Building your first low-wing model - Pt.1

The Great Planes Venus 40 is an ideal first low-wing model

In the February to May 2006 issues of RCM&E I told you how to assemble and fly your first ARTF high-wing R/C trainer. Judging by the response I received, that information proved useful. But, several readers also posed another question: How do I tackle my first-ever low-wing ARTF sports/aerobatic model?

Wonder no more! Your wish is my command, and this time Im back armed with an attractive low-wing ARTF design that Ill be assembling over three build tutorials. The box of bits will be transformed into an exciting and flamboyant aerobat!

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Remember that ARTF models employ similar construction methods so whilst I’m using the Great Planes Venus 40 as an example, you should find the techniques I’ll describe transferable to most other low-wing ARTF models.

GREAT STUFF!

The Venus 40 is a Don Anderson design for Great Planes. Its claimed to be an ideal introductory aerobatic machine thats both pleasing to the eye and easy to fly.
The big, bright, colourful kit box certainly sets the pulse racing! At 55 span this sleek, slippery creation is destined for a four channel R/C system with five servos (independent aileron servos operated via a Y-lead) and a .40-.51 two-stroke or .52-.70 four-stroke engine.

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The airframe is a traditional balsa/ply construction of high quality, wrapped up in Monokote heat-shrink film. The fuselage, wing panels, horizontal and vertical tail parts, plus the control surfaces, are integrated with a nine-colour décor scheme that many of us aspire to but few can achieve. The GRP nose cowl, GRP wheel spats and clear canopy are pre-decorated to match the main colour scheme, and a set of self-adhesive decals add that finishing touch.

The photo-illustrated, 28-page assembly manual is typical of stateside aeromodelling thoroughness – every kit part is listed. Additional items required, such as suitable glues and tools, are fully documented. When youve assembled the model and installed the engine and radio gear according to the pictures, accompanying text and tick-boxes, the crucially-important airframe balancing and flying aspects are explored in adequate detail.

Very little other than the appropriate glues, some simple tools, plus the engine and R/C system, are needed to complete the model as the good quality poly-bagged accessory package is ultra-complete. But what about the specifics of the build? Im coming to that

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FIRST CONTACT

Before assembling the airframe components, any lifting covering panel seams and/or trim pattern edges must be re-sealed, the covering material should be tightened up and bonded firmly to the wood. Do this with a modellers iron set at medium heat. A press and slide action works well; move the iron from the middle outwards over the trim pattern edges.
Crank up the iron to hot (Monokote can take it) and go over the main covering expanses so that all visible sags vanish. Prick stubborn bubbles with a pin and press flat, if required. Avoid the just-sealed seams and trim pattern edges as the increased heat could cause distortion. Follow the hot iron as you go with a Top Flite Hot Glove or wad of kitchen towel to thoroughly press down the covering over the solid sheet areas. A heat gun works well too for the overall taughtness, but again dont linger over the previously-sealed edges.

WELL HUNG?

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Now we attach the ailerons and fit the independent aileron servos.
Drill a 3/32 hole in through the centre of each pre-cut hinge slot on the trailing edge of both wing panels and the leading edge of both ailerons. This hole enables thin cyano to be injected in with a narrow-tip applicator to zap the hinge when the wing panels and ailerons are correctly dry-assembled.

Cutting out the cyano’ hinges

Cut the hinges from the supplied fibrous material with a sharp scalpel or scissors. Marked out with a Biro and straight-edge as recommended in the instruction book, you get a total of twenty-four hinges, eight of which are used for the ailerons. Snip off the corners of the cut hinges at 45-degrees to allow easier insertion into the slots.

Using a sharp scalpel, cut the covering material from around each hinge slot and re-seal for a neat effect. Loosen each slot slightly with a broad number 18 chisel-tip blade in a heavy-duty modelling knife handle and test fit the hinges in the wing panels and ailerons. When hinge fits are smooth and accurate, but not sloppy, dry-fit the ailerons to each wing panel to see how everything lines up. When satisfied, take everything apart again.

Re-insert the hinges accurately half-way into both ailerons, making sure they project at right-angles to the hinge-line. Using thin cyano dispensed through a narrow-tip applicator, zap each hinge solid in-situ through the previously-drilled holes. Dont use too much cyano and ensure that it doesnt seep far out on the wing side of each hinge.

When the cyano has hardened, attach the ailerons to both wing panels. Once more fettle for the best fit and alignment possible, aiming for a minimum hinge-line gap (just enough to allow a strip of paper through) combined with full-and-free up and down movement. Zap the ensconced aileron hinges through the holes on the wing trailing edges. Bingo perfectly mounted ailerons!

