Scottish Aviation Pioneer

The Pioneer could carry four fully equipped soldiers, or 2000 lbs of freight, and drop them into very small airstrips on hilltops or in jungle clearings. To this end, the undercarriage and pilot’s seat were stressed for arrivals of 20g! As a modelling subject, the Pioneer makes a characterful and interesting alternative to the usual high-wing types like the Piper Cub. If you want to add more scale detail than I’ve given on the plan, the RAF Museum at Cosford has a Pioneer that you can plunder for information. If Cosford’s not convenient for you, then Martin Photos (www.ivyandmartin.demon.co.uk) can supply you with a CD containing pictures of the aircraft.

Begin by making two ladder-side frames from 3/16″ square balsa. Then cut out all of the ply formers, and mark the 1/4″ ply engine former (F1) with the engine crankshaft centrelines. Remove the side frames from the plan, and add the ply gussets for the undercarriage and wing-strut fixings – not forgetting that you need to make a right and a left version! When you’ve done this, sheet the outsides of the frames with 1/8″ medium balsa.

To assemble the fuselage, glue formers F2 to F6 into one fuselage side, slotting them onto the longerons, and butting them up to a vertical member; take time to ensure that theyre perpendicular to the formers. When firm, glue on the other side frame making sure that everything’s parallel and square.

As you can see, the full size machine was a big aeroplane.Before fitting former F1, prepare your engine mounting; drilling holes for the mounting bolts and fuel lines is much easier at this stage. Since the cowl’s circular, you can rotate the engine around its centreline to mount it at the angle that best suits your exhaust / silencer arrangement.

I used an ASP .46 two-stroke with the silencer at the bottom, but a four-stroke silencer would be more discreet. Next, bring the fuselage sides together at the tail and nose formers (F1 and F10) to create a symmetrical tapered fuselage. When these are secure, glue the remaining formers into position, and add the airfoil-shaped pieces of liteply that form the wing mounts. You’ll need to reinforce these joints with small pieces of strip at the front and rear as they carry the flight loads. Fit the two stringers at the top and bottom of the fuselage, and then attach the pieces of 1/8″ ply for the main undercarriage mountings and tail wheel. Also add the piece of 3/16″ sheet balsa for the tailplane mounting, not forgetting the slot for the fin.

Insert the snake outers for the rudder and elevator; these should be the Golden Rod type with a hollow inner, so that you can screw wire pushrods into them later. Using tapered strips of 1/8″ sheet balsa, plank the upper fuselage from F2 to F4, and from F6 to F9, and the lower fuselage from F4 and F5 to F10; leave a gap at the top between F8 and F9 to slot the fin into later. Planking in the front cockpit area is a bit fiddly, but don’t worry about seeking perfection here as most of it will be cut away later. Remember, though, to leave a gap between F4 and F5 for the hatch that allows access to the receiver and servos.

The aircraft doors are cut from 0.3mm ply and stuck on top of the sheet fuselage sides. Add some pieces of 1/8″ sheet balsa to fill the odd gaps, and provide mounts for the windows where necessary. At the sharply curved front of the fuselage, fill in the gaps between F1 and F2 with soft block. Similarly, add more blocks in front of F1 to create the sections that extend into the cowl, which fits around the engine mounting. Tack-glue the cowl block in place and sand it to shape, then remove it, cut out the middle to suit your engine mounting, and then glue it back permanently.

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The underbelly hatch hides the sero and receiver positions.The bearers for the servo tray are made from two strips of 1/4 x 1/4″ spruce; the tray itself is cut from 1/8″ ply.

The radio access hatch, meanwhile, is made from two frames that match F4 and F5, these being planked into place. I used small magnets and pieces of tinplate to hold the hatch in position. At the back of the fuselage, another piece of soft block is used to form the tail cone, while the rear hatch is cut out of the planked area between F9 and F10; the hatch only extends 23/4″ aft of F9. Insert a small piece of block at the rear of the hatch space for the tailplane strut mounts. Once again, I used a small magnet to secure the rear hatch, which is positively located in F9 by two toothpick pegs. At this point, you’re ready to give the fuselage a basic sanding to shape.

The undercarriage is formed from two pieces of 10swg wire linked by a triangular sheet metal joiner; only one of the wires is used to form the wheel axles. Assemble the gear by setting up some commercial undercarriage clamps at the correct separation on a board. Clean the area of wire to be joined thoroughly, and set the pieces in the clamps. Bring the wires together and add the joining pieces; they go underneath the wires as you assemble them, so that they’re on top when the undercarriage is on the aircraft; the joiners can be crimped in place with pliers to hold them. Using a large soldering iron, flood the joint with solder.

