zipper [ziper] n zip; that which moves with energy vigour.

I first became aware of the existence of the Birdworks Zipper whilst watching the excellent Lift Ticket video from Reeseproductions.com. Transfixed by the power of Dynamic Soaring, I was suddenly confronted by this diminutive whirling dervish, shrieking like a Banshee around the circuit at the back of the hill. It was difficult to determine the lines of the model as it sped up across the boundary layer. The caption beneath merely hinting that I was viewing a "Zipper". The noise it made as it passed the microphone and the sensational roll rate being demonstrated by the able pilot proved more than sufficient to prompt an online session which turned up the Birdworks company web address in Oregon USA (http://www.harborside.com/~birdworks/bwhome.htm). There she lay. The Zipper. It Zips!… say's the advert. Well we'll have to see about that!

Long time flying buddy Julian Beckett had noticed the Zipper too, largely for the same reasons and it wasn't long before E mails were being exchanged and deals struck to get two of these semi kits across the Atlantic and into the UK.
We took the decision early on to order the 48" span version, going for compact performance over aspect ratio, although there are some awesome pictures of 60" versions on the website. The 48" sub kit is advertised at $57.60 plus shipping ($67.60 for the bigger one) with a note that shipping overseas would be more costly. When we had finalised the price I dispatched my share to Julian to cover the International Money Order. The kits arrived well packaged within nine days of placing the order. Importing from the USA to the UK almost always carries some kind of penalty and HM customs and excise had got wind of the contents and charged us a whack for the privilege of flying something a little different on the UK slopes. The final cost to each of us was now £57. Thanks very much!

Don't let the term 'sub kit' put you off. The philosophy of the Zipper build is that you can completely finish it using one sheet of 4' x 1' x 1/64" Birch Plywood available from any good model shop, plus a few scraps from your bit box. However, an ARTF model is most certainly is not. The ply is basically used for skinning the wings and control surfaces. The Fuselage and Canopy are decent epoxy glass mouldings and very, very strong. They are certainly well up to the job required of them, if a little pin-holed on the surface. The self latching canopy mechanism is ingenious relying on the accuracy of the mouldings to work correctly. By simply squeezing the sides of the canopy you can install or remove it. Much better than mucking around with catches or magnets. The fuselage has trim lines very clearly marked into the moulding and simply requires a little sanding back to get the correct profile. The remainder of the build is restricted to the various bracing's and reinforcements. Spruce strips for the leading edges and fin base, a little scrap 3/16" ply for the former and wing bolt plate, some odd bits of scrap balsa for the fin, and a little miscellaneous hardware, are all you need to finish the job at little further cost.

The kit basically consists of the Fuselage, Canopy and the low density white foam wing cores plus a detailed eight page instruction booklet which also doubles as a plan for the formers, servo tray and fin which are reproduced at full size. In addition it contains a listing of all wood and sundry components required to complete the kit. It is true that some internet reviews of the kit have damned the poor (undersized) fit of the fuselage formers, but this helps to prevent distortion of the fuz to ensure that the canopy remains a good tight fit.
The wing cores themselves deserve a special mention, they are after all the bulk of the model. The cores are intricately cut to a slightly weird looking EH 2-10 (modified) section. This is basically very thin, semi-symmetrical and reflexed upwards at the trailing edge which produces an unusual bulge on the underside about two thirds of the way towards the rear. The thin trailing edge of the wing requires a little care in handling if it is to survive undamaged at the skinning stage. The EH 2-10 is well proven and extensive Zipper testing with modern DS aerofoils has reinforced this with the designer eventually deciding to keep it unchanged.

Construction of the model starts with the wings and the application of the spruce leading edges. The use of thick UFO (User Friendly Odourless) Cyano is promoted for this application although epoxy would do just as well. This cyano (and most other odourless types) are foam safe and non aggressive. However you would do well to check yours on a piece of the wing bed first. There is some question over the necessity to install the spruce leading edge first, as it would have to be shaped before the wing skin was added. Birdworks recommend overlapping the wing skin onto the leading edge to improve the overall strength of the wing, but as I was to glass cloth the finished wings anyway I chose to add the leading edges after the skins had been put into place. I don't often like to stray from the manufacturers instructions but the build of this particular kit somewhat relies on a good knowledge of construction methods and some considerable forward planning on the part of the builder. An example of this is the need to consider at this early stage, any centre section reinforcement you think you may require if you intend to really show the model a rough time in the air. Anything from uni-directional carbon 'spars' to reinforced wing joiners would all be possible although nothing more than a wing bandage is referred to.

