In this plan builder's article, Loris Goring updates the classic two-channel soarer
Over the last 50 years nothing’s given me more pleasure than building and flying disgustingly simple balsa gliders, that cost very little to make. In fact, they’re so cheap they even make my bank manager smile!
Many years ago I built a Monterey, a first-class glider design by Bob and Roland Boucher that’s still available from the RCM&E plans service. It was superb as both slope soarer and winch-launched glider, but my original came to an end when I had a remarkable Uri Geller type accident. At nothing more than cruising speed the hefty wire wing joiner suddenly folded, consigning the model to history. When I recovered the joiner there was no sign of fatigue and, in fact, I couldn’t bend the joiner back again, no matter how much pressure I applied; very odd.
Back in the workshop I resurrected the model (Mont-1, as I called it) using the same fuselage but with a built-up tailplane; the 1/4” sheet balsa version of the original was a bit on the heavy side for my liking. From here I developed Mont-2, which had a new polyhedral wing and, finally, completing the 15-year evolutionary phase, I present Mont-3, which has had almost everything fractionally changed, except the tail feathers!
Whether full-size or model, there are two prime design considerations in the development of any aircraft, that is to make it lighter and stronger than before. Thus, during the evolutionary development of the all-balsa Mont series I’ve shaved weight by strengthening critical parts such as the fuselage and wing spars, using carbon tows and light, glass woven rovings. On the Mont-3, Kevlar and carbon are extensively used, both impregnated with epoxy resin. The original Monterey fuselage featured 1/8” balsa sheet, yet that of the reinforced Mont-3 uses 3/32”.
Having read this article, if you fancy building your own Mont-3 (and I strongly recommend that you do!), just be aware that in addition to the usual tools required for a traditional build, you’ll also need a special pair of scissors to cut the Kevlar. You’ll also need suitable containers for measuring and mixing the 2-part epoxy. Old 35mm camera film containers are ideal, as only small quantities of resin need to be mixed at a time. Okay, eyes down, let’s get bashing that balsa.
Construction starts with the tail surfaces, which are easily built over the plan. It’s essential, of course, that your building board is dead flat, so double check that all’s well in this respect before pinning down any wood. When dry, remove the components from the board, sand smooth and round off the leading edges and trailing edges ready for covering. It’s up to you how to hinge the elevators (I use small, round pivot point hinges), but however you do it make sure that everything moves freely. There’s nothing more disconcerting than hearing a servo strain under the load of a stiffly-hinged control surface. A good elevator hinge line allows the surface to drop (when unconnected) by gravity alone.
The Mont-3’s fuselage is slimmer than its predecessors, meaning that the two servos (Futaba S9601s on the prototype) have to be slightly offset, one in front of the other and not positioned side-by-side as before. Also, the nose is longer than that of earlier versions, allowing the battery further forward to assist with the C of G. In lengthening the nose I had to be careful not to change the Centre of Lateral Area (CLA), as too much modification in the side area might have affected the excellent turning ability of Bob and Roland’s original design.
Start building the fuselage by cutting the sides. These are longer than stock 36” sheets so you’ll need to glue additional pieces to make the required length; place the additional (shorter) length of sheet over the full-length sheet and make a diagonal cut as per the plan, providing a perfectly cut joint between the two, ready for gluing. Without further reinforcement this joint would be extremely weak, but the addition of a 1/16” birch ply doubler at the top, 1/4” triangular balsa and (later) Kevlar / epoxy nose reinforcement will more than eliminate any weakness. Use a round needle file to cut out the elevator and snake exit points, and at the tail end use a sanding block in a vertical position to taper off the triangular balsa so that the sides join properly at the extreme tail end.
Cut all the fuselage formers, then glue numbers 1 and 4 in position to one fuselage side. When dry, add the other fuselage side and hold the tail end together with masking tape. Before the glue sets it’s vital to check that the fuselage is still symmetrical about the centreline of the plan view - no banana-shaped fuselages, if you please! When dry, slip in formers 2 and 3, but don’t glue them in place until later, when the fuselage is ready for the Kevlar (or glass roving) reinforcement. Glue in former 6, holding the fuselage to the correct shape using masking tape.
Next, cut the balsa bottom to size (grain running across the width of the fuselage) and glue it in position. Best practice here is to cut slightly oversize, then sand it back to the fuselage sides, rounding slightly at the corners. You now have a U-shaped fuselage, open at the top with easy access for the reinforcing materials. Apply Kevlar (or glass) from formers 1 to 4, and behind 4 to 6; run it right up the 3/32” sides and bottom planking and cut oversize so that masking tape can be used to hold it in place outside whilst the inside is being dabbed with two-part epoxy resin. Aft of former 6 to the tail I used four carbon tows at the four corners of the triangular pieces to give them extra strength. The snakes and aerial tube are then slipped into place, and while the resin is still green, glue in formers 2, 3 and 5 (doing this whilst the resin is ‘green’, i.e. only partially cured, means that the bond will be stronger) and cut off all overlaps.
Catching this green stage is the only problem as different resins go off at different temperatures and time. Try too early and the mat will stick to the knife, or even pull away from the balsa; too late and it’s hell to cut. It slices cleanly at the proper green stage. When everything is good and set, the aft top deck planking (grain running width-wise) can be fitted.
Tow hooks are bent from 16swg piano wire, epoxy glued to the ply platform. The forward hook is for initial use to hone your launching skills, and then, when you’re ready, you can move to the rear hook for a truly high launch.
