The idea to build a small Side arm launch (SAL) or a Discus launch glider came from the need to replace my aging DreamFlight Alula. That got me looking round the web for a kit that was small and light, I found the Opteryx but that was no longer available. There where others but at a cost and shipping made it prohibitively expensive, so designing one was a better option.
There was some design criteria as I wanted I glider that could a slope soared and be discus Launched.
- Must be light,
- Small enough to leave in the car
- Needs to fly in very light lift conditions
- Fast to build
- No need for plans to build
- Obviously cheap.
So I sat down and started drawing, the design inspired by the Alula, another SAL and the Opteryx.
What I ended up with was kit that is Self aligning, uses less then a role of plastic heat seal covering (aka Solarfilm, Oracover, coverall). All up flying weight of the prototype is 109grm. Everything is modular and it interlocks and so no plan is need to build the glider.
Whats with the name - "Shark SFG"? Well, its pretty simple Small ******* Glider or Small Fun Glider, with a shark in the fin, because the fin was made to resemble a sharks dorsal fin. The nose from the side was shaped to resemble that of a shark.
- 1 X 1.5mm Balsawood sheet
- 2 X 3mm Balsawood sheet
- 1 X 5mm Balsawood sheet
- 1 X 900mm X 5mm carbon fiber tube
- 1 X 900mm X 3mm carbon fiber tube
- 1 X 180mm X 1.2mm X 3mm flat carbon fiber rod
- 1 X 1.2mm carbon fiber or FR4 sheet (115mm X 100mm)
- 1 X Energizer 9v battery pack
- 200mm X K&S Piano Wire 18 SWG / 1.2mm (for control horns)
- 200mm X 1.2mm or 1.5mm carbon fiber rod (for control linkage)
- 100mm X 1.5mm heat shrink tubing
- 2 X 1mm X 3mm diameter neodymium magnets
- Cyanoacrylate adhesive
- Razor plane (optional)
- Balsa Knife or Craft Knife (with snap off blades)
- 1.2mm, 1.5mm drill bit
- Cordless drill
- Sand paper (100gr, 220gr, 400gr)
- Sanding Block
- Hot Melt Glue gun
- Zona saw, Hacksaw or Blunt Balsa knife blade
- Heat sealing Iron or Household Clothing Iron
- Paper toweling
- Soft cloth
- Engineer square
- Soldering iron and Sol
- Access to CNC router or Laser cutter
- CNC Router requires a 0.8mm or 1mm Corn roll cutter for cutting the Balsa and Carbon Fiber sheet
- Laser will need enough watts to cut through the carbon fiber
NB: You can always build a CNC as I did but this will take a hole lot longer then ;-)
- 1 X 4 gram or lighter park flier Receiver
- 2 X 3.6 gram Servos or lighter 500grm torque is ample
And lastly about 2-3 hours to cut, assemble and cover
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Step 1: Drawing the Plans
This was done using AutoCAD and took about a week. Everything used in the glider is simple and available from any local hobby shop. The drawings where then laid out to fit the balsa wood sheets.
The Airfoil for the prototype is the AG03 from Mark Drela, in his description "Built-up and solid-balsa small HLGs; Built-up small electrics needing large speed range". it seems to suit the glider perfectly. I ended up scaling the plan to a larger version at 900mm wingspan versus the 750mm wingspan.
Attached are the CAD files, these can be used to cut the kit, if you have any amendments please fell free to make them but remember to give credit where its due. And this project is not for resale or profit
Step 2: Cutting the Kit
The Balsa cutting is a simple process it was done using a 1mm corn roll / fisheye cutter. These can be purchased on sites like AliExpress and Banggood for next to nothing and they really do a superb job. The Carbon Fiber sheet was cut using a 0.8mm cutter.
The cutting time will vary depending on your machine. Once the kit has been cut using a balsa knife cut the holding tabs and trim any parts should it be needed.
