Hollow moulding - a LOW TECH method…

[Jolly Roger]

Late evening and the second skin comes off the foam plug and looks….











Fantastic!

In a wave of euphoria I decide now is the moment to score a hinge line and carefully file it until it starts to bend freely.

The whole process takes about 10 mins and it is a total joy. The peel ply layer, sandwiched in the middle of the layup, faithfully does the hinging thing. A few strokes with a needled file is all it takes.

Yep, I’ll be using living hinges again.

[Jolly Roger]

It’s Wednesday morning and I have a wobble. Flexing the two fin skins in my hands, I think how incredibly floppy they are. When joined at the leading and trailing edge, will they really make a rigid fin? Maybe I should just sandwich the blue foam core in the middle after all, to be on the safe side. I mean, the foam fits perfectly, and it only weighs 26g. Mmmm.

In the end I cut a big lump out of the middle, but leave an inch band along the leading edge to ensure the glass joining tape gets pushed tightly against both sides. I decide to leave the foam section inside the rudder.

I also finalise where the rudder servo will go, and cut a slot for it.

[Jolly Roger]

Show time!

Time to close up the fin, joined at the leading edge with a 2” wide strip of 200 gsm glass cloth (cut on 45 degrees) and 4 carbon tows pressed into the very leading edge. I think a really strong join is essential here to create the structural integrity of the whole fin. If it cracks open at this seam, the whole strength of the skins is totally wasted.

I be mixing epoxy again.

The whole sticky mess needs remarkably little clamping (unlike the sheeting of a traditional wooden wing). The skins seem to naturally cling to the foam core. I “hinged” the two sides together at the leading edge with parcel tape, and after that it only needed clamping at the trailing edge.

[Jolly Roger]

Today, 8am and the workshop door swings open to reveal an absolutely gorgeous fin.

A quick clean up of the edges and a test fit onto the tailboom (which I moulded over a male plug at Christmas).

I’m especially chuffed with the leading edge which is immaculate. No filler/bodging required.

Total weight is just over 120g, less than the original fin which was a traditional balsa/spruce structure covered in 50gsm glass cloth.

The fin feels unbelievably strong in both bending and twisting.

[Jolly Roger]

With nothing else to do, I cut along the non-hinged side of the fin to release the rudder, opened it out then sanded out some foam to allow it to swing both ways.

[Jolly Roger]

The last job to do is to line the inside of the fin/rudder recesses with carbon cloth.
For the rudder, this is more or less cosmetic as it is amply strong. However for the fin, this is essential for structural integrity as it completes the “tube”. How many times have we seen a plywood fin post break away from the sides of a fin moulding?

I may post photos of this if I get a chance but I’ve basically achieved what I set out to: a quick and cheap hollow-moulded structure, and my first living hinge.

Thanks to all who’ve offered tips/encouragement.

Rog

[Mike F]

Hi Roger

Very interesting reading (for me), particularly the construction of the live rudder hinge. I am sure that your approach could be used in repairs too.

Thanks for sharing!

Mike