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1/4 scale T-21 CW

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chris williams
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Re: 1/4 scale T-21 CW

Post by chris williams »

Uncovered balsa-sheeted wings are very susceptible to the damp and best kept dry...! Not sure why you're sheeting the wing in strips of balsa sheet rather that the widest possible sheet available (4" usually)?

Also, if you turn the plan over, you should be able to faintly see the lines on the other side. You don't need to re-draw the entire thing, just draw over the salient points: TE, spar and rib positions etc.

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SedB
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Re: 1/4 scale T-21 CW

Post by SedB »

Thanks, I will flip the plan over! Indeed this is not the easiest way! :oops:
Believe it or not, the sheeting is out of one piece. With the end of Graupner, here it was also the end of good quality balsa. The piece you see was quit flexible in the middle.. I am ordering balsa from TopmodelCZ now, I hope they run a better quality!

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Peter Balcombe
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Re: 1/4 scale T-21 CW

Post by Peter Balcombe »

chris williams wrote:
17 Jan 2021, 19:44
Uncovered balsa-sheeted wings are very susceptible to the damp and best kept dry...! Not sure why you're sheeting the wing in strips of balsa sheet rather that the widest possible sheet available (4" usually)?
Vincent,
As Chris mentions, I find it best to glue whole 4” sheets together to make a balsa sheet as wide as the wing, then splice lengths together to make a sheet the size of the complete panel & glue on as a single piece.
It’s best to use Aliphatic glue to join sheets as this dries hard & allows you to easily sand back the joints and give a nice smooth sheet to fit. Cyano will be too hard to sand easily & PVA glue leaves a rubbery surface which is hard to sand cleanly.
A good way of joining sheets is to butt joint the long edges (after cleaning up if necessary by us8ng a long straight edge). Tape the sheets together using cheap masking tape, then turn over & use the tape as a hinge to open the joint 180 degrees & apply a smear of glue along the length. Fold back & place flat with tape on underside so that excess glue comes out on top side. Wipe off excess glue & apply a bit more masking tape & weights to hold sheets flat whilst glue dries fully.
Make up sheets to required width & then splice the resultant wide sheets end to end to get the required length by overlapping the sheets by say 50mm. With the overlapped sheets taped together to prevent movement, cut a zig-zag line right through both sheets together. Once you remove the various triangular pieces, you will find that the two zig-zag ends should mate perfectly. Glue this joint to fix the sheets together, again using tape to temporarily hold the sheets together whilst the glue dries.
Sand the joints & sheets on a flat surface to give a good surface all over - it is much better to do this before fitting the sheet to the wing as the sheet can be fully supported whilst sanding.

If you trim the resultant large sheet roughly to the wing planform (allowing for the extra width due wing curvature), you can then fit as a single piece, pinning or taping the LE & TE & using lots of weights (or piles of old magazines) to hold the sheet in contact with the ribs.

Chris has shown this sheet joining technique in build threads long ago.
I use the above technique on all my balsa sheeted wings.

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SedB
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Re: 1/4 scale T-21 CW

Post by SedB »

Thanks a lot Peter for the clear explanation! It didn't come to my mind to sheet the whole length of the wing at once. I thought doing it in two obvious parts would be the (easiest) way to go. But indeed, doing the whole at once must be much, much stronger.

Till now I used medium ca to join sheets of balsa, and only zig'd or zag'd once, so just made one 45dec cut at the end of the sheet. The technique you mention is new to me and I for sure will give it a try!! Don't be mad if it doesn't reflect in the pics straight away; the build is a bit further ahead as show here. I appreciate all the help I'm getting here a lot, so many thanks!

Dan

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Peter Balcombe
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Re: 1/4 scale T-21 CW

Post by Peter Balcombe »

A pleasure Dan (sorry I got the wrong name in my post!)

If you do fit the sheeting in multiple pieces span-wise, then don’t forget to join at a rib and also add additional rib surface thickness doublers to increase support & glueing area at the sheet join.
If the joint is unsupported then it will crack & break through at some point when handled or due to flying loads.
Peter

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chris williams
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Re: 1/4 scale T-21 CW

Post by chris williams »

I drew this plan back in the last century, before I had access to digital photography. However, I do have some build pics if the T46 wing from 20 years ago, if they might be helpful...?
Attachments
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SedB
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Re: 1/4 scale T-21 CW

Post by SedB »

Perfect! Thanks a lot!
I should be fine Chris, you already send me the whole folder on the T-21 it self which had been really helpful.

Dan

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chris williams
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Re: 1/4 scale T-21 CW

Post by chris williams »

Whilst trawling through memory lane I remembered that at a later stage I modified the T21's struts, using 10mm Ali tube as a core with the ends flattened and drilled out. This became my go-to method for all the later strutted models..
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SedB
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Re: 1/4 scale T-21 CW

Post by SedB »

Man, why do I do things so complicated :oops: I just finished these, wondering about the best approach..
First I bend some test strips:

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Which later turned out to these:

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I did reenforce both with a carbon strip, but now I'm considering to make new ones as yes, they look stronger made out of one piece! :D
Thanks a lot!!

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SedB
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Re: 1/4 scale T-21 CW

Post by SedB »

So as suggested I flipped the plan and got on with it ;) ;

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I have to cheat every ones in a while with filler;

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It almost starts to look like a wing :D :

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