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May 02, 2021, 02:25 AM
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gliderguide's Avatar
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Build Log

RESpite. The BRF project...


Moved this as I’d put it in wrong place.

So, this is probably fundamentally a bad idea. A 4.2m electric RES poly built up wing....

Panel sizes
Centre panel 1300 mm. Twin spoilers. 16mm carbon main spar, 7mm sub spar
Mid panels 950 mm. 14mm carbon spar. 6mm sub spar
Tip panels 500mm 9mm carbon spar, 5mm sub spar.
3mm carbon LE.
Balsa sheet TE with lamination of carbon tow sandwiched between upper n lower surface. Washout incorporated into mid panel. Carbon joiners in tube between panels. Chord around 275mm at centre. Centre bolt beam 4 laminations aircraft ply, 2 balsa. Will wrap w Kevlar after sanding.

Fuse either CLM Pro carbon set 136 with 1150mm boom, or a spare E Maxa fuse n tail I have. Aiming for a flying weight between 1400-1600g. Bought the carbon. Had the ribs cut. Now it’s play time. Big thanks to Greg Potter at Glidergear for assistance with the drawing, and yet another stunning set of ribs cut.

One off build for my own use, but sharing the journey, however good or bad it turns out to be
Last edited by gliderguide; May 03, 2021 at 09:25 AM.
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May 02, 2021, 02:26 AM
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Carbon tube where I am is only available in 1m lengths. Not ideal for a 1300mm panel. I decided to cut 2 x 1m down to about 650 each. It’s 1mm wall thickness. I then used a 500mm length of 14mm carbon tube centred inside the join. This was followed by a 300mm length of 12mm tube.

I have some 10mm solid. Is it worth adding a 150mm bit inside that for an electric launch floater? I will use an offcut of prob 14mm through the centre out a couple of bays to assist with load sharing too. The skins in the centre will be 1mm aircraft ply, with lightening slots. I’m trying to keep weight low without compromising strength.
Last edited by gliderguide; May 02, 2021 at 07:07 AM.
May 02, 2021, 03:42 AM
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I think this is a good idea. Should result in a nice, easy flying model.
I do have some reservations about torsional rigidity on a wing that big. I would be tempted to have a second tube spar in the Dbox, at least across the centre panel. But I tend to over design/build.

I will be interested in the final result that's for sure.
May 02, 2021, 04:07 AM
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There’s a second spar at about 2/3 chord. And a doubler inside the main spar for almost half the length, and a third layer for 1/4. Hopefully that helps taper the strength and avoid and major stress riser. I will do some strength stress testing on the main spar before I add ribs.

The spars are not parallel. The rear one follows the line of the TE toward the tip. Hopefully this triangulation will help reduce any flutter. Maybe.

I could probably add say a 10mm spar forward but at 1m, it wouldn’t reach the ply ribs in the last two bays before the mid panel join. It would mean I’d need to add more ply ribs but logically it would be better to carry that spar to the join.
Last edited by gliderguide; May 02, 2021 at 08:35 AM.
May 02, 2021, 04:28 AM
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I suspect you are fine on span wise strength, even with the joint in the middle of the outer tube. The torsion is where my concerns would be. Build and see is probably the answer. You can always add another tube in the front later (not simple but doable).
May 02, 2021, 05:06 AM
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May 02, 2021, 05:45 AM
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I agree on the torsion. Flutter would be bad!! The spar is about the biggest diameter I can comfortably fit with enough meat left on the ribs. The big diameter should give a fair amount of gluing area, coupled with the 2/3 sub spar, and reinforced TE. I did look at the CLM pro carbon D box on their website but itís neither cheap nor really big enough. My skills arenít sufficient to produce my own carbon d box, nor could I justify the expense. A Super Ava Pro 4m is roughly this with a D Box and probably $1500 au for an airframe. Tempting. But Iíve been there and done that. Was great. However this is just an experiment for my own curiosity. Itís been in my head for ages. Iím not technically smart with design work, however Iím taking my ideas from a broad range of planes Iíve built, flown, bought etc in 40 years in the hobby. Proportions and foils predominately from the Bubble Dancer and BD derivatives. With some mods to make construction easier and to suit the non winch launches. Might work. Might not. Iím ok either way so long as if it doesnít work, I can understand why it didnít then either try again, or just do something different.
May 02, 2021, 08:31 AM
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Centre spar done. All tube 1mm wall
2 x 16 mm tube cut at 660 to allow some trimming.
Sleeved with 14 mm tube cut at 500 mm.
The 14 mm was reinforced in centre 300 mm with 12mm tube. All sorted into one half, the other bit of 16 mm is ready to go on tomorrow. I had to slide ribs on to check it out. Not spaced, or glued, or the doublers done, or the TE spar, or even the TE!! But you get the idea. Good sized spoilers hopefully. Roughly 45 cm long and 5 cm deep. Yes. The desk at work is too small for the centre panel but will be good enough once one side is glued, it can hang in the air a bit.
Last edited by gliderguide; May 02, 2021 at 08:39 AM.
May 02, 2021, 08:40 AM
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mhodgson's Avatar
What rib thickness are you using?
Rib spacing?
May 02, 2021, 08:59 AM
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Ribs are 1/8”. Spacing on the full ribs is roughly 65 mm with a half rib in between. Yes I mix up my dimensions. I was taught to build by an old English art teacher in school so plans and wood where always imperial. Ribs are not correctly spaced or glued in that pic. I still need to laminate the ply rib doublers to the two outboard balsa ribs..
May 02, 2021, 10:32 AM
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Metric inch by inch. I grew up using metric until I began model model building when I used imperial. Now I mix and match, which ever is easier.

Now I would have gone for 1/16 riblets and put two in each bay.
May 02, 2021, 02:34 PM
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Using very lite balsa, the 1/8 seems fine. Having just watched a mate built a Purito, which does exactly that, yeah, may well have been better. This was cut 12 months ago and has been waiting on the shelf to get to the top of the pile. The BD uses thicker ribs and Drela rationalises it nicely for strength. Like I said, this is V1. I’m doing this build log to document this journey, but also to plan for the next one. Double riblets noted. May end up with some lite 1/16 sheet over LE to back of riblet for torsion anyway. In V2.
Last edited by gliderguide; May 03, 2021 at 09:27 AM.
May 02, 2021, 04:55 PM
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Decent sized spoilers by the looks of it, and if she flies it's going to need them. It's a really fun project by the looks of it. 4.2m is going to be massive on that desk.
May 02, 2021, 06:20 PM
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Itís a big IF Mike. The centre spar feels very strong though. Still undecided on the clm pro fuse (really sexy!) or the older e Maxa Iíve got. That has tails, servos and motor mount plus a few repairs. Might actually be a smarter option bc if I break it, itís no great loss. Already has a few dodgy bits. Iíd just need to sort the wing interface as the Maxa runs 4 bolts from underneath, up into nuts in the wing. The BRF is designed to bolt down into the fuse with two M4 bolts. Nothing some creative dodgy followed by bog n paint canít fix....

Maybe a couple of dowels and rubber bands? Proper old skool kool!!
May 02, 2021, 06:41 PM
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Would be a fist full of rubberbands for a 4.2.
You got to go the old fuse you have for the prototype i reckon.


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