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Les Kramer's L-3 Certification Project
 
FIN ATTACHMENT
The following parts make up the fin attachment components:
• (4) 3/8” thick, 7 ply, plywood fins with routed leading and trailing edges PIC
• 6 oz. 2" wide Fiberglass Tape
• 6 oz. Fiberglass Cloth
All parts are assembled with West Systems epoxy (105 resin / 205 hardener / 404 filler) where required. PIC
Laminating the Fins
As mentioned in the INTRODUCTION, one cause of certification failure is fin breakage upon landing. To help prevent this, the fins are laminated on both sides with 6 oz. fiberglass.
Testing Centering Ring Spacing
The space between centering rings #1 and #2 create the space where the fins are installed. Proper alignment of the centering rings is critical. The first step was to test fit each fin to make sure that the centering rings were aligned and that the fins "fit" in each location.
Marking the Fin Guide Lines
Prior to installing centering ring # 2, I used the fin alignment guide created from FinsFun to mark the location of the fins on the forward face of centering ring #1 and the aft face of centering ring #2. I also scored a small slot in each line the width of the fin. PIC
Sanding the Fins
The root edge of the fin tab was sanded with a heavy grit sandpaper as well as 1 1/2" of the face. This will allow proper epoxy bonding in the following steps.
Tacking the Fin in Place
The first fin was tacked in place using 5 minute epoxy. Only a very small amount was used on the root edge of the fin tab. Once the epoxy was applied, the fin was placed in position using the "slots" in the aft face of centering ring #2 and the forward face of centering ring #1. Carefully aligning the fin on the line on the motor mount tube ensured the proper alignment.
Attaching the Fin
Once the fin was in place, I checked to make sure that the fin was aligned properly. I also slightly turned the assembly and used a spare centering ring to hold it in place. The angle of the assembly was such that the epoxy would "rest" against the fin and not run down the motor tube. Next, I mixed a generous amount of epoxy that was only slightly thickened with #404 filler. I poured about 1/2 of the mixed epoxy along the root edge of the fin tab. Quickly, I placed a pre-cut piece of 6 oz. fiberglass tape along the root edge of the fin tab. One half of the tape was placed on the motor mount tube and one half on the fin. Once the tape was in place I used a small wood dowel to press the tape firmly to the fin and the motor mount tube. I took the balance of the epoxy and poured it over the tape and wet out the glass. The area behind the fin tab was also filled with epoxy. I also placed a small amount of epoxy at the joint of centering ring # 2 and the motor mount tube.
Each fin attachment point was allowed four hours to cure before proceeding to the next joint.
Completing the Fin Attachment Process
All areas of the centering rings were secured to the motor mount tube with epoxy. With the completion of this step, all centering rings are now attached to the motor mount tube. The completed fin assembly was set aside to completely cure.
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