Racing and Slope Planes

Birdworks Zipper

More Photos
Type: Flying plank
Wingspan: 48"
Wing Area: 260 sq in.
Airfoil:Modified EH 2-10
Construction: Fiberglass "taco shell" fuselage, ply sheeted foam core wings, balsa fin.

(I've updated the flying part of this review since I first wrote it. You can skip right to the new part here).

The Zipper is a 48" span tail-less sailplane, with a stubby fuselage, nearly straight wing, and vertical fin hanging off the back (this configuration is often referred to as a "plank" rather then a flying wing). The wing uses a modified EH 2-10 airfoil, with an area of 260 sq/in. The planform is a simple taper, with the leading edge sweeping slightly back and the trailing edge sweeping slightly forward.

Construction: The Zipper comes as a short kit, with only the fuselage and white foam wing cores supplied. The builder has to supply the 1/64" ply wing skins, hardwood leading edge material, balsa for the vertical fin, and ply for the rear wing mount and forward former/wing mount. The instructions that come with it are clear and easy to follow, and they include templates for the wood parts the builder needs to fabricate.

Fuselage: The fuselage is a "taco-shell" design, meaning that the upper part is not closed out, except for the bit of the nose forward of the canopy. The canopy, wing, and tail eventually cover the open parts of the fuse. The canopy is unusual in that it does not require any sort of latch to keep it attached. Instead, it has a flange molded around the bottom which keeps it in place on the fuse. To put the canopy on, you squeeze the sides together and push it in place. Underneath the canopy is a fairly roomy area for the battery, mini receiver and a couple of micro servos.

Zipper Zipper Zipper

The Zipper is a bit unusual in that the elevon servos are mounted in the forward fuselage, with push rods extending back to the elevon horns*. At the rear of the forward fuse is a ply former that includes a hole near the top which accepts the wing hold-in dowel. The mid-fuse is also roomy, and relatively empty (aside from the aforementioned push rods), making it easy to install ballast. At the rear of this part is a ply wing mount block, which gets drilled and tapped to accept a nylon screw. Behind that, the top surface of the fuselage slopes downward, providing an opening for the elevon control horns. Finally, it narrows quite a bit, providing a slot for an internal reinforcing spar and the vertical fin, which sits on top of the spar.

Wing: The stock wing is a white foam core sheeted with 1/64" ply. Since my intent was to use the plane for DS, I opted for some gray foam cores that Steve at Birdworks had available. To better withstand the rigors of DS, I added a 1/2" wide carbon strip to each side of the wing (at the thickest part). Each strip expends down about 3/4 of the span, on both top and bottom. Between these strips, near the center of the wing, is a vertical grain, 1/8" thick balsa piece, which extends about 6" from either side of the wing center. This acts as sort of a shear web between CF strips.

The instructions call for a hardwood (I used basswood) leading edge, which is supposed to be attached and sanded before the wing is sheeted. This is somewhat unusual, but it is supposed to result in a much tougher leading edge by preventing the LE from getting bashed back between the skins and into the foam core if it hits something hard. I was a bit nervous about sanding the leading edge to shape while it was attached to the bare foam, though, so I opted for the more traditional method of sheeting the wing first, then adding the LE.

Once sheeted, the ailerons get cut out of the leading edge. The exposed foam at the rear of the wing and the front of the elevons gets capped off with ply. Now, it was at this point that I noticed that the wings weren't completely straight. I don't have a vacuum bag setup, so I sheet my wings using the old fashion method of putting lots of weight on them. I have a couple book shelves that I use to sandwich the wings, but they are a bit old, and I didn't notice that they were a bit curved. Unfortunately, this curve carried over into the wing as a slight warp (more on that later). Getting on with it, the next step is to glue the wing halves together and reinforce the joint with glass. The wings are joined flat, with no dihedral. Finally, you drill the hole for the rear wing mounting screw, install the wood dowel in the leading edge, and attach the control horns to the elevons.

Radio gear: I used a couple Hitec HS-81 micro servos for the elevons, a Hitec 555 receiver, and a cube shaped 350mAh NiCad for power.

Final flying weight is 20oz. This gives a wing loading of 13oz/sq ft. Not exactly floater territory, but it's not too heavy either.

