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Bending wood for a DLG wing (Read 2555 times)
kevin m
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Bending wood for a DLG wing
Jun 9th, 2006 at 2:58am
 
Whilst thinking more about the Dlg wing that I intend to start (its been a busy week in the office and I have no energy still), I was looking at the weak points / areas that need glass cloth or carbon to strengthen the joints.  This seems to me that its always the dihedral breaks.
Thought - build a wing with no dihedral breaks - ie all in one sheet with the dihedral to each tip from the centre with no joins at all.  Assume youknow what I mean, the wing tapering all the way out.
Question: 
- How?  Could the wood be soaked and set in a jig for as long as it takes?
- Can you bend with the grain?
- How would the wing stay shaped once released from the mould? How would I eliminate the potential to stop the wing reverting back to a flat sheet?
- Once made and assume you can get to the above stage, cover it with what to ensure strength and shape?  Glass cloth with epoxy?
- If cant be done from balsa, then anyone tried a foam core with balsa sheet over the top?

It would eliminate the potential for breakages round the joins - no?  Give a stronger wing? 

Over to you.

Kev
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Pijuvwy
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Re: Bending wood for a DLG wing
Reply #1 - Jun 9th, 2006 at 6:34am
 
It's funny you should be thinking along these lines:  So was I.

But my conclusion was that we could try a wing with no dihedral breaks AND no balsa.

I mean, a curved CF skeleton covered with film.

But perhaps, if total rididity was not achieved in the skeleton, then a shell of some kind may be needed.  Perhaps balsa, or perhaps CF gauze plus dope.

Kevin, Geo's method of ironing the balsa sheet would probably be suitable for the large wing you have in mind, and I think when the curved wing is doped, it should prevent any tendency to straighten.
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Re: Bending wood for a DLG wing
Reply #2 - Jun 9th, 2006 at 6:44am
 
Kev

Soaking balsa in an ammonia/water mixture softens the lignum(?) which effectively binds the wood fibres together. I suspect that 1/4" would need some hours immersed in the solution - probably overnight is best. Then bind it to your eliptical dihedral form and leave it to dry. I'm told that when dry it takes on a permanent set.

I've got an article on this in an old model mag which I'll dig out & see if there is anything else to add.

Regards

PeterT
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tom johnson
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Re: Bending wood for a DLG wing
Reply #3 - Jun 9th, 2006 at 7:48am
 
Jeff Raskin had a method of bending Balsa with the grain using a clothes Steam Iron.

http://www.smarttoolsinc.com/Model_Kits/TigerFly/TigerFlyConstruction.html

It might work for all balsa wings.
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kevin m
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Re: Bending wood for a DLG wing
Reply #4 - Jun 9th, 2006 at 9:10am
 
Ok Peter - so how do we decide on how to make a jig with the smoothness that is required on the underside.  Furthermore, now thinking about the airfoil shape, Wonder how we decide the airfoil shape?  Should it be changed as every airfoil we use is based on a wing with dih breaks.  This one I propose to not have any dihedral breaks at all, should this have any affect.  It would keep the airflow smoother from the centre to the tips, therefore, should be change anything?
Will the airplane fly better, could we introduce undercamber and give a better glide?
All questions to get us all thinking.
Lets hear from you all and that includes anyone new here.
Kev
sat working in his office VERY very hard - NOT!
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iflyhlg
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Re: Bending wood for a DLG wing
Reply #5 - Jun 9th, 2006 at 10:42am
 
Hi Kevin, this is Bruce Kimball. I have built curved wings before by laminating thin sheets of balsa. The bottom two layers were 1/32 sheet, then multiple lams of 1/16 till I was up to the thickness I needed. The upper layers do not need to be full chord since they will be carved away. I laminated them up on a curved form using a vaccum bag since it was the easiest way to apply pressure. It worked but was a lot of effort.
  I have done curved wings out of foam and glass and they are easier to do than a regular wing since I don't have to cut and sand dihedral breaks. I just cut out the foam flat using airfoil templates and a hot wire and when I vaccum bag the glass on I just bag it on a curved form.
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JoshuaF
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Re: Bending wood for a DLG wing
Reply #6 - Jun 9th, 2006 at 10:56am
 
Kev,

Stephen Poterala built a pair of elliptical wing models...the CLG flew like gangbusters, but warps plagued the larger one and I don't think he ever has got it trimmed out.
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« Last Edit: Jun 9th, 2006 at 10:57am by JoshuaF »  


Sand it down to nothing, then cut it in half
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Re: Bending wood for a DLG wing
Reply #7 - Jun 9th, 2006 at 10:59am
 
The larger wing. Steve cut out the blank and water formed it, jigging until dry, then sanded it to shape. It personally think all the shaping could be done first. A layer of glass cloth or silkspan would be more than enough to hold the shape.
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Sand it down to nothing, then cut it in half
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applehoney
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Re: Bending wood for a DLG wing
Reply #8 - Jun 9th, 2006 at 6:25pm
 
>Soaking balsa in an ammonia/water mixture softens the lignum(?) which effectively binds the wood fibres together.

That's the theory .. but some say adding ammonia does little more than using plain water, as balsa has very little lignin in its cells.

Does it? I dunno, just what I read here and there ...
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Re: Bending wood for a DLG wing
Reply #9 - Jun 10th, 2006 at 4:40am
 
Hi Bruce (Iflyhlg)

Welcome to SFA, although I guess that the welcome should come from Kev as maestro moderator!. It was good to see you at the Nats again, pity about the weather! What a difference a couple of weeks makes; here in England it is now in the high 70s with just enough breeze to make things interesting. We have a local combined HLG/CLG comp here this evening and tomorrow, and I'm hoping that Mark & Mick will fly as in previous years.

