West Wings Skylark build thread

[angry_muppet]

So, the latest update...

I manufactured and fitted the nice additional bits as suggested by Little Keith...



It all looks nice and clean and well repaired.  Lets see how it all fits together...

The answer?  It doesn't... 



Ain't that just sweet!

A 1.5mm ply brace that snaps when you try and fit the elastics to the wing. 

My question is: "what now?"

I have enough ply to remake one brace, but this is likely to happen again.  I have 2 carbon rods from a previously crashed plane which could be used...  My issue will be getting more 1.5mm balsa/ply to resurface the wing again.    If anyone is placing an order or can direct to anywhere in the SE that will have a little in stock, let me know.

Positives?  It's all learning for No 2... 

[johnfireball]

Hi,
    That looks like lightply which is not suitable for the wing joiner. Get proper hard plywood and make a new one.
John.

[Happy Days]

I'm wondering why the joiner broke just by putting the bands on the wings?

David, when the wing halves are joined together the underside of the them should look like a rather flat "V". The very bottom (or point ) of that "V" should protrude into the cavity of the fuse and the underside of the wings should rest on the top of the fuse..

I'm thinking that the only way for the bands to have snapped the joiner is if the bottom of the "V" was resting on something causing the wing sit too high and 'rock' from side to side. The bands would then have been trying to pull the dihedral of the wing  'flat' and so snapped the wing joiner

Keith

[Spanner]

Hi,
I would suggest that you use some good quality 3mm hard ply / Birch ply and make the joiner full depth or perhaps 1mm less. To install simply cut slot out of both top and bottom skins and using saw remove just enough of the ribs to allow it slot in. Glue to both top and bottom spars and leave a slight gap at the top and bottom which can then be capped with balsa and sanded back. Also ensure that the grain is running along the brace which is normally the direction that provides most strength, from your photo it looks like the grain was vertical in the joiner that you made. To spread the load at the joint between the two wing halves onto the skins you could also use some 50mm wide white bandage and lay across the joint between the two wing halves it can be glued down using white glue rubbed in. This would reinforce the but joint at the wing root.

Hope this helps

Joe

[Spanner]

Hi,
Just one more observation, are you actually gluing the two wing halves together as the brace is just that a brace to reinforce the joint. The joint along the two root ribs looks very cleanly broken if it was glued.

Joe

[angry_muppet]

The answer is, no I hadn't glued the brace.  I wanted to check the fit and alignment before I would do that.

There isn't anything that is protruding that is causing the wings to rock.  Everything seems to be A-OK.

A full depth brace sounds like a reasonable idea; I have 1.5mm ply blank which I could use.  Obviously 3mm is stronger...  And heavier...  The question is does the build warrant a 3mm brace when the ribs are all 1.5mm?

[Spanner]

Hi,
Its the spar that takes the wing load not the ribs they are there to give it shape and support leading and trailing edge. You could use 1.5mm good quality ply as that is what is proposed on the plan. However if you are unsure of the quality of the ply use 3mm hard ply not light ply. The root ribs should also be glued together as the dihedral brace is intended to transfer the wing loads along the spar, it is not intended to take torsional loads or prevent the wing from twisting, this may be the issue you were having while braking the braces that you have already installed. The use of bandage that I suggested would reinforce the but joint at the root ribs without going overboard on weight or strength.

Joe

[Happy Days]

If the wing joiner had broken due to a heavy landing or such like I could understand it breaking, but this model has never flown.

Before modifying the plane, my desire would be to find what is causing suffieient stress to snap the joiner while the model is still on the bench.

K.

[angry_muppet]

One question, which may be irrelevant...

Does it matter how the retaining elastics are fitted?  I had them perpendicular to the span.  Would it make a difference if they were fitted diagonally? As in, forming an X across the centre of the wing?

[Happy Days]

You'd normally use 4 bands David. One, (on each wing half,) would be parallel to the fuse, and the remaining two would form a cross. The amount of "stretch" of each band is normally somewhere in the region of 20 - 25%. So if the root chord of the wing is 10 inches (distance from the front to the back of the wing ) use a band that was 8 inches long when 'relaxed'. There is no hard or fast rule on this but for a non aerobatic model like yours all the bands are really doing is holding the weight of the fuse when the model is in flight.

One of the benefits of banded wings is that on a heavy landing the bands snap rather than the airframe breaking.

K.

[angry_muppet]

Forgive me for using metric measurements...

The wing chord is 158mm
The distance between centers of the wing mounting dowels is approx 147mm
The relaxed length of the supplied band is 105mm...

As the band needs to "wrap around" the wing to the dowel, we're talking a stretched length of 170mm... That's triple the figure suggested by Little Keith

Is it possible the bands are creating too much force on the wing?

[Happy Days]

I suspect that it is the bands that are supplying the force that broke the first joiner, but I'm having trouble understanding how. The tension that the bands apply when simply holding the wing to the fuse shouldn't stress the wing joiner,........ provided both wing halves are level. Which you've already said they are.

David, when the two wing halves are joined together on a flat surface (kitchen table I believe in your case ) with the wing joiner in place..................is the top of the L.E and top of the T.E of each wing half level. Or does the front (or rear) of one wing half stand up proud above the other wing half?
I'm thinking that if one wing half is twisted or warped, or the joiner isn't sitting flush against the side of the spar, then when you fit the bands, they might be applying  torsional stress (or twist) to the joiner which will cause it to snap!

K.

[billscottni]

Slightly surprised that the distance between the dowels is less than the wing chord? It would usually be the same or very slightly more. With it being less you're effectively "pinching" the wing with the bands, as they have to go back under the LE or TE to get to the dowel. Having said that I suspect it's mainly the band tension and the use of Lite ply that are the issue. If the band is going from 105mm to over 158mm thats over 50% stretch and I'm surprised you're not damaging things just getting them on! Couple that with a lite ply brace and somethings bound to give!

[angry_muppet]

Kudos to Rogallo who kindly provided me with a few off-cuts of birch ply to reconstruct the brace! 

A pair of braces have now been manufactured to fill the full depth of the wing; I can now report back on the state of affairs...


There is a small amount of difference in the underside of the wing (about 2 - 3mm).  This appears to be due to a misgluing of the TE on the port wing with a slight upward inflection.    (No...  I didn't glue the LE in upside down...)  There doesn't appear to be any difference in the majority of the underside; the LE and balsa sheeting are flush.  I would doubt that such a minor deflection wouldn't cause excessive torsion or create an issue with control?  It would (theoretically) slightly affect the attitude of the aircraft by altering the wing's angle of attack... 


Any ideas before I glue the lot together?

[rogallo]

Loads of epoxy and cover it up. It will be fine, ever see any of my models!
Twill be grande