I've always had a soft spot for the Zivko Edge. With a couple of successful smaller prototypes already under my belt I wanted to raise the bar a little, to see how far I could take a few sheets of foam and a little carbon fibre... Yes folks, were going Big Time!
By keeping the construction fairly basic, the airframe can be completed within just a few days using a combination of 3, 5 and 9mm EPP (Expanded Polypropylene), a few lengths of carbon fibre and off-the-shelf canopy and undercarriage assemblies. An inexpensive and straightforward build for anyone with a little building experience.
For those unfamiliar with EPP, note that not all foam is created equal; similarly specified sheets can vary in density and thickness. To minimise errors during the build, try and physically match the material of paired components (e.g. fuselage sides) wherever possible. Another little tip: For parts such as bulkheads and decking, cut slightly oversize and trim to ensure a snug fit. Don't be tempted to fill gaps with glue, as youll end up with a heavy (and weak) airframe. Okay, lets get cracking!
AT THE CORE
The first task is to cut the wing core. Take a length of 5mm EPP sheet and, using a straight edge, accurately cut the wing shape shown on the plan. Next, using UHU Por, glue a strip of 3mm x 0.5mm (or 3mm x 1mm) carbon along the leading and trailing edges. Don't be tempted to butt-join carbon offcuts, as this will significantly reduce the overall strength and rigidity of the wing.
The removable wings require both locating tubes and anti-rotation tubes to be fitted, to accept the inner tubes that pass through the fuselage. Three T-Rex 450 helicopter tail booms (Align part no HZ018 T) are ideal for the outer tubes, these being one for each wing and one for the fuselage. A 25mm length cut from two of the tubes (the end without the slot) form the anti-rotation tubes. Cut slots in the wing as indicated and glue these tubes in position, noting that the main ones are inset by 1 from the wing root. Spars are fitted next, these being 15mm wide strips of 9mm EPP sheet, reinforced with carbon strip glued to the side face of the foam. Glue the spars in position and leave to dry on a flat surface.
The wings are held against the airframe using Velcro cable ties, i.e. those with a belt-type D-loop at one end. Position and glue the straps on the underside of the wing core as indicated on the plan, leaving an adequate length of Velcro to overlap its opposing strap. Once dry, cut and fit the ribs as shown.
The wing now needs to be skinned, using a single piece of 3mm EPP sheet. Loosely wrap the foam around the wing, cut slightly oversize to leave an overlap and then carefully glue into position pinning as you go. Note that starting at the l.e. and smoothing the foam out gradually towards the t.e. prevents the wing from warping. Fit the tips once the wing has dried, then carefully trim away the excess foam at the t.e. and sand the tips to a nice, rounded profile. The overhanging foam at the root can be roughly trimmed at this stage but Id advise leaving the fine trimming until later in the build, when the wings can be squared up against the fuselage.
Chamfer the bottom edge of the ailerons along the hinge line to give adequate clearance for large control surface throws, then glue a length of 3 x 0.5mm (or 3 x 1mm) carbon fibre strip along the opposite (trailing) edge. For hinging, place the wing and aileron on a flat surface, leaving a 3mm gap between the two. Ensuring both surfaces are right side up, run a length of 12mm wide 3M Blenderm tape (see Fly Electric on page 60 - ed.) along the full span of the hinge line, joining the two surfaces together, and press firmly into position. Fold the aileron back onto the wing so both upper surfaces are touching and apply another length of tape to the exposed hinge line underneath the wing. If the tape refuses to stick to the foam, run a thin bead of Por onto the foam surface beforehand. Ensure there's sufficient control surface movement in both directions and that the tape is free from creases or air bubbles.
To complete to wing fixings we need to make the inner wing tube and anti-rotation tube that slide inside the locating tubes. Here, then, take a 1m length of 10mm O/D carbon tube, with a 1mm wall thickness, and cut a 750mm length for the main spar. The remaining 250mm is used for the ant-rotation tube. Once cut, lightly chamfer the tube ends to prevent them from splintering.
The fuselage sides are cut from 9mm EPP. If you have to butt-join two sheets together, make the joint at the pseudo cowl line; besides being heavily reinforced in this area, any imperfections in the join will simply look like the fuselage has a detachable cowl. Clever, eh? Next, cut 12mm wide fillets from 9mm foam and inset them 9mm around the inside edge of the fuselage sides as indicated on the plan. Cut the (9mm) central former and glue a length of 3 x 1mm carbon strip on the top side along the centreline. For the prototypes radial-mounted Hacker motor I made a firewall from 1/8 ply, glued directly to the 9mm EPP bulkhead B1, which is then attached to the front of the central former and reinforced with a few EPP fillets. The tray to retain the Li-Po can either be from carbon fibre sheet or liteply.
