Balsa Workbench Primary Trainer Step 1: Wing, Part A

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The Balsa Workbench Primary Trainer was designed to teach the new builder the basic skills of building RC planes from balsa and ply, and to be a good primary flight trainer.  To give credit where it is due, this design owes a lot to a plane called the Trainer 5 by Joe Bridi, which was published in RC Modeler magazine.  Starting with the Trainer 5, I developed a new design that is easier to balance, is more durable in a crash, and is easily adaptable to either electric or 049 glow power.  Thanks to the sizable cooling scoop behind the propeller, overheating shouldn’t be an issue for those who are not confident about keeping their batteries and speed controls cool.  I added that feature because of my own adventures with overheated wires.  I also changed the construction details to make it easy for a beginner to see how the plane goes together and to end up with a straight airframe.  The plane can easily be built without a printed plan.

 

What you will need to complete this kit:

  • 6 to 8 pieces of 1/16″ x 3″ x 36″ balsa sheet.  You can use 4″ wide sheet if it is available, in which case you will need about 5 of them.  Don’t buy super soft balsa that breaks easily, and don’t buy rock-hard balsa.  Imagine yourself handling the fuselage or wing, and look at the balsa available in the store.  You don’t want your finger to go through it with normal handling.
  • One sheet of 1/8″ balsa.  You won’t need much, so if you can buy a piece that’s 2″ wide by 18″ long, that’s enough.  But if you get one 3 x 36 sheet you’ll have a lot of leftovers for future projects.
  • About a foot of 1/8″ x 1/4″ balsa stick.  You can cut this from your 1/8″ sheet.
  • One 1/4″ x 1/4″ x 36″ hard balsa stick.
  • 1/4″ x 5/8″ x 36″ medium balsa stick for leading edge. (1/4 x 3/4 will work if you can’t find 5/8.)
  • One piece of 1″ triangle stock for wingtips.  If you prefer to build flat wing tips you can just glue a piece of your 1/8″ sheet to the tip and sand around the edges.
  • One piece of lightweight glass cloth 4 inches wide and about 16 inches long.  You can also use lightweight cotton weave fabric or other suitable substitute.
  • Thin and medium CA glue, or Titebond wood glue, or Elmer’s white glue.  You may want to use epoxy to attach the fiberglass at the wing joint and to fuel proof the nose if using a glow engine.
  • Sanding block and 120 grit and 180 grit sand paper (or something in that range).
  • X-Acto knife, razor blade, scalpel, or other hobby blade.
  • A small square and a long straight edge.
  • Seven inches of 3/16″ dowel for the wing retaining rubber bands.
  • About 3″ of 1/8″ dowel, or preferably 1/8″ x 1/8″ square spruce or basswood stick for joining elevators.
  • Nylon tail skid from the hobby store if flying from hard surfaces.
  • #64 rubber bands to hold the wing on. Use 2 to 4 per side.
  • Pins if using slow setting glue.
  • About 12″ of 3/32″ music wire for landing gear.
  • One pair of 1 to 2 inch lightweight wheels and 3/32″ wheel collars.  If flying from a rough field you can use big 2.5″ lightweight foam tires if desired.
  • Push rods.  You can make these from one nylon tube push rod, in which case you would use some of your 1/8″ sheet as an anchor for the front end of the outer housing.  Or you can buy an extra 1/4″ balsa stick and some mild steel push rods with 2-56 threads on the ends to make stick-type push rods.
  • Clevises and control horns for the elevator and rudder.
  • Your choice of covering materials and the associated paraphernalia for applying it.
  • Radio system and either electric motor or 049 engine, and suitable propeller.

Step 1:  Wing Construction

Wing construction starts with the wing skins.  Select five sheets of 1/16″ x 3″ x 36″ balsa.  These sheets should be medium balsa, not super hard and not super mushy. You want the top skins to be able to curl easily enough to follow the airfoil contour without cracking.

