REVIEW OF THE ZAGI SI
GENERAL HINTS WHICH APPLY TO FLYING WINGS.
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On my "wish list" of gliders, to be able to fly at any site in any condition, was a decent flying wing. I say decent, because many of them fly, but some not really well. My research had discovered that the Zagi was commonly the benchmark against which other flying wings were judged.
There really is not much to them, just a wing, elevons and vertical wingtips, with no fuselage or tail. Generally they are designed for flying off the slope, supported by the updraft of air. Despite their apparent simplicity, they can fly brilliantly, be extremely stable and an absolute hoot. And, if combat is your game, they are ideal.
The Zagi SI has a wing span of 1200mm (47 inches) which allows it to do rolls particularly well. Wings much larger than this tend to wallow around the sky unless they are heavily loaded and are flown in a decent wind at high speed. Lighter models will fly in lesser lift and can catch lighter thermals, but because they don't have the momentum, they tend to stall when looping or if they are inadvertently given a little too much up elevator and they don't have the penetration to allow the high speed required for quick rolls.
Your popular size flying wing does not lend itself to launching off the field as a lighter and/or larger flying wing might, because they often out-run the chute and drop off the line.
Being extremely damage resistant, newer pilots can gain skills very quickly as a result of them being able to be put back into the air immediately after an unfortunate (crash) landing. Reducing the fear increases the fun and the ability to learn. In addition, they are easily transportable, don't require assembly at the site and don't suffer much from hanger rash (i.e. damage in transit or storage).
Flying wings are generally made of foam, either (a) polystyrene, (b) EPP, which is heavier but more resilient, or (c) a combination of the two, giving the best of both worlds. I was pleased that the ZAGI SI was in the third category, which meant it would be well weighted for a range of winds and that I could go heavy on the tape and not be anxious about it being too heavy, and I would have the additional option of adding ballast should I wish on those days when a gale was blowing.
Some models have wooden, fibreglass or carbon fibre (such as arrowshaft) leading edges - which are first to hit (and breakable) in a crash and generally unacceptable in combat. Others have spars in the wing, back from the leading edge. However it is now generally accepted that the appropriate use of tape is an easier way of going about things and the model will have sufficient rigidity to fly, and enough give to bounce back after a prang. (Note that if your wing flexes it will interfere with the movement on the elevons, if this occurs, a little extra reinforcement of the trailing edge will be required. I did not have this trouble with the Zagi using the my chosen covering technique described below.)
THE ZAGI SI KIT
The design originates in the USA however this kit is made in England by J Perkins Distribution Limited, under licence. The kit consisted of two polystyrene foam wings, two EPP foam leading edges, corflute or correx tips and elevons, rods, clevises and horns and a roll of coloured packing tape for covering.
Most polystyrene wings are hot-wire cut, as were all Zagis until relatively recently. The polystyrene wing sections for this kit were fully moulded, complete with holes for receiver, battery pack and two standard sized servos. There is even a gap at the front on one side (the opposite side to the battery pack hole) for ballast to trim the model for correct position of centre of gravity. All this really took away the opportunities for error.
PUTTING IT TOGETHER
The instructions are easy to follow and include photographs of the important steps, with total building time being about a day or less. In brief: sand off the moulding marks on the foam; remove the stringy residue off the wire cut EPP leading edges then glue the leading edges to the wing halves. I used five minute epoxy, but only mixed up enough to do one at a time, as five minutes disappears pretty quickly! The edges are to be held on with tape as the epoxy sets. This can be masking tape, packing tape or some of the coloured tape supplied with the kit. However I kept a close eye on it to ensure no misalignment occurred as it set, which would be very unforgiving.
One thing not pointed out on the instructions was to watch you didn't inadvertently glue the left leading edge on to the right wing and vice versa. Having recognised this, I double checked numerous times as I was gluing as I would have been pretty cross if I had to rectify the error.
With a third batch of five minute epoxy, I glued together the two wing halves and used tape to temporarily hold the halves together - again watching to ensure they did not fall out of alignment as the epoxy set.
A hint here - before mixing your epoxy, tear off plenty of strips of tape and stick them from the edge of the table ready for immediate use when required. This saves a lot of panic when things become sticky, the epoxy has started to set and everything seems to be happening at once. And after gluing, immediately pick away any dollops of epoxy before it gets too hard.
TOW HOOK PLATE
Although built for the slope, the instructions suggested the plane could be bungee launched, but gave no hint as to how the tow hook might be attached. On a piece of thin 25mm x 40mm ply I glued two smaller pieces, to give the tow hook something to be screwed into. I then dug an indentation into the underside of the foam and epoxied the ply into to the wing, almost flush, with the back about on the CG mark. I installed the tow hook after covering. The hook needs to be quite large as flying wings fall off tow hooks very easily.
