Sopwith Pup (oz1710)

About this Plan

Sopwith Pup. Radio control scale WWI biplane fighter. Wingspan 12 in. AUW 12g. For electric power and micro RC.

Modern (foreign language) plan, this is a free download from the author's homepage at http://www.mjd.se/ it's an old-timer style stick and tissue balsa-framed model, designed for Parkzone micro RC electrics.

Quote: "I've always been fascinated by small, free-flight scale aircraft. Those that model them are masters at building light airframes and really understand how to trim their designs so that they fly majestically. I fancied building something similar, but not free-flight, oh no, I'm a staunch R/Cer! With small, powerful electric flight powertrains and R/C systems now freely available - they don't get much smaller than the Kyosho Minium range - I reckoned I had a decent chance of success. What's more, having flown ParkZone's superb ultra-micro Piper J3 Cub, I realised I just had to do it.

The aforementioned Cub is fitted with a Spektrum AR6400 Rx, which has a
built-in ESC and two small linear servos for elevator and rudder control. Up front is a miniature electric motor and gearbox mated to a 100 x 60mm propeller. Putting all this, plus the Cub's 70mAh Li-Po on the scales yields a reading of less than 10g1 Perfect for a model with an AUW of around 30g. The Cub costs about £70 and I reckoned it would make a great donor for my design. But what to build?

Thoughts of a really small biplane started buzzing around my head, and after consulting one of Sweden's foremost free-flight gurus, Johan Wallin, my preference started leaning towards a Sopwith Pup. Sopwith's small bipe has a rather unusual combination of generous wing area, large stabiliser and good dihedral. What's more, it's a very straightforward and uncomplicated design. I'd talked myself into it, then, a Sopwith Pup it would be!

When building very small models such as this you need some additional equipment beyond the usual hobby-type tools used for larger aircraft. Sharp scalpel blades are essential, and you'll need a small set-square and a syringe with which to apply adhesive. Good lighting is important, too, indeed a desk lamp with a magnifying glass is essential for really small jobs. In addition, some scales capable of weighing tenths (or even hundredths) of a gram is useful for comparing materials. We need to keep the weight down, so be careful with your wood selection. Use hard, straight balsa for the longerons, slightly softer for everything else.

In terms of adhesive, I recommend Deluxe Materials Superphatic for all the wood parts. Right, you've got the tools, wood and glue, let's build!

CONSTRUCTION: Anyone having a go at the Pup will have previous building experience so I won't give a full account, rather, I'll focus more on the micro-build techniques. Being able to build straight is the biggest single challenge for the success of small models, so bear that in mind at all times with this one!

The first job is to make the wing ribs, using the sandwich method. Make two 0.3mm aluminium templates, then cut 38 pieces of 1mm and six pieces of 1.5mm medium hard balsa of sufficient size for the ribs. In fact, it's a good idea to make a few more than the 44 required, since they're so easily broken or lost during construction.

The ribs locate over 0.5mm x 1mm rectangular carbon fibre spars, whilst the wing tips are made from three pieces of 1mm x 25mm balsa, laminated and sliced into four pieces about 6mm wide (the tips are fitted when the wing panels are complete). The wing centre-section is built separately, the outer panels being glued to this with 8mm dihedral under each tip. Glue small pieces of 3mm sq. balsa where the wing struts locate, and drill 0.5mm holes as shown on the plan. The struts are made from 0.5mm carbon fibre rods, faced with 0.5 x 3mm balsa.

TAIL FEATHERS Built flat over the plan, laminate the contour of the fin and rudder using three 1mm x 3mm balsa strips in the same way as the wing tips. Control horns are from 0.3mm aluminium, epoxied in place, whilst the 2 x 6mm hinges are cut from drawing film.

FUSELAGE: No surprises here - the fuselage is completely conventional. Build two identical sides, then drill the two 1mm holes in the wing beam ends. Position the sides (upside-down) over the plan and glue together, making sure everything stays parallel. When dry, bring the ends together and glue, then add the transverse strips. The cabane struts are bent from 0.5mm piano wire (exactly as plan) before being added to the fuselage. Note that any deviation in the length of these struts will give the wing an incorrect angle of attack, which will in turn affect the model's performance.

Ok, cut F1 from 0.4mm plywood and F2 - F8 from 1mm balsa, then plank over the forward fuselage with 0.5mm sheet. Balsa pieces, 1mm thick, make up the pushrod exit guides at the rear. The cowl, meanwhile, is made from laminations of 3mm balsa. When finished and shaped, soak it with cyano and cover with paper or thin fibreglass; this is the only detail on the model that can't be too heavy! I used two pairs of neodymium magnets to secure the cowl to the fuselage.

Since the model cries out for a dummy radial engine, we've given you one on the plan! Pretty good, eh? So, cut out the engine detail from a photocopy if you want to keep your plan intact) and glue it to the propeller shaft, ensuring that it doesn't foul the engine cover as it rotates. Finally, to complete the front end, the underside access hatch is made from transparent plastic, secured with a magnet in the back.