Application of the skins to the foam panels utilises a method new to me whereby the skins are attached using a strong, double-sided sticky tape. Whilst sounding a little improvised, in practice it does work very well. It is quick and easy to do with no mess or warping of the skin whatsoever. You could if you like use colloidal silica thickened wing skinning epoxy or even contact adhesive to do the same job, but build time is considerably shorter and less messy utilising the suggested method. Be aware though that any adhesive you do use should be 'non foam aggressive' or your beautiful little wing cores may melt away before your very eyes.

With the panels cut to size and the core covered in B&Q heavy duty carpet tape, the skin is gently rolled onto the wing whilst it is supported on a flat surface in its original shuck. Success however is wholly dependant on the absence of dust, and cleaning both the core and the skin with a tacky cloth before application of the tape helps tremendously. All that then remains is to apply pressure (and lots of it!), to the wings overnight as they rest in the core beds on a flat surface. This ensures that the skins are well and truly immovable and that the wing sets straight. After that I added the spruce strips to the front and planed them to shape. In the event I ended up skinning Julians wings as well but this time I used thickened epoxy. Whilst this was much more time consuming and considerably messier, it did seem to produce a stiffer wing overall.

The trailing edge of the two skins is now joined where they meet at the back of the wing and the whole thing then given a final sanding to shape. Simple balsa block wing tips are added and the control surfaces are removed from the wing to be faced with 1/64" ply along with the 'new' wing trailing edge. The foam faces are reinforced a little with epoxy fillets formed by the melting away of some of the foam just under the wing skins in advance of the ply facing. I used an old soldering iron to melt back the foam and these epoxy fillets really do add quite a bit of strength to the back of the wing. The improvements in the rigidity of the elevons is marked. Once sanded smooth the wings can be joined flat and inverted in their core beds and the centre section reinforced. I made up a simple joiner from plywood and uni-directional carbon fibre sheet which would looks less unsightly under a glass cloth finish than a piece of bandage around the centre.
Construction of the wing is now completed save for the addition of the leading edge dowel, a hole for the wing bolt, covering or painting and hinging of the elevons. The plan shows a 3mm wing dowel and a small 4mm wing bolt but with bad experiences on a similar model I chose to upgrade these to a 5mm carbon dowel and a 5mm nylon retaining bolt.

Whilst film covering would be more than adequate to finish the model, I was going for the glass cloth finish. I eventually used 50g cloth with the weave at 45º to the chord to improve the overall torsional rigidity and then a further layer of 20g cloth at 90º over the top of that. After a further weave filling coat and a rubbing down, a final coat or two of Halfords Primer/Filler cut back with 1500 wet and dry saw the wing and Elevons ready for the trip to the paint shop.
The construction of the fuselage is simply a fitting out exercise. The formers are made up and epoxied into position with the various other bracing pieces, wing bolt mounting plate and servo tray. The fin was made up from hard 3/16" balsa and a 8" long 3/16" square spruce strip added to its base which also serves to strengthen the rear of the fuselage and after shaping and glassing it was simply epoxied in place square to the mounted wing. After fairing in and sanding the fuz was primered and cut back slightly again with the 1500 sheet ready for the top coat.

One trip to the paintshop later and my Zipper looked resplendent in its iridescent candy yellow 2 pack epoxy finish and with the application of natty silicon hinging, black 'carbon effect' trim and some Julian Beckett Originals 'Zipper' decals, it had a lovely 'moulded' look about it.
With Hitec HS81MG servos installed in the fuz, a JR R700 rx, a 600mah nimh pack in the nose, the control throws and CG set as per instructions (only 3mm of up elevator!) and last but not least, the lateral balance checked, the finished all up weight was 24oz. Bang on the designers target weight. Not at all bad for 2 layers of glass cloth a beefy wing joiner and multiple layers of paint. With its 260in² wing area the final wing loading had now sneaked under the recommended optimum of 14oz/ft².

First test flights were done out of desperation rather than common sense. You know how it is when you have a new model ready to go. 10mph up a small hill in poor lift just doesn't cut it with the Zipper. Most of the flight was spent trying to stay above both eye level and stalling speed. Whilst it would be stupid to claim this as a success it was sufficient to prove three things. The Zipper is no floater; it covers ground very quickly even in light lift and it makes the most awesome howling ripping sound heard since the cross wing fighters of Star Wars.

The Zippers next outing and test flight proper came a few days later at the Yorkshire Slope Pylon Racing event. With 46mph blowing up Levisham hill, the sun blazing down and cameras at the ready the Zipper was given a hefty chuck into the azure.
The reflex on the elevons is diagrammatically depicted within the instruction sheets. This had been set accordingly but proved to be slightly more than necessary as the model shot up and over my head and the first few seconds of the flight were performed looking straight up at the underside of the model. Easily overcome however by a gentle push on the stick to bring the model into the lift at the front of the slope. With the trim adjustment made the Zipper started to cover ground really quite quickly. The airframe is very clean and penetration into the high wind effortless. A quick hands off check and a small adjustment to the aileron had it settled and ready for trial.

