A few people have asked me how much weight is really saved by milling the Depron for indoor models and is it worth it ?. Well here is the answer with my Index3s. As the with battery ready to fly weight is only 105 grams, 17 grams is a big saving.
Once the ailerons and elevator are hinged and all parts are still flat its time to paint.
I use water based acrylic available from the bargain shops as well as Bunnings.
Thinned with water I use a Hobby King spray pen and off cuts of Depron or Foam Board as a stencil. On a hot day it will dry in minutes, but if you do make a mistake you can quickly wipe it off with a damp rag and try again. A good effect on the canopy is made by spraying the outer boundary darker and leaving the inner lighter, this works well with black as in above picture.
In the depron aircraft I cut out I provide two types of wheel pant profiles to use. One has a wheel profile at the bottom and the other none at all. With the wheel profile you can glue carbon tow around it to protect it as a skid.
With the other you can bend some thin piano wire, glue it in place and use it as skid.
A third option I have just found is to stick a thin strip of sticky soft velcro to slide on the indoor floors.
Always glue the rod between the undercarriage leg and the top of the pant, no matter how thin as it makes it much stronger. Also backing up the inside with carbon tissue helps too.
Most of the building is done upside down on a flat surface, preferably glass. Using a 90 degree set square glue the lower fuselage to the wing and tail.
1 , 0.7 or 0.5mm carbon rod or tube is used for the wing and fuselage bracing and 1.5 mm rod for the under carriage, Carbon fibre tissue can be used to strengthen the rod to Depron joins.
6K Carbon tow is glued around the bottom of the skids to form a make believe wheel. Its also good to reinforce the skid with carbon fibre tissue.
Next step is to turn the model upright and install the control gear.
With the underside completed we can now turn the aircraft over. Now time to install the servos.
For this model I use Dymond d47’s, 2 x d47″s” (red arms) for elevator rudder, and a ordinary D47 (white arms) for the extra load on the aileron. Tie Kevlar thread long enough to reach the control surfaces to both sides of the large servo arm.
Centre the servo and screw the arm on. Push the servo into position with the thread both sides. Glue the control horn that slots into the elevator followed by a short carbon rod that runs thought it. Slot the ends of this rod with a knife for the kevlar pull pull thread. The thread will stay in the rod while you adjust the surface to centre. Apply glue and whine the end of the thread around it once or twice.
For the aileron I use carbon and heat shrink push rods. With a dremel cut slots in the servo arm for 2cm carbon rods slightly angled forward. This is to make up for the main rods angled out to the surfaces. Glue these to the arm then centre and screw to the servo. Shrink the joins away from the model(Depron melts) leaving them slightly loose. Also be careful as the carbon rods can melt with enough heat. Temporarily centre the ailerons using sticky tape then place the servo, push rods and joins into position. Then just apply CA to the ends of the heat shrink.
Push the rudder servo into position with its arm and pull pull threads attached. Hang them through the outer tail-elevator hinge while the upper fuselage half is glued on. The rudder is then hinged and connected.
For the motor mount angled doublers are glued to the nose with thin 6mm wide plywood strips forming horizontal and vertical mounts.
For hinges I use the Mylar out of old 3.5 inch floppy disks. Just break them open and cut into strips. Slot the ailerons and elevator at the hinge points and glue the Mylar hinges in with HobbyKing foam safe CA. Mark the wing and tail hinge points, slot, test for maxim movement and glue together. You can now paint the top surfaces of the aircraft as they are still perfectly flat. Join both tail and wing together on a flat level surface ready for the main assembly.
To stiffen up the wing I use carbon fibre tow as light weight spars. You can pull some out of standard carbon fibre cloth, divide a length of 12k fibre or use the 6K flat ribbon tow I supply with my sheets . But to keep it light I only use Hobby king CA instead of epoxy. Fibre tow works the best as you have a flat ribbon of fibre for better bonding.
Start by gluing one end of the tow to the Depron. Once it is dry run a line of CA along the wing. Then pulling the carbon straight lay it into the CA. Mop up any excess CA with a tissue, but be quick as this stuff starts to dry in about 30 seconds because of the fibres. The trick is to spread the fibres out flat. If the carbon turns over or is pulled too tight, it will not happen.
Same again near the leading edge then turn it all over and do the same for the top. You now have a very stiff 3mm Depron part.
The next step when building the Index is to hinge the ailerons and elevator.