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Author Topic: Building sheet balsa wings  (Read 673 times)
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marcin_pl
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« on: September 26, 2019, 06:06:08 AM »

How do you build sheet balsa wings with ribs? Do you glue ribs to a bottom of the sheet just in hand or on flat surface? Maybe you use some kind of a jig? I'm afraid of gluing just "in the air" because my wing may get warped.
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« Reply #1 on: September 26, 2019, 02:00:55 PM »

Marcin,

Fully sheeted wings were popular for F1A, B, and C in the 1960s through the 1980s. With the advent of carbon fiber D-box construction, and more recently full carbon covered foam wings the balsa sheeted wings are les often used.

But sheeted balsa wings do offer a way to build a wing with an accurate airfoil (no tissue sags between ribs!). Construction is quick and easy (no covering!).

Yes, you do need some sort of jig to build the wing. It the airfoil has a flat lower side, than a smooth, flat board will work. For a wing with an under cambered airfoil, then you will need to make a building board that matches the wing under camber. A simple way is the use the hot wire technique to cut a block of foam. (Check with a Radio Control glider flier for help.) Another technique is to cut a number of balsa "ribs" that match the underside of the airfoil and are straight on the bottom. Glue these to a flat board. Then attach a piece of 2 to 3mm plywood on top. Use a slow epoxy glue and rubber bands to hold the plywood in place until glue sets.

Next the lower sheet is positioned on the form, leading edge and spar added, then the top sheeting is added. Again, a slow epoxy is necessary to give sufficient open time to coat the top of all the ribs, spar, etc. before the glue drys. Again rubber bands are used to hold the top sheet tight agains the ribs.

What size model are you building? That will determine the thickness of the sheeting used. Typical thickness for the F1B wings I built was 1/32 inch (0.80mm) balsa.

At the leading edge you can either shape the leading edge and extend the top sheet over top of the leading edge or you can add a strip of balsa against the rear of the leading edge and glue the front of the top sheet to the top of this strip so the top sheeting backs up to the leading edge.

The finished wing can be covered with tissue, light fiber glass & resin, or simply painted with a non-shrink lacquer.

Louis
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strat-o
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« Reply #2 on: September 26, 2019, 03:07:30 PM »

Marcin, I haven't done what you are trying to do, but if you are building something smaller than an FAI competition model, I bet you could solve it by pinning the balsa wing down upside down on a nice flat building board, along the line of max camber.  Use a lot of pins and put some of them angled to the left and right.  Then carefully wedge spar stock under the balsa wing, front and rear.  Double check that none of the pins have pulled out.  Adjust the camber by moving the spar stock as needed.  Use the ribs as a guide to ensure you are achieving the correct camber shape.  Once it looks true bond the ribs to the wing. 

Also check out the Jedelsky wing as an alternative.  This wing has a two-part flat camber that looks like a very flat "V" shape and can be built without bending.

Marlin
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ZK-AUD
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« Reply #3 on: September 26, 2019, 06:12:55 PM »

You may also find that if you build your jig on a narrow board the same size as the wing that masking tape is a great option for holding down the upper surface while it is drying - being wider it won't crease the wood
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« Reply #4 on: September 26, 2019, 07:54:33 PM »

Marcin,

I should have mentioned that wash out in the wing tips can be accomplished by skewing the tip panel on the jig. For wash out the tip can be moved forward on the jig. You can also achieve wash-out in tapered tips by having most of the taper in the trailing edge.

Louis
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lincoln
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« Reply #5 on: September 27, 2019, 02:21:18 AM »

If I was doing a sheeted wing, I'd take the sheeting that was going to be used on a significantly curved surface and wet it. Then dry while attached to an appropriately curved surface. An Ace bandage might be good for this.  You can get at least a little sagging between ribs after some time if you just use flat balsa.
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marcin_pl
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« Reply #6 on: September 27, 2019, 03:59:09 AM »

Thaks for all answers. I think I was not precise in my question. I didn't ask about fully sheeted wings with normal airfoil (with sheeting on upper and lower surface), but about a simple curved plate airfoil with ribs on lower surface.
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billdennis747
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« Reply #7 on: September 27, 2019, 04:15:56 AM »

simple curved plate airfoil with ribs on lower surface.
Unless it i small, it will lack torsional rigidity
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« Reply #8 on: September 27, 2019, 08:32:58 AM »

Marcin

The Jedelsky wing was typically built with a thicker piece of balsa for the front 30% and a thinner, curved piece for the rear portion. For small towline gliders that I built back in the late 1950s and early 1960s the front portion was 3/16 or 1/4 inch balsa (roughly 4 or 6mm) and the tear portion was 1/16 inch (1.5mm).  The upper surface of the thicker front part of the wing was sanded to an airfoil shape; the rear portion of the wing was simply curved dry and glued to ribs. The ribs were exposed.

The ribs were slightly longer than the wing cord. This allowed them to be pined down to a flat surface with the pins at each end of the rib. Then the front part of the wing was glued down, next the rear piece was glued in place, butting against the back of the front piece.

The purpose of the thicker front portion of the wing was to provide extra bending strength and to fill in the front portion of the curved plate to reduce drag. Eric Jedelsky developed a series of airfoils using this construction technique. These were used on both A-1 & A-2 towline gliders of the period (now known as F1H and F1A gliders) as well as small gass models.

Louis
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lincoln
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« Reply #9 on: September 27, 2019, 07:46:38 PM »

I'm assuming this is for a small model. 

I seem to recall doing this freehand without too much trouble. If using solvent based glue, you could always dissolve the glue and do it over.

Another idea, which I haven't tried, might be to glue the ribs onto a board with rubber cement and then glue the sheeting to the ribs with something stronger. The rubber cement might be weak enough to break away without damaging anything else. I'm guessing that working some dental floss under the wing would help to remove it. It would probably be a good idea to use some kind of tape,to mask the board in between the ribs so the wing doesn't get glued down permanently.

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marcin_pl
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« Reply #10 on: September 30, 2019, 05:41:44 AM »

I plan to use it in small scale model as a "scientific" test. This airfoil (bent plate) seems to be more efficient than typical airfoils for low Re numbers (small models).
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Tapio Linkosalo
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« Reply #11 on: September 30, 2019, 07:14:39 AM »

For most accurate results, (hot-wire) cut a piece of foam concave to the shape of the airfoil top. Then wet the balsa and pin down to the foam to dry. Once dried, you can glue the ribs in place assuming that you put the ribs to the bottom side). I used to build Jedelsky wings (Swedish "Balsar" F1H) in similar manner, the jig enabled to make an accurate seam between the leading edge part and the trailing edge sheet (taping the top side together, opening the seam for adding glue from beneath, then pinning the parts to the jig to push them tightly...)
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Larry R.
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« Reply #12 on: October 01, 2019, 11:01:51 PM »

I have had success with bent flat plate sheet wings on the order of 18" span.  Take a look at the Flying Funtique, designed by master modeler Bill Hannan (http://www.airplanesandrockets.com/airplanes/flying-funtique-article-plans-apr-1969-aam.htm).  Two or three sheets of 1/16" balsa is more than enough to build this model.

You might also take a look at John Blankenship's peanut scale Bristol Scout, an all sheet model with curved airfoil wings.  This model can be found on Outerzone.  I'm currently building this model, making slow progress as time permits.
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