Flights of Fancy

[ElyDave]

yes, the dihedral brace is balsa.

Do you really think that a single sheet of ply will be strong enough against the twisting/fore and aft forces?  There is nothing else in that gap.

[TheLurker]

The gap troubles me.  Have a thought,  but need to find a copy of the plan to see if feasible. 

In nutshell.  Glue root ribs together, losing gap altogether.  A "standard" construction.  Would still use a ply brace.  Need to see plan to confirm wing still fits on fuselage.  Scribble more after work.

ETA.
OK, should be working, but this is an emergency.
The plan* shows dihedral setting on the centre section root ribs.  This makes me think they /should/ be glued together and that the centre of the wing should sit just under the top line of the wing bay on the fuselage.


*If it's the plan pcolbeck linked a few pages back.

[JonBuoy]

Looking at the plan that Pat linked to the root ribs should indeed be glued together.  The spar then gets a chance to work through the centre section.

Try measuring the span - if it is greater than 36" that is a pretty good indicator 

[ElyDave]

Thanks guys, that gap bothered me from the start. My thinking was exactly the same as yours, but then if the wing root ribs were gluesd together, there would be no need for that brace.

I'll post a couple of shots of the box pics and instructions to show the ambiguity here. The wing root rib has a slot for the brace, which is cut to go to the next rib and butt up against it, leaving that gap.  The gap also ends up leaving a bit of slightly saggy tissue in the middle as you cover it.

I guess there is always a bit of adaptation with these things

[JonBuoy]

I can't see the amount of dihedral specified on the plan in which case the brace isn't a brace it is a jig.

[ElyDave]

Instructions here, possibly ambiguous.  No slot in R2, only R1, also used for another jig between the inner and outer wing sections.



Box-front image here, not really clear enough to see ribs joined or not



My build, two inner ribs seen between the elastic bands.  The wings sit on top of two bits of 3/32" square glued to the top of the fuselage longerons after covering. That seems to accomodate the centre of the gull-wing

[TheLurker]

I can't see the amount of dihedral specified on the plan in which case the brace isn't a brace it is a jig.

Yeah, I know I should be working. 

It is (sort of) shown, just in a horribly non-obvious way.  It looks like you're supposed to align the tops of the root ribs with the dotted lines shown on the plan.  If so that's ugly.

I think the phrase, "...join two wing halves at centre with full dihedral gusset J." is what we're looking for.

Could you post a photo of the wing mounting area of the fuselage - sans wing?  For this style of thing I'd *normally* expect to see a shallow V cradle or something that looks like if you squint.

Cheers,
Lurk.

[ElyDave]

Yeah, me too, but this is far more interesting.

[TheLurker]

Yeah, that looks suitably cradle like.  For this style of mounting the fuselage bay is usually left uncovered so that the wing root can settle into the bay, but if the dihedral angle is quite shallow it won't be an issue.   I'll post a photo of one of mine this evening to show you what I mean.

[Steph]

https://twitter.com/MWStory/status/1300788254418010121

[TheLurker]

Coo.  That's impressive. Annnd after the Lord Mayor's Show ...

Dave, this is the sort of thing I would normally expect to see.  The V shaped former doesn't have to support the wing, it just has to provide clearance for it.

[ElyDave]

OK, so mine took the opposite approach and built up rather than taking away to achieve the same effect for the clearance.  I'm OK with that, but is there a consensus view on the wing brace/jig/gap fiasco?

[TheLurker]

is there a consensus view on the wing brace/jig/gap...

Without the wing in front of me, and please accept my apologies in advance for anything that is blindingly obvious, my suggestion is...

0.  Measure the current tip dihedral.

1.  Cut away the tissue on the underside of the root panels on each wing.  Leave a flap of about 1/2", errr 13mm, on the tissue that's left in place so that you can glue the cut edge down and get enough purchase on it to keep it taut.