LANDING GEAR: Made from 0.5mm piano wire, ensure that you build the landing gear as accurately as possible, otherwise it'll be a poor fit. The crossbar is a length of 0.5mm x 3mm carbon fibre. To make the wheels, laminate throe pieces of 1mm balsa (crossgrain) using cyano, then drill a 2mrn hole in the centre. Shape the wheel cones frorn 3rnrn soft balsa, and glue to the wheels. For the hub, simply glue a 7mrn long plastic tube, then paint and cover the wheels with Japanese tissue.

Cunningly, the 0.5mm carbon fibre rod axle attaches to the middle of the crossbar with thread soaked in cyano, resulting in functional suspensionl As for the wheels, these are retained using the Insulation from a standard servo cable - fiddly, but effective.

DETAILS: Adding a few scale details really gives life to this little model. A Vickers machine gun can be made from 2mm aluminium tube and balsa, using 0.3mrn carbon fibre for the detailing. The pilot is carved from soft balsa, with Japanese paper providing the obligatory scarf. A carbon fibre and balsa pitot tube looks good too, and you can also make the scale control horn with control cables if you wish, but I didn't bother. I guess you have to draw the line somewhere!

COVER AND PAINT: I covered the prototype with Japanese Esaki tissue, using plenty of dope on the underside of the ribs to make sure that it stuck. The tissue is then carefully tightened with water and coated with three layers of diluted dope. One very important point here is to keep all the covered parts (except for the fuselage) clarnped down after water tightening or doping, to ensure that no warps appear as the panels dry.

Spray paint the parts using the smallest amount of Humbrol enamel, diluted with cellulose thinner. Almost all British WWI warbirds had the upper sides painted with PC 10, a shade that can be mixed using equal parts of Hurnbrol No. 98 and 155. The underside is sprayed with No. 121, the nose and cowling with alurninium No. 56. As for the roundels, these were sprayed with No 25, 34 and 60, with masks cut from Frisket film. Lastly, the fin text and registration numbers are water decals, printed from an ordinary colour printer. The propeller was sanded with 600 paper and painted brown with a little dry brushing of light yellow and dark brown to simulate wood grain. Meanwhile, ink lines mark the ailerons, while adding some rivet detail adds much to the overall scale impression. To finish, I gave the Pup a thin coat of satin varnish No 135.

WEATHERING: To emphasise the ribs and framework under the covering, watercolour pencils can be used to very good effect. I also sprayed a weak mix of dark brown acrylic paint onto the underside of the fuselage nose and wings, and to simulate running castor oil I applied a well-diluted mix of varnish mixed with brown, brushed on and blown (a tip I picked up from none other than World Scale champion Pete McDermott!).

FINAL ASSEMBLY: With all the major components built, covered, painted and detailed, we can now assemble what is, in effect, an ARTF. Start by gluing the stabiliser and fin into place, then mount the engine and secure the R/C gear (glued in). Make up the pushrods and add Z-bends to connect them to their respective control surfaces before fitting into the fuselage and through the exits at the rear.

Glue the wings in place using epoxy and then add the struts, carefully checking all the rigging angles as you go. When dry, mix up some more epoxy and fit the landing gear. The wing rigging is simulated using elasticated thread (supplied by SAMS, www.samsmodels.com), painted grey and secured at the ends using PVA.

Getting the CG right on a model as small as this is crucial, indeed a few millimetres either way really can be the difference between success and failure. The Pup has a short nose, so by design tends to be a little tail heavy. Nobody likes adding ballast to a model, particularly one this small, but needs must. As a result I had to add 2.5g in the nose of the prototype, some 10% of the model's entire weight! So, take heed and build the rear of the fuselage as light as possible. With the control deflections set to ±4mm for the elevator and ±5mm for the rudder, I dialled in about +30% expo in order to get a well-mannered, smooth control response.

FLYING: When it comes to flying this little 'un it should come as no surprise that nothing other than breeze-free conditions will do. Indoors is best (hall size at least 20m x 40m), however she can also be flown outside on a flat calm summer's evening. With the CG spot-on and the control surfaces at the aforementioned settings, the Pup is smooth handling and well-mannered in flight. I've been flying her for a couple of months now, and she continues to perform really well every time, cruising happily at around 50% throttle. Keep the power on during the landing phase, gently easing her down to the ground. The only part of the Pup's characteristics that's really tricky is the ground handling, so don't say I didn't warn you! Aerobatics aren't part of the remit, although when flown outdoors or in a bigger hall she will loop and stall turn.

This has certainly been a very exciting project. It's very stimulating and educational to build small. The need to carefully monitor material weight and hone the fine motor skills of your fingers could prove very useful for future projects. The Pup's material cost is negligible, whilst the construction and flight enjoyment is immense. Go on, challenge your building skills - are you big enough to build a small model?"

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