F3K is “a multitasking contest where RC gliders must be hand-launched and accomplish specific tasks.” Which means that these airplanes must be super light weight to be able to “absorb” the lift of rising air and really stiff to cope with the violent launches. These models are commonly known as DLG’s, which is short for “discus launch glider”, due to their unique launching style. One wingtip is equipped with a “launch peg” which is gripped with two fingers. To launch the airplane you spin one revolution around your own axis and throw the DLG, just like a discus (hence the name). A skilled pilot can launch a DLG to an altitude of 70 meters and then stay up indefinitely if there is enough thermal. This airframe I’m about to build is called the “A2” and is made by two very skilled F3K pilots that that just happens to be friends of mine.
The kit. An oval carbon fuse and glassed wing.
2.4Ghz compatible kevlar pod.
Drilled holes for the nuts that’s going to hold the wing on.
Made a slot for the ballast compartment.
3mm plywood to get some surface for the nuts.
Glued the nuts in place as well as the ballast chamber.
Drilled holes in the wing. This was a bit tricky to get perfectly straight and true but if you’re careful and take your time it shouldn’t be any trouble.
Cut a slot in the boom for the tail fin.
Glued the tail fin on using CA. Be very careful to get it mounted straight.
Mounted the control horn for the elevator. Be sure to use foam safe CA.
Hmm the boom needs a slot for the control horn.
Tailplane glued on. Make sure it’s straight to the wing.
Time to cut the ailerons. I used a exacto knife. I was extremely careful only to cut trough the top layer of glassing.
I then used a hacksaw to cut all the way through on both sides of the wing root and the tips of the ailerons.
Beveled the the hinge using sandpaper.
The tip of the ailerons doesn’t extend all the way to the wingtip. This is to get rid of unwanted vortexes created by the sharp edges ailerons usually have.
I used a dremel to machine out the servo bays.
Easy as pie.
Added carbon fiber control horns.
Servos test fitted.
Servos glued in place and covered using clear packing tape.
Servo cables comes out on the bottom of the wing.
Made a hole in the fuselage for the cables.
Time to mount the elevator servo. I first glued it to the balsa piece supplied in the kit.
Glued into the fuselage.
Linkage done and hooked up. It’s made from a super light weight string not unlike dental floss and it’s the strongest sting I’ve ever come across.
One Hitec optima 7 that needs to loose some weight.
Since I’m going to use 2S LiPo to power my DLG, I needed a voltage regulator. I choose to build my own using a L4940V5 low voltage dropout regulator and a couple of capacitors. This regulator is rated at 1.5A continuous with a drop out of 450mV.
To be able to use as short cables as possible, I mounted the regulator on the side of the receiver. I’m powering the receiver through the “SPC” port which means that I get the voltage of the LiPo sent back to the transmitter. That way I always know exactly how much power I have left in the onboard battery.
Receiver done. I also exchanged the heavy boda antenna for a light weight whip.
Time for the lipo to get a work over. Original weight: 32 grams
New weight: 28 grams. Removed the long and heavy power cable, unnecessary padding and tape.
Jammed into the nose.
Throwing peg mounted and the edge of the wing rounded to prevent cutting my fingers open.
Canopy “latch” system – a 1 mm carbon rod glued in the middle.
Simply slide one end of the carbon fiber rod underneath the edge of the fuselage and slide it back until the front end of the rod can be pushed under the front edge of the fuselage. Then slide the canopy in place.
I wasn’t quite happy with the first battery as I couldn’t get it far into the nose to get the CG right. So I decided to make another one out of loose cells.
I paralleled the cells two and two and then took those cells and put them in series, making a 2S2P 480mAh battery.
Size of the first battery: 54*31*10mm
Size of the second battery: 28*21*20mm
Weight: 22 grams.
I had to exchange the tail fin I originally mounted for a lighter one in order to get the CG a bit forward.
I also moved the tail servo forward a bit. Since the new battery is smaller I can still get it out without any trouble.
Despite all this I had to add a couple of grams of lead to the nose to get it properly balanced.
Time to fly!
Up into the sky!
Here is a short demonstration video:
The elevator came loose just as I was throwing. Ooopppss.
I bought a new carbon fiber wing from a friend that crashed his DLG and broke his fuselage.
Time to fly.
Flies like a dream.
This is still my favorite line of site plane of all time.
Here is another video filmed just for fun: