Big thanks to www.FrSky-RC.com
for providing the Receiver unit for this project :)
This is my 6th attempt to make a DLG (Discus Launch Glider). The first 4 versions simply didn't work, the 5th version did fly lots of times but it's performance wasn't the greatest. With each new version I change elements of the design as I learn, the last time I learnt my wings where to narrow and didn't provide enough surface area. Also using a regular KE profile wasn't so good but also the thickness being just 5MM on version 5.
So in version 6 I'm stepping up the size of lots of things, ignore the tail configuration in the diagram, however all sizes are correct:
This is 1.5MM plywood with template for a fuselage marked out, fuselages will vary pending your own electronics.
This is the center plate for the whole fuselage section, servos slot cut out next to where the carbon fibre boom will go.
To create a round fuselage shape I opted to make ribs out of the same 1.5MM plywood.
Now all glued in place and sanded flush.
Testing out how well the CF rod sits in place.
The servo leads come through a hole in the center bottom rib, so testing out its easy to get in and out before adding anything more!
To create the round fuselage shape I soaked some 1MM thick balsa wood in hot soapy water submerged. I use dish washing liquid to help relax the fibres. Only needs 30 minutes or so and then its very pliable.
To create the nose I added some blocks of balsa in place and simply sanded it down to shape, use a sharp knife first to slice away chunks then I suggest a 60 grit sandpaper to start really bringing down the excess. Use a 500 + grit for finishing.
The CF rod now glued in place.
More re-enforcements on the bottom of the fuselage along the center and also glass fibre covering the back over the CF rod and back rib, I use CA glue to bond it. Remember to ventilate.
Quick check in on the scales shows my fuselage with full CF rod at 28 grams which I was happy with.
Now you can see the balsa wrap on the fuselage with a cut out top to act as a canopy cover / access.
The tail sections use 1.5MM balsa also, very thin.
To save more weight I cut out circles in the wood, you will see this on other models - why circles? Because there is no single stress point (no corners).
The wings and tail fin will all be mounted on and through the CF fuselage boom, so I use electrical tape to mark out positions. Here you can see the wing drill hole points.
The tail will slot into the rod and must align with the wing bolts and fuselage! This is why you use tape!
The CF rod needed cut to create the slot for the tail fin, at the beginning of the cut I use carbon fibre tow wrapped around then CA glue bonded, this will prevent the cut putting stress on the rod and splitting.
Once covered the tail pieces where put in place, the hinges are made from fibreglass tape which I also covered with the film to hide it while being strong.
Can see I used a 8MM high balsa block to raise the tail section, a piece of glass fibre cloth is wrapped under the bottom and to the other side. Once set with CA glue the balsa block is very securely attached to the CF rod.
The fuselage now being covered a nice blue theme starts to take shape.
Now with the rudder and elevator servos fitted I got to see how easy it was to thread the wires through to find it wasn't so bad.
The main wing was not so straight forward. Balsa sheets here and most places come in 1 meter by 10CM by the thickness you want, in this case 6MM. So to make a 14.4CM solid wing I had to bond a strip of 2nd sheet to the 1st.
Once done I marked out the bottom wing sand point, this will create the under camber shape of the wing which reduces weight and increases lift / glide potential. Look at the piece of paper in the photo for exact placement measurements.
A wing tip can be as fancy or as basic as you want but I opted for a nice lead off angle only slightly rounded. The measurements again are in the photo on the piece of paper.
I start by sanding down the leading edge followed directly by sanding down the trailing edge to just thicker than I want it in the end. I then sand out a basic KE profile shape, for the tips I sand them down further.
The bottom starting from the marked line I sand out on the line itself about 3MM deep then begin sanding each side out in a rounded shape, this creates the under camber shape.
Just testing how the wings sit side by side.
To give the wings strength I use epoxy in a bowl and use a sponge to apply the epoxy. I do this much like how you would wax and polish a car, using a moderate amount of epoxy and rubbing it on in circles to make sure parts of the wood grain don't get missed.
Once the entire area is covered squeeze the sponge of any epoxy and use it to wipe off excess on the wing, this time run the sponge from the tip of the wing down to the end following the wood grain, then you won't rip off little bits of sponge. You can see once epoxied the wood grain really stands out and you can now see the camber shape on the bottom.
Once the epoxy is dry use the same fine sandpaper 600+ to sand it smooth as you can. I wanted dihedral in this wing so I put the left wing flat and raised the right wing on the tip by 2CM and bonded the halves back together.
Because the wing will be removable it needs to be quite strong in the center. This is the same 1.5MM plywood used on the fuselage. With the drill holes in the same positions as on the carbon fibre boom naturally.
I just used some scrap CF rod to check the wing sat in the right position lol, it did :)
To fix the wing for flight it has to be pretty solid, to do this I used 4/3MM aluminium pipe which at these small lengths are very strong.
To stop the rods twisting and to stop the CF rod from snapping I use CF tow to bind the pipes in place and then used CA glue to bond the whole thing.
To fit the wing support plates I used a full coat of CA glue then pinned them down until fully dry. It's all wood on wood bonding so once the CA is dry it's very strong and the whole wing will become more rigid.
Now the wing is in place by slotting on the aluminium rods. Only 3M bolts are left to simply keep it down, the bolts should not be stopping side / twisting forces only to prevent the wing lifting upwards.
The fuselage now all rigged up with 1.5MM carbon fibre rod control lines to the tail.
Control horns fitted in place and trimmed down to the smallest possible size. You can also see the tail glass fibre cloth better in this photo.
Finally the receiver is mounted in the bottom of the fuselage after being stripped down itself and servo lines soldered directly to the RX so no servo plugs. The 1S lipo however does use a servo plug from other systems I use, you can also see I re-enforced the canopy cover piece with glass fibre cloth inside otherwise it'd be pretty weak at just 1MM thick.
The AUW (All up weight - flying weight) is 185 grams / 6.52 ounces
The wing has a surface area of 1300 cm sq / 1.40 sq ft / 201.15 inches sq.
Wing load ability (weight/area):
6.52 oz / 1.40 sq ft = 4.67 oz / sq.ft
for other Europeans like me:
Converting square CM into square decimeter ( 1 > 0.01)
1300 cm.sq = 13 dm2
185g / 13 dm2 = 14.2g / dm2
Part 2 will show the maiden flight when the weather gets better and also any tweaks I made if any between now and then.