Velocity is easily varied with the utilisation of elevator trim. The incipient stall being characterised by bobbing of the nose. A typical 'plank' characteristic easily remedied by a click of down. Showing no tendencies to drop a wing the model merely porpoises along as if sniffing for new lift to get it shifting again. As the speed increases the reflexed surfaces tend to promote a zoom and the requirement for constant trim variations across the speed range never departs. The minute control throws required to elicit a massive response indicate the pureness of the flying machine. It is easy to fall into the trap of setting the model up with much more throw than required. 3mm of up elevator and 6mm of aileron throw looks ever so boring on the bench but are really all that is necessary for the first try at least.

With any new model I always try the flat flying foremost. Flying lazy eight circuits in front of the hill the Zipper demonstrated little variation through the upwind and downwind portions. Smooth turns and a quite accurate aileron response encouraged me to come a little closer for the cameraman. I did find however that despite the use of full down trim I still had to hold the nose down as the model got faster. The feel of the model was also a little off putting as I am a fan of exponential but appeared this time to have used too much. I tried the roll which was a little too slow for my taste but smooth and easily controllable. After a few half pipes and Farnborough turns for the camera the trim adjustments were beginning to annoy me so I set it up for an uneventful landing on the nice flat slope top. The adjustments made and the servos re-centred to avoid differential throw I tried again. Much better this time. The speed was much improved with the re-setting of the elevator trim. I had increased the rates a little and dropped the exponential to only 15% and immediately it felt like a different model. Responsive but not twitchy. Time to see what the Zipper could really do. I gained height for a speed run and some aerobatics. The height gain had the little model getting extremely small very quickly and a mental note was made to not get it too far away again. The nose was pushed forward and the rate of acceleration was at first very impressive. Then it became excellent. Then it became alarming. It was just about at this point that the grin started to spread across my face from ear to ear. Smoothly pulling back on the stick the model shot skywards again into a nice big round loop of around 200feet diameter. A full stick roll out of the bottom at top speed had the model whip round in a fraction of a second, and that noise! I tried a few pylon turns and found that I could really crank it round. No tip stalling or degradation in speed whatsoever. Exploring the potential of the model further indicated its energy retention through successive evolutions to be exemplary, and this without ballast.

The flight continued in this vein for around 15 minutes and with a buzz matched only by a double espresso it was time to head back to terra firma where the onlookers had a new found interest in the model.

The next session was on my favourite local hill with a 25mph South Westerly forecast. I love this hill. It's my panacea. Worth every minute of the trip just to lay in the heather and speck a model out before screaming earthbound in a long dive with the inevitable aerobatics sortie until the energy expires. That wasn't on the itinerary today though. Low, fast and furious was the plan.
This was the third and best slope that the Zipper had been tried on and I was not to be disappointed. The lift was ballistic and it was possible to punch the Zipper over 200 feet vertically for a predictable stall turn before screaming back along the slopeface for the return punt. Flying big cuban eights and endless loops and bunts I now had the model really well sorted. By this time a little negative exponential had been introduced to prevent proverse yaw through the turns and the CG had been moved back another millimetre (yes it did make a difference). Attempting my landing however proved the Zippers first flaw. I simply couldn't slow it down enough to hit the tiny spot that is the landing zone. Successive circuits saw me going further and further back behind the hill just to deplete the kinetics. After some time I found a line I could use and the model skidded to a stop almost sideways. I was well satisfied with my new toy.

So? I ask myself. Is the Zipper a paragon of slopeside aeromodelling? Well, overall I think it could well be. I enjoyed the build tremendously. It awoke many aeromodelling skills that I'd forgotten I had in these days of ARTF and EPP, and I found the whole affair very rewarding. The Zipper is by no means a novices model in either its building or flying aspects. Moreover the slope soaring hot ship that Birdworks appertain it to be. Fast (occasionally very, very) smooth, easy to fly, well behaved, if a little difficult to slow up for landing, certainly an unusual head turner, nippy, and at 48" span compact with it. It is however, no floater. Stick with its knitting and put it in the car when the wind gets up.

The designers at Birdworks have recently been undergoing extensive testing on many aspects of the Zipper. They have explored alternative wing sections and heavier ply for the wing skins (resulting in the need for a larger fin to increase yaw damping). Servos installations have been tried in the wings but are felt to degrade performance and introduce failure points under high g loading. The main result however is that they have chosen to leave the Zipper exactly as is. Proof enough that this is already an optimised machine.
So does it Zip like the ad said?
Let's have us a race and I'll show you!

Andy Ellison.