The undercarriage oleo struts are shaped from 5/8 x 1/4″ hard balsa with sheet metal ends, which are wrapped with a strip of paper and filled to simulate the oleo. It helps to make the strut slightly oversize at first and then trim it to fit the fuselage and undercarriage attachment points; I used 8BA bolts to join mine. The tail wheel, meanwhile, is a commercial item, though its strut is made from wire bent as shown to create the cranked shape and the hook for the rubber band link to the rudder horn.

Finally, cut out the planking to the shape of the front cockpit, and put in all the cabin window frames, with the exception perhaps of the diagonal one in the roof, as this will make cabin access easy for the radio fit. As you can see from the photos, the tops of the front window frames butt up to some leading edge-shaped pieces of block which fill the space between the wing mounts and the cabin window frame taper. Once the cabin frames are in place, the curved rear cabin sides can be added. These are made from 0.3mm ply with appropriate window cut-outs. Similarly, at the front, a 0.3mm ply piece fills the gap between the curved fuselage sides and the straight lower window frame. Finally, add two small blocks to fill the gap where the wing mount trailing edge meets the top of the rear cabin.

Being circular, the cowling is simple to make. Mine is from two rings of 7/8″ good quality ply turned on my metal lathe and joined with two layers of 0.3mm ply strip, but you could build yours using ply rings and medium block balsa. I turned a hole in the front of my cowl so that it’s a snug fit on the front bearing housing of the engine, and fretted out seven holes to let the air in. The resulting spokes formed the supports for dummy cylinders that were made from a black Depron sheet (a meat tray) with pushrod tubes formed using wire bent into a V-shape. Note that the top cylinder is vertical on the real engine. The fuselage is pretty well finished at this stage, and can be put to one side for the moment.

The plan only shows the port wing, so you’ll need to build the starboard wing from a reverse image. You can make this either by spraying the plan with WD 40 so that the paper becomes transparent, or by using carbon paper with the transfer side uppermost. Lay the plan on your clean sheet of paper with the carbon paper below and trace the image with a pencil. The carbon image will transfer to the back of the plan. The wing slats and flaps are optional, by the way, as you can fly the model ‘clean’, noting that the section for the slatless wing is slightly longer than the slatted variant. The high-lift gadgets add a lot to the aircraft’s character, however, and I recommend that you fit them, even if they do make for extra work. As a matter of interest, the slats and flaps on the full-size aircraft were ganged on to one selection lever, so deploying them was very much a case of all or nothing.

Construction of the wings themselves is straightforward. The ribs and spars are completely sheeted to simulate the original metal skin; the only exception being the ailerons, which were fabric covered – Solartex on the model. Start by preparing all the ribs from 3/32″ sheet balsa and liteply. Don’t forget that you’ll need to make holes for the servo wires, and for the wing joining tubes in the liteply end ribs. Next, pin the 1/8″ balsa trailing edge (t.e.) strip on the plan; you can cut the aileron t.e. strip and build the wing and aileron simultaneously if you wish. Then pin the lower main and secondary spars onto the plan, and raise them 1/16″ to allow for the sheeting. The wing t.e. in the aileron position is a piece of 3/8 x 3/4″ balsa notched for the ribs. The wing leading edge (l.e.), on the other hand, is 3/16 x 5/8″ hard balsa, and this needs packing up 1/8″ in its position.

The aileron l.e. is a piece of 1/2 x 1/2″ balsa rounded to a quadrant shape. With the spars in place, set the ribs into their correct positions, starting at the root end. The liteply root rib is tilted 1/32″ off vertical to allow for the minimal dihedral. The front of each rib needs to be packed 1/16″ above the bottom of the l.e. to allow for the wing sheeting. When you get to the aileron area, cut the tails off the ribs so that youre able to fit the false trailing edge. The cut off pieces can then be trimmed at their front end to fit the aileron leading edge; a riblet cut from 1/4″ sheet adds strength at the aileron tip. Add all the gussets, the hinge reinforcing pieces, and the liteply gusset for the strut mount. Finally, install the upper secondary spar.

When all the glue has dried, remove the aileron and wing from the plan and repeat the whole process for the other wing, again taking care to build a port and starboard versions. She was never fast, and she was no beauty, but Scottish Aviation’s Pioneer had character in spadefuls, as we saw last month when we built up the stocky fuselage and wings. This month, we’ll finish the build with that powerful tail, and the slats and flaps that helped to get the Pioneer in and out of make-shift airstrips on hilltops and in jungle clearings.

