IN THIS CONTINUING study of ARF trainers, last month we built the Hobbistar 60 MK III's wing according to the factory design. However, a few nonstock design features could be seen in the photos. The most obvious modification was the use of two aileron servos—one in each wing half instead of only one housed in the wing's center-section.

The Hobbistar MK III ARF trainer is already a great product, but a few simple modifications can make it even better!

Employing two wing-mounted aileron servos, each controlling one aileron, is becoming the norm for most sport aerobatic and contest RC aircraft. Although this feature has long been a favorite flutter eliminator, the advent of modern "computer" transmitters permits this arrangement to reach its full potential. With these transmitters, the ailerons perform double duty as flaperons.

Flaperons are ailerons that can be lowered (or raised) to act as flaps (or spoilers) while remaining in control of the aircraft's roll performance. When lowered, usually 35°-40°, flaperons add drag that slows the aircraft while increasing the wing's lifting force at slower airspeeds.

If the flaperons are raised 20°-25° as spoilers, extra drag remains, but the wing loses lifting force. Such "spoilerons" (my term) have several uses that are not generally recognized by most RC pilots.

Precision Aerobatics pilots have discovered that deploying spoilerons in high-wind conditions makes it easier to perform precision landings when the wind howls. 3-D pilots have found that spoilerons mixed to the elevator and working as CL Aerobatics flaps would make for some exciting aerobatic stunts.

However, flaperons are the most common use for twin-aileron-servo installations. With flaperons deployed, most models, including the Hobbistar 60, exhibit better slow-speed handling, have steeper and more easily controlled landing approaches, and fly the approach at slightly lower airspeeds while touching down at significantly slower speeds.

Flaperons and computer transmitters also make it possible for the RC pilot to adjust the aircraft's "aileron differential." Last month I wrote about "adverse yaw," which is the tendency for the airplane's nose to first swing in the direction opposite the desired turn before finally heading the right way.

This annoying behavior is most obvious at slow airspeeds and high angles of attack (when the nose and wing point steeply upward). Most RC pilots notice this problem on final approach, when their airplanes seem to have a mind of their own as they wallow along on the approach path.

Adverse yaw happens because the aileron moving downward has more drag than the upward-moving aileron. This forces the nose to swing toward the down aileron's side and then turn correctly as the wing starts to bank.

Differential means that the ailerons can be adjusted so that the downward-moving aileron moves down less, reducing its drag, and the upward-moving aileron moves farther, increasing drag on its side. Once differential is correctly adjusted, adverse yaw is minimized or possibly eliminated. Aileron differential is possible using a single servo by drilling off-center holes in the servo output wheel, but fine adjustment is nearly impossible.

Considering the many flaperon benefits, installing twin aileron servos is well worth the extra cost and work. The extra building time is temporary, but the performance improvements are permanent. Twin-aileron-servo installation begins before the wing halves are joined.    

Locate each aileron-servo's position. Mounting the servo in the middle of the aileron's span is best but is unnecessary on trainer and sport ARF models. It works well to locate the servo on the inside of the wing bay that allows the servo's wire to just reach wing center.

Position servo one bay past bottom wing sheeting. Front mounting lugs are just behind main spar. Mark servo's dimentions on covering.

Carefully cut covering the width of the wing bay with sharp hobby knife or small soldering iron. Make sure covering remains over each capstrip and front sheeting.

Using a 6-inch servo extension makes it easy to connect the ailerons at the flying field without making the servo wire so long that it causes radio problems. You should tie the connectors together to prevent separation during installation and flight.

Position the servo next to the inside wing rib. Place the forward servo mounting lugs just behind the main spar. Mark the back of the rear mounting lugs. Use a sharp hobby knife to carefully cut away the covering to a point roughly 1/2 inch behind the rear mounting lugs. Use a piece of low-tack tape to prevent the covering from tearing.

Cut a piece of 3/16 scrap balsa approximately an inch higher than the wing is thick and roughly the length of the opening. Position it against the wing rib. Draw a line matching the wing rib's curvature, and put this piece aside for now.

Use five-minute epoxy to make the forward servo-mounting rail. This is fabricated from two pieces of 3-inch-long 1/2 x 1/4 spruce and 1/4 triangle-stock spruce that is the same length. After the epoxy sets, cut this assembly into two parts. Epoxy each to the rear of the front wing spar and against the wing rib. Make sure the assembly is mounted under the capstrip, if present, and against the wing rib.

Make the rear servo-mounting rail from a piece of 1/2 x 1/4 spruce rail. Reinforce the mounting area using 1/4 spruce triangle stock. The rear rail spans the entire distance between the wing ribs, so be careful when determining this measurement. Position the servo on the front rail, which is already installed, and use it to position the rear mounting rail. Glue the rear rail in position with five-minute epoxy.