I’ve progressively refined the design of an engine block. This is a device that sits inside the air-frame, about 1/2 way along the air-frame, either glued or screwed in. To it we have a hook for the shock cord and a the bottom we have a point to screw in the motor using Aeropack adapter.

Design 1

The initial design was using a 30mm Aluminum tube section with Aluminum ends and an eye bolt.

Design 2

This then morphed into a 40mm 3-d printed tube section with Aluminum ends with small shackles.

Design 3

This then morphed into a 20mm 3-d printed tube with G-10 ends and a bolt with a welded nut.

Design 4

This then morphed into a 20mm 3-d printed tube with G10 material ends with much smaller bolt, with the one of the ends recessed 10mm from one end and a 3/16″ eye bolt screwed INTO the main bolt. Take note of the “channels” in the exterior of the block.

The last one can easily take 20kg load using the “hammer” test in BOTH directions.

 

On the final version, I was able to exert 20kg loads in either direction without failure. Failure being movement of the Engine block. i.e. this design is adequate for the task.

Expected Loads

The thrust of the motor is expected to get up to ~800N in the UP direction. The force of the ejection gases is expected to be about 300N. i.e. considerably less.  Remember, we are unlikely to use shear pins in the retention of the upper segment of the air-frame.  Instead, we will use strategically placed tape to provide interference fit that requires certain level of force to overcome.

How it was glued in

I marked distance down the air-frame where I would glue the Engine block. This was to be sufficiently far in so I could install a 6GXL motor case in the future, if I desired.

Next I sanded the internal surface with GRIT-60 sand paper on a stick and then I cleaned it with a damp rag.

Gluing it in was not a simple task. I had to apply glue to:-

• The block itself, in the external channels using  a blade to ensure there was minimal excess
• The inside of the airframe.

I wanted so smear enough epoxy in the RIGHT place in the air-frame. I created a contraption that I would guide along a 5/16″ rod to the correct distance along the air-frame and then I would let it touch the inside of the air-frame as I rotated the air-frame.

 

How to assemble it

It is difficult to assemble without some special tools. Below are some photos of some tools I created to get access to the parts.

I had to weld a socket onto the end of a large steel tube. The tube was sufficiently large so that the bolt could easily go into the pipe, so I could be sure to completely tighten the nut onto the bolt.

The other device was a solid piece of dowel in which I cut a slice into it, so I could friction-fit the it over the eye-let bolt.

I’m constructing my own air-frames.

The set-up

I have on my work-bench two posts at either end held in place using G-clamps. Then I have a sanded down  length of wood with a Aluminium Mandrel that slides over this wood. The Mandrel is 38.10 mm in diameter and is 3 mm thick.

Mandrel preparation

The Mandrel was sanded using 120 right down to 1500 wet sanded. I then cleaned it with Methylated spirits and then I used Brasso to bring up a really nice shine. Then I cleaned it again with Methlyated spirits and paper towels. I did many goes over this to ensure there was absolutely no grime at all. On the left hand side of the Mandrel I beveled the edge to ensure there was nothing sticking out that might stop me sliding off the air-frame.

NOTE: Brasso is a Coles product in Australia. It is supposed to be for Brass, Steel, etc, but Not Aluminium. It’s sister product Silvo is meant to be for Aluminium, but I found Brasso did a much better job.

After the mandrel was cleaned up, I got some Glad Bake grease proof paper and cut to the following dimensions:-

1300mm x 280mm

This was wrapped around the Mandrel length-wise so that it went around twice + ~10mm. It was then glued to itself using a GlueStick. I confirmed that it was reasonably tight and could slide. The left hand side is the end I get the Air-frame off.

The Peel Ply

I use Nylon Peel Ply.

It has a red-trace about every 50mm through it. I cut approximately 300mm of it off the 1270mm reel of material. I then used my USB heat gun device (that came with the 3-D printer) to cut off the 20mm off one of the long edges to give us a straight edge with no loose threads.  I then did the same with the other long side so that I had the Peel Ply with dimensions 1270mm x 265mm – with no loose threads.

The Carbon Fibre fabric

The fabric I used is Twill 2×2.  It is a weave that a lot of other people use.

Then I cut the Carbon Fibre fabric from the 5meter roll. I took GREAT care when doing this work. I did this all on the ground and because tidiness is so important, I vacuumed the ground before hand. I then measured out the fabric to look like the following.

After it was cut, I weight it. It came in at 83 grams. You will notice that on the right side, the fabric is a bit wider. This is so the base of the air-frame is slightly greater diameter, so the rear closure is smaller diameter than the Air-frame. This will make two stage (where this is the second stage) a lot easier.

The epoxy

I used K3600 Renlam epoxy. I made three lots of epoxy because I ran out. Batch sizes were:-

• 132grams
• 40 grams
• 35 grams

After making the first batch, I wetted out the first 120mm of the CF fabric. This was done to minimize dry spots that appear. And it worked!

 

The CF was sticking to the Glad Bake I had carefully laid out on the bench beforehand, but I was able to pull it away from the Glad Bake and put it on to the Mandrel. It took a few goes to get it all lined up on the Mandrel. Not an easy process. Then I was able to start applying more Epoxy.

I spent a fair amount of time examining the tube after the rolling. I wanted to make sure I hadn’t missed anything.

 

Removal of the Air-Frame.

That evening (23:00), 12 hours after applying the Peel Ply, I removed the Air-frame off the Mandrel. It was quite easy. I removed the Glade Bake and I then removed the Peel Ply. The Glad Bake stuck to the inside of the air-frame, but I was able to tease it off with a piece of aluminium right-angle. I did a test fit of the motor casing. It fit well!

 

Then I set it aside in my office in vertical orientation to cure for about 48 hrs.

Here are some photos of the finished tube.