As a safety note please remember to wear a mask or respirator as there will be a particles in the air that can cause lung damage when using a CNC, this goes especially for the carbon fiber dust.
Step 3: Building the Fuselage
Preparing to build
We also need to focus on cutting some parts for the assembling the fuselage at this point while we are cutting stuff. Using a Zona saw, hacksaw or just rolling the 3mm carbon fiber tube on a balsa knife blade, cut length a 28mm length and 110mm. This will form the pins that will align the fuselage, secure the tail in place and align the wing.
DO NOT GLUE Anything until instructed else sanding and covering will become very difficult
Locate the tail, that is the elegant part with the small shark cut into it. Taking the 200mm length of carbon flat bar, measure it against the long slot in the tail and cut it to suit the lenght of the slot. Lightly sand this carbon with 220grain sandpaper and wipe it off to paper toweling. On a flat surface, push the carbon into the slot and CA glue it on place.
Push the 28mm and 110mm length tubes through the 3mm holes in the tail. The short on at the back and long one at the front. Locate 4 X 3mm sheet parts as picture and 2 X 1.5mm parts. first slide on the 1.5mm former and then the two 3mm parts, they are in descending at the trailing edge from 1.5mm being longest. Butt these up against the tail on either side as pictured.
Locate the two 3mm fuselage sides and slide one of them onto the tail section pins. We will now focus on the center section and the wing spar support. This is a series of 1.5mm and 3mm balsa rings and two supports. Using the 5mm Carbon rod, gently feed that though the fuselage side, first slide on the on support, then feed on the balsa rings the order of the rings is irrelevant as it is there to build up the thickness between the two supports. Then add the second support feeding it down the tube, now add the second fuselage side.at this point you should have a reasonably square and stiff fuselage with no nose.
You can at those point using a engineer square check for squareness and using CA tack glue the supports and rings in together, but not to the fuselage sides. DO NOT glue the carbon rod in place.
Assembling the nose
Once again the nose is made up of various parts, locate the 7 parts that make up the nose. make sure to line them up in the correct order, there is one central one with the deepest cut out and 3 that go on each side. This where you will be able to add nose weight (if required) and the battery.
Line up all the parts from longest to shortest on either side as pictured, tack glue them with CA. We can now mate this to the fuselage, note that the angled part is at the top of the fuselage, once you are happy for fit you can now tack this to the fuselage sides, and remove the carbon rod it should all stay together.
Fuselage to and bottom sheeting
There are three 3mm balsa rectangular sheets, this the kit, two are for the top and bottom of the fuselage, the third is the access hatch. They can only fit one position, start by adding the one butting up against the tail former's. You will note that this will push the wing support into the correct position and prevent the sheet going lower then it should sit on the side of the fuselage once you are happy with the fit, CA glue this former in place, and the supports in place. Repeat the some process for the top and bottom pieces.
Step 4: Shaping the Fuselage
Start by pushing out the two pins holding the tail in place, remove the tail. At this point you can use thin CA and glue everything on the fuselage together properly. Block sand the fuselage rear smooth using 220grain sandpaper, and then 400grain sand paper
Carve the nose
To carve the nose draw the rough shape with the pencil, and then roughly carve the nose to shape using a razor plane or craft knife. Then proceed to final shaping thing nose with 100grain sand paper. once happy with the shap smooth with first 220 and then 400grain sandpaper
Making the hatch
Find the hatch sheet and gently sand the 45 degree angle to make it fit, place it on the fuselage and sand the entire fuselage with 400grain sand paper. Remember the better you sand smooth now, the better the finish will be at the end.
Step 5: Building the Wing
After having cut the tabs and pushed out the parts. Lightly sand the parts as this will make it much easier at a later stage.
