Balsa Workbench Primary Trainer Step 3: Sanding and Assembly

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Step 3: Final Assembly and finishing

 

This is the part where it all comes together.  Let’s start with something easy and put the two elevators together.  You’ll need a piece of 1/8″ dowel or better yet, of 1/8″ x 1/8″ square spruce or basswood.  This piece should be 3 inches long.  Before you charge ahead and glue it into place, lay out your stabilizer and elevators so the elevators are flush with the ends of the stabilizer.  Put the joiner in the slot where it belongs.

You can see that my joiner is too long, and the elevator protrudes past the end of the stabilizer.


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In this case you would want to trim or sand your joiner until it’s the right size.


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Now move the stabilizer out of the way, and glue the joiner to the elevators.


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Now it’s time to sand everything.  I left the sanding until the end so it would all be done at the same time.  You can make one big mess and then vacuum it all up.

Sand the edges of the tail surfaces.  All edges of the horizontal stabilizer should be rounded, except the trailing edge.  The vertical stabilizer should be rounded on the front and top edges.  The rudder and elevators should be sanded to a V shape at the leading edge (hinge line) and rounded on the other edges.  For aesthetics I like to leave the elevators square on the edges facing the rudder.

Here’s the leading edge of the elevator, and you can see that I also rounded the outer end.


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The stabilizer stays flat on the rear edge.


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I personally think the elevator looks better if you leave this edge square.


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All edges of the fuselage need to be sanded flat.  When you finish, the corners are all sharp.


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Then go back over the whole thing and round all of the corners.


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If you want to save time sanding the leading edges you can use a planer to take off the extra balsa wood.

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Sand the top of the leading edge so it continues in the direction of the wing skin where it meets the leading edge.  Do not round it yet, just sand only one facet.  Hold your sanding block so that when the excess is sanded away, the block will be resting flat on the wing skin.


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Here’s what it looks like when this step is finished.


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Also, sand the bottom of the leading edge flush with the bottom wing skin.


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While you’re at it, sand the wing tips flush with the top, bottom, leading edge and trailing edge of the wing.

 


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Round the edges of the wing tip, the same way you rounded the corners of the fuselage.


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Sand the root flat.


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Sand the top and bottom corners of the leading edge so they are round.  Make the curve consistent along the entire leading edge.


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Cut a piece of lightweight glass cloth 15″ by 4″.


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Glue the two wing halves together.


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Place the cloth on the bottom of the wing so it hangs over the trailing edge about a quarter inch.


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The cloth can be attached with thin CA (you’d better get it right the first time), epoxy thinned with alcohol, or Titebond or white glue thinned with water.  With the exception of CA glue, spread  the glue with a disposable acid brush.  I used white glue thinned with water on mine.  I’ve used thin CA a million times to do this, but honestly I can’t recommend it.  It’s fast but not easy, and it makes a huge noxious cloud that burns your eyes.


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Pull the cloth around the leading edge and glue it to the top of the wing. (If you’re wondering about the balsa patch in my wing skin, it’s because I stuck my thumb through it.) Leave the extra cloth hanging over the trailing edge.


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Leave the wing to dry, or let the epoxy cure if that’s what you used.


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After your glue has cured, the excess glass cloth needs to be trimmed off.


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Use a sharp blade to cut the glass cloth at the trailing edge of the wing.


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Sand the edges of the 1/16″ ply rectangles.  Leave the root edge and the trailing edge square.


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Attach these two pieces to the top of the wing at the trailing edge.  These will protect the wing from the rubber bands that hold the wings on.


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Make a pair of wing dowels out of 3/16″ dowel stock.  They should be 3.75″ long.  Round the sharp ends off so they will go through the holes more easily.  Temporarily install them, but do not glue them in place until after the fuselage is covered (because if you glue them before covering, they will be in the way when you are covering).

Trim the top of the windshield flush with the top of the bulkhead.


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Finish it with a sanding block, keeping in mind that the surface should be horizontal to match the bottom surface of the wing.


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Mount the wing and see if it leans to one side.  If it does, sand the wing saddle on the opposite side until the wing sits straight on the fuselage without leaning.


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Turn the plane around and set the tail assembly on the stabilizer saddle to assure that it sits parallel to the wing.  Adjust the stabilizer saddle with a sanding block until the stabilizer sits straight. Do not permanently attach the stabilizer at this time.


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All that remains now is to cover the plane with your choice of film, fabric or paint.  The part of the stabilizer that attaches to the fuselage should be left uncovered, as should the portion of the vertical stabilizer that is inserted into the slot in the horizontal stabilizer.  When these parts are attached, you want bare balsa glued to bare balsa.

After you cover the fuselage, permanently install the 3/16 wing dowels.

If you’re flying this plane from a hard surface, add a nylon skid to the bottom of the fuselage under the tail.

When you permanently attach the tail, be certain that the vertical fin is parallel to the center line.

Add 2″ wheels.

If you built your plane with a removable hatch, secure the front edge of the hatch with strong magnets, a latch, or a rubber band around the nose of the plane.

Attach your rudder and elevator with CA hinges or other method of your choosing.  Install the motor and radio.

Set the center of gravity 2 3/8″ from the leading edge.  To help you set the center of gravity, turn your finished wing upside down and put a small dot of medium or thick CA glue on each wing, 2 3/8″ from the leading edge.  Spray a shot of accelerator, and you have little bumps that you can feel with your fingers while balancing the airplane.  Set your rudder travel to 3/4″ left and right, and set your elevator travel to 1/2″ up and down.  Use a 1/16″ shim under the left side of the motor mount to create a few degrees of right-thrust.

If you have any questions about building or finishing this airplane, send me an email and I’ll help you figure it out.  Below are some photos of the airplane ready for covering, and then some photos of the plane ready to fly.  As you can see, I equipped my plane with cheap gear from ebay.  I have an EMax 2822 motor, three cell battery, 20 amp speed control, and a couple of Tower Pro servos.  The photo shows the propeller held with an O-ring, but if you are using three cells I recommend using a good hub with a nut to hold the prop.  The O-ring allows too much vibration, and my propeller came off at high speed and took the cowl cheek with it.  So make sure yours doesn’t do that.


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