ROLL CALL

The Monokote hiding the independent aileron servo bays and the wing root servo cable exit holes is cut away 1/8 inside each opening with a sharp scalpel and sealed down with a mini iron equipped with a contoured shoe tip before fitting the aileron servos. At this stage, be careful not to disturb the factory-installed string running between each servo bay and the root ribs; this is used to pull the servo cables through on the next step.

I will describe how to sort one aileron servo and its linkage. The other servo is mounted and connected up in exactly the same manner. Tie the outboard end of the string to the servo cable end-plug, then carefully feed the end-plug up and out through the hole in the wing. Untie the string and keep the servo end-plug in place on the wing panel top centre section covering with some masking tape. Accurately align the servo in its bay and mark the location of the mounting holes on the lower wing skin with a stout modelling pin or fine-point felt-tip pen. Remove the servo and drill at the marked points with a 1/16 bit a hand-held pencil drill is especially useful for this job.

Take your time fitting servos, it’s time well spent that’ll be repaid in the air

Sparingly inject thin cyano into the drilled holes. This toughens the wood and enables greater screw purchase. When the cyano has cured, permanently screw down the servo, being careful not to over-compress the grommets.
With the radio system switched on and the aileron servo neutralised, fit a straight servo arm. Use a plastic set-square placed at right-angles to the hinge-line to mark a line on the aileron underside that is in line with the hole in the servo arm. The line-intersection point beneath the aileron is where the control horn will be centred when its mounted. Mark this position with a fine-point felt-tip pen.

Mount a control horn on the line you marked on the aileron. Ensure that it sits centrally on the line and that the holes on the upright arm remain centred over the hinge-line – this is to ensure equal angular deflection. When its correctly positioned, hold the control horn steady and lightly drill through its mounting plate holes into the aileron; this is just to mark the surface. Remove the horn and accurately drill through the aileron fully. Ensure that the drill remains at right-angles vertically to the aileron as you bore. Trim away the covering around the drilled holes with a sharp scalpel and permanently mount the control horn in place.

Screw a clevis half-way on to the aileron kwik-link wire rod thread so that equal amounts of screw adjustment will be possible later. Snap the clevis onto the horn and lay the kwik-link rod back over the servo arm. Tape or pin the aileron at neutral, making sure that the radio system is live with the aileron stick and trim neutralised. Mark the kwik-link rod fractionally behind the servo arm hole (towards the wing trailing edge) with the fine-point felt-tip pen, then remove the rod and bend it downwards at 90-degrees. Clip excess wire away with a good quality pliers about 3/8 beyond the bend. File the clipped wire end clean and re-fit through the outermost servo arm hole using the supplied nylon swing keeper. Any slight adjustment to the linkage is now easily achieved by screwing the clevis in or out. Done properly, no more than two turns either way is required.

Follow the same mounting and setting-up arrangement with the other servo and aileron.

CORE PRINCIPLES

Stick three or four masking tape strips chord-wise adjacent to each root rib top and bottom; this is to prevent excess epoxy spoiling the covering when the wing panels are glued together. Also ensure that both servo cable end-plugs are firmly anchored with masking tape to stop em flopping about.

Dry-fit the wing panels together, familiarising yourself with the insertion orientation of the stubby ply dihedral brace. Make any wood trimming adjustments now to ensure a gap-free wing panel fit. When satisfied, separate both panels and remove the brace.

The wing brace in place, note aileron servo lead exit

Mix up some twenty-minute epoxy and apply it generously inside the right-hand wing root brace cavity and to the right-hand half of the brace. A length of 1/4-square hard balsa, tapered with a razor plane to form a spatula, is good for applying the epoxy. Feed the brace into the prepared cavity and wipe away excess epoxy with kitchen towel. Ensure that the brace is fully pushed home and allow the epoxy to cure.

Next, cook up some one-hour epoxy. Thoroughly coat the interior of the left-hand panels brace cavity, both wing root ribs, plus the projecting left-hand half of the brace. Use the (cleaned-up) spatula stick again and ensure that the epoxy is thickly but evenly spread on. Bring both wing panels together firmly, checking for proper alignment. Ignore minor depth-wise differences between the wing panels, but ensure that both the leading edges and trailing edges are accurately centred. Use modelling pins to hold correct alignment here, if needed. Wipe away the squeezed-out epoxy along the top/bottom root rib interface with meths or surgical spirit and a kitchen towel.