Before covering, I dusted the wings then sprayed them with adhesive (adding only 5 grams to the weight). I know that tape (or Solar Film heat shrink covering or its equivalents) will stick better to foam if it is first given a quick spray of what used to be called 3M77, but is now sold in Australia as Multi Purpose Spray Adhesive in theGREEN spray can. (Do not use 3M Hobbyist and Trade spray as it may eat into your foam.) In a warm environment of 19 degrees Celsius (my new shed has heating!), drying took about thirty minutes before I started with the tape. The instructions make reference to using "bullet tape" to add strength before using the coloured tape supplied. This is in fact fibreglass reinforced adhesive packing or binding tape.
The choices then became: (a) just use the coloured tape, (b) use some fibreglass packing tape as reinforcing and then the coloured tape or (c) cover the whole thing with fibreglass packing tape and then cover it with the coloured tape. Note that the fibreglass will go off because of the UV from the sunlight if it is not covered with coloured tape. I chose option (b) so as to get a decent amount of strength but not weigh it down so much that it would not fly if the conditions were a bit marginal.
I put fibreglass tape on the trailing edges, folded over the top and bottom, then layers from the outer tip along the top of the trailing edge and CONTINUING ON ACROSS the other wing right through to the leading edge of the wing opposite. I did the same on the other wing and the underside. Then I did another two runs of the tape (both top and bottom), overlapping each previous run by about 4mm. Following this was a layer (or was it two) on the leading edge and another two across the nose, going to about 100mm either side. I figured I was going to be putting extra weight on the nose so I may as well do this with tape to provide strength before I started adding lead. Total weight of the fibreglass tape was 74 grams.
Next came the coloured tape provided in the kit, again starting at the back and working towards the front, overlapping by 3-4mm. The instructions suggested that in the absence of the fibreglass tape, the first line of tape extend right across to leading edge of the opposite wing, with remaining strips simply extending 50mm across the centreline. From discussions with other builders I established that I had a further choice here, to continue every strip of covering right through to the leading edge of the opposite wing. If you are not using the bullet tape I would recommend this as it would add considerable strength and most of the additional weight would be ahead of the centre of gravity.
I wanted the bottom to be a different colour from the top as it is very easy to become disorientated with a flying wing, confusing top with bottom. So I used the yellow tape supplied in the kit on the top, and some red tape at the bottom.
Hint - heavily coloured tape - (blue/red) may be subject to fading and can get to look real scruffy - so put this on the bottom of the wing, not the top.
Total weight of the coloured tape was 50 grams, so I could have easily added some strength with only marginal weight gain by continuing the tape across the wings rather than stopping in the middle.
These are made of corflute, also known as correx, which is that "plastic cardboard" that small real estate signs are sometimes made of. These also get covered with tape and taped on to the wing so as to be able to freely hinge up and down, but not so sloppy as to be flopping around.
The control surfaces are simply the two elevons at the back of the wing. ELEVONs are a mixture of ELEVator and ailerON. Both up gives up, both down gives down. To turn, one goes up and one goes down. To do this, mechanical or electronic mixing is required. Mechanical mixing can be achieved by having the "left/right" servo (which is attached to the elevons) being slid along rails or rocked or pivoted by a fixed "up/down" servo. The problem with this is that it requires a number of linkages (thus giving more room for slop in the controls) and the servos to be behind each other in the centre of the craft - weakening it structurally and requiring the second servo to be placed further back, and in consequence extra ballast is required in the nose. The better arrangement (and this is a requirement of the Zagi SI kit, since the servos are in a fixed position) is the use of electronic mixing, either by an electronic mixer placed between the receiver and servos, or, if you have it, the use of the electronic mixing function in your computer radio.
Electronic mixers come in two types - those with the ability to adjust the amount of up/down compared to left/right, and the cheaper ones that don't. You are likely to be disappointed if you go for the non-adjustable model as flying wings require a considerable amount of left/right but not too much up/down.
Computer radios with an inbuilt mixing facility usually also have the ability to adjust the travel/sensitivity of the respective functions, allowing both good up/down stability but plenty of left/right manoeuvrability. If you also put in some exponential into the left/right function, you can reduce some of the sensitivity of your turning in the middle ranges of the stick whilst still having the craft being able to roll real fast with full stick movement.
Sumarising; your basic 2 channel or multi channel radio is fine provided a quality electronic mixed is included, or if you have computer mixing on your transmitter then a separate on board electronic mixer is not required.
PUTTING IN THE GEAR
Diagonally cut the covering tape across the holes for the gear, and push the tape down into the holes. With a drill bit, I "drilled" holes from one gear cavity to the next so that the servo wires would not need to be mounted on the surface. I took the plugs off the servo wires, pushed the wires through the holes and then put the plugs back on, being careful to ensure they were in the right order. I was going to use double sided tape to hold the servos in but they were so snug in their "beds" that I didn't bother, relying on their snugness and the covering layer of tape to hold them in. The plan correctly suggest using standard weight servos. You could use lighter (more expensive) servos but you would then have to add extra lead to achieve the required centre of gravity position anyway. Before putting in the servos, I plugged them in and ensure that they were "centred", as you do not have the opportunity to adjust the arm on the spline after they are in place. And make sure the screws are nice and tight!
