Scratch Designed/Built Sailplane
UPDATE: This build is now complete. The following pages include every step of the build, including the design, construction & maiden flight. I had a lot of help from the community here & want to thank those that offered advice. I think you'll find this an entertaining & educational read. Enjoy...
Hey everyone. This is build log for a scratch-built, scratch-designed, gull-winged, V-tailed, electric powered sailplane.
A friend gave me an electric glider balsa kit to build (thanks Ned). Not being one for leaving well enough alone I immediately started thinking of mods. A rounded fuselage would be nice, maybe a V-tail, a fancy high-performance airfoil & so on & so on. By the time I was done thinking up mods I realized I couldn't use the kit anymore & returned it. She'd be 100% scratch built, awesome!
So here's my initial design roughed out in CAD...
Wing Span: 108" (2750mm)
Airfoil: SD 7023
Motor: Turnigy G15
Propeller: APC 13" x 7" folding
Battery: 3S 1800-2200mAh LiPo
Channels: Aileron, Elevator, Rudder, Flaps, Throttle (with the crow, flaps down/aileron up thing)
Construction: Balsa, Spruce & some Carbon Fiber
I chose a subtle gull-winged design mainly because I love the classic look but also to help with ground clearance for the big flaps. For the same reasons a V-tail was also chosen. My main flying field is mowed grass but I like the idea of being able to take this "off-road" & land safely in weedy, long grass. If it was a foamy job I wouldn't worry but this bird being mainly balsa will be a tad fragile. Will be nice to have all the weaker bits up out of the way & let the fuselage take all the landing abuse. Besides gull-wings & V-tails are just plane cool.
Although the construction will be old-school built-up balsa I'll be using some modern tech as in carbon fiber here & there. For example the tail-boom will be a combination of old-school stick-balsa construction surrounding a CF tube. At least that's the plan.
Anyway enough babble, here's the build...
Check out my YouTube videos: Nodd RC
The fuselage "pod" is made from big chunky 1/2" balsa planks. Yeap, 1/2 inch...
My dumbbell finally came in handy, can't remember the last time I exercised with it...
To get her to thermaling altitude I chose a Turnigy G15 motor turning a 13" x 7" folding propeller.
The motor mount is epoxied in position with 4° of down-thrust. As the nose is pretty narrow I chose not to add any side-thrust. If necessary I'll mix a little right rudder to the throttle channel...
Sheeting is added to the bottom of the fuselage...
Before I close off access to the rear the carbon fiber tail boom is well glued into the structure...
With that done I complete the bottom sheeting...
A total of four layers of 1/8" balsa were laminated together. Again that's a 1/2" thick!..
Some of you may be wondering what the heck is this guy doing using 1/2" balsa to make a glider fuselage? Well here's why I chose such thick lumber...
Most of the wood is sanded away to eventually produce a nice rounded cross-section...
This is why I love working with balsa, its a real joy to shape. (For those who might ask, that's a 4 meter Discus sailplane in the background but that's for another blog)...
Time to make a canopy. After making a paper template I slip a two liter soda bottle over the nose & make it snug with some scrap wood...
I love this part, a few passes with a heat-gun & the bottle shrink-wraps perfectly around the fuselage...
Next, using the paper template as a guide, I gut away the canopy area...
Now she's starting to look like a glider. I laminate two sheets of wood together to form the base for the canopy...
Trim to fit, a coat of paint & some scrap wood to help align the canopy in the fuselage...
Gluing the soda bottle canopy to its base...
I wanted the fuselage to transition smoothly into the carbon fiber tail-boom. Falling back to my old-school stick-balsa fuselage days, I started working on the transition area...
Once covered this should look pretty good & who knows, might even add a little structural strength to the CF tail-boom...
Fuselage is more or less complete. It came out so-so lightish but I can always hollow out the inside more if I want to shed a few more grams. Love how that spinner is looking...
UPDATE: The following wing design failed during one of the early test flights. Although there's still plenty of useful & interesting info here, I'd advise skipping ahead to post #14 unless you're interested in how not to build a wing.
Time to make some wings. I'm no expert when it comes to airfoils so I did some research online. Taking advice from several sailplane forums I ended up concluding this puppy was the ticket...