Use long strips of masking tape stretched span-wise across the centre section top and bottom to maintain joint tension. Most likely, yet more epoxy will be forced out along the joint when doing this, so wipe that away once more. Place the joined wing panels flat on the bench and allow to dry.

When the epoxy has set, remove the masking tape strips, plus the aileron servo cable end-plug tape-fasteners. Re-bond the covering if needed as the tape removal may have locally loosened it. Clean the centre section one last time with meths and kitchen towel and admire the tidy job!

Now, we’ve got to mount the assembled wing onto the fuselage

ATTACHMENTS

Two leading edge hardwood dowels and two trailing edge nylon bolts hold the wing in place on the fuselage. The hardwood dowels are epoxied into pre-drilled wing leading edge holes, while the nylon bolts screw into pre-installed metal captive nuts on a transverse-mounted fuselage ply plate.

Sand the dowels lightly with glass-paper and mark each one 1/4 from one end. Insert some twenty-minute epoxy with a cocktail stick into the wing leading edge holes and coat each dowel with glue the long distance back from the marks. Employ a push and rotate action to ease the dowels in to the holes as far as the 1/4 marks and wipe away excess epoxy. Ensure that the dowels fit easily prior to applying the glue, as theres nothing more panic-inducing than trying to force-fit dowels that are too tight for the holes with the epoxy setting fast!

When the dowels are set solid, burn through the centre section Monokote with a flame-heated brass tube to reveal the pre-drilled bolt holes. You may prefer to simly cut away the film for a slightly less tidy effect. Mount the wing on the fuselage by engaging the dowels into place and check the fit of the bolts into the captive nuts through the pre-drilled wing holes. Use a round file on the wing holes, if necessary, to ease clearances and allow a smooth but snug bolt fit.

An anti-crush ply plate stops the tightened-down bolts from compressing the wing surface. The distance of each bolt hole from the wing trailing edge is first marked on the wing underside covering with the fine-point felt-tip pen. The bolts are then removed and the plate is centred with this span-wise line, and also the root rib joint-line. The plate outline is now marked on the wing underside with the fine-point felt-tip pen and the film is scored 1/8 within this perimeter using a sharp scalpel, being careful not to cut into the underlying wood.

With the covering patch removed, the plate is epoxied and clamped in place. Clean up epoxy seepage as required. When firmly bonded, drill through the plate via the pre-formed wing bolt holes and tidy up the drill-perforated Monokote with a mini iron and contoured shoe tip.

FAIRING FINALE

A large belly-pan covers the wing centre section underside. This is put in place with the wing securely attached and the model lying upside-down on the floor.
Open up the covering over the wing bolt holes on the belly-pan and heat-seal the edges with your mini iron. Locate the belly-pan accurately over the wing centre section underside so that it aligns evenly with the front and rear fuselage bottom surfaces. Tweak the fit so that its centred with the fuselage and get the front and rear gaps to roughly the same clearance. When its sitting comfortably, temporarily tape the belly-pan to the fuselage front and rear bottom surfaces and mark with fine-point felt-tip pen its external outline on the wing underside.

The belly pan – the covering needs to be cut away where the glue is required – wood-to-wood adhesion is essential

Remove the belly-pan and cut chord-wise with a super-sharp scalpel 1/16 inside the marked lines. Make an additional chord-wise cut 1/2 further in from the cuts already made so that a strip of Monokote may be peeled away each side this is where the belly-pan will adhere to the wing. Be extremely careful to only cut through the covering material and not into the sheet balsa underneath! Heat-seal the cut covering edges on the wing centre section underside, if necessary.

Trim excess Monokote from the wing-facing edges of the belly-pan before sticking it in place, leaving a 1/16 overlap to seal down. Mix up a batch of half-hour epoxy and carefully coat both the belly-pan edges and the areas of bare balsa on the wing centre section underside. Dont use too much glue, but ensure that the mating surfaces are evenly coated. Carefully reinstall the belly-pan, checking one final time for correct alignment. Weight it down with a (sealed!) bag of sugar, ensuring that it doesnt go crooked in the process. Keep the wings level span-wise while the glue dries. Wipe excess epoxy away with the balsa strip spatula, followed by meths and kitchen towel.

Remove the ballast when the glue is dry and admire your nicely aligned belly-pan, which streamlines and unifies the fuselage underside contours.

Its a good idea to place polythene sheet (Solarfilm backing is ideal) over the wing seat and in front of the fore/aft radio bay formers before bolting on the wing to do the belly-pan attachment job. This protective measure prevents accidental epoxy emissions from making the wing-to-fuselage relationship more permanent than intended!

She’s the perfect model for a low-wing novice and indeed for any flyer