The hole for the battery pack is suitable for either a flat or square battery configuration. The receiver hole is also large and it is suggested that the aerial go down centre of the plane (and be covered with tape) and then thread down one of the flutes of one aileron. I did this and it worked well.
Control linkages are easy enough and clearly illustrated however I chose to have the z-bend at the servo end rather than at the tail end, as it is easy to stick holes in other planes in transit or storage. It is also easier to make adjustments to the clevises on the control horn end rather than at the servo end. I usually replace the horns and clevises provided in kits with something more solid however I was happy with the quality of what was provided and didn't change a thing.
More corflute / correx, this time a little lighter. Firstly I covered the open "flutes" with coloured tape and then, following the instructions, cut a slit in the corflute to slip some tape through to hold it on from the top, as well as the tape wrapped around the bottom. However I also put a small flathead nail through the corflute into the foam at the rear before putting on the bottom tape, so the fin wouldn't start flopping around and loosening the tape.
The centre of gravity is 200mm back from the point of the nose and the instructions suggested temporarily taping a pencil across the CG line and adding lead to the slot that is in the leading edge for that purpose, between the styrofoam and the EPP at the nose. This worked well and I ended up adding 90 grams to get it just right. Total weight of the plane came in at 656 grams. I was initially confused as the front page of the instructions suggested 450 grams and the back page 750 - 800 grams. However the manufacturer advised that the 450 grams is correct and that subsequent editions of the instructions had been rectified.
Although in theory I was some 200 grams above the recommended weight, I was not concerned as, for the reasons explained elsewhere in this review, I wanted a craft with plenty of penetration and momentum and I was within the weight parameters of similar wings flown by friends. Had I wanted a lighter craft, I would have put less fibreglass tape on and this would have also meant less lead required in the nose. If I had a second model I would make it light so as to be able to fly in really marginal conditions when this one wouldn't.
In order for the wing to fly at the correct "angle of attack", the bottom of the elevons need to be parallel with the underside of the wing, and this is explained in the text and photos in the instructions. This pointing upwards of the elevons is called "reflex". The instructions advised that "the elevon throw should be 10mm (measured 25 mm from the tip)". Now it wasn't clear to me whether that meant a total travel of 10mm or up 10mm and down 10mm, so I went for the latter.
To create the elevon mixing on my transmitter, I set it up under the "delta" mode. However I am aware that by giving full elevator and full turn, particularly if the trim happens to be adjusted away from centre, it is possible to turn your poor servos around to a point that they are not capable of achieving, thus causing them damage. So I reduced the travel settings in the transmitter for both elevator and rudder so that coincidentally giving both elevator and aileron would not cause damage. Had I not had sufficient travel in the elevon I would have moved the clevis down to a lower hole in the elevon control horn to compensate.
First flights were off the bungee, just because I was on the field, not the slope. As I mentioned, this model is not really designed for the bungee and flights were short, but it was fun and gave us the opportunity to test out the trim - which was found to have too much elevator. So I reduced this and it flew in a much more manageable fashion. Up of 8mm, down 8mm, left 16mm and right 16mm gave a good result.
First flights off the slope indicated that that the trim and balance were perfect. Rolls, loops and general hooning were easily executed and with a great deal of enjoyment. The craft is remarkably stable yet also remarkably responsive.
As Humhrey Bogart once said, this was the beginning of a beautiful friendship.
I had actually been aware to the Zagi's reputation for some time and when a trainee asked me what he should buy as his second glider (he already had a very docile 2 metre polyhedral balsa trainer), I had no hesitation in recommending the Zagi. I told him that it was a bit more expensive than some similar kits but all reports were that it was second to none.
I later watched him take the Zagi for its maiden flight, executing loops and rolls without any previous experience of doing so. When he landed he turned around with the biggest ever smile on his face - and knowing he had a craft that would be giving him enormous pleasure for a long time to come. Now I too have a Zagi - and an equally big smile!
They've got the equation right and I have no hesitation recommending the Zagi SI.
Make it any colour you choose with coloured tape. But note that some colours will fade.
The kit came with a single roll of coloured tape and all the hardware - but not radio servos, epoxy or spray adhesive.
Don't mix up the leading edges!!
For extra strength, cross at least a couple of widths of tape completely across to the other wing.
My red tape fades to an awful pink, so I used the yellow tape provided for the top.
I saved the red for the bottom - which is what I'll be seeing most of when it's in the air.
Method of launching.