The plan build article by David Stevens - 25/2/11
Ok, you’ve made the decision to start R/C flying and want to get into the air as soon as possible. You have little (if any) modelling experience, family and work responsibilities render you short of time, but you can set some cash aside. How very tempting it is to take the ‘Almost Ready To Fly’ route! So you check out the offers and duly return home with a big, beautiful box and all the support equipment you need. Inside that box lie the superbly finished components of a magnificent aircraft that’s virtually guaranteed to fly. With expert assistance it certainly should teach you to fly - and quickly and safely, at that. But should you pile it in (as all R/C fliers inevitably do from time to time!) it’s likely to prove difficult, if not impossible, to repair - even if you do have the ‘inner’ knowledge. And apart from that, it’ll have denied you the thrill of seeing your own skill and handiwork take successfully to the skies. No, what you want is a plan-built design that will give you the full R/C aeromodelling experience... and it just so happens I’ve one for you!
Enter Swot, a smaller-than-usual trainer for .18 - .25 two-stroke engines that can serve as basic trainer, intermediate trainer and as a surprisingly athletic fun machine for the advanced flyer. The large amount of wing dihedral means that it can be steered by rudder only (if that’s the way your instructor insists you have to start), or through the combined use of rudder and aileron. Contrary to some predictions, the roll rate has proved excellent and inverted flight is no problem, even with the flat-bottom wing section.
Construction is straightforward (if slightly unusual in places) and requires most of the techniques a committed model maker should get to know about. As ever with built-up models, extreme care taken at every stage will repay you with reliability and good handling qualities. But if your time really is at a premium, perhaps you should use a very simple ARTF to begin with and treat Swot as your second, longer-term model.
Before working our way through the assembly, just a quick word about a material I recommend for most of the ribs and bulkheads. It’s a lightweight foam board, backed on both sides by paper, often used by architects for models of their intended buildings. Some hobby shops stock it and I suspect many art suppliers do, too. You can cut it with a very sharp knife, but a power fretsaw makes it a doddle. Apart from its excellent strength-to-weight ratio the material is inexpensive, has no weakness along the grain, no tendency to warp, plenty of area for white glue to grip and a useful elasticity that absorbs knocks rather than splitting or shattering. And if a section is pushed in or creased, you simply cut it out and replace it.
KIT OF BITS
As you’ll need to pin the plan to the board and protect it with a sheet of clear polythene, it makes sense to first trace and cut out the bulkheads, ribs and other shaped components.
F1. Cut this from of 1/4” ply, mark the holes for the engine mount and glue the blind nuts to the back using epoxy (Araldite). I used the Great Planes 6-32 x 11/4” blind nut bolt set, and advise you to do the same - the reason for this choice will become clear later. Temporarily line up the engine, mount the fuel tank on the plan and mark the holes for the fuel piping and the throttle rod (or snake) onto F1. Drill out. Chip out a ‘trench’ as shown to half-accommodate the plastic undercarriage bolt, and treat the engine mount similarly.
F1A. This takes no serious load and can be made from balsa. Glue to F1.
F2 / F3. Cut from 1/8” ply. Locate and drill holes for throttle snake.
F4 - F8. Cut from foam board or medium grade balsa, gently sand or file the top ‘slope’ as shown.
C1 / C2. Soft balsa. Fix 1/2” dowels on the inner side of C2 for the rubber bands that will hold the canopy down.
These are all made from 5mm spruce. Cut L1 (which runs from F2 to F4) to length, and epoxy (on protected building board) to L2, making an angled joint as shown for maximum gluing area. Remove when set. Cut the others slightly over length.
3. Fuselage side sheeting
Ply doubler. This runs from F4 forward to the nose, and vertically from L3 to L4 as noted by the symbol ▼
Balsa sides. These are from 1.5 or 2mm medium balsa. The shape is defined by the plan’s side elevation (less the top and bottom decking) except for the forward end, where the upper edge is defined by L4 and F2, thus leaving a cut-out for the cockpit canopy. Should you wish you can also make a cut-out for the cockpit side window. If your balsa sheets aren’t deep enough, end-butt the extra as necessary whilst temporarily pinned down.
Unusually the wing is built up either side of a solid main spar, the aim being to create a genuine ‘D’-section at the front for maximum stiffness. Thus the ribs will be in two parts, front and rear.
W1F. Cut from foam or medium balsa (14 off).
W1B. As above (8 off).
W2F. Make two in foam and two in either 2mm or 3mm ply.
W2B. Foam (8 off), ply (2 off). The innermost need holes for aileron servo cables and plugs.
WX. For the centre-section, and the only full-length ribs (2 off).
5. Dihedral brace
Cut from 1/8” ply. If you’re a skilled pilot already, you may wish to reduce the dihedral or eliminate it entirely!
