Sunday, August 14, 2011

Reworking the upscale Cluster Bomb

At this weekend's launch, the 'chute on my Upscale BLU-97B Cluster Bomb tangled and it came in hard..  One foam board fin was totalled and, upon closer inspection, the nose cone (thin glass over pink foam) had a crunch and there was a slight compression crack along the tube spiral.  The Cluster Bomb has had a hard life.  The body isn't in the best shape after landing in a pond in Middletown and one of the surviving fins had previously been cracked.  Since the new fin will have to be painted and I don't have matching paint, I decided to rework more than just the fin and repaint the whole thing.  It won't be in really great shape, but should look a lot better.

I cut the replacement fin from 3/32" plywood.  This is pretty light but not as light as framed foam board.  I don't think having one ply fin should affect its flight.  The original foam fins were 'trough the wall'.  Luckily, I had the foresight to glass the fillet area (actually glass and some scrap carbon tape).  I essentially dug the foam out until it was flush with the tail cone, leaving the composite reinforcement on each side.  I reattached the fin with polyurethane glue on the new fin's root and a liberal amount of wood glue on the composite tabs.  I chose wood glue because the inner surface of the tabs was the paper coating from the foam board.  The fin seems stiff and should be stronger than the remaining foam fins.  It won't break as easily, but it will tend to break at the root.  Time will tell how it holds up.

I reworked the other cracked fin by digging out the cracks, wicking in thin CA, and then backfilling with wood glue.  It was clamped straight and will be smoothed with Fill'n'Finish.  As long as future damage isn't too bad, I guess I'll continue to patch them in this manner.  The penalty of the foam board is that, even with a huge 'chute, one fin is slightly damaged on every landing.

The body crack was treated with thin CA and then touched up with F'n'F.  I did the same to the top of the tube where it was crunched slightly by the shock tether.  The Quaker Oats tube is not the strongest and was made less so by its swim.  I was shocked that it was salvageable. The water also messed up the label stock 'decals'.  So, the remaining ones were peeled and sanded.

The crunched nose required that I remove a patch of the glass.  To get the surrounding area into shape, I inserted balsa shims between the glass and the foam.  I then filled the open spot with F'nF. 

I'll use primer on the naked fin and all areas with exposed F'n'F and will repaint with Rustoleum OD.  I hope to have her done by next weekend, weather permitting.