2.  Cut the brace at the mid point to separate the wings. 

3.  Carefully remove the bits of brace from each half. 

If you haven't got a v. fine scalpel saw. Sand down the brace where it lies along the spar until all gone. Cut it as flush to the outside of the rib as possible, then san flush.  Use a fine drill bit in a pin chuck, or your fingers, to drill out most of the brace where it goes through the rib then using a cut down emery board sand out the slot.  If the root ribs suffer a bit splint them on the inside with some scrap sheet.  Won't make a significant difference to the AUW.

4.  Cut a new brace.  I would use 1/16" ply but it's not mandatory. 
If you are using balsa use the hardest you have to hand.  Nick some scrap from the Bird Dog kit if needs be.

5.  Re-assemble wing with root ribs glued together making sure you get the tip dihedral close to what it was.  Don't worry if it is not exactly the same.  As long as it's symmetrical and not wildly out it'll be OK. 

Clamping the root ribs together while the glue sets might be a bit tricky with the top root bay panels still covered.  You could prop the wing up on blocks to give clearance underneath for clothes pegs/clamps or use fine (silk or dress-makers) pins through the top covering to hold them together.  Do a dry run first to see what works best.  If push comes to shove cut the tissue away from the topside root panels. 

6.  Re-cover the underside panels. 

7. Sit back and enjoy the glow of a successful repair and the re-issue of its CoA.

Andy or JonBuoy might have better thoughts.
Cheers,
Lurk

[ElyDave]

Thanks, that's pretty much what I thought if the consensus was to stick the wings together at the root.

Based on experience with these wings I think it will be much easier to uncover top and bttom back to R2 (minus about 1/2") to allow access and then recover that central section in one go.  I've already removed the very centre section to assess the damage so some tissue repair is required to the top as it is, and I think I'll be neater working between ribs.

[andytheflyer]

I haven't seen the plan, but generically, I'd strip the covering off the panels R1 to R2, glue the root ribs together making sure that the dihedral is correct, and add a 1/16 ply brace to the spar and the LE and TE before re-covering.  Maybe only 1/32nd ply for the TE and LE as your wing loadings will be very low. You could glue the braces to one wing first once you are happy with the fit, and then glue the other wing to the root and braces. That removes a bit of opportunity for things to move while the glue is setting.  Since you need to re-slot the root ribs to accommodate the braces, maybe do one brace at a time?

If the dihedral is a bit out, then I'd add some softish sheet to one or both of the root ribs and re-profile to get the correct dihedral, before then joining the wings.

I say 'generically' because that's the usual way of doing it - unless this plan's different.

You won't need the 1/4 ply here. Trust me, I've done a few of these.   Even my big models never had braces more than 1/8 ply.

[ElyDave]

So, having stripped back the covering to R2, removed the existing brace/jig and set up a couple of props for the inner wing dihedral, R1 does not go vertical. The two vertical ends of the centre wing spar are however a good reference

What I did find however is that the bottom of a camembert box is ply of exactly the right thickness to make a brace.  Tracing around the existing brace gave me a pattern to work from that I was able to trim at the outer edges to fit widthways.  I've glued it to one wing spar, once that's dry I'll do the other and also get some strip inbetween the ribs at the right point to keep the gap as well. 

I'm quite enjoying this avante-garde approach to the ambiguity of the instructions

And here it is, with a few fillets along the line to both stabilise and keep the gap consistent at the base, need to trim them to the line of the ribs once dry.  It looks like I've managed to keep the dihedral nicely.

[TheLurker]

...the bottom of a camembert box is ply of exactly the right thickness to make a brace.

Consider that idea well and truly nicked.

I'm quite enjoying this avante-garde approach to the ambiguity of the instructions...

Aero-modelling as improv. jazz.  Discuss. 

That fix looks very neat.  Were it me I would pack that gap with some medium block shaped to fit, or perhaps run some sheet infill into the top of the gap, to be sure that stress is more evenly distributed across the joint.  Mnd you I'm a bit of a worry-guts.