6. Tail surfaces
These can be cut from light or medium 5mm balsa sheet. Note that the fin piece continues down to the full depth of F7 and F8, requiring a notch at the rear to accommodate F8. The edges must be reinforced with hard 5mm square balsa as indicated to discourage warping and damage.
Swot’s fuselage is of square section, fully sheeted and with a straight bottom to allow easy, accurate construction on the building board. An important feature of the design is the arrangement of the fuel tank, which is totally separated from the fuel-vulnerable radio gear and servos but visible through the cockpit transparency for easy and regular checking. So, here we go, step by step:
1. Place bottom longerons L3 on the top plan elevation, using either side of the spruce spars to locate them.
2. Using white glue, stick all the bulkheads in place and make sure that they’re fully seated and vertical. Allow the glue to set.
3. Glue in the L4s, L5 (spruce), L6 (hard balsa) and the L7s. Be patient - allow them to set.
4. Glue in the ready-joined longerons L1 / L2, clamping as necessary to hold the slight curve around F4. Glue in the L8s, shaping as necessary to match in. Allow time to set.
5. Stick on the liteply doublers that run forward from F4. Improvise pressure pads with books, tins of paint, gold ingots or whatever, to make sure that full contact is made with L4, L3 and the bulkheads.
6. Do likewise with the fuselage sides - oh for inexpensive, deep, wide-jawed clamps!
7. Block in the extreme nose. Later, when it’s sanded to shape, you’ll need to cut out a ‘U’ shape to clear the front of the engine and two holes for a tool to get at the lower engine mount bolts.
8. Y2 and Y3 are from 5mm ply and run full width between the lower longerons and the bulkheads. Parts Y1 are triangular shaped to allow easy insertion and removal of the tank. Epoxy in place.
9. Remove the assembly from the board and sheet in the bottom with either 1.5mm or 2mm balsa sheet, cross grain. Don’t sheet in the upper rear section just yet.
10. Sheet under L1 between F2 and F3, allowing 2mm at either end to act as a cabin window seating. Fill in the three other sides of the seating using strips of scrap.
11. Cut the two strips of 1/8” servo mounting ply, that rest on L4, between F3 and F4. Check that the servos fit okay, drill holes for the mounting screws and epoxy in place. Temporarily fit a servo and work out the line of the snakes to the rudder and elevator horns. Cut the snake outers over length, position them through the bulkheads and locate them firmly through a holed, balsa crossbeam on F4 and through F7. Incidentally, impact glue grips the slippery plastic very well.
12. Sheet the upper rear fuselage in a cross grain direction.
13. Mark out the 1/8” ply plate that lies under the rear of L1, then drill the holes and epoxy in the blind bolts. Epoxy the plate into place and rest the fuselage upside-down to set.
In the interest of adding weight only where it’s needed for extra strength, the main spar of the wing doubles up from centre to root - we’re in good company, R.J. Mitchell did much the same with the Spitfire! Beware though - the wing panels don’t join directly together as the space-saving layout on the plan might suggest; a centre-section fits between them!
14. Reposition the plan on the board so that the 1/2” square leading edge spar will hang clear just over the edge. This is necessary because the spar is slightly deeper than the ribs so as to provide a gluing face for the forward wing sheeting.
15. By temporarily pinning to the board, glue together the two components of each wing spar. Use 1” x 1/8” medium balsa.
16. Locate the main spars on the board using long pins driven down and across them at a steep angle. Make sure the spars are absolutely vertical then pin down the rear 1/2 x 1/4” balsa spars.
17. Secure all the ribs to the main and, where appropriate, rear spars using white glue and allow to set.
18. Glue in the 1/2” square medium balsa front spars, making sure all are correctly seated. Allow to set.
19. Deck in the forward section of the wing and the rear section across the three innermost ribs. Shape and glue the upper, more rounded, full-width wing tip decking. Fit the rear rib cappings. Ideally you should also fill in between these cappings above the rear spar with similar strips; it may seem fiddly, but the covering will go on much better. Sand as required.
20. Remove the wings, turn over and pin down on the board using scrap strip packed underneath to achieve a rigid, warp-free stance. Deck the forward section.
21. Build up the box and mounts for small wing servos, allowing threading space for cables and plugs. Deck in around it and then add the remaining cappings. Deck in the underside of the curved wingtips using vertical strips. Sand smooth.
22. Over the plan, build up the centre section. This uses one-piece ply (2mm or 3mm) ribs with slots for the dihedral brace and cut-outs for the front and rear ply cross-plates (the extreme rear is made of triangular aileron strip.) Deck over with cross grain 1.5 or 2mm medium balsa sheet.
23. Remove the centre-section from the board, turn it over and deck the underside, leaving a hole for the aileron leads.
24. Thread the wings onto the centre-section then glue and weigh down onto the board so that dihedral angles can be checked and held exactly.
