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How to Make Lattice Wing Skins for Flying Model Aircraft

May 02, 2015



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Airfield Models (http://www.airfieldmodels.com/)How to Make Lattice Wing Skins for Flying Model Aircraft

At the time of this writing I have built lattice skins for only one model, Thwing!  I do not consider myself to be an expert at this.  I am very pleased with results, however, so I plan to use lattice on more models in the future.  If you have done this type of work before please feel free to contribute your tips.

I learned enough from this experience to have a good idea how to use the same idea with other types of wings.

A lattice skin can be used as a replacement for any wing that has a skin.  For example, it can replace a full-sheet wing skin and it can also replace a D-Tube type skin (leading and trailing edge sheeting with cap strips).

I have some ideas about using the skin in those applications but I don't have photos to illustrate because I haven't actually done it.  I will discuss those ideas later in this article.  First I will cover what I actually did.

 
 

Getting Started

Thwing!'s wing having a lattice skin from balsa wood.Before I made the lattice I contemplated a variety of ways to accomplish the task.  I could have used my table saw to cut grooves across the sheet.  That would have required very accurate measuring and cutting because multiple passes would be necessary to cut the grooves to the correct width.

If you have a real table saw with a dado blade then you could set up a runner as I did on my router table.  I expect the table saw will cut more cleanly than my Dremel did.

Sketch the lattice on the wing plan view.  The more closely it is spaced, the more wood that will be needed.  It will also be stronger and heavier.

The lattice I made is 3/32" thick x 3/16" wide.  The grooves are cut 3/64" deep (half-lap joints) and 3/16" wide.  I spaced the lattice 1-3/4" between centers.  The wood is medium density balsa.

I suggest you prepare more lattice than you'll need.  Wasting $5.00 worth of wood is cheap insurance against having to set up all the tools again later while trying to get the settings identical.  It's also not a bad idea to have some on hand for repairs.

Before you start

  • True one long edge of each board.  This edge will be the side strips are cut from after they are grooved.

  • Make one or both ends exactly square to the trued edge.  This is very important because the first cut  will be the template for all others.

  • Test your setup using a board the same material cut into smaller pieces.  Make a few test runs, strip the wood and assemble it.  Make adjustments to the tools as necessary.  Do not start on the boards you will use until you get the whole thing working properly.

 
 

Making the Lattice

Attach a guide to the router table and use a sanding block to push the board through the cutter. Cut a guide from hardwood, thin aluminum or hard plastic.  It should be the exact width of the grooves, no thicker than the depth of the cut and extend before and after the cutter at least the width of the board.  In this case I used a piece of 3/16" x 1/32" x 9" basswood.

Be sure the edges are smooth.  I rounded the feed side slightly to allow the board to slide on without catching.  I glued the guide to my router table using wood glue.  Lightly sand along the edges to remove excess glue.

The wood can start to cock even with a good fitting guide.  It is a good idea to draw some lines on the table that are perpendicular to the cut.  Use these lines to help keep the board aligned as it is fed through the cutter.

I used a coarse sanding block to hold even pressure on the board as it was fed through the cutter.

The guide ensures that the grooves are spaced consistently. The first cut is made by pushing the edge of the board against the guide while feeding the board through the cutter.

This is why it's very important that the end of the board is exactly square to the long edge.  I used a machinist's square to true the end.

It is very easy to have a mis-cut here.  Trim off the end and try again if that happens.  Work slowly and deliberately.

After the first cut is made, place it on the guide and continue with the second cut.  Repeat until the entire board is grooved.

It is best to sand the board to remove as much fuzz as possible before cutting the strips.

Cut the board into strips and block sand them to uniform width. Strip the board into sticks using whatever method you like.  I used a fine blade in my table saw, but a hobby knife and straight edge will work too.

I cut the strips just slightly too wide so I could stack them on their sides and block sand them to the correct width.

Assemble the lattice oversize and trim to fit. Important!  Cut all the longest pieces first.  Do not cut for one panel at a time.

For example, if you are skinning the left and right wing panels separately, then there are four skins (top left, top right, bottom left, bottom right).

Identify the longest piece and cut 4 of them.  Place them in three stacks and put the fourth piece over the drawing so you know which pieces have been cut.

Continue with the next longest stick and so on.  If you cut one panel at a time you may end up with enough wood that could have made all the skins, but no pieces are long enough.

Gluing the Lattice

Because the wing curves along its chord but is straight from root to tip, I glued the lattice so that the bottom pieces ran spanwise and the top pieces ran chord wise.  The reason being that the curve will open or close the joints depending on which side is up.  Putting the chord wise pieces on top closes the joints.

Keep in mind that you will need to make two lefts and two rights (in this example) they are mirror images of each other.

Make the skin over-size and then trim to fit.  I suggest you pick one corner, such as the corner at the root and trailing edge to use as a base point.  Cut all other edges to fit the panel.  This will ensure the skin is consistent on all panels.

I used Weldbond adhesive that was thinned slightly with water to join the lattice.  I started by laying all pieces in one direction on the table and then gluing on the cross pieces one at a time.

After it was all glued up I put a large board over it and weighted it down until it was dry.

I suggest you make a special sanding block to sand the lattice skins.  Cut a bevel on all the edges and make the sandpaper go over the bevels up onto the sides.  The reason being that if your sanding block is flat it will catch in the lattice and make gouges or take out chunks.

If you sanded the sheet before you stripped it and you cut the grooves to the right depth, it should take very little sanding to even out the joints.

The Center Section

Most wings have center section sheeting.  I mentioned at the beginning of the article that I haven't done this because Thwing! didn't work that way.  As I see it there are two ways to go about adding the center section.

The method I would use is to make the lattice large enough to cover the entire wing panel in one piece (top or bottom).  In the center I would cut balsa squares of the same thickness as the lattice and fill in the openings out to where ever the center section sheeting should stop.  If you assembled the lattice accurately it should have perfect squares.  A table saw can make these squares quickly and easily.

If you don't have a table saw then it is probably best to make a plywood template and use a hobby knife.  Note that the grain of the squares will run diagonally to the square so that it is parallel to the span of the wing.

The other method is to cut the center sheeting to the correct length.  Lay the lattice over it and cut away the center sheeting so that the lattice locks into the sheeting.  This method seems like it would be more tedious and weaker.

In any case, I think that butt-joining the lattice to the center sheeting would be a really bad idea especially if the wing depends on the skin for any strength at all.

Attaching the Lattice

After the lattice was trimmed I glued it to the wing structure along one edge and allowed it to dry.  Be sure that it is aligned properly before it dries.

I used moderate pressure to stretch the lattice across the wing and glued it to the opposite edge.  A generous number of pins held it in place.  A syringe was filled with Weldbond adhesive thinned about 25% with water.

I went around the wing lifting the lattice slightly and applying the adhesive to every spot where it contacted the wing structure.  More pins were added and it was allowed to dry.

The lattice adds tremendous torsional rigidity to the wing, but I doubt it adds much spanwise strength.  It also will not be very resistant to crashes.  I would expect the stuff to make a pretty spectacular, yet non-repairable, explosion if it hits the ground with much force.

Build to fly not to crash. Planes built to crash do.

 
 

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How To Edge-Join Balsa Sheeting to Make a Skin
How to Build a Wing for a Flying Model Airplane

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Copyright 2004 Paul K. Johnson