Sunday 2 August 2015

Balsa / fibreglass tails

Ages ago I came across this article by John Gallagher on wax paper bagging DLG tail surfaces, and ever since I have wanted to give it a go.

I'm working on a Fusion style pod and boom sloper designed by Leadfeather on RCGroups and thought bagged balsa tails would suit. My foam elevator is proving to be a bit delicate for repeated rough landings.

2.5mm x 75mm balsa sheet. Elevator will be 100mm wide so an extra strip needs to be glued on. 
I'd probably use thinner balsa for DLG tails or sand down the 2.5mm. 
Later I realised 100mm wide balsa is also sold at Bunnings.

Medium CA glue on baking paper to prevent sticking to the cutting mat.

Marking out the elevator using professional draughting aids. 
Wonder why my curve radius is reducing over time?

Elevator cut out

Sanding to shape. Rounded leading edge and thinned trailing edge

Sanding balsa is very dusty so use a face mask if you value your lungs

Cut out some light weight 18 gsm fibreglass cloth with a rotary cutter, a bit larger than the balsa

and another one

Place the cloth on a sheet of baking paper 

Mix up some epoxy finishing resin and spread it through the cloth using a razor blade as a spatula.
Lay a sheet of paper towel on top and press down to blot up the excess.
Repeat once more with a clean sheet of paper towel. According to John it has to be twice, not once, not three times but twice to get the right amount of epoxy.
(Aargh - I forgot to blot the excess on the first side, we'll see how it goes)

On a clean flat board lay down 8 layers of paper towel then a sheet of baking paper. 

Lay down the balsa, then carefully position the wet cloth and baking paper over the top.

Lay down 8 more layers of paper towel

Then another clean flat board and heaps of weight. 
If you're a scuba diver you will have plenty of lead weights at hand. 
Otherwise use heavy book, tools, bricks or whatever. 
John used about 22lb or 10kg. Leave to cure for 3 to 4hrs.

 So far so good. Repeat for the other side.

The next morning - Trimming the excess with scissors

Hmmmm........ 15g is heavier than I expected, but it is plenty stiff and strong enough. 
All the weight is in the balsa so I might try thinner balsa next.

The very next weekend
 I bought some 1.5mm balsa thinking the weight would be much less than the 2.5mm. BUT the thinner balsa is made from harder and heavier stock so it's not much lighter. 

Trimmed to size and axle fitted - 13g. 
Trying 3mm CF rod through a 3mm hole in a short piece of spar (rather than solid CF ferrule)
OK I'm happy with that. Same weight as laminate covered 3mm depron but much stiffer.

Next idea was to try 3mm depron. 
The uncovered depron h-stab was much lighter - 4g compared to 9g for 2.5mm balsa.
Today I fibreglassed one side BUT used different baking paper which proved to be non "non-stick". The resin glued to the paper permanently. Back to the original Multix non-stick Bake and try again.

Trying shaped 6mm depron with carbon tow spar, painting on thinned resin with no pressure. 
Uncovered 6mm stab is only 5g - looking good.

And 3mm depron using the wax paper bagging method.
Air temp is only 11C so curing takes all day.

The weekend after that
The 6mm shaped depron with Carbon tow spars and painted on thinned resin is super stiff and really nicely shaped but again just a little heavy at 13g. If I could get it down to 9g I would be happy. 
Next I'll try a thinner version of the same core with thinner carbon spars and baking paper bagged fibreglass for a DLG elevator.

For the Fusion sloper full flying elevator I tried 2mm balsa covered with 75mic laminate and it came out at 11g including the axle and stub boom. Excellent, happy with that, and it's stiff enough and works well. A very quick and easy build. Would probably be OK for the DLG too.

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