Secrets to perfect control-surface construction

Model Airplane News, Mar 2003 by van Mourik, Dick

The design and construction of an airplane's control surfaces, specifically the ailerons and flaps, really help to distinguish a scale model from a sport flyer. This month, let's go back to the building board and take a closer look at these particular elements.

BUILT-UP AILERONS

An aileron can be anything from a simple triangular piece of solid balsa to a completely built-up structure. The most common aileron and flap construction method involves making the wing as a single piece and then cutting the control surfaces out of it. Though this isn't likely to produce the most scale-like surfaces, the method does have its advantages, especially when the wing features washout; the correct amount of washout will have automatically been built in to the control surfaces when the wing was constructed. This ensures a perfect, flush fit when it's time to assemble the pieces. Figure 1 details the building sequence of a typical aileron.

Also shown in Figure 1 is a small section of what is known as a "Frise aileron." Though slightly more difficult to construct, the Frise aileron more closely resembles that of a full-size plane. With a Frise aileron, the LE drops below the wing when the aileron is deflected; this causes a disturbance in the airflow that minimizes adverse yaw.

SOLID AILERONS WITH WASHOUT

On models such as the P-51 Mustang, which has small ailerons and wing washout, the ailerons must be built separately from the wing. To do so, I first cut a piece of 1/64- or 1/32-inch plywood into the shape of an aileron. I then lighten it as much as possible and cover both sides with hard-balsa sheets of a sufficient thickness. Next, I make two, thin plywood end ribs with the required washout, and I glue them onto each end of the aileron, as shown in Figure 2. Last, I use a razor plane to shape the assembled aileron, and then I sand it with a long sanding bar; the correct washout will automatically be built in.

FLAPS

Flap deflection causes two things to happen:

* The wing's angle of attack increases, and this, in turn, generates more lift and enables the aircraft to fly more slowly.

* Drag is increased-behind the flaps and especially at their outer ends. This drag causes the aircraft to descend at a steeper angle. To avoid such a situation, the flaps must be fairly rigid. Unfortunately, the section of the wing in which the flaps usually are is often too thin to support true-to-scale, rigidly constructed flaps, so compromises must be made. One solution is to use split flaps; these are generally built up in a box-like manner to make the lightest, stiffest structure possible. For the stiffest flap, add a laminate made of balsa and plywood. When you're dealing with a very thin airfoil such as that of the Spitfire's, though, it's best to use a laminate made of 1/64-inch plywood glued to an aluminum plate with contact adhesive. I also usually add a 1/4-inch rod and a fiberglass or aluminum tube to the flap's TE to serve as a torsion rod. One advantage of this technique is that it allows the entire structure to bend-a significant benefit considering that the Spitfire's rear wing section is curved.

Another common type of flap is found on the Mustang, the Thunderbolt and many other military and civilian aircraft. Similar in design and construction to the solid ailerons mentioned earlier, these flaps are built right into the wing itself, and when lowered, they affect the appearance of the wing as a whole. To make these stiff but light flaps, I first construct a central core out of 1/64-inch plywood, and then I glue the upper rib halves to it. Then I glue the false LE into place, and this requires total accuracy. It's extremely important that it sits absolutely parallel with the wing and the building board. Next, I tack-- glue the flap to the building board with a few drops of CA; then I apply the upper sheeting, and I sand the TE down to ensure a flush fit with the ply core. Next, I attach the LE and TE to the building board with double-sided tape, contact cement, or something similar. Make sure that everything is secure; it's at this stage that a twist could form, and the flap will lose its shape. Now apply the lower sheeting to the underside of the flap to give the structure a definitive shape. After you've sanded and applied the true LE, your flap is ready to be installed.

To accommodate higher flight loads, the flap's hinge blocks and those that support the control horns must be stronger than those used on the rudder and elevator. To attach the blocks, I recommend that you use a good-quality, slow-drying epoxy or thick CA; it's less sensitive to humidity and temperature differences than aliphatic glue.

SLATS AND SLOTS

Another way to increase the amount of lift a wing is able to generate is through the use of slots. A slot is a small, secondary "wing" in the forward part of the wing's LE and mounted at a lower angle of attack than the wing itself. The gap created by this slot forces air to flow over the upper side of the wing and maintains a linear airflow even after the plane has reached its usual stalling angle. In effect, slots enlarge an aircraft's critical-angle-of-attack range.