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Mar 21, 2019, 01:27 AM
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It's looking great RobtP.

I usually put a small doubler/gusset inside the fuselage where the dowel and the tube enter, just scraps of hard 1.5mm. However the local stresses on the fuselage are quite low and I have never had failures - and my landings can be a severe test for any u/c.
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Mar 21, 2019, 03:58 PM
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Thanks Footrrot. I'll do the same.
Regards
Bob
Mar 24, 2019, 01:38 AM
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Looking back through my posts on the u/c I notice that you asked a really good question that I didn't answer very well. The highest stress most likely occurs not at the ends of the tube but in the middle; a hard landing will try to bend the tube down in the middle.

I should have mentioned that I usually epoxy the tube to the bottom sheeting and/or to a 1.5 x 5mm cross-piece set upright. In the 30 years or more that I've used this u/c no part of it has ever broken nor has it caused damage to a model. On light models I've even used paper tubes when I couldn't get the right size in aluminium braced as above of course.

I think you'll find it tough.
Mar 24, 2019, 05:31 AM
'Douglas' to his friends.
OK, OK, you've made your point. You can lift your foot off, now.
Mar 24, 2019, 07:03 AM
I like real wooden aeroplanes!
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I've had the odd model in the past which has made me feel like doing that!!
Mar 24, 2019, 03:56 PM
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Actually, I'm rather fond of the model; I built it nearly 30 years ago and it flies as well as ever.

The point I'm making (I'm sure you got it but it's important) is that this u/c will take such deflection with no damage to itself or the model and recover completely - over and over again if you wish to be brutal.

Another point I'm reminded of - the model in the picture shows no signs of fuel soakage. Most u/c s that have wires passing through the fuselage develop tiny cracks at this point which with every deflection of the u/c serve to pump fuel into the structure. At least partly for this reason old "f f sport" models like the Tomboy (massive bulkhead with u/c lashed to it - crazy) always become fuel soaked even when the engine bay has been slathered with epoxy.
Mar 24, 2019, 04:02 PM
..down is easy...
I am so stealing that method!!!!
Mar 24, 2019, 09:22 PM
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Footrrot - that's impressive! Many thanks for the additional info and pic. I see now how, on landing, the wire would tend to rotate under the peg and try to force the end in the tube downwards (took me awhile to get that ). I was thinking of laying a piece of fibreglass across the tube to fix it securely to the bottom sheeting, which hopefully will do the trick. Also I'm giving myself a couple of options for the tube - have ordered both aluminium and carbon fibre tube (latter square section but round inner). Have never used carbon fibre before but assume it would be stronger/stiffer than aluminium and maybe less likely to bend down in the middle. BTW I'm using 2mm wire.

Oops just realised I might have ordered the aluminium tube a size too small
Mar 25, 2019, 02:15 AM
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Your idea of securing the tube to the bottom sheeting with fiberglass is excellent and should provide more than enough strength - I've never had the tube bend anyway.

The wire size on the red model in the last photo is 1.8mm supporting about 20oz. I'm not sure what weight you are expecting but the same wire size that you normally use will be OK. The length of the legs makes some difference - longer legs need to be thicker. Don't be too fussy - there's a lot of leeway - if you go with your gut feeling it will be fine..
Mar 25, 2019, 09:07 PM
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Target weight is 16 oz as suggested by BMatthews but I suspect will be a little heavier as I seem to have difficulty building lightly so could end up nearer 20 oz. However I am trying The next size down wire I have is .062", about 1.6mm which may be a bit light but will make a final decision when the tubing arrives.
May 20, 2019, 10:37 PM
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Thread OP

Progress report


Back on the case, after stalling when none of the tubing I ordered fitted the u/c wire I am using! One of the pitfalls of ordering by mail order. However picked up everything I need in a well stocked model shop when visiting relatives in the South Island last month, so full steam ahead.

The u/c is now fitted using Footrrot's method and am very pleased with the result. Not difficult to do, apart from thinking carefully about the angle of the main leg before doing the 3D bend away from the fuse side, but managed to get it close each time with only minor tweaking required. I like the nice springy action when pushed down, although haven't been game to give it the boot test! Thanks again to Footrrot for posting this method.

Tail fin has been lightened by cutting out the existing sheet and fitting ribs. Total saving on the tail assembly 5.2g.

A start has been made on the wings. Fuse just needs pushrods fitted and finishing around the nose. Still hoping to make it by June
May 20, 2019, 11:39 PM
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Glad the u/c turned out OK. You need quite a good eye to get those bends right. Well done.


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