Mark these sheets in the middle and use a square to cut them squarely in half, so you have 1/16″ x 3″ x 18″ sheets.


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(Please note that if you make a mistake and accidentally cut the sheets a half inch short, it’s not the end of the world.  The plane will still fly.  Similarly, you can start with more 36″ sheets and cut your wing skins as much as 21 inches if you want to.  The plane will fly more slowly and will possibly even be a better trainer.  I built one of these with a 42″ span and it was able to catch thermals and stay up long after it ran out of fuel.  If you go beyond that size, you’ll need to start enlarging the tail and putting it farther back, etc., because it has become a large plane.  At any rate, the point is that the length of the wings does not have to be super precise.)

Because you are making the bottom skins first, if your sheets vary slightly in length use shortest ones first so the top skins will be longer than the bottom ones.  If you don’t understand why, read through the directions and you’ll see that you don’t want to build the bottom structure and be left with a skin that’s too short to cover the top.

Balsa sheets are frequently warped or curved along the edge, so you may have to straighten the edges with an X-Acto knife and a long ruler.  I can usually put them together so two adjacent sheets are curved the same direction, and they fit well.  You can join the sheets with thin CA, medium CA, titebond, contact cement, or white glue.  I consider edge-joining sheets to be a foundational balsa building skill, so I’ll post an article on this site telling how to do it.

Join three of these sheets along the edges to make a big sheet that is 1/16″ x 9″ x 18″.


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Mark a line 5″ from the trailing edge of your sheet.  This will be the bottom wing skin.


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Place a wing rib so the front edge of the spar notch is on the line that you marked.  Make a mark about 1/16″ to 1/8″ beyond the leading edge of the rib.  Make one of these marks at each edge of your sheet.


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Use a long straight edge to cut the sheet on these marks.


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Cut a 1/4″ x 1/4″ x 36″ balsa stick in half to make two 18″ spars.  Glue one of these spars behind the line, so the line is at the front of the spar.


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When finished, everything should fit like this:


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Follow these steps again to make another bottom wing skin for the other wing.  The second skin should be made from two 3″ wide sheets plus the excess that was trimmed from the first sheet.

Next we will mark the locations of the wing ribs on these sheets.  Make a mark on the spar 1.125″ from the edge of the sheet (that’s one and an eighth).  Make another mark at 3″.  Then continue making marks every 3 inches all the way to the other edge.  Then move to the trailing edge and make the same series of marks.

Mark the other sheet, but make sure you go in the other direction so you have one left and one right.


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The end with the closely spaced ribs is the root.  These go in the center.


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Glue your ribs to the bottom sheeting, but don’t add the root rib yet.  Try to place the ribs so they are perpendicular to the spar and standing straight up.


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Now you need your pre-cut dihedral gauge.  It looks like this.


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Use the dihedral gauge to set the angle of the two root ribs, and glue the root ribs into place.  Don’t be tempted to reduce the amount of dihedral, because a plane with no ailerons will fly well, only if it has enough dihedral.


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These wings will have a top skin attached.  We want the top surface of the wing to form a smooth curve from leading edge to trailing edge, so we will have to sand the trailing edge of the bottom skin to a bevel.


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This sanding technique is one of the foundational skills of balsa model building.  The trick is to place the wing at the edge of your building table and position your sanding block so it follows the tops of the ribs and sands the excess from the top of the bottom skin.  As you can imagine, there is a risk that you will accidentally sand the ribs away as well.  If you do not trust your skills yet, you can protect the ribs with masking tape.


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Now place your wing at the edge of the table and sand a bevel into the bottom skin.  After you do this a few times you will develop the skill to safely sand a wing without destroying the ribs.  The sanding block will give you feedback.  It feels and sounds different when you are on the ribs.


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Don’t sand it too much.  This is what it should look like when finished.


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If it isn’t perfect, the plane will still fly.

 

To keep these pages reasonable, I’m going to split the wing building instructions here.  Go to the top menu and look for Balsa Workbench Primary Trainer Step 1: Wing, Part B