Construction Guide

Part III

After the complex job of fitting the firewall in last months article the installation of the servo tray is relatively easy.  The servo tray fits horizontally between the cockpit and the wing saddle and will house the elevator servo, rudder servo, throttle servo, receiver, switch, main tank and header tank.  A very important former that needs fixing securely and once again good preparation makes the job easier.  I have heard many stories of servo trays coming loose in flight causing the demise of the model, so it is vitally important to get the former fixed securely into the fuselage.  This former is a carbon nomex laminate.  I was introduced to Nomex a few years ago.  It looks and feels like very thin, stiff brown paper but has a very high specification.  One application is in the manufacture of fire suits for Formula I drivers.  The form useful to modellers is a honey comb structure.  The honey comb is one of the strongest structures know to man, with a layer of carbon on each side you have a very strong and light structure.  This is perfect for application in models, fitting a model with formers made of carbon nomex laminate can save ounces over the conventional ply ones and be a lot stronger.

I have found it best to prepare the former for the tank and radio gear before it is installed as cutting it for servo holes inside the fuselage can be problematic.  This is where having built a Synergy last year pays dividends as I know exactly where everything needs to be fixed.  The tank bay area is already cut but usually needs opening up slightly to accept the tank.  I have found it necessary to protect the tank as the carbon edges are sharp and the vibration can gradually cut into the tank.  So a strip of thin foam is stuck round the tank and header tank cut out.  The tank will be secured with Velcro so two slots on either side need to me made to slot the Velcro through.

The carbon nomex laminate does not secure servo screws well so it is a good idea to add some ply to take the screws.  The elevator servo also needs to be raised slightly so the pushrod will clear the rudder servo; this is easily done with a single thickness of light ply.  The throttle servo can also be positioned; this is as far forward as possible to reduce the length of Bowden cable push rod used.  A mini servo is used here to save weight as torque is not an issue.  I have found some excellent servo screws available from www.modelfixings.co.uk that I think are better than those supplied from servo manufacturers.  The switch and receiver also need to be mounted on this former and the location is up to the individual.  The attached photograph shows my chosen layout that I have found works well.  I changed the receiver position on this model to take it away from the rudder servo.  A hole was cut in the servo tray for the receiver and it will be secures with Velcro.  This takes the receiver away from the rudder servo and makes the installation neater. 

When the servo tray is complete it needs to be fixed in the fuselage using laminating resin mixed with cotton fibres.  It is important to sand the area in the fuselage and a thin strip on each edge of the carbon/nomex sandwich to help adhesion.  A small fillet can be created similar to the one around the firewall to ensure nothing comes loose.  Once the former has been stuck into position the throttle linkage can be installed, nothing special and is just done using the standard method of Bowden cable running in a sleeve.  A small former is usually required to hold the sleeve in place, I use a small amount of light ply fixed perpendicular to the former, with the sleeve glued through the firewall as well, the Bowden cable should run smoothly.

The wing is the next area to be tackled, with this build I have had the wing sheeted, shaped and joined by Probuild.  So on delivery the wing is basically ready for covering.  The wing has a foam core and sheeted using very light contest grade balsa.  At the same time as skinning the aileron hinges made from Kevlar are glued into place, the Kevlar cloth is sandwiched between the balsa and the foam.  This forms a very light weight strong and surprisingly flexible hinge.  Each wing panel is finished with a balsa leading edge and tip.  The servo box is also to cut to suit your choice of servo with the servo lead hole cut through the wing core. 

I received my last set of wings un-joined and I have joined them.  The joining of the wing needs due care and attention as it is easy to get the panels misaligned.  The centre of the wing is a high stress area especially at the junction of the fuselage and wing.  It is imperative not to try and save weight by using one small central reinforcing layer of cloth; the wing would not survive long term in high stress manoeuvres.  A flat surface is vital and the wing tips should be parallel to each other, a wing incidence meter is helpful as well as a rule.  I have found it very useful to lay the wing panels in the foam the core was cut from, this provides support for the wing and is easily adjustable with small amounts of packing.  Alternatively you can make a couple of ply templates and fix them to the centre lines of the wing tips, the depth of the template should be equal to the dihedral of each wing panel from the wing tip.  The root of the wing should also be sanded so alleviate any deviation in the panels.  Both panels should meet exactly.  If all is correct after checking and re checking the two panels can be joined using Cherbourg laminating resin.  When the epoxy is dry the wing can again be checked for alignment and checked to make sure the centre joint is sealed.  A light coat of dope is then applied to the centre section of the wing; this seals the balsa to prevent the epoxy soaking in adding unwanted weight.  When dry the dope can be lightly sanded and all dust particles removed.  The surface of the wing can then be wetted with epoxy.  Three layers of 80g glass cloth will be added on the top and bottom, one of width 500mm, one 400mm and one 340mm.  The largest piece is placed on the wing first; the cloth can be wetted using an old credit card or stippled using a brush.  It is important not to use too much epoxy, there should be just enough to wet the cloth and make it go transparent, the second and third layers of cloth are then added to the centre section.  Little or no extra epoxy should be needed to wet these layers of cloth.  Any excess epoxy should be scraped off.  The process is then repeated for the other side of the wing.  Once dry the glass cloth can be sanded to a smooth finish.  If the correct amount of epoxy has been used you should be able to feel the weave in the cloth.  Filler may be required at the edges of the cloth and a weave filling layer of epoxy and fairing compound can be applied using an old credit card.

My wing came joined but with woven carbon fibre cloth used instead of the glass cloth which provides a lighter and stronger wing.  I decided to get Probuild to join the wing, as not only was I looking for it to be lighter but I also wanted greater accuracy.  I have always found that the tips have been slightly out when finished so hopefully having Probuild join the wing will get rid of this inaccuracy. 

The wing is attached to the fuselage using four M6 nylon bolts and two ply cross members.  The ply cross members need to be fixed into the fuselage to accept the blind nuts that the nylon bolts screw in to.  With the moulded fuselage the wing saddle comes ready formed to the correct airfoil section along with wing fillets so no sanding or shaping has to be done here.  The wing comes ready drilled from Probuild so the wing is placed onto the saddle and the positions of the holes are marked onto the fuselage.  At this marked point the two wing saddles are fixed into position across the fuselage.  These are fixed with an epoxy and cotton fibres mix.  Once the glue has dried the wing can be placed onto the fuselage and checked for alignment.  Five measurements need to be made; wing tip to fuselage on the leading and trailing edge and wing tip to the back of the fuselage all ensure the wing is square.  If all is correct, check again just to make sure, then the holes can be drilled through the wing and into the formers I have fixed into the fuselage.  It is a good idea to drill two holes and put the wing bolts through and check the alignment again to make sure nothing moved.  It is also easier to hold the wing down to drill the last two holes.  The wing is removed and the holes can be enlarged for our manufactured blind nuts which are then glued into position.

Next article we shall move onto the tail plane.

Click here for Part IV

The Original Series of Articles was published in RC Model Flyer

It is published here with the kind permission of Ken Sheppard. Editor - Model Flyer