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Old Jul 02, 2009, 10:03 AM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
Build Log
L-39 scratch built foamie

I want to share my design of a L-39 Albatros I just built. It all started with adding retracts to my Phase 3 EF16 and upgrading it with a 64mm EDF to make up for the increased weight. Now I had a very good 55mm EDF left over that needed a new home. So I looked for a good candidate and since I really liked the look of the L-39, I started to make this design.
The EDF gave about 470g of thrust in the EF16 at max 25A (3cells). The EF16 (originally 620g) flew very well with it, so I wanted to stay below this weight.
Here is a great side with pictures: http://www.l39.com/pictures.html

Edited: Updated the plans to latest changes.
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Last edited by Low Pass; Jul 11, 2009 at 09:29 PM.
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Old Jul 02, 2009, 10:14 AM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
I still had plenty of Blue Core and wanted to use it. It is quiet soft, which allows bending very easily but it also needs more stabilization if you need it stiff in the wings. It also needs to be flattened and smoothed on the one side that doesnít have the thin plastic foil.
After smoothing by sanding; the thickness is not 6mm anymore so I used 5mm for the design. However it should be possible to use 6mm Depron with little adaptations. But Iím not sure about the bending part of Depron. Maybe heat application and/or lamination of 2 layers of 3mm could do the same trick.
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Old Jul 02, 2009, 10:22 AM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
I used 3M 77 spray for laminating the blue core parts together, 6 minute epoxy and 30 minute epoxy with micro balloons and sometime foam safe CA + accelerator.

The paper templates are temporarily glued to the plastic foil side using only a little bit of 3M 77 spray. I use cutter knifes and keep them as sharp as possible. At one I dremmeled the back side to get a double sided blade for some parts and for hinges.
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Old Jul 02, 2009, 10:23 AM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
You can cut out all parts at once or piece by piece. Especially for the thin doublers I recommend to straighten/flatten and smoothing the material first before cutting out the parts. Otherwise youíll have to be very careful not to break it.
You also have to be careful when sanding a part in one direction only, not to stretch the material in that direction too much.
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Old Jul 02, 2009, 10:46 AM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
Start with the main wing section. First straighten the material by bending the waves into the opposite direction. I used a piece of a broom stick to roll the foam over it at the wave tops from each side until it is completely flat. Then sand the rough side with 80 or similar sand paper until smooth. Use 180 sand paper to get the surface really smooth.
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Old Jul 02, 2009, 10:48 AM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
Cut the groves for the carbon fiber rods. I used 3mm outer diameter hollow spars. They will strengthen the wing but will also be used to run the wire for the ailerons. Make the groves deep enough to get the spars in flush to the surface, but be careful to not separate the parts.
Cut out the service hatch opening for the aileron servo and cut the spar channels through in the middle section. Prepare the spars and 30 minute epoxy with lots micro balloons to lighten the mixture and burry the spars with the glue into the groves. Keep the cut-through middle section free of glue.
Place the whole wing on a flat surface with a wax paper layer below and above. Place a board on top, press it with some old book or whatever you find until it cured.
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Old Jul 02, 2009, 10:55 AM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
missing picture for above
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Old Jul 02, 2009, 10:56 AM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
Cut out the aileron pieces, mark and keep them for later.
Prepare the fuse bottom layer by cutting out and sanding a tapered slope in the middle section where the wing will meet the bottom of the fuselage.
This part is then glued to the bottom of the wing section and the inner bottom part by spraying just enough 3M 77 on one side (look 2 pictures below for final part).
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Old Jul 02, 2009, 10:59 AM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
To achieve a dihedral of about 3ļ cut the long spar open in the middle, place a bent music wire piece into the hollow spar, fill the remaining gaps in the groves at the center with epoxy + micro balloons. Wax paper below and above will protect again the parts from gluing to the wood. But this time a board only presses the middle section while 3 layers of foam at each side where the ailerons start will create the dihedral. Again press it with a pile of books or some other weight until hardened.


After the epoxy is hardened the bottom part should look like this.
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Old Jul 02, 2009, 02:22 PM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
To protect the leading edge of the main wings slit the front and glue a flat carbon fiber strip (5mm wide, 1mm thick) using 6minute epoxy + micro balloons. Foam safe CA + accelerator didnít work well; it actually melted the blue core foam, even so it was foam safe.
After the carbon fiber reinforcements are glued in place the front edges can be sanded round and the trailing edges tapered.
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Old Jul 02, 2009, 02:42 PM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
Now it is time to start the side walls of the forward fuselage. Prepare the doublers and glue them on the side wall piece and on top of each other using 3M 77 spray.

Glue the bottom doublers to the bottom fuse using 3M 77.

And then glue the side walls one after another to the doublers and the bottom.
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Old Jul 02, 2009, 02:49 PM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
Laminate the nose parts together and then to the fuse. Cut to a triangular shape following the lines of the side walls.
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Old Jul 02, 2009, 02:53 PM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
Glue the side wall ends together with epoxy + micro balloons or foam safe CA + accelerator.
Laminate the rectangular bottom doubler to the end between the side walls (not shown here, see below)
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Old Jul 02, 2009, 03:04 PM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
Prepare the ESC holder by cutting out the space for the size of your ESC and gluing one of the rectangles with cut out to the bottom. Sand the bottom until the ESC fits in flush. The top side with the metal heat sink has to be cooled with the streaming air in front of the fan. Create a slope to the front for the battery cable and control wires.

Glue the second rectangle to the bottom and cut/sand a slope to make room for the aileron wires later. Make sure, the servo will fit and epoxy a little piece of plywood as servo mount on this bottom side (not shown).
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Old Jul 02, 2009, 03:13 PM
Some epoxy & it flies again...
Low Pass's Avatar
Santa Cruz, California
Joined Sep 2008
507 Posts
Finish the ESC holder by laminating the small rectangle pieces as stand offs and sand the bottom one to the correct height. The ESC top should be just flush with the opening.

Glue the ESC holder in between the funnel center piece and the fuselage bottom.
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