Zipper Zipper

Flying: The first flight occurred on a day with very good lift. The Zipper seemed to immediately take to it, despite having no ballast. It was fast, though I could see it could obviously be lots faster with some ballast. It can turn fairly tight, though it does suffer from the typical "nose-bob" problem that afflicts many flying planks: pull too hard on the elevator, and the nose will quickly get into an up-down oscillation which slow the plane way down (it will stall/spin if you hold the up elevator). You need to keep the elevator travel fairly low because of this. The roll rate was great, in fact the only plane I have that rolls faster is my TurboFly wingeron.

One thing that became quickly apparent is that they Zipper gets very small, very fast. I've flown other flying wings and planks before, so I'm used to the unusual visual cues you get from them versus a standard-tailed airplane, but the Zipper is even more extreme. The thin wing, pointy fuse and fast roll rate mean you have to keep a really sharp eye on it, especially while it's flying directly away from you. A bright paint job and contrasting stripes on either the top or bottom of the wing are almost a must.

Zipper Zipper
Stripes and bright colors are a must for good visibility!

As good as the first few flights were, I did notice some oddities. Sometimes, when rolled into a turn, the tail would slide way out, like it wanted to go into a skid. I almost looked like the vertical tail wasn't effective enough. It was also very apparent that control goes away fast when you slow it down a bit. Sometimes, while entering a tight turn, it would violently tip stall and go into a nearly vertical spiral dive. Other times, it would sort of rock back and forth from one wing to the other, at seemingly sufficient flying speeds. Still, overall the plane was well behaved.

Several months later...

UPDATE!: If you've read this review before, you know I had some problems sorting out the flying characteristics and quirks of the Zipper. Well, I've since been able to fly it on some days with good lift, and I think I've got this little bird figured out...

Light lift: It doesn't like it. Perhaps someone else could build the plane lighter and that would help, but this plane really wants good lift. The performance and stability just degrade too badly when the lift gets light, which can be an issue on inland slopes. At the medium sized hill (around 200 feet high) I fly at, I'd say it would take a consistent 15+ mph wind to fly well. If the lift is really cycling and the wind drops to less then that, you are going to find the plane struggling to stay airborne.

Handling oddities: Even in good lift the strange "proverse yaw" behaivor still occurred when anything but a small amount of aileron was applied. The problem is magnified at low speeds. I finally got fed up with this and tried a HUGE amount of reverse differential. I set up my radio so that the ailerons move down 3 times as much as they move up. It sounds ridiculous, but it worked! The plane now behaves in turns in a much more normal fashion...quite a bit smoother then it was with zero differential. In general, the nose tracks through the turn in what appears to be a coordinated manner. At high aileron rates, though, this setup goes from just canceling out proverse yaw to creating adverse yaw, so rolls can be ugly. Still, I think I can probably reduce the amount of reverse differential I'm using and find a happy medium.

High speed: A lot of people tend to think that a tailless airplane like the Zipper would be hand full at high speeds. It just looks like that it should be very unstable in pitch. In fact, its very stable and easy to fly in a high speed dive. I must confess that I still haven't ballasted it up yet. Conditions haven't been consistently good enough to give me confidence to do this. Still, I think it should work very well in high-wind, high-ballast conditions.

Dynamic soaring: Well, I've seen video of Zippers, or Zipper-like airplanes doing the DS thing really well. While my normal DS spot is no Cape Blanco or Parker Mountain, it's reasonably good, with speeds of 120 mph attainable by composite planes in average conditions (as I write this, the local record is 154 mph). I assumed the Zipper would work pretty well, but so far that hasn't been the case. On one hand, if you try and do a big circle, the plane will just gradually start losing speed, and you'll have stop after only 5 or 6 circuits. On the other hand, you really can't do a small circle, because pulling too hard on the elevator just causes you to lose speed, and you definitely can't yank it through a tight turn on the top and bottom of the hates that! It also seems to get really disturbed when it goes through the boundary layer, to the point that it can really get slowed down quite a bit.

The only thing I can figure is that the plane really wants to make a big circle, but just doesn't have the momentum to get through. My guess is that it needs a combination of big wind and lots of ballast to have the momentum to go through a large DS circle and generate a lot of speed. Time will tell on this one.

Conclusion: The Zipper is a fun, if a bit odd, little airplane. It's compact, fast, looks cool, and definitely offers something different!

*Although the manufacturer suggests that mounting the servos flat in the wing will weaken the wing (because it's very thin), I've heard from a few people (including a well known DS/plank expert) that say it works just fine.

(Another version of this review can be found on LiftZone).


The Zipper is now flying in paradise!