Add triangular balsa fore and aft of the servo tray as per the plan, then cast the nose block weight (or simply fill the bored-out balsa with lead shot and resin). Don’t sand the nose until the hatch is complete. The latter has a 1/8” base, three semi-bulkheads and is covered using shaped balsa strips; sanded and glassed over to finish, it’s secured to the fuselage with a plastic dowel in the front and a removable pin at the rear. The 1/4” triangular balsa gives a good platform for the wing to sit on, capped with foam plastic. Finally, all that’s left to add to the fuselage are the dowels to retain the wing bands. Traditionally these are wooden items, but I used an old bit of carbon tube that had been kicking around my workshop.
Mont-3 uses a two-part, polyhedral wing, the panels being linked with wire joiners. To start the build, cut two ply rib templates from 1/16” birch ply, as per the plan. The larger template is used to cut all the 3/32” constant chord ribs (used in the centre section) plus, in conjunction with the smaller template, the ribs for the tapered tip sections.
If you cut two large templates then you can make both lots of ribs using the sandwich method. Here’s how: For the inner wing panels, cut two batches of 14 balsa pieces, each 1” deep with a length that’s equal to the chord width. Sandwich these between the two template ribs, pin them all together from either end, shave off the surplus balsa with a razor plane and sand right down to the templates. Whilst still pinned together, cut the slots for the spars. Note that the basic Clark-Y section has been modified in the centre panel, with a slight undercamber whilst the outer wing panels remain unmodified.
When laying down the lower l.e. sheeting, note that tiny balsa wedges are used under the front edge so that it curves up to meet the 3/8” square balsa l.e. The latter must also be wedged up so that it’s flush where it lands on the bottom sheeting. This is the hardest part of the centre-section construction, as the rest of the wing construction involves cutting the holes for the brass main tube joiner and aluminium locator joiner. The brass tube is right under the main spar, where it spreads the maximum stress during a launch. Note the infill above and below the tubes, the webs between ribs 1 - 4 and the lengthways grain 1/16” ply webs, glued from ribs 1 - 6, back and front of the main spar. Don’t glue the joiner tubes in just yet, they’ll be secured in due course when setting the dihedral.
With the two centre panels complete to the same level of build you can focus on the tip panels, again using the templates for sandwich method rib creation. It’s important to keep the weight of construction at the tip as light as possible to help prevent tip stalling, which is why the spars, l.e. and t.e. are tapered, as is the top and bottom sheeting. The design requires no washout at the wing tip, so all parts of the wing are built on the flat. Meanwhile, the wing tips are 3/8” square balsa, reinforced with 3/16” triangular gussets at their corners. The tips will later be sanded to the same aerofoil section as rib 11, and then rounded off.
We now need to join an outer panel to a centre panel. When bringing the two together you’ll notice that the top spar will have to be shorter than the bottom spar in order to join properly, and that the l.e. and t.e. will need sanding to the correct angle so that they join tight up against each other. Lay the centre panel down dead flat, secured to your building board, then offer up the outer panel, raised 41/4” at the tip to give the correct dihedral. Remember that we don’t want any washout, so make sure the panel is parallel. Use a medium (12-minute) setting epoxy to glue the l.e., t.e. and spars; the joint will be very weak until the plywood webs are glued front and rear of the main spar and the triangular pieces added as shown. Right, that’s one wing done - repeat the process to build the other.
With both wings now to hand, insert the wire wing joiners in their respective tubes. There should be 15/16” dihedral at the end rib of the centre panels; if the wing tends to spring up, then the joiner holes are too tight and need easing. Re-check and adjust as necessary until all is well and the joined, polyhedral wing is good and equal.
The webs fitted and glued behind the main spar are 3/32” balsa, but note that there aren’t any fitted at ribs 8 to 11 on the outer panels. The upper wing sheeting can now be completed, landing on the top spars. This means that every rib must be capped top and bottom, using 1/4 x 1/16” medium grade balsa strip; if it won’t easily spring to the curve of the rib, put it under a rolling pin. This will crush it slightly, but if you use PVA glue then the water in it will compensate by swelling the balsa.
Covering and finishing is up to you, just make sure that you don’t add too much weight. I like to use either 2-can polymer or 2-can epoxy based paints, sprayed over a filler / base coat that’s sanded back until the balsa is almost exposed. The object here is to achieve a good, shiny finish, but with minimum weight of paint for the final coat.
Mont-3’s maiden flight was off a 150m winch line into a force 3 wind, a launch that resulted in an eleven-minute flight. She behaved herself impeccably, but the flight time wasn’t much to write home about. The next outing, a hand-launch from a sloping field near the windward side of a Devon hedge, resulted in a climb-out of a few hundred feet and a flight that a few club members shared - they didn’t want to bring her down! When I finally managed to wrest the Tx from them so that I could ‘solo’ with her, Mont-3 was more than happy to hang around in the air for ages, in fact after a number of 20+ minute flights it was my neck that gave out. The next modification will be to plaster my neck with Kevlar and epoxy!
On the line, Mont-3 goes up as straight as a die and flat turns for thermal flight are a doddle. Despite knowing that the wing is immensely strong, getting her down from a dot in the sky makes me regret not fitting airbrakes; she has to be clawed down out of the sky by doing continuous loops to lose height.
Simple to build and a real fun model to fly, I guarantee you’ll enjoy Mont-3 as much as me.
Model type: Thermal soarer
Modified by: Loris Goring
Wingspan: 98.4” (2489mm)
Fuselage length: 47.5” (864mm)
Wing area: 801 sq. ins.
All-up weight: 43oz (1219g)
Wing loading: 7.7oz / sq. ft.
Functions (servos): Rudder (1); elevator (1)
Plan details: Two sheets
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