After suffering the complexities of both fully built up and foam RC Plane wings for about eighteen months, I decided to investigate the possibilities of designing and building wings from solid balsa sheet. The initial inspiration for these came from the Mini-Phase—many other readers must have wondered, as I did, at the simplicity and efficiency of those solid balsa wings. Since then I have built and flown several different slope rc soarers having solid wings, and have experimented with both parallel and tapered wing planforms and sections. The culmination of these experiments appeared in the shape of Poltergeist, a design that has inter-changeable wings of 50 and 73in. for use as either a fast aerobatic fun model, or a pretty effective soarer.
As may be imagined, the density and strength of balsa is proportionate to its weight, and the weight of a 3 x 36in. (12 x 76 x 915 mm) balsa sheet used in the wing construction is shown. Also illustrated are some design studies for different types of soarer wings, along with the density of balsa required to give the wings enough bending strength for the normal flight stresses that each type of glider is likely to encounter. The less mathematically minded will want to know that the weight of a sheet of balsa with a density of 51b ‘cubic ft. weighs 2.oz. (70gm), and the weight for a cubic ft. sheet would be 31oz. (91grn). I discovered that the advantages for this type of construction are numerous. The rapidity of construction is notable as a set of wings need take no more than three evenings to complete including covering. The cost of a pair of medium to small soarer wings is comparatively economic, and they are simple to repair. A dented or crumpled area can be cut out and a block glued in and carved to shape, while a completely broken (sheared off) wing panel may be simply glued together
Right: the Poltergeist 73 sailplane has an all-up weight of 22oz. (625gm) and uses 61b/cu. ft. density balsa. A sheet k x 36 x 3in, therefore weighs 3oz. (12 x 915 x 76mm sheet weighs 85gm). Below, the Poltergeist 50 has an all-up weight of 23oz., and uses 5/cu. ft. balsa (sheet described above weighs 2-2-,oz.-71gm).
Left: example ‘A’ has aerofoil thickness of 6.25%, uses 51b./cu. ft. balsa, maximum loading 160oz. to the sq. ft. ’13’ also has an aerofoil thickness of 6.25%, with 51/cu ft. balsa, but the wing is over twice as heavy and is not capable of withstanding as high a `g’ as A. Example ‘C’ employs a 8.33% aerofoil and 6.51b./cu. ft. balsa, and can withstand a breaking strain of 60oz. per square foot.
Again using the exposed fibres of the break as a natural key. A designer using solid wings can afford to be adventurous as the problems of tapered, swept and even crescent wings are consider-ably eased by the absence of spars and braces that have to be lined up. In fact, tapered wings are to be preferred for this type of construction, as this planform concentrates the bulk of the wood (and its strength) at the point where it does the most good, at the wing root. Medium to low aspect-ratio wings are best to start with as, although they are heavier for a given area, they are also stronger. (Mathematics applied to this problem proves that a RC aircraft wing of 6:1 has a loading strength of almost nine times that of a wing of equal area and an 18:1 aspect ratio.) The aerofoils used on the Poltergeist design are ideal for this type of construction, being not too thick, basically flat-bottomed with a low cambered mean line, and having a sharp leading edge. Other thin (5 to 10 %) aerofoils suitable are the Gottingen 795, Sigurd Isacson 3306, Eppler 387 and Eppler 193. After deciding on the density of balsa required, the modeller may descend on the local shop armed with a spring balance and a sheet of masking tape (the latter for providing an attachment point on the balsa for the balance—shopkeepers, the exact weight of wood cannot be found, choose some slightly on the heavy side, as lighter wood could fail under a lighter loading, and prove to be a false area for saving weight. It is better to take the excess weight out of the aircraft fuselage, where excessive loads are more likely to be caused by a crash rather than flight stresses. The dimensions of most wing panels will require that a number of sheets have to be joined to pro-vide a wing blank. Any sheets heavier than usual should be concentrated at the root and at the centre chord position, to provide the best effect in the strength of the wing. A combination of edge and scarf joints using PVA glue gives a suitable amount of strength with-out too much complexity, and these can be held together with masking tape while drying. When dry, the wing “blank” may be cut to plan outline with a stout bladed knife, using a steel straight edge, preferably of sufficient length to reach from root to tip. Trim the root and tip ends, and mark on them the chosen aerofoil.
Check and re-check at this stage to ensure that a right and left wing will result, and not something like a right wing and a left wing with a swept forward trailing edge. If the wing has a tapered thick-ness, this has to be dealt with next. Fig. 1 shows the wing at this stage, with a taper line marked on the leading and trailing edges. Saw-cuts are made, passing through the waste wood down onto the line, and these prove a great help when carving down onto the taper. A razor-plane will be found invaluable during this, and following, stages. If a definite procedure is followed while carving the aerofoil, no problems should result, and I have found the method about to be described works well. First, mark lines tangential to the aerofoil drawn on the root and tip, as shown in Fig. 2. Using the straight edge, join the points where the tangents emerge on the wing blank surface and edge, and carefully carve down to this flat area outlined. This procedure is repeated, as in Fig. 3, until a rough aerofoil appears, at which point the razor-plane is swapped for a sanding block in order to complete the shaping. Templates providing the aerofoil at intermediate stages along the wing are not essential, but may assist those who are doubtful about their ability to carve down to a straight line. Very little is left to do in order to complete the wing. Wing tip blocks are the same as any other type of wing construction, having the grain running across the chord. The ailerons are really simple as all that has to be done is to cut them out and provide the clearances for the movement. No more false trailing edges or hinge block to be fitted, the completed aileron is just cut from the wing. Joining the wing panels should cause no problems, as modern glues are strong enough to ensure that a well made joint is stronger than the wood surrounding it. Small models only require a glued butt joint, while the Poltergeist designs have a butt joint with the addition of a thin ply cover strap on the underside that acts as a reinforcement and a rc plane wing, locator. Should any additional bracing be required, ply joiners may be simply inserted into the root. Should it be thought necessary,harder balsa spars may be laid into the wing although, if the wing has been correctly built and stressed, such a measure should be unlikely. The wing is suited to any form of finishing method, although the use of heat-shrink film can be recommended, as this gives a very professional finish with the minimum time expended. Why not give this type of construction a try? There is every chance that the wings you will be building will be more pleasing in their efficiency and ease of repair than any other you have used.
All materials used in this guide to construct the different types of RC Plane wings have been purchased from www.r-c.uk – They are our preferred supplied for anything related to remote control aircraft.