Wing structures and construction tips

Model Airplane News, May 2002 by van Mourik, Dick

Everyone agrees that wings are a vital part of any airplane; most aircraft don't perform well without them! Compared with fuselage construction, for which a variety of construction methods can be used, there are far fewer ways to build a proper wing. A good wing structure should be able to withstand the forces that act on it during flight, but other features should be considered, too. The wing's span, average chord length and the model's intended airspeed all influence its design. It is also very important to keep the wing's weight to a sensible minimum. The main parts of a wing are its ribs, spars, and its leading and trailing edges, but the construction possibilities for these depend on the particular model. A fabric-covered, wire-braced biplane wing, for example, requires a much different construction method than does a fully sheeted cantilever wing of a WW II fighter, which might additionally be equipped with flaps and retractable landing gear. Let's take a closer look.

SPARS

The spars are the basic foundation for each wing, and it is vital not to skimp on them. When you look at drawings of full-size wings, you'll notice that the main spar is positioned at the thickest point of the chord-generally, around 30 percent when measured back from the leading edge. This is done for a purpose. Regardless of the material used, it is much easier to bend something when it's lying flat than it is when it's standing upright. Plainly said, the thicker the wing section, the more difficult it will be to bend. Figure 1 shows various spar constructions with their relative moment values. To achieve the maximum bending resistance of the wing, it is important to properly connect the two spars with a shear web. A thin wing such as those commonly used on biplanes is often insufficient to allow the spars to be spaced far enough apart to produce a cantilever structure; this is why functional bracing is required. With a braced biplane, I usually omit the webbing to save some weight. From a stress-distribution point of view, it is better to brace a wing with flying wires. but aerodynamically, the biggest disadvantage of doing so is the enormous drag caused by this spider web. For smaller modelsup to about a 60-inch span-I often install an auxiliary spar in the center of the ribs or make the main spar from very large balsa, onto which the ribs are then slid. This way, the wing gains sufficient strength, and the flying wires do not need to be functional. I usually make my spars out of basswood or spruce, although I have used hard balsa in the past. For highly loaded models, I add carbon fiber or Kevlar to the spars with some epoxy resin. This adds tremendous strength to the structure.

During normal flight, the upper and lower surfaces of a wing suffer from opposite forces; the upper side is pushed toward the center (compression), while the undersurfaces have to cope with a pulling force (tension); see Figure 2. To prevent the spars from breaking, a joiner web should be glued between the spars; this is generally made of vertical-grain,1/16-inch balsa sheet, but very thin, 1/64-inch plywood can also be used. If everything is properly glued, adding shear webbing increases a wing's moment-againstbending strength by a factor of 24!

The wingspan also directly influences the spar's dimensions and the thickness of sheeting that's needed. The bigger the span, the more the structure's weight comes into play. As in every construction, however, it is best to avoid stress points. Figure 3 shows the difference between a full-size aircraft's tapered spar and an acceptable modeling spar structure. Tapering the spar out toward the wingtip ensures a good distribution of the flight loads.

Last but not least is the model's speed. It is easy to understand that slow-flying biplanes or light planes require lighter spars than do racers that have to endure higher wing loads. The bigger and faster the model, the larger and stronger its spar needs to be.

RIBS

Wing ribs serve mainly to give shape, and therefore, they do not need to be very strong. Their thickness, however, should be sufficient to allow proper gluing to the wing sheeting. The bending forces described earlier act almost completely on the spars and sheeting; the ribs are hardly affected. Ribs can usually be cut from soft 3/32- or 1/8-inch balsa sheet. Depending on the stiffness of the wing sheeting used, I keep a maximum distance of about 2 to 3 inches between each rib; this avoids the "starved horse" sheeting sag. With a one-piece wing, it is also a good idea to increase the thickness of the center section ribs that support the sheeting where it contacts the fuselage at the wing saddle. I use 1/4-inch balsa here.

LlEADING AND TRAILING EDGES

Leading and trailing edges don't add much stiffness to the wing; they serve mainly to give shape to the airfoil section. There are several different ways to make leading edges, as shown in Figure 4. To make them more ding-proof, always use hard balsa. Trailing edges are a different matter, especially with scale models. One of our goals is to make trailing edges a scale thickness-that is, as thin as possible-without sacrificing stiffness. Figure 5 shows some of the methods I've used. A common construction method is to insert thin plywood stiffeners and glue them to the rear sheeting using epoxy, as shown in Figure 5B. In more recent designs, I have also used thin carbon-fiber profiles to stiffen the trailing edges. The thin metal cable trailing edges that Anthony Fokker used on his famous early designs are of special interest. To avoid unnecessarily complicated model designs, it is best to duplicate these fragile structures with thin plywood and glue them to the bottom of the ribs as shown in Figure 6. When covered, this deviation is hardly noticeable.