When you’ve completed the wings, you can get on and add some suitable mounting plates for the mini aileron servos (I used Ripmax SD 200s). Bear in mind, though, that the output arm of the servo needs to be positioned such that the arms will be clear of the lower surface of the wing after you’ve added the sheeting. The time for sheeting the wings isn’t yet, though – you’ve got fit the 3 tubes that house the wing joiners, first. The joiners themselves are made from two 10″ lengths of 5mm carbon fibre or steel rod – either is fine, so long as said rod forms a snug sliding fit in the aforementioned lengths of tube. When you’ve fitted the tubes, offer each wing up to the cabin top’s airfoil section and transfer the position of the holes in the wing ribs to the cabin.

Note that its the trailing edge thats used to line up the wing; on a slatted wing the leading edge falls short of the cabin top’s leading edge section. Also, when marking out, remember to allow 1/16″ all round for the wing sheeting. Next, set up the wings against a straight line on the bench, and thread the rods and tubes into their approximate positions. Prop up each wing tip by 1/2″, and align everything accurately so that the tubes sit snugly against the main spars. Now tack the tubes to the spars with five-minute epoxy and when dry fix them permanently to the spar by binding them with bandage or fine glass cloth and epoxy; two layers of cloth are enough. When set, smooth off the underside of the binding preparatory to fitting the wing sheeting. Finally, add the hooks for the wing joining bands.

Finish the ailerons by sanding them smooth and drilling them for the Robart-type barbed hinges. The centreline of the hinge is within the aileron leading edge, so make a slot to permit full hinge movement. Drill the corresponding holes in the wing trailing edge. On the full-size aircraft, the gap between the wing and the aileron is a curved slot. You can replicate this by adding a triangular strip to the top of the wing trailing edge and shaping it to extend back over the aileron leading edge. Sand the wing trailing edge and aileron leading edge to get 45 degrees of up and down movement, but don’t fix them in place yet. Finally, slot the aileron leading edges for their 1/16″ ply horns, but again, don’t glue them in place just yet.

SHEETING

The wings are sheeted with 1/16″ balsa panels, which are made by butt-joining pieces of 4″-wide sheet to achieve the necessary width and length. Sheet the lower part of each panel, beginning by butting the sheet up to the 1/8″ trailing edge and the leading edge. When the glue has set, cut out the area of the servo bay and fit the servo, extending its lead through the wing to give about 2″ spare beyond the inner rib. Also add the strut mount, bolting it through its gusset. Repeat this process for the upper wing skin before adding the 1/2″ sheet wing-tip block. Note that on the slatted wing this extends forward of the leading edge to allow for slat retraction. Now is the time to fit navigation lights if you wish, using moulded acetate covers and coloured LEDs. Once you’ve sanded the wings to shape, set them aside and finish off the slats, flaps and struts.

The flaps are made from pieces of 3/16″ sheet balsa sanded to an airfoil shape. They’re fixed with liteply mountings that slot into the wing trailing edge and the flaps. Cut the slots in the t.e. and flap so that they’re a tight fit on the mountings to allow dry assembly. Use the jig from the plan to get the 20-degree depression angle, and trim the mountings so that they sit flush with the wing surface. They extend above the flap surface, however, to simulate the track mechanism of the full-size. Do not fix them in place at this stage. The slats, meanwhile, are made by gluing together 1/2 x 1/2″ strip and 11/4″-wide trailing edge stock, and using curved sanding blocks to form the undercamber. Sand the slats until they’re a close fit to the wing in the retracted position.

The 10 slat mountings, meanwhile, are cut slightly oversize and individually shaped to fit their wing and slat locations; mark the positions with a code so that you don’t mix them up. Give the mountings a final finish with sanding sealer, taking care to keep it off the glue area, and then cement each mounting to its location on the slat before offering up the whole assembly to the wing leading edge. The fit probably won’t be perfect to start with, but it should be close. Now wrap the wing leading edge with fine sandpaper, and gently slide the slat from side to side to remove any high spots. When the slats fit well, put them aside until later and start on the wing struts, which are made from strips of 3/16 x 3/4″ hard balsa sanded to a streamlined shape. They have wire end-fittings that engage with the tubes of the strut mounts on the fuselage and wings; a simple push-aside spring clip keeps them in place. I recommend that you cut the struts oversize, and trim them to fit your aircraft when assembled.

The (tail) end is in sight now! The fin and tailplane are cut from medium 1/4″ balsa sheet. The tailplane has an aerodynamic slot in the leading edge which is formed from strip balsa with suitable spacers. Add some discs of 0.3mm ply, top and bottom, at the strut attachment points. The rudder and elevator are made from a sandwich of 1/16″ sheet balsa with 3/32″ strip detail to give them some shape when the control surfaces are covered with fabric. Two small plate-and-pin hinges take care of rudder movement, while five larger ones handle the elevator. The rudder is operated by a 5mm dowel post that extends down into the tail wheel hatch where it’s moved by the same 1/16″ ply horn that works the tail wheel steering. The elevator, on the other hand, uses a commercial horn for movement. Sand the pieces to a streamlined shape, but don’t assemble them until after covering the model.

