April 8

Stage Four – Building Balsa RC Airplanes

Steady progress is being made on my Hawklett andJoining Wings I now have the second wing panel built and joined to the first one. The retracts are fitted into the wing panels and connected to the centrally mounted servo.

A certain amount of patience is required when building balsa rc airplanes as glues have to be given adequate time to dry. Some people are happy to use fast setting CA glues but I prefer to use White or Yellow aliphatic glues for the majority of wood to wood joints and slow setting Epoxy glue for high stress joints. These do require that the joints be left for the glue to penetrate the wood fibres and dry completely.

You will see in the photo right how I clamped the wing panels to my work table and used some old six cell Lipo packs to weight the joint down while the slow set epoxy glue dried. The trailing edges  are blocked up  so that the centre line of the ribs are horizontal all the way along each panel.

The circular item near the joint area is one of the wheel wells for the retracts. there is another on the other panel hidden from view in this shot.

Strength Considerations

Parallel chord wings are relatively simple to brace across theWing Dihedral Brace centre join line using what is known as a ‘dihedral brace’. This usually takes the form of a piece of plywood that is glued to the front or rear of the main spars, full depth across the centre joint line and extending outward several rib spaces either side

When joining swept back (or forward) wings  there is no straight line available between the two wing halves to which the brace can be glued flat. Instead the brace has to be cut to a length that fits across the centre glued ribs and is slotted to accommodate  one or two ribs before terminating at the junction of a pair of outer ribs and the main spars. Hopefully the illustration here will help you understand this arrangement.

Further strength is ensured by gluing a web of 1.5mm (1/16″) balsa between the top and bottom main spars with the grain running vertically the full length of the wing apart from the outermost three bays on either side.

Installing The Retracts In The Wing

This process is a little involved and care had to beWing Centre section With Retract Servo taken when cutting the pushrods and outer support tubes to the correct lengths. The linkage to the central servo is via ‘z’ bend 2mm rods set into Sullivan ‘Gold-N-Rod’ look-a-likes whilst the linkages to the retract activator rods are via metal clevises. These are mounted on threaded rods so that fine adjustments are possible.

When retracted the wheels fit into round wells that I made from rolled cardboard cut from the sides of a cereal packet. These are rolled round a suitable diameter spray can and glued. once dry they were mounted on false bases glued between the appropriate pair of wing ribs. The retracted Retracts & Servo Installationwheels would actually interfere with the inner ribs so cut outs had to be made in these ribs to accommodate the false bases and wells.

Before connecting the pushrods to the servo I connected up my trusty servo tester to both the wing servo and the fuselage servo that drives the nose leg. This enabled me to determine the correct way to connect these push rods to the servo and match the operation of the nose leg.

In the past, I have used a single wing mounted servo to operate both wing legs and nose leg on a trike retract arrangement. The only problem with this is that there has to be an easily connectable linkage for the nose leg when fitting the wing to the fuselage. This can be quite difficult to arrange so I decided to go for two servos connected by a ‘Y’ lead.Nose Wheel Retracted A much simpler arrangement although carrying a small extra weight penalty of the additional servo.

You may have noticed in the last two photos that there is a cut out in the centre leading edge where the wing sits against the fuselage. This is to accommodate the nose wheel where it extends beyond the limit of the fuselage underside when retracted. The sketch on the right shows this more clearly.

Adding Trailing Edges & AileronsWing Tip Block

Referring to the pictures above, you will see that I have attached the central trailing edge section and the wing tip blocks. The next step is to carve and sand these to shape prior to cladding the wings with the 1.5mm (1/16″) balsa sheeting.

The photo on the right shows one of the wing tip blocks glued in place. The full depth of the end rib at its widest part is 35mm (approx. 1 3/8″). The depth of block I required was achieved by gluing together three laminations of 12mm (1/2″) soft Balsa.Aileron Servo Plate Mount

My favoured approach to installing the aileron servos is to fit two plywood rails across one of the rib bays. The servos are attached to 1.5mm (1/16″) ply plates on their sides so that the output arm protrudes through slots in the plates. The plates screw down to these rails.

All that needs to be done before sheeting the wings is to face the frontal area that fits up to the fuselage bulkhead with 1.5mm (1/16″) plywood prior to drilling and fitting the wing locating pegs.Aileron Servo Plates

The rear wing bolt holes will be drilled through a ply reinforcing plate. This will be fitted once the wings have been sheeted and the underside fuselage fairing has been glued in place.

The ailerons are cut from 12mm (1/2″) medium balsa and sanded to the correct profile, tapering to 3mm at the trailing edge.


We’re Getting There

Next time I hope to be able to show you the model in some of here colours. The covering has arrived and just as soon as I have finished the woodwork I will be getting down to some serious heat shrink covering. This is not a scale model so I have adapted an early RAF/ Swiss Air Force colour scheme for my plane.

I have to say that I really enjoy scratch building balsa rc airplanes and so far this build has not disappointed. There have been a number of modifications necessary and these challenges add to the enjoyment of creating the finished article.

This is a totally traditional build process using wood and other materials common to models going back to the earliest days of rc model planes. Maybe one day I’ll get round to using more modern materials but for now I just love the feel of these traditional materials.

Don’t forget to check out my main website, www.rookiercflyer.com, especially if you are a newbie. Everything you need to know about getting started is there. This is the fourth post in this build series so if you want to go back to the beginning and follow it through from the start, go to “How To Scratch Build RC Planes“.

Come back next week for the next episode of this build blog.



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Posted April 8, 2016 by Colin Bedson in category "Modelling Skills

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