Trailing edge and ailerons:
This step is best performed on a flat surface. This section is comprised of 5mm sheet parts, press fit the trailing edge into the wing tip, this is drawn to be a snug fit. Make sure but taking the aileron and butting it up against the trailing edge, the Gap between the aileron and the wing tip should result in a parallel joint. Add a drop of CA to the joint.
From here we will first build one wing half, as we need to put the fuselage in place before we build the second wing half. I chose to start witht he right hand side wing.
The first step is to add the outer most wing rib (you can at this point do this for both wing tip ribs), the importance of this is that is will hold the carbon spar in place. Everything from this point will be square to this wing rib. As shown in the one image, I used a scrape piece of balsa to square it up and then CA glue it in place.
Taking the full length of 5mm carbon tube, cut the tube to 690mm long, this is to become the gliders main spar. Sand the entire spar lightly with 220grain sand paper, and slightly round the ends of the spar to prevent splintering, and lastly wipe it down with a piece of paper toweling. Gently fit this through the tip wing rib as we don't want to break the wing rib and push it all the way till it reaches the hole at the end. DO NOT go into the hole area. If you are worried you will just push the piece of 5mm carbon you cut off in to the hole, as this is used later for the launching peg.
Slide the wing ribs in the correct order onto the main spar. Once they are in place you can press fit them into their corresponding slot on the trailing edge. You can now use the same block of wood to square them up, you don't have to glue them in place yet, and it will make life a little easier. One the last two ribs you need to fit in the carbon servo tray. using 220grn sand paper lightly sand the tabs that will be pushed in to the wing ribs. This servo tray helps to square up the ribs and the wing. Press fit it in place, I have added an image to show how it will look when it's not on the wing spar.
For the Leading edge we use 3mm carbon tube or rod, cut two equal lengths of 315mm. Lightly sand the leading edge with 220grain sand paper and wipe it down with a piece of paper toweling. If you have noticed it's lsight longer, then it needs to be you are correct. This will extent into the side of the fuselage helps to correctly position the wing on the fuselage.
Mating up the Fuselage
At this point we need to slide the fuselage we assembled earlier to the wing. Slide the fuselage down the spar till the fuselage butts up against the root rib.Take the balsa knife and cut away the covering for a secure glue joint.
Also make sure to gentle slide in the carbon rear locator rod.once this is in place you can push the Leading edge carbon into the fuselage and then slide it to snug up against the slot in the wing tip. If you have down everything correct and not glue it you can now easily see how the wing is self aligning. Tack glue to leading edge to the wing rib. place it on a flat surface and glue it all together.
Since this is a prototype at this point it may be worth while mentioning that the image shows the prototype wing tip, and the one in the CAD drawings has a slight change. The leading carbon edge butts up against a 90degree stop. On the original I was planning to use full carbon but it would have results in an indentation on the wingtip.
REPEAT FOR THE OTHER WING HALF
Servo installation and cover:
push the servo lead into the fuselage, and using hot melt glue, glue the servo in place ensure the correct orientation of the servo and the horn. I would recommend plugging the servo's in to the receiver and centering them before installing them is the air frame as once they are installed they cannot be removed or re-centered without cutting.
Once you are happy use hot melt glue to install the servo then let it cool. There after we can install the servo cover, makes sure this piece of wood is sanded smooth with 440grain sand paper before installing the sheet.
You can now also go through the entire aircraft and glue each and every joint, be 100% sure not to miss anything.
Step 6: Sand and Cover the Tail and Fuselage
Focusing on the tail, round all the edges sand if smooth on a flat surface then cover the tails with your choice of colour. I chose transparent green to highlight the shark in the fin.
Cover the Fuselage, once again use your choice of iron covering. it's easier doing this now as well will cut the covering from the wing rib location later. cut out the holes win a balsa knife or a soldering iron
I chose to do it later for photo purposes.
Once it's all covered replace the tail in and insert the tail locator pins. No need to glue it we can do that later.
Step 7: Finishing Up
At this point you should have a pretty need complete air frame, there are only a few minor items left to complete.