Carefully pin the fuselage sides to the central former and glue in place, checking for alignment. Cut a 4 (100mm) and 6 (150mm) length from the remaining heli tail boom for the wing locating tubes. The 4 rear anti-rotation tube should fit virtually flush to the fuselage sides, whilst the 6 main tube should protrude approximately 1 either side. Tack-glue the tubes in position and test fit the wings (not forgetting to slide the inner wing tube and anti-rotation tube in first) to ensure the distance between the fuselage tubes is exactly the same as those in the wings. When happy, glue the locating tubes firmly in position, then add EPP fuselage doublers around the immediate area and leave to dry. Next, remove the wings and then cut slots in the fuselage sides for the Velcro wing straps and aileron servo cables.
Moving to the front of the fuselage, secure the motor to its mount and attach the ESC beneath the central former. Cut a small slot in said former, feed the ESC battery wires into the Li-Po bay and the Rx lead towards the radio compartment.
Sheet the top of the fuselage up to the front of the canopy and then, working around the lower part of the fuselage, carefully sheet the bottom of the cowl, bending the EPP into a gradual arc. Double up the foam in the u/c area and sandwich a few strips of filament tape between the sheets to prevent them from ripping in the event of a heavy landing. Fit the foam nose piece, leaving a gap around the motor to ensure sufficient airflow, then carefully contour the corners of the nose with a sharp knife, finishing off to shape with a 400-grit sanding block.
Fashion a small air intake in the bottom of the cowl and a couple of small exit holes just in front of the u/c mount to let air through the ESC bay. Cut the remaining section of carbon tail boom into a couple of small exhaust stacks and push them into the exit holes. Open out the Li-Po bay hatch in the top of the fuselage and secure in position with either Velcro or some small neodymium magnets. Likewise, form a hatch in the bottom of the fuselage just behind the u/c plate to gain access inside the fuselage for the wing fixings and aileron servo connection.
Glue B5 to the centre former, chamfering the bottom edge to ensure correct alignment with the rear of the canopy line, then trim and fit the remaining formers except for B4. Carefully bend the fuselage sides around the bulkhead to form the turtle deck and test fit the foam fillets in the top opening. Trim and chamfer the fillets as required until the approximate shape of the turtle deck is achieved, then glue in position. Trim and sand into shape, removing just enough material to give a nice contour to match the canopy line.
Tail surfaces are from 9mm EPP sheet, the lower edge of the elevator hinge line and both sides of the rudder hinge line being chamfered to ensure adequate control surface movement. Add carbon strips to the fin and stabiliser trailing edges and reinforce the narrow centre-section of the elevator with additional strips of carbon or liteply / fibreglass. Hinge the tail feathers and check for sufficient movement in both directions. Slit the rear fuselage, then slot the tailplane in position checking for correct alignment against the inner wing tubes. Add the fin / rudder, this time using a set square to ensure alignment.
My search for a suitable canopy ended with Vortex (www.vortex-vacforms.co.uk), whose Tucano canopy (part no. CN32) can be made to work with very little modification. In truth you simply fit the thinner tapered end of the canopy to the front, initially cut oversize, then trim to suit. As the turtle deck bulkhead is slightly narrower than the canopy, a useful tip is to gently heat the plastic with a hairdryer and glue B4 into the canopy. This will ensure that the moulding has the correct profile for test fitting and is easier to position for final cutting.
The prototypes dashboard is a Photoshop tweak of a dashboard image found on the internet, printed onto a sheet of card and laminated in clear plastic. Adding a cockpit floor makes provision for a 1/5 scale pilot - my guy is from Extreme Flight (available from www.freestyle-rc.com). With the cockpit decked out, place the canopy assembly into position and glue B4 to B5. When set, run a bead of canopy glue around edge of the canopy to finish.
The u/c tends to be a weak point on foamies; too stiff and it might rip from the fuselage, too flexible and it will bottom out. A bit of Teutonic engineering from Multiplex saved the day for the Edge, in the shape of the u/c from an Acromaster (Multiplex pt. no. 224206). This consists of a tough plastic mounting plate, pre-formed piano wire legs, spats, wheels and all the fixings and fittings, for just £10! To reduce the chance of ripping the fuselage, glue a few strips of cross weave filament tape (packing tape with embedded fibre stands) around the u/c area. Cut a small slot through the tape and into the foam for the plate locating tabs, and glue the mounting plate in place using Por.
