Airfield Models - How To

Repairing Thwing!

May 03, 2015



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Airfield Models (http://www.airfieldmodels.com/)Spar Reconstruction

This is part 3 of an article describing the repair of Thwing! due to damage caused when the motor came loose from its mount in flight.

This is not a typical spar reconstruction in that the spar that broke is not critical to the strength of this wing.

If a spar breaks in a conventional wing then it should be joined back together with a piece of plywood on both the front and back of the spar.  The plywood should extend several inches past the break in each direction.

The thickness of the plywood will vary depending on the size of the aircraft and the location of the break.

For example, on a .40 size model having a spar break near the root I would use at least 1/16" plywood on the front and back of the break.  I would make the new braces about 6" long.  It should be epoxied in place and clamped securely for the strongest possible joint.

 
 
A new section of spar is made to splice to the existing piece. The spar was actually the easiest part of the repair.  The break occurred at a place where it was accessible.  No surgery was necessary to gain access.

I used a sharp knife to cut a bevel on the end of the existing spar.  A new piece was cut to match and left over-length at the root end.

After checking the fit several times and ensuring I would be able to put it in place, the new piece was set aside.

The shear web is glued back in place. The shear web broke loose from the bottom spar taking the top spar with it.

The bottom of the shear web was left alone because it fit back to the bottom spar.

The top spar was cut away.  Excess wood and glue was sanded from the top of the web.

The web was then glued back into place.

The new spar is added. What can not be seen in these photos is a joiner piece in front of the top spar.  I used the same material as the spar 1/8" square balsa about 2" long.

The joiner piece was added to make the repair easier.  It did not add any significant strength to the spar.  As I already mentioned, this spar's strength isn't critical.

The new piece is glued into place and the spar reconstruction is complete.

 
 

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Repairing Thwing!'s Leading Edge
Repairing the Sheeting and Lattice Work

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Copyright 2004 Paul K. Johnson