Aerotech Mega Initiator #89541, Part 11, Gluing In The Engine Mount

Test fit, test fit, test fit -  before gluing on the centering rings.

The mount is slid into the airframe tube and the fin tabs slipped into the fin slots into the centering ring notches.

Here I've marked the position of the bottom edge of the ring with masking tape.

This allows me to apply the epoxy to the upper side of the ring. After that dries, the tape is pulled and an epoxy fillet is applied to the underside joint. Repeat the epoxy fillets on the other centering rings.

You are given the choice of gluing in the mount then attaching the fins or - 

Extend and cut the fin slots down through the bottom of the body tube. The fins are glued onto the mount then the entire assembly is slid into the tube thru the extended fin slots.

I decided to not extend the fin slots. Sometimes this extended cut allows the tube end to flare out.

Before you use epoxy - 

Some people are allegic to epoxy. Use a gloved finger to smooth out epoxy fillets.

With the fins dry fitted in their slots, 

this puts the low centering ring just under 7/8" from the end of the airframe tube.

To glue the upper ring in place,

Slide the mount down, about 1/4" below the final position. 

This lowers the mount and upper ring. Epoxy can be applied from above, then the mount slid up into the epoxy glue ring.

 

I used a dowel to reach and apply a ring of epoxy glue.

With the upper ring in the lowered position, the dowel was marked to be used as a depth guide.

To reach in with the epoxy,

Roll just the end of the dowel in the glue. This is set into the tube up to the mark and rolled around the tube wall. 

Continued next post -

Aerotech Mega Initiator #89541, Part 10, Final Fin Fill & Dry Fits

The fins on the flight version were left square.

That inside leading edge angle still wouldn't match up. I added some CWF for a better flow.

Here's the same fill after sanding. Some 400 grit was wrapped around a pencil to sand the angle.

Another dry fit of the fins showed the ring slots were too tight.

The sides were shaved down, widening the slots.

Here's how far the engine mount is recessed, the ring face is just under 7/8" from the end of the tube.

With the fins in the slots set the placement depth of the  engine mount.

Aerotech Mega Initiator #89541, Part 9, Assembling The Engine Mount Adapters

In the last post, I did some dry fits until the rings were tight on the tube, but still moveable.

Here I'm using the flat face of my sanding block to get the lower ring flush and square with the end of the motor mount tube.

Here's the engine mount ready with the upper ring 1" from the top of the tube.

A dry fit ready for the epoxy fillets.

Epoxy fillets were applied with a gloved finger and smoothed with alcohol.

In the picture, I've applied a fillet and am removing most of it with a Q-tip. This is the ring that "sockets" into the larger engine mount tube. You don't want excess glue to hinder the fit.

 

The finished mount.

The other two mount assemble and glue up the same.

Aerotech Mega Initiator #89541, Part 8, Ring Diameter Sizing

Both the inside and the outside edges of most rings need to be sized to slide into or over the tubes.

I used a combination of a sharp knife and a half round diamond file to slightly increase or decrease the diameters. I found that removing the laser ash brought them to almost the correct size.

The additional motor mounts need some sizing.

That smaller (inside) ring should slide into and almost "lock" into the largest mount tube.

That required a slight angle sand.

Sand a little, test the fit and continue until it friction fits into the larger motor mount tube.

This is the adapter bulkhead.

The instructions don't mention it, but put a washer on both sides of the ring. The screw eye and threaded bolt are locked in place with some epoxy.  

Aerotech Mega Initiator #89541, Part 7, Prepping More Parts!

I know there is a LOT of parts prepping - 

Ask any wood worker. The secret to a great finish is good surface preparation.

   

Here's that inside corner on the leading edge of the fins.

On the top picture - 

With the primer shot, you can see the mismatch of the rounded leading edge at the center of the angle. Some corrective sanding was done.

Here's the same corner after a directed shot of filler/primer. More careful sanding should get things in line.

The fin after the second coat of filler/primer - 

There were some small lines that needed some CWF and more sanding.

It seems every slot was a hair too narrow.

The root edge notches and the fin slots in the body tubes were widened for a better fit.

I could slide my sanding block into the slots using 220 grit.

The fins were test fitted. 

The joint of the body tube and root edge was very good now, tight enough with no distorion of the tube..

Aerotech Mega Initiator #89541, Part 6, Fin Filling

One of the fins had a knot hole fill.

It won't be noticeable with an easy CWF fill and sanding.

This fin will go on the flying build.

Here's that inside angle trouble spot I mentioned earlier.

When sprayed with filler/primer you can better see what needs to be cleaned up with better rounding.

The root edge and fillet areas were taped off for the filler/primer spraying

The fin in back is before sanding,

The fin in front is after sanding with 400 grit.

Here's a close up of the sanded fin surface. 

It's easy to see the gray filler left in the shallow grain.

Aerotech Mega Initiator #89541, Part 5, Fin Shaping

The fins are 1/4" thick, five ply plywood. The plywood is very high quality.

The instructions don't mention it, but I'm going to round the edges on the presentation model.

I drew a pencil line for reference all the way around the exposed outside edges.

I started by knocking down the edge at a 45 degree angle with some 220 grit on a block. Sanding with the block helps keep the width consistent.

The tricky part is where the concave edge meets at the break in the leading edge corner. This took the most work.

You won't be able to judge how well the rounded edges meet until you spray it with some filler/primer later on.

On the left,

is the first rough knock down of the right side trailing edge.

On the right is a good rounded edge on a different fin.

I wouldn't sand the blackened laser cut root edge until you check the fit before gluing the fins thru the wall and against the motor mount tube.