Add DLG fittings
So unless you are only going to throw the glider off a cliff to slope soar, you will need to locate the U-shape parts there are 4, you will only need two for the top of the wing tips. Glue these in place at the end of the wing tips, these are designed to accommodate the carbon fiber peg holder. Because the spar sits more proud at top of the wings it's permissible to sand it down to that there is no bump when the 1mm carbon fiber part is glued in place. Also the Carbon plate must sit flush touching the carbon spar for strength, that it doesn't get crushed. The bottom one will sit flush. You can use the 5mm off cut carbon tube the line them all up. Repeat the process for both sides.
Attached the Hatch
The hatch which was sanded earlier gets a tongue in the front and a small plate at the back the tongue and plate are made from scrap. Using a 3mm drill bit gently drill holes in each surface and install the 3mm neodymium magnet. The hatch is drilled at the back to line up with the plate at the rear, install the plate at the correct position and CA in place.
Cover the wings
Covering the wings is probably the hardest thing about building the plane, because if you don't shrink it evenly you may end up warping the wings. Start from the root rib and tack it all around. When you get to the trailing edge, add the aileron and cover the hinge line with the aileron 45 degrees to the bottom, this is you hinge line. Once you have done this heat and push down the covering as you go with the soft cloth. Every modeler will have their own method so I won't elaborate any further. Repeat the covering process till all 4 sides are covered and sealed
Install the control rods and horns
The original control arms where to short and hence forth too sensitive these where updated to the kit and are shown, these are installed just behind the hinge line on the aileron, and CA'd in place.
The control rod are joined by a z-bend in the 1.2mm piano wire these are cut to about 25mm long, you will the hold the wire in place with a 1.5mm heat shrink tube. This works well and once you are happy and have flown the glider you can put a drop of CA on each side to keep it permanently anchored. You will need to repeat this 4 times.
Step 8: Final Steps Before Flying
Making the battery
Making the battery was a very interesting one because we need to disassemble a rechargeable Energizer 9V pack. Gently pry open the top end of the Energizer battery, then pull out the contents. What you will see is 7 beautiful round 1.2v 150mah cells. These fit perfectly into the aircraft. Cut away three and solder the positive and negative wires to the correct tabs. I wrapped mine with insulation tape, this helps to keep it snug in the nose too. And that's pretty much all there is too it.
Install the receiver
This is as, as plugging two servo leads, there is no switch in the plane, so the battery gets plugged directly into the receiver. Because of this there is no need to Velcro the receiver in place either.
Balance the aircraft
If you built the plane using everything I did, then the plane will balance perfectly, I would recommend always going for slightly nose heavy and this will make it more stable. the aircraft's balance point is just in front of the main spar.
It's super sensitive on the elevator if it's tail heavy, so make sure you get the balance right. My radio is on 50% exponential, limit control throws to a maximum of 8-9mm up and down. and work from there. Add some nose weight if you are unsure, the flying weight of mine is 130 grams and 210grm with lead Ballast.
Step 9: Does It Fly... Proof Is in the Pudding
To do the video, Find a stunning slope, beautiful view and enroll the help of your 6 year old son, and then get him to hold the camera while you fly. At some point after this we had kelp gulls flying with, and we couldn't have been more happy with the result.
Max the 6 year old photographer took us a video and did a sterling job. He never lost it once. It flies like a dream, there was some mods, these were all applied to the attached CAD files. It's quick too, flies in light wind, discus launch so far I have got to about 25 to 30m. It will take some practice due to the weight being so light, throw it to hard and you will back flip every time... we also discovered that you can omit the pin because the lightning hole on the wing tip is plenty to grip on to during lauch.
Thanks for reading and I really hope you build one and let me know how it goes. I am already working on a larger one with about a 950mm wing span, by request from my fellow modelers.
Second Prize in the
Make It Fly Challenge