What is a Discus Launch Glider (DLG) ?
A DLG is in comparison to other glider types a rare breed mostly due to the build materials or cost. A normal glider like thermal, soaring, tow line and bungee all use wind or a line to get airborne while a DLG is launched by spinning around and releasing the glider.
Named directly after the sport type Discus where you hold a disc at arms length, run then spin to release with maximum velocity possible. This type of gliding allows you to reach some very impressive heights without needing a line of any type, also because of the flight characteristics which is generally very calm and gentle you can can catch the model. Once the technique is perfected you can stand in 1 spot > spin to launch > fly > catch > spin to launch and so on.
I have to tip my hat to Yasu from RCgroups.com for giving me guidance on this very unique glider. His models are amazing with some weighing less than 27 grams!
On with the build!
Used on my DLG:
- Frsky TFR4 FASST 4 channel Rx Bought from giantcod.co.uk
- Nine Eagles 120mAh <> 150mAh 3.7v 1 cell lipo batteries.
- HK S0361 3.6g / .45kg / .12sec Micro Servo Bought from rc-addict.nl
- Competition balsa, 1000MM * 900MM * 6MM used for wings. Your looking for a VERY fine wood grain with a very light colour.
- Competition balsa, 1000MM * 900MM * 2MM used for tail pieces.
- Epoxy the thinnest you can buy.
- UHU Por glue
- CA glue
- Carbon fibre plate: 50MM * 2MM * 1000MM
- Carbon fibre rod: 1000MM * 2MM diameter.
- Heat-shrink pipe in various diameters.
Competition balsa in Europe just isn't as light as in Asia/USA. The wings untrimmed weighed in at 22 grams, after trimming and sanding profile I got it down to 12 grams.
Each wing perfectly weighing 6 grams. The root CORD is 65MM.
The tail fin is 90MM * 40MM I just freehand drew out the shape. The horizontal tail area is 120MM * 36MM again hand drawn shape. Elevator is 16MM deep while the rudder is 18MM deep.
Tail boom is 4MM carbon fibre rod which is 430MM long. Wings, tail and rod total 19 grams prior to coatings.
The tail gets 1 coat of very thin epoxy while the wings need 2 coats more towards the trailing edge to strengthen the thin balsa areas. In order to save time and be able to coat both sides of each piece I tested using tack pins inverted to keep the pieces of the surface.
First coat of epoxy applied, you can see how the normal very light grain of competition balsa gets darkened as a result which I like.
Many people spray or paint over the finished wing, instead I mixed some red acrylic paint into the epoxy before applying second coat on the wings. First I did a quick rub over with light red then added a little more red for a 2nd pass on the trailing edge. No paint needed and will be visible in the air.
The wings gained 4 grams for the duel epoxy coats which equates to 1 gram per coat per wing!
On to electronics as mentioned above the electronics list is all for lightness. I removed the FrSky receiver case which dropped 2 grams. The total at this stage was 42 grams.
The tail areas are secured in place with very thin glass material using CA glue. The horizontal tail is raised 6MM from the rod for down elevator throw movement.
I'm using good old fashioned medical tape for hinges again after it worked so well on the Crazy-3D project.
Wings finally dried (each epoxy use requires 12-24 hours to dry), wings have an exact 600MM wingspan.
Wings are joined using the same thin glass across the top center of the wing using epoxy this time to fix. Note the nose has a 15MM excess.
This is to mount 90MM carbon fibre plates using CA glue at first. I trimmed the screw mounts from each servo side to make them sit together.
The opposite side carbon fibre plate will require a balsa piece trimmed to width of servo so each side has a connection to the center rod.
I used heat-shrink pipe to encase the RC receiver which I then glued used UHU Por glue so it has a rubber mount in case of shock impact. See how the wood block at beginning of the carbon fibre plate makes up the gap.
The other side of the receiver I used sponge packed in to provide yet more shock resistance.
My first problem arises, most DLG people remove the servo pins from the receiver and solder direct but as this is a prototype I don't want to have to resolder later.
You can see how the Ariel leads run out of the top towards the wing.
The constructed weight at this point was 45 grams, up 3 grams from last weigh test.
To prevent having to modify the receiver I tested an idea, I popped the contact sleeves out of the servo lead plastic plugs (the black plug on the end). I then bent the contacts 90 degrees on the thin center area, you can see how much shorter it is this way.
The servo arms are cut down to the shortest possible length. The rods are carbon fibre (2MM) with a piece of heat-shrink pipe connecting metal arm ends. The rods weight didn't register on my whole gram scale so it's erm.. light lol.
Adding in support rod area's up the fuselage boom will help keep tension in the control line, not doing this will convert servo movement into the rod just bending instead of moving your control surface.
The 1C 120/150mAh 3.7 Lipo battery on a custom loom using the same trick as the servos. The loom including battery connection weighed 1 gram.
I had tested a different set-up with the battery beside the servos but it required 14 grams in lead to get COG correct, this pushed my build weight over 60 grams. Instead I added an extension plate on the nose which then only required 3 grams lead.
Another view, can see how thin it is :) I also added some sponge between the carbon fibre plate extension and where the battery slots to help guide the battery into place.
The whole nose needs covering, but what to use. Many people use carbon fibre or glass constructed pods. Others like Yasu use heat-shrink pipe over a balsa mould heated until hard which is pretty clever. I went for the Yasu technique but instead used a broom stick end! It's worth noting I first coated the broom stick with soap from a bar rubbed on to help release the heat-shrink pipe. This is the back-end view of it.
The front is left open for the battery to go in, also ensures the battery cannot come out/free while in flight. The servo tops come through the tube so only the arms are visible. This is just a case of test placing the pipe in place and marking where they are.
The final result is 55 grams, note the throwing peg on the wing tip, that's 2MM carbon fibre same as push rods using carbon fibre strand CA glued in place to provide strength. Centre of Gravity (COG) is 28MM in from leading edge back.
Video will go live with a review project when the weather is good enough, it's been fog for almost 2 weeks now... Anyone got a giant fan I can borrow lol
600MM balsa carbon fibre - Discus Launch Glider aka DLG. 55 grams of gliding fun