25. Fabricate the 1/8“ ply forward mounting tongue, then glue and screw it into place.
26. Offer the wing up to the fuselage and carefully locate the hole positions for the wing bolts. Drill out and pray you’re right!
27. Cut slits in the rear spar to suit the aileron hinges.
28. Match these slits in the ailerons (1” x 1/4” medium balsa). If you’re worried that the ailerons feel too heavy, drill lightening holes - I did!
DOWN AT THE BACK
29. Build the fin, rudder, tailplane and elevators over the plan, gluing the strengthening edges to the softer ‘inners’. Are lightening holes strictly necessary? I would suggest that any weight saved at the rear is a job well done, but it’s up to you.
30. Having cut out the slots in the rear fuselage, offer up the tail surfaces and adjust if necessary to fit (the fin must go in first). Make slits for the hinges.
BACK UP FRONT
31. Screw the engine to its mount. Offer up to the fuselage, and chip out the ‘U’ at the extreme front of the fuselage to make room for the crankcase of your chosen motor.
32. Make the holes that will give tool access to the lower bolts of the engine mount, then reinforce the extreme front of the fuselage with glass fibre matting - this could save your engine from expensive damage!
33. You now need to make a slim-fitting extension handle for the Allen key supplied with the Great Planes blind nut set mentioned earlier. I simply chopped off virtually all the short section of the key, slotted it into a brass tube filled with plastic steel, and pushed in a 4” wire handle at the other end. It may not be the ultimate in strength or sophistication, but it works!
34. Temporarily screw in the engine and mount, cutting away as necessary for clearance of the exhaust stack and throttle arm. Build up close-fitting decking behind the engine and over F1. The latter may not be strictly necessary, but it looks much more professional.
35. In situ, but with masking tape between to avoid uncalled-for sticking, build up the cockpit canopy frame using C1, C2 and 1/2 x 1/4” strips of soft balsa. Shape to fit.
36. If necessary, build up fairings (from scrap) to merge the wing centre-section into its forward and rear fuselage surroundings.
Cut a piece of transparent material well oversize, and neatly glue it over the canopy frame using impact adhesive or an alternative canopy glue of your choice.
Unlike many trainers, Swot is a tail dragger. Why? Because the bumpy field I fly from makes short work of those weak, steerable nose legs! If you have the luxury of flying off a bowling green or similar then you may prefer to reorganise things and add a tricycle undercarriage arrangement instead - nose wheel behind the engine mount slot, and the main legs sweeping rearward to take the wheels behind the centre of gravity.
A steerable tail wheel may not be strictly necessary on a trainer - a skid would suffice - but you’ll enjoy the easy ground handling it offers when you become more proficient.
The nylon screw that locates the radius arms is designed to shear off in the event of a heavy arrival. All you have to do is loosen the engine mount bolts with that customised Allen key tool, remove the stub and replace... far better than rebuilding the forward fuselage!
FINISH TO BE PROUD OF
By nature of its built-up construction you’ll have spent many hours constructing Swot. It’s therefore only right and proper that you wrap up your creation in the finest possible livery. You will, of course, have thoroughly fuel-proofed all vulnerable areas previously...
The prototype is clothed in green and cream Profilm, with a red fin of the same material. The handsome self-adhesive, fuel-proof lettering and numbers were provided for a very reasonable sum by Geoff Sparey of Amberley Signs. Give him a call on 01252 836436 if you want a similar set.
In the interests of good visibility and orientation, the green and cream are reversed top / bottom wherever possible; but this does present a problem in getting a straight line where the one meets the other, as along the wing and tail leading edges. I therefore suggest you ignore Profilm’s instructions of ironing from the centre outwards (it’s virtually impossible to cut a truly straight line with the film in situ), but seal the straight line first and work back from it. Similarly the top and bottom deckings - cut the film precisely to size, seal the outer edges then work inwards.
THERE AND BACK
If you’re a raw beginner it’s absolutely essential that you join a club and learn to fly under expert tuition. In fact I would say it’s dangerous, irresponsible and a total waste of time and money to try and do it on your own; flying an R/C model is far, far more difficult than driving a car!
I’m told by the experts who have flown her that Swot is responsive and vice-free, but it must be flown correctly. Master her, easy stage by easy stage, and you’ll be well on your way to a lasting relationship with this most satisfying of hobbies.
Model type: Four channel trainer / sport
Designed by: David Stevens
Fuselage length: 37''
Wing area: 353sq. in.
All-up weight: 31/4lb
Wing loading: 21oz / sq. ft.
Rec’d engine: .18 to .25cu. in. two-strokes
Radio: Four function (five servos)
C of G: 47mm from leading edge
Control functions: Aileron, elevator, rudder.
a 25 siyse engine high wing trainer any one built one By John Whoriskey
by John Whoriskey
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