Thanks for the info on laminated eliptical dihedral wings. Vacuum bagging is a bridge too far for me at the moment, but the process sounds interesting. More importantly, how did the finished wings compare with the traditional "hard" dihedral break type? Did you notice any improvement, or was there little difference for more effort? I have to say that I've never had a wing break at a dihedral break, but that might just be me not trying hard enough.
Regards

PeterT
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« Last Edit: Jun 10th, 2006 at 4:41am by PeeTee »  
 
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Re: Bending wood for a DLG wing
Reply #10 - Jun 10th, 2006 at 10:54pm
 
Don't know if bending wood is such a good idea...after time it will want to relax to whatever twist/bend it wants to & will do this in changes in humidity & temp. As far as  building a wood wing with strong poly breaks why think of the wing as one sheet? Wakefield flyers from the 60's & 70' would sometimes build solid wings but in separate longitudinal strips glued together. As they could size the strip as close to the airfoil shape there was less to carve off & heavy wood could be used in the TE portion & light wood in the LE. We have the advantage nowadays in that we could use lighter wood & put carbon strip inbetween some lengths. Carbon makes great poly break braces. I have just finished a 24" Butterfly wing ritzed out with carbon strip put INSIDE the ritzed out wing before ribs are glued in & braces at all poly joints doubled from carbon...Light wing that could almost be discus launched! In the end I would make a wing out of strips (probably 4-5) with carbon inbetween 2-3-4 with braces in those areas. Such a wing will not bust on a launch & you could probably lift a car with it!
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Re: Bending wood for a DLG wing
Reply #11 - Jun 12th, 2006 at 12:37am
 
I soaked my Quail Egg (2in. wingspan) in a 20/80 bleach/water bath for 24 hrs. It came out white as a ghost, but more importantly, was very pliable. I put the wingtips equally on two small wooden blocks covered with duct tape. I kept adding little bits of water to a shot glass placed on top of a small block in the centre of the wing until I got the bend I wanted. I left the wing for another 12 hrs., until it dried.
The bend stayed in and the glider flies wonderfully. Pad the wing with a piece of cloth so as not mark the balsa, which tends to be very soft initially. I've done similar procedures with larger wings, sometimes bending just wingtips, sometimes the entire wing. I also use a good steam iron to steam out warps, as well as bend wingtips. The key here, though is to dope or coat the wing immediately upon drying, or the bend will eventually begin to straighten out. As far as bending a dlg wing, I think it would be a very tricky procedure, without a precise form as a guide.
Almost forgot to mention that I use ziploc bags of varying sizes filled with water, placed at strategic points to bend entire wings.
Cheers
George
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« Last Edit: Jun 12th, 2006 at 1:12pm by geo »  

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Re: Bending wood for a DLG wing
Reply #12 - Jun 12th, 2006 at 1:27pm
 
I know that the weather got nice just when I was leaving. On the balsa laminated wing I just covered it with tissue and dope. On my composite wings, they have a mirror smooth finish which I feel ( my opinion only) that they are too smooth for the size of gliders we are flying. As far as the dihedral goes, on the composite wings it is much easier to bag in the smooth curve than to cut them apart and rejoin them. The reason is that with the foam and glass wings the skin is only .002 inch thick over the foam core and trying to line up the skins after sanding the dihedral brake is a bit tricky so I just do the simple thing by applying a thin strip of glass around the joint to tie the skins together. As far as flying I haven't seen a big difference between the curved wings and the poly wings since most of us seem to be going to the six panel wing.
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Re: Bending wood for a DLG wing
Reply #13 - Jun 13th, 2006 at 8:25am
 
What a fascinating thread!  Lots of great items.

2 kinds of laminating mentioned: 

Two or three horizontal layers glued together - sounds like a good way to keep that curve in there.

5 or more vertical slabs - I'd like to try something along those lines.  Perhaps some slabs need not be complete - could leave gaps for lightness, without too much weakening, especially if carbon is also used.

Re: Lignin.  I also believe there is little of it in balsa.  When you are buying balsa, and you hold it up to the light to reject sheets with too much "dark stuff", you are rejecting lignin.  Just my opinion.

This is interesting:  http://en.wikipedia.org/wiki/Lignin

Although balsa is strictly a "hardwood" http://en.wikipedia.org/wiki/Flowering_plant
the timber seems to have more in common with the "softwoods" http://en.wikipedia.org/wiki/Pinophyta

All trees must have lignified cell walls, in the vascular tissues at least, or water could not be transported to tree-height.  But do the lighter-coloured timbers have far less lignin in the non-vascular tissues?

The wiki reveals that as well as its properties relating to waterproofing the vascular tissues, lignin is STRONG and HEAVY.

So, if the "softwoods" and balsa both have less lignin, then why is pine heavier than balsa?  I'd say it's the RESINS in pine that make it heavier, but not as heavy as lignified timber.

What does that leave us with?  I think it means that balsa is essentially a cellulose-only timber.

We make our balsa into a resin-and-cellulose timber when we apply dope - but only in the outer "shell" layer.  Very similar to a foam core with resin (eg epoxy) coating, except that our cellulose core has directional strength.

My opinion, that is.
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« Last Edit: Jun 13th, 2006 at 8:27am by Pijuvwy »  

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Eliptical Dihedral
Reply #14 - Jun 17th, 2006 at 7:15pm
 
The Ritz Wing is a classic for eliptical dihedral for other FF models, but if you were to build one, then fill it in with soft balsa, it MIGHT work.

Good luck.

http://www.smallflyingarts.com/Archives/Feature_Articles/Evolution/evolution.htm

http://www.antiquemodeler.org/adl/adl-gas/Invader.jpg

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