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Author Topic: Alternative to sliced ribs  (Read 1464 times)
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OZPAF
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« on: December 19, 2011, 04:37:55 AM »

Faced with producing multitudes of sliced ribs for the Hanger Rat kits I was producing for our Air League group's entry in the recent Hanger Rat postal organised by Marcelo, I remembered John Barker's(Hepcat) approach with his Penny Plane build
http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=872.0
As, I was contemplating producing a min of 15 kits with 10 ribs per wing, I decided to mould the required balsa laminations over a hot wired foam core.
This was not difficult due to the relatively short section of foam required (about 10’ or 250mm). I used a fairly dense white Styrofoam around 2lbs/ft3 and blue foam would have been fine.
I wet the laminations and prebent them on the foam, holding them down tight with cotton strips(from old bed sheeting) and leaving to dry overnight. Using the foam offcut or jacket as a clamping medium will not work as it doesn’t allow the moisture in the balsa to evaporate in a reasonable time(guess how I know).
After drying the laminations overnight – the next problem was what glue to use for the laminations. If I was to use the cotton strips to pull the balsa into shape ( an old idea that works really well) then I had to be careful that the glue didn’t stick to the cotton strips.
I ended up using another old method based on PVA. I wet out the contact sides of the balsa laminations with undiluted PVA (Aliphatic would no doubt work – Titebond etc) and then let them thoroughly dry.
The laminations were then reattached to the foam core, bound on again with the cotton strips, using around 3 layers and pulling down tightly, paying particular attention to the LE.
I then ironed over the cotton strips using a fair bit of heat (almost on a linen setting), stopping when the cotton became too hot to handle.
The glued moulded balsa sheet was then sliced using a similar simple slicer to that shown by John.
Thanks to John’s initial idea I was then able to slice of the ribs (0.8mm wide) very quickly after a little practice.
John
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Alternative to sliced ribs
Alternative to sliced ribs
Alternative to sliced ribs
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Warhawk
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« Reply #1 on: December 21, 2011, 05:04:34 PM »

John,

Any difficulty in maintaining rib width slicing through the glue?  I've tried this before, and had problems getting a smooth cut.  I had a number of "bumps", which of course messed with the next rib stripped.  I worked through it by sanding the edge smooth after each cut, but it ended up being a lot more work.  I was working with a laminate using thinned Titebond, too.

Justin
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Bruce McCrory
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« Reply #2 on: December 21, 2011, 06:51:58 PM »

Why are you laminating a second sheet to the bent rib sheet(s)?

I don't know the thicknesses you are working with here. I have soft (light) 1/16th (1.75mm) A-grain easily mold to some awesome looking chord depths.

I slice nearly all my indoor ribs. Besides the glue interference, it is easy to gain a rocking motion in the formed stock while cutting.

hbm
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OZPAF
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« Reply #3 on: December 22, 2011, 09:39:41 AM »

John,

Any difficulty in maintaining rib width slicing through the glue?  I've tried this before, and had problems getting a smooth cut.  I had a number of "bumps", which of course messed with the next rib stripped.  I worked through it by sanding the edge smooth after each cut, but it ended up being a lot more work.  I was working with a laminate using thinned Titebond, too.

Justin

Actually No I didn't Justin. The glue line formed under pressure and heat is very thin and is barely noticeable when looking at the finished rib. For this application I would fell that the PVA(Aliphatic - Titebond etc) would provbably work hust as well


Why are you laminating a second sheet to the bent rib sheet(s)?

I don't know the thicknesses you are working with here. I have soft (light) 1/16th (1.75mm) A-grain easily mold to some awesome looking chord depths.

I slice nearly all my indoor ribs. Besides the glue interference, it is easy to gain a rocking motion in the formed stock while cutting.

hbm
The ribs are formed from 2 layers of 0.8mm(1/32 sheet) with the grain running in the chord wise direction, and sliced to 0.8mm wide. I prebent the sheets after soaking in warm water and on drying and removing from the form – there was significant spring back from the shape required By laminating the rib – the shape seems far more stable, especially as the balsa is being bent along the grain.
The main reason for making the ribs this way was the large number of consistent ribs required. I sliced off around 150 in around 1 hr. There was a little spring back from the shape at the LE, so I wouldn’t feel that they are more accurate than well sliced ribs.
They are probably heavier at the same size as well but for many small models such as Peanuts and larger they may be a good way to go. The weigh increase maybe marginal in practice as all the glue solvent has evaporated and as they are stronger than sliced ribs – they could actually be made thinner.
The slicer was very easy to use with normal care required, and the glue had no affect on the cutting that I could determine. After a little practice - it was very quick and consistent.
John
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Bruce McCrory
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« Reply #4 on: December 22, 2011, 11:06:35 AM »

John,
It sounds like you are using fairly dense wood. Also, if it worked, that's what counts.

In my own experience:
1. The mold is a harder balsa, or even hardwood.
2. Glued laminations of stock are messy; but engineering prospects open.
3. My favorite method is to slice a bazzilion strips to dimension, wet them, and stack-lock them to the mold. Water acts like a magnet, so the fettucini-like sticks align, cuddle, and drape over the mold fairly easily. Then wrap and dry. Ribs thicker than 1.75mm can be molded in this fashion.
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OZPAF
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« Reply #5 on: December 22, 2011, 05:32:54 PM »

Bruce that sounds like an interesting apprach as well Bruce . Its obviously a lot easier to bend the individual strips along the grain.The hotwired foam would work just as well doing it that way.How well do they retain their shape after drying?
The main reason for using heat to activate the PVA was to eliminate a mess and wet glue that could cause it to stick to the cotton binding. It works well.
This form of moulding may have many applications as well.
John
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