Using my CAD software I printed then contact-cemented the rib shapes onto my wood. I used a scroll-saw to rough cut each then went to the belt sander to finalize the shape...
Next I printed up the wing center section plans & began assembly...
As this is not a flat bottomed airfoil I added tabs to the ribs to help keep them at the correct AOA. They will be sanded off once the basic frame-up is complete...
Seeing as I went to all the trouble of choosing a fancy airfoil I thought sheeting would be a good idea. Saggy covering can do terrible things to a wing's shape, especially on the top leading edge area...
As much as I like this SD 7023 airfoil, it is one thin SOB. There's not a lot of room in there for joinery so once again I opted for some CF reinforcement. I also made these end ribs from plywood to help support the CF rods...
The two center sections of my gull-wing will be glued together. The outboard sections will be joined using CF rods indexing these brass tubes. Again with the thin airfoil thing, I was limited in how much dihedral could be built in & still use rods to connect the sections. Best I could do was 7° dihedral in the middle & 4° down where the outboard gull-wing sections join. As this will be an aileron ship so I'm not too worried about built-in roll stability or rudder only turns anyway...
Once again with the thinness issue I'm a little worried about structural strength. For the inboard sections at lease I chose to fully sheet the top. I'm debating whether the bottom could benefit from sheeting too. I don't want to end up building a flying brick either, weight is always concern...
The center sections are clamped & glued together, now we're getting somewhere!..
Servo wire is run for the ailerons & flaps...
The outer wing panels. Again I printed plans from my CAD design, taped the 8½" x 11" pages together & started to lay things out (gotta love computers)...
Nothing too special here, more of the same, glue the ribs to the spruce main spars using their tabs to keep everything aligned...
Well that's all I have for now, will post more as I go. Comments & advice would be appreciated...
Nice work! Looking forward to follow the progress.
I want to learn from you =)
Glad you're enjoying this build.
Someone in a mirror post on another forum expressed some concerns with my carbon fiber wing rods. Specifically he didn't like that they weren't mounted between the main spars. He felt my current setup that splits the wing loads between two CF rods then transfers that to the spars through multiple ribs, wouldn't work. He said he'd never seen that setup & that it'd likely fail. I'm not convinced this is the case but he had me sufficiently paranoid about wing failure that I opted to add some extra structure that'd tie at least one of the rods to the main spars directly. To do this I added spruce shear webbing to the front of the spars where the forward wing rod is, cut a slot in that to fit the rod then epoxy everything together. I'm pretty happy with this additional reinforcement & kinda glad this guy gave me some grief about it. Here's how that looks...
I also added spruce shear webbing around the center wing section wing rod tubes...
With the wing joints beefed-up I got back to building the wing. Here's the top D-tube sheeting on an outboard wing panel waiting for the glue to dry...
Will have more to show soon.
Lovely build and an interesting design.
I have to say I agree with the guy who commented on the joiners. Ideally they should be in between the spars for max strength. At the very least I would put in some spruce or vertical grain balsa infill between the joiner and the sheeting. It all really depends on what stresses you're going to put the wing under.
The one thing I would have done though would have been brass tubes in both panels of the wing. Gluing to joiner in is mistake I made only once. If you break one of those joiners it will be major surgery to replace it.
I put the three wing panels together today so I could admire the wing as one piece, looked pretty awesome. When I set her down on my workbench I noticed she tipped to one side. I kinda expected this, one wing panel is slightly heavier than the other. I still had some sheeting to do so I figured this was a good time to see if I could equal things up a little. I mixed & matched my balsa stock until I found a combo that was close in weight...
I could have just added weight to the lighter wing but using slightly lighter balsa on the heavy panel achieve the same result without adding additional weight. I'll likely need to do this again once everything is complete but at least the wing is fairly close for now, give or take a few grams.
More reinforcing, I added gussets to the rear of the ribs in the non sheeted areas...
Just before the last of the sheeting went on I ran a length of string through the ribs so that I can later fish my servo wires though...
Outer panel sheeting is complete, woohoo!...
That's all for today, time now to watch some telly.
very nice,you'l have time for tely later lets get to work,,,lol
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