The lightweight airframe means that superb performance can be found using a relatively small motor (in my case a Hacker A30-16M). Turning a 12 x 6 prop and fuelled by a FlightPower EvoLite 3s 2100mAh Li-Po via a Turnigy Plush 40 ESC. This combination generates around 350W (around 160 watts / lb) of power! Although the Edge is relatively light it has fairly large control surfaces, so I decided to use Hitec HS65MG (metal geared) servos all round.
The aileron servos go in first, these mounted in holes cut into the bottom of the wing outer skin (take care not to cut into the central wing core). Note that the servos are mounted mid-span, to reduce control surface flex. Don't be tempted to move the servos nearer the fuselage, as this will seriously affect the models handling. Carefully feed the servo wire inside the wing and pass it through a slot in the root rib. To make life easier, make a wire fish to pull the cable through and prevent it getting caught inside the wing. Glue each servo flat to the wing core and seal any gaps between the servo case and wing outer skin with a bead of glue. Attaching a lightweight control horn and push-rod completes the job.
Rudder and elevator servos reside in a 9mm EPP tray, mini snakes connecting them to the control surfaces via lightweight horns. The Rx (Spektrum AR6100 in my case) can be positioned just below the canopy area, secured with a little glue and filament tape. Connect all the radio, and when happy that everything works okay, you can box in the fuselage base. To reduce tail flex, the rear base section of foam has a carbon strip glued to its inner surface that locates into a slot in the bottom of the fin. After the initial flying shots were taken I added doublers to the base of the fin (see plan) to increase the stiffness of the tail. Attach the tail wheel to the underside of the fuselage using a mini Dubro bracket, ensuring adequate ground clearance for the rudder. Assemble the model and adjust the Li-Po position until the C of G falls around 75 - 90mm back from the l.e. - ideal for general sport flying and aerobatics, although you may want to move this slightly rearward for 3D.
BIG UN AWAY
The day of the maiden flight saw perfect flying conditions, but with our divot filled flying field being less than suitable for a conventional take-off, a hand-launch was needed. Throttled up and way with a gentle underarm push, the Edge pulled near vertically into the sky - the Hacker was clearly more than adequate! A decent altitude was reached in seconds, at which point I throttled back and flew a few trimming circuits. Here, a few of clicks of aileron had her tracking as straight as a die. Checking out the stall, she just gradually lost altitude with virtually no wing rock or nasty habits. Flipping her inverted resulted in a slight climb (usually indicative of the C of G being too far back). Upping the revs once again, the Edge whipped up a fair bit of pace at which point the EPP surfaces began to flex. This is quite normal although, clearly, a little throttle management pays dividends to get the best from her.
Tipped on her side, knife-edge felt very locked in, needing just a hint of rudder to keep the nose up. Slow axial rolls and four-pointers looked positively elegant, a touch of rudder keeping things on an even keel. Upright and inverted spins proved very controllable, the flat spin needing a trace of aileron to keep things level while the big rudder kept the back end revolving at a brisk rate of knots. Snap rolls are quite impressive; with the tail stepping out nicely and immediately pulling back into line on command, it really will snap on a sixpence!
Throttled back for landing, the Edge slowed up well, and steadily lost altitude before a quick flare brought her home. Mission accomplished!
CLOSE TO THE EDGE
With many wringing-out sessions under its belt, the Edge has proven to be a cracking flyer. Its large size presents well in the air, and its slower flying speed and forgiving nature instil the confidence to throw it around the sky. If the worst should happen and your ambition outweighs your ability, most minor dings and scrapes can be fixed at the field with glue and TLC. Go on, have a go!
To get away from the stark foam look, add a dash of spray applied colour and finish off with a few home-made decals. Most acrylic paints should be fine on EPP, and RCStyro spray cans (from BRC Hobbies or Robotbirds) give a great finish for around £3.50 each. For the more ambitious or artistically inclined, those big flat surfaces make an ideal canvas for a spray gun (see RCM&E April 2008 issue for some great pointers). Use a few light coats to keep the weight down and for a nice, even finish. Print a few decals using waterslide decal paper, glued in position with 3m77 spray adhesive and sealed around the edges with RCStyro clear gloss spray.
Sport / 3D aerobat
495 sq. in.
35 - 40oz (1 - 1.2kg)
10 - 12oz / sq. ft.
Hacker A30-16M brushless motor / 2100mAh 3s Li-Po / Turnigy Plush 40 ESC
Aileron (2); elevator (1); throttle (via ESC); rudder (1)
Deflections: +/- 45° all surfaces (3D); +/- 30° all surfaces (sport)
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