Add Carbon to Spars ???
Sky Bench Kit's Wood Spars ...
The decision to add reinforcing carbon and or wrapping the spar on Sky Bench kits starts with the question of how are you going to launch the sailplane.
If you are going to use a proper size hi start or electric motor, no carbon or wrapping is necessary.
If you are not going to zoom launch from a very strong electric winch, no carbon or wrapping is necessary.
If you are going to launch from a mild winch with a wimpy ping, no carbon or wrapping is necessary.
If you are going to fly sanctioned contests with very strong winches ( AMA Nats, OVSS, Etc. ) to zoom launch, adding carbon or wrapping is the right thing to do. But, doing a poor job of gluing the carbon can result in failure. Also, not all Sky Bench kits require adding carbon to zoom launch. The Sky Bird is very strong right out of the box.
I have seen and flown other persons Sky Bench kits that carbon and or wrapping was applied to the spar structure that were off the wing loading chart due to excessive weight. My kits use one piece plywood root shears on both sides if the spars and vertical grain balsa share blocks between the spars that result in a very strong inner wing panel structure at a proper wing loading.
How thick and where to place the carbon is good stuff for debate and equally important is how to glue the carbon.
Since most of my kits have 1/8" deep spar notches in the ribs, I have settled on this procedure for making carbon spars. I cut 1/16" plywood strips the width of the spar to the length of the inner panel. The plywood goes down on the bottom sheeting with wood glue. Use a rib to locate the spar's position on the sheeting.
NOTICE: ... ON MY NEXT BUILD, I AM GOING TO USE 1/16" BASS WOOD IN PLACE OF THE 1/16" PLYWOOD. THE BASS WOOD WILL WEIGH LESS AND IS EASILY ASSESSABLE.
Next, 3/8" x .060 (1/16") carbon is epoxied to the plywood and weighted down over night. I mean really weighted down to squeeze out excess epoxy. Be sure and clean up excess thoroughly. I apply the slow cure epoxy to the carbon very thin.
Next are the ribs and vertical grain balsa shear blocks. The blocks are epoxied to the carbon between the ribs and weighted down, same procedure as the bottom carbon strip. The top carbon is applied to the top of the shear blocks with epoxy and weight down same as the bottom carbon.
Next the top plywood spacer is epoxied and weighted down to the top carbon. If your good, you can lay the top carbon and top plywood at the same time.
Next, cut the necessary ribs away from the spar and epoxy the front plywood root shears in. Before adding the tip panels, install the wing rod and rear alignment tubes and then the rear plywood root shears.
BTW ... the guys that transitioned from woodys to moldies without experiencing carbon woody spars and the guys that started with moldies just don't get it when they watch me dive straight in from spec out height. It is part of the fun of flying contests againts the moldies and AVA's, etc. with a woody.
Ray Hayes LSF 803
Sky Bench, Ft Wayne, IN 46804E Mail: SKYBENCH@COMCAST.NET