I covered the fixed flying surfaces and most of the fuselage of my aeroplane with silver Solarspan, using some white film for the cabin and upper fuselage, and black film for the nose. The control surfaces of the full-size aircraft are fabric covered, so I used silver Glosstex for these components. Although I used film for the wing struts, the undercarriage struts and cowl were sprayed with aluminium and black car paint, and then fuel-proofed. When covering the fuselage, remember to leave a bare area under the tailplane so that it can be glued to its platform, and also on the lower part of the fin where it slots into the fuselage.

The undersurface of the slats between the mounts can be covered with pieces of white film, using silver on the topside. The slat mounts themselves should be painted black, taking care to avoid the glue area where they fix to the wing. When you’re ready to attach the slats, simply remove narrow strips of wing covering where they’re to be mounted, and glue them on. The top and bottom of the flaps are covered with silver film, which needs to be removed from the saw cuts that you made earlier for the mountings. Once you’ve glued the flaps to the wing at a 20-degree angle, you can paint the mountings silver, and make up some small covers from 0.3mm ply for the brackets on the wing trailing edge. Note that these covers are also painted silver. I asked my local sign-maker to produce the roundels, lettering, and what have you, while Solartrim took care of the remaining odds and ends of covering and detailing.

You’ll doubtless have to decide how much interior detail you want to add to the cabin. I painted the interior of mine with Humbrol cockpit green, and made five identical seats from my memories of flying with the RAF. These were mounted on a shallow box that covered the servos. If you’re going to fit a pilot, he’ll need to be 7″ high to be in keeping with the model’s scale. One piece of work you’ll have to do in the cabin before glazing is to install a Y-lead in the top wing mounts to hook up the wing servos; I glued my lead into small rectangular slots that were sized to suit its sockets. The cabin windows can be framed using white Solartrim strips that are applied after the acetate windows have been glued in position. Incidentally, you might find it helpful to leave the cabin roof glazing until after you’ve inserted the radio gear.

To complete the assembly, slot the fin into the 3/16″ balsa tailplane mounting and formers F8 and F9, before laying a bead of 1/8 x 1/8″ balsa around its base. The tailplane is glued onto its platform, and then strutted with streamlined brass tubes that are joined at the centre using a metal strap. This strap is then screwed to the block in the tail hatch, and the ends of the tubes are secured to the tailplane with 10BA bolts. The radio installation starts by fixing the throttle servo to the top of the servo tray in a way that suits your engine installation, and then fitting the receiver and main servos to the bottom of the tray. Adding the snake inners calls for cunning.

At the tail end, I used metal pushrods with a threaded end that fitted the snake inner, and Z-bends at the horns. At the servo end, the metal pushrods have clamp-on clevises to join them to the servo arms. The trick is to insert the snake inners from the tail end, and attach the horns afterwards by threading them onto the Z-bends. The battery on my model was secured behind the cabin using Velcro pads and a strap. This makes it a moveable feast, so that you can alter its position to set the C of G, which should be at 25% of the wing chord measuring back from the leading edge (or, if you’ve built your wing with slats, measured back from the front of the slat). The tank, meanwhile, was fitted in the usual place immediately behind the firewall. Hooking up the throttle may require you to fit the mini plastic snake inner to the servo first, and then thread the inner into its tube as you insert the throttle servo tray. Set up the control throws to give 3/4″ up / down movement for the elevator, 1″ left / right for the rudder, and 1/2″ up / down for the ailerons.

My ASP .46 provided plenty of power for the model’s 5 lbs 4ozs all-up weight, and she took-off briskly without making any undue calls on that big rudder for directional stability. Once in the air, the model is stately rather than dynamic, but this reflects the appearance of the original very well. The elevator and rudder are powerful, but the ailerons are sluggish, so it’s essential that you use the rudder to initiate turns – the full-size aircraft was just the same. There isn’t a stall as such, just a mushy wallowing, and the landing is equally undramatic, largely because the model will fly very slowly. Again, the rudder is essential for lateral control, and you should remember that the only undercarriage springing is in the tyres – unlike the original, the model won’t survive a 20g arrival! Have fun.

Datafile

Name:   Scottish Aviation Pioneer
Model type:  Scale transport aircraft
Designed by:  Chris Reid
Wingspan:  58” (1473mm)
Fuselage length:  42” (1066mm)
All-up weight:  5lbs 4oz (2381g)
Wing loading:   21oz / sq. ft.
Wing area:  580 sq. in.
Control functions:   Aileron, elevator, rudder and throttle
Rec’d no. channels:   Four (five servos)
Rec’d engine:  .46 two-stroke, .40 four-stroke