumites, I bought one recently and found it to be excellent for fine work. It’s a bit like a small fretsaw with very fine but sharp blade.

[Cliff Bastow]

Hi Vince,

I built one of these from the plan and it was fairly straightforwards as I remember,

I will try and dig out my old plan and see if i can identify the bits you are struggling with.

It was a good flier until I crashed it on launch one day!

All the best with the build.

Cliff.

Edited By Cliff Bastow on 19/01/2019 08:14:23

[Piers Bowlan]

Hi Vince, Excellent choice of scale model from RCM&E Oct 2012. An unusual scale model, with a few building challenges.

The second page of the J21 plan showing the wing has some details of the Corby Starlet on the lower half. You can safely ignore W15, W16 and F4T and F3T as they are parts for an entirely different model!

For thicker balsa and ply parts a coping saw works well, particularly if you buy a fine blade for it. Just cut round the outside of the marked line and sand carefully to shape with a permagrit block or similar. That is what I would do at any rate. I usually photocopy the small parts like bulkheads and ribs and you can temporally fix them to the wood with 3M Promount or Fast Tak spray adhesive. A glue stick would work too.

If you have some spare cash you could invest in a Scroll saw like one of these but a coping saw will work just as well.

Good luck.

 

Edited By Piers Bowlan on 19/01/2019 11:07:13

Edited By Piers Bowlan on 19/01/2019 11:16:01

[Bob Cotsford]

For curved cuts in balsa your X-Acto/scalpel/Sawn-Morton knife should cope quite easily if you make the cut with several light strokes. Razor saws come into their own for straight cuts in thick sheet and thinner ply. Scroll saws and coping saws should only really be needed for curved cuts in thicker (6mm and up) balsa and plywood.

Of course if your 1/8" balsa is really hard stock it might be easier with a coping or fret saw with a fine tooth blade but you won't often get to use balsa wood that hard unless it's strip wood!

[Vince Consentino]

Thanks for all the input. I found the perfect exacto handle and blade combination for those tight curves. Pictures coming soon once assembly commences.

[Nigel R]

I just cut oversize and sand back with a sanding block.

If its a circular part I'd probably do a few straight cuts, like an octagonal shape or similar, to cut down on the sanding a bit.

[Vince Consentino]

So as I plan the next step after assembly I am looking for suggestions regarding servos and wiring. I have a Futaba 72.62 controller I inherited from my dad but not sure if it is current with latest technology. Any input on hardware or suggested resources is greatly appreciated.

[kc]

Coping saws usually have blades with 14 or 15 teeth per inch which produces rather ragged cuts in balsa. Coping saws are often used with teeth pointing the same way as tenon saws etc, however they cut much more accurately if used with the teeth pointing to the handle ( i.e as a pull saw or japanese saw ) due to the blade being in tension on the cutting stroke and therefore not buckling too much. The same way as fretsaws or jewellers saws (piercing saws) which have even finer blades and need to be used as pull saws. Finer toothed fretsaw blades are much better for balsa and the smaller jewellers saw frame is easier to use.

Fretsaws, jewellers saws and coping saws used as pull saws are all best used with a traditional Vee cutout. My photo shows a Vee cutout of 9mm ply mounted on a block of 4 by 2 timber which raises it to a handy height when fitted in a Workmate or carpenters vice. Slightly less height is better if used in an engineers vice. The saw shown is a jewellers saw.

[kc]

Vince are you in Britain? The Futaba 72 62 would appear to be a 72 mhz transmitter and that's not a legal frequency for aeromodelling in Britain. But used elsewhere in the past and possibly still OK there.

Nowadays almost everyone uses 2.4 Ghz because it doesn't need the same frequency control ( 'Peg' system etc) However in Britain we could still use 35mhz or even 27 mhz if we wanted, but the gear will be past it's prime now and not really worth buying new nicads for a set that has been unused for years. Nicads need to be in top condition for model aircraft and old nicads probably won't - not worth risking. So your Tx and Rx may not be up to it now but 2.4 ghz sets are cheaper than ever and have lots more handy features than old sets. So that's the preferred way to go I suggest. Old servos might be OK though if fully tested first. Again servos are cheaper than ever and much lighter weight too in many cases.

[Vince Consentino]

So I am planking wing of my j-21 and I am curious about mounting and then accessing the servos in the wing. Do I plank the wing and then cut out and access panel? Or do I need to install the servo first? Any insight is appreciated.

[kc]

It would be usual to make all the provisions for the servo mounting and servo leads before sheeting wing, then remove servo but leave extension wire before sheeting right over. Then cut aperture for servo. Of course you need to make provision for the servo extension leads at a very early stage in wing construction - usually when cutting the ribs to shape a large hole for paper tubes is cut into some ribs.