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Zagi 5C Build

Posted by Tom Benedict on 22/04/2013

While waiting for the new ESC for the Raptor, and while I figure out what I want to do about ventilation to keep the motor cool, I built a new plane. Friday while I was at work, my Zagi 5C flying wing arrived from Trick RC.

Trick RC makes a number of flying wings, some powered some unpowered. The 5C is an all-purpose unpowered glider that’s really better suited to slope soaring than riding thermals, but there are people who have adapted their 5C to thermalling as well. My plan was to build it as a slope soarer, but to be as careful as I could about weight along the way. You can always add weight in the form of ballast to give a plane better penetration in higher slope winds. But it’s a lot harder to take weight out.

The Zagi 5C comes with an excellent instruction manual, which is also available in PDF form on the Trick RC web site. Trick RC updates the manual as new building techniques come along, so it’s pretty current. For example, the original Zagi was reinforced with fiber strapping tape. The Zagi that arrived at my house came with flat carbon fiber spars that are glued into the wing for reinforcement. As new glues have been developed, the instructions have been changed to reflect best practice there as well. Even so, I found a couple of build logs that provided information that wasn’t in the manual that came with my plane. The build posted by Mokulele808 to the RCGroups forum is the one I based a lot of my build off of.

The only thing I really changed from Mokulele808’s build is how I covered the wings. For that I took inspiration from surfimp’s build of the Swiss Fish, also on RCGroups. surfimp, aka Steve Lange, the designer of the Le Fish aerobatic glider, used laminating film to cover the wings rather than the tape covering used in the Zagi manual, or the covering that Mokulele808 used. (As as side note, Mokulele808 mentions CP laminate covering at the tail end of the build, and said this is probably the route they’d go if they built another one. See? I really did read all the instructions before beginning!)

Friday night I glued the wings together using 3m Super 77 adhesive, as per the instructions both in the manual and in Mokulele808’s build. Super 77 never completely hardens, so it’ll flex with the foam without cracking. Once the wings had set, I installed the carbon fiber spars. This is where Mokulele808’s build branches off from the build in the manual. The build in the manual uses Super 77 to install the spars, but Mokulele808 used Gorilla Glue, a polyurethane adhesive. This provides a stiffer connection between carbon and foam, and results in a more rigid structure.

Polyurethane adhesives foam up when applied. Gorilla Glue is no exception. In both Mokulele808’s build and surfimp’s build of the Swiss Fish, they leave the spars uncovered to let the glue foam up and out of the slot. Once the glue has set, it can be trimmed back with a sharp blade. I took a different approach, and taped over the glue line with blue painter’s tape. This let me lay the wing in the wing beds to keep them from twisting as the glue cured. After an overnight cure the wing came out quite straight and amazingly stiff. The painter’s tape came off with no problems, leaving a relatively flat glue joint behind. A little sanding, and the wing was ready for the next step.

When I ordered the kit, I was under the assumption that two servos came with the plane. I was wrong, of course, and was left with the question of what to do. Because of my kite aerial photography habit of the past six years (six years?! (yes, six years!)) I have a stash of full-sized servos. I’ve really come to like the metal geared Hitec servos I started using on my DSLR KAP rig, but they’re heavy, and for this they really are overkill. Rather than weigh the plane down with these, I opted to install two Futaba S3003 servos instead. These are plastic geared, they’re relatively light, and because I’ve been swapping out my KAP servos for the Hitecs, I had several of the S3003 servos that were looking for a new home.

The Zagi 5C, like most flying wings, embeds the servos in the wing during the build. They do this by having a die-cut “cookie” of foam where the servo goes. You press the servo into the die cut slot, and the cookie pushes out the bottom of the wing. Once the servo is flush with the top of the wing, the cookie is cut flush with the bottom. Voila, two perfectly depthed servo pockets!

The kit came with fiber strapping tape, which is still used to reinforce the trailing edge of the wings and the nose, even with the carbon fiber spars. I opted to use biaxial fiber tape instead. This shows up under a couple of different names: “biax”, “crossweave”, or in the case of the brand I’m using, 3M Extreme Shipping Tape. In appearance, it’s very similar to a 3D mylar sail for a racing boat. It’s wider than the supplied tape, so I took a weight penalty in using it. But with the CP laminate wing covering rather than a tape covering, I made most of that weight penalty back.

The manual suggests cutting a recess in the electronics bay to house the excess servo wire. I really don’t like excess wire hanging around, though. It’s unnecessary weight that’s easy enough to remove if the servo leads are built to length to begin with. I do this with my KAP rigs, and did this on my Raptor as well. The servo leads on the Zagi only need to be a couple of inches long. Rather than wad the wires up at the back of the electronics bay, I cut them to length and re-terminated them. (Actually, I’m getting short on connectors so I unsoldered them from the servo, cut them to length, and soldered them back on. No need to crimp on new connectors that way.)

After I had the basic wing put together and had the trailing edge reinforced, I decided to add some color to the plane so I would be able to tell its orientation from the ground. I bordered the top of the wing in orange tape, and the bottom in black. The whole wing was then covered in 3 mil CP laminate film. To do that I wrapped the laminate film around the leading edge, and trimmed it at the trailing edge. This left me with a really clean leading edge that had no seams. I used the same color scheme on the control surfaces, so the entire wing is bordered in color. It really is easy to tell if the wing is right side up or upside down.

My zeal for lightweight building wound up biting me in the rear in the end, though. The build weight for the wing is 17 ounces, battery installed. Mine came in considerably lighter than that. Unfortunately, the geometry of the wing means that most of the wing area is behind the center of gravity. I lightweighted the covering on the wing, so it unbalanced things and made it tail heavy. In the end I had to add 36g of ballast to the nose of the plane in order to get the CG right. So much for lightweight builds! I still came in under weight, but not by much.

The real treat was when I took it out Sunday evening for its maiden flight. Before taking it out I double-checked the CG, checked the control throws (1/2″ up, 1/2″ down, just like in the book), checked the placement of the antennas, etc. When I finally tossed it out for the first time, all the care and attention paid off. Its first flight wasn’t long, maybe thirty feet or so. But it flew hands-off without any tendency to pitch up, dive, or roll. It flew beautifully.

Zagi 5C

Now I can’t wait to take it out for a real flight!

– Tom


One Response to “Zagi 5C Build”

  1. Jason said

    I love the idea of one color tape on top and another color tape on the bottom. It look great AND purposeful…I will pass this along to our customers.

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