Construction 3-16-04

Today's job was to get the first fin attached.  It's a straight forward procedure.  Insert the motor tube assembly into the aft airframe, place a bead of epoxy onto the fin root, insert through the fin-slot, give it a little wiggle into position and secure until the epoxy cures.  To strengthen the epoxy and thicken it both 403 Micro fibers and 404 High Density filler from West Systems was used.  First one pump each of the Resin and Fast Hardener were mixed in a small cup.  A small amount of this was brushed onto the fin root to allow it to soak into the wood.  Then 2 teaspoons of each the 403 and 404 fillers were mixed into the remaining epoxy.  The end result is an epoxy with the consistency of peanut butter.  Getting it this thick is important as it keeps the epoxy from running all over the place and keeps it where I need it.  Also these fillers add strength to the epoxy joint.
No Pictures for today, some idiot forgot to charge up the dead battery pack, but since I have to repeat this process three more times for the aft airframe I think I can manage to get some pictures tomorrow.

Construction 3-17-04

Same as yesterday, attach another fin.  This time my un-named assistant remembered to charge the camera (OK, I admit it, it was me).

Once again you can click on the thumbnails at any time for a larger picture.  Here is one of the main fins with some wax paper applied.  This way if some of the epoxy gets between the fin and the slot it won't glue the assembly together.  The thickened epoxy is buttered across the root edge and the section of tab which butts up against the aft centering ring.

Then the fin is placed into the slot, once it contacts the motor-tube I give it a little wiggle then slide it firmly against the aft centering ring.  The the wait for the epoxy to cure.

Here is the fin from yesterday.  The assembly was pulled out of the aft airframe for a clear view of the joint.  When all the fins are done they will get proper fillets along with some fiberglass tape reinforcement.

Yet another view, this one with the middle centering ring pulled down to its eventual position at the front of the fin.  Between these two rings is where I will foam it.  Between the tape and the foam the fins should withstand any impact that doesn't blow the whole aft end apart.

So now we're at 2 down 2 to go.  Not visible in these pictures is the index marks on one fin and the corresponding slot on the airframe.  In theory if everything is perfectly perpendicular the assembly should slide together with any fin lined up with any slot.  Since I doubt this is the case the index mark will let me know which orientation is the right one when I slide the assembly back into the airframe.

Construction 3-19-04

Finished remaining fin allowed to cure overnight.

Construction 3-20-04

Today's goal was to coat all the fins with epoxy and install the fiberglass joint reinforcement.  The epoxy coating on the fins doesn't really add any appreciable strength to the fins but serves to seal the wood.  The fiberglass tape isn't really needed since I will be foaming them into place, but better safe than sorry.  Besides if I have enough fun flying an M motor I may get a hankering to kick it up a notch in the future.

Step one was to cut off 11 inch pieces of the fiberglass tape
(4inch wide 7.5oz) and trim the width slightly

Here is the fiberglass dry fitted

The fins and motor tube wetted with plain west systems epoxy

The fiberglass reinforcement in place and wetted down with epoxy

All four fins are now reinforced with fiberglass.  Each section was allowed to partially cure for around 2 1/2 hours before rotating the tube and doing the next section.  Although the fast hardener was used the shop area is cold enough that it is still quite tacky after 2 1/2 hours, but cured enough that it shouldn't run.  The completed assembly will be allowed to cure overnight before it is mounted into the aft airframe and foamed.

Construction 3-21-04

Reinforced fillets were added to all the centering ring motor tube joints and allowed to dry.  A test fit showed that a couple of drips of epoxy on the fins were causing the aft airframe to spread a bit.  A quick sanding of the offending areas another test fit and this time the fit is perfect with the aft airframe sealing tightly around the centering ring. 

Un-thickened epoxy was painted on to the edges of all centering rings and allowed to partially cure. A batch of thickened epoxy was prepared and applied not just to the rings but liberally spread to the inside of the airframe.  The motor tube / fin assembly was slowly slid into the airframe, once the forward ring was past the fin slots more epoxy was added inside the tube again.  The assembly was slid in some more allowing the center ring to push the epoxy  ahead of it.  One last time epoxy was added inside the airframe again this time for the rearmost rings.  Once it was fully seated the fit was checked one last time and the airframe placed in an upright position for the epoxy to cure.  Again it was necessary to allow it to cure, then time to fill the fin slots to keep the foam where it  belongs.  Due to the amount of force expanding foam can generate I have decided to allow the assembly to cure at least 24 hours before proceeding.

Fin assembly installed into airframe.  This pic is before I filled in the space between the fin and the slot.