[ElyDave]

I did think of a piece of strip down the centre of the formed by the ribs but thought that would be a stress concentrator. Now that I think its secure I'll add a few more wedges evenly in the gap, just to spread the load evenly as you say

[andytheflyer]

Aero-modelling as improv. jazz.  Discuss. 

That fix looks very neat.  Were it me I would pack that gap with some medium block shaped to fit, or perhaps run some sheet infill into the top of the gap, to be sure that stress is more evenly distributed across the joint.  Mnd you I'm a bit of a worry-guts.

Ahem.  That was my point about adding balsa to the root ribs and re-profiling.  The wing joint needs to be strong so 100% glue contact is handy.  That is (well, was) my usual technique in dihedral wing joining.

[ElyDave]

Not wishing to pooh-pooh the experience, but there is such a thing as over-engineering - ask IKB.  I've added another three or four wedges and I can't detect any play right now.

Come the first "positive landing" I stand ready to be told "I told you so"

[andytheflyer]

Not wishing to pooh-pooh the experience, but there is such a thing as over-engineering - ask IKB.  I've added another three or four wedges and I can't detect any play right now.

Come the first "positive landing" I stand ready to be told "I told you so"

[TheLurker]

This might not look like a success, but it is.  Why?  Let me count the ways...

0)  The damage was due to a truly atrocious launch by a mate. He's forgiven. He's a trimming guru for rubbber models and I'm quite definitely not.

1)  It did fly.  Max was only 8 seconds because we were sorting out trim while stepping up winds on the motor (11", 3/16" x 2 strands, 3.75g) and only at 250/300 or so (1/4 - 1/3) max turn count.

2) It did not require any additional ballast, no tail weight or nose weight.  Haven't checked my notes yet, but IIRC that puts it comfortably sub 40g which is pretty good for a model of this type.

3) It sat "right" in the air which means that the design is right (wing & stab incidences, stab proportions etc) which is why it's a success as it's my first ever build from 3 view.

What happens now?

0) Disassembly of both wings roots.  Could be a bit tricky, but has to be tried.
1) Improvement in wing root anchoring.  As currently drawn it's good enough for Keil Kraft grass, but not real world (footy pitch) grass.
2) Knock off or "sprung" UC legs.  The fixed design puts additional stress on the wing when "landing" on real world grass and a CFIT with a fully wound motor... well you can see the result above. 
3) Modify the plan to reflect the changes.

[rafletcher]

Today I was talking to a colleague during an office move, and he showed me a 35 year old 0.75cc diesel aero engine, With adjustable compression, which he ran on ether and castor oil mix.

[TheLurker]

...showed me a 35 year old 0.75cc diesel aero engine...

Oh.  Oily jobs.  *Sniff* 

There are certain, sadly deluded, people hereabouts who like them and there's a whole sub-culture in aero-modelling given over to the care and nurturing of old (as in 50, 60 years and even older) IC model engines.  They even get to write about it in Aero Modeller!  Utterly baffling to all us right thinking (and you know we are) silent flight types. 

*puts spoon down to sit back to wait for Andy's expostulations and defence of noisy, smelly, recalcitrant and _expensive_ diesel engines*   

[andytheflyer]

...showed me a 35 year old 0.75cc diesel aero engine...

Oh.  Oily jobs.  *Sniff* 

There are certain, sadly deluded, people hereabouts who like them and there's a whole sub-culture in aero-modelling given over to the care and nurturing of old (as in 50, 60 years and even older) IC model engines.  They even get to write about it in Aero Modeller!  Utterly baffling to all us right thinking (and you know we are) silent flight types. 

*puts spoon down to sit back to wait for Andy's expostulations and defence of noisy, smelly, recalcitrant and _expensive_ diesel engines*   

Nah.  Not going to bite.   

I will, however, have to get a cloth to wipe the cornflakes and coffee off my computer screen.............