Minimum IMPACT

Model Airplane News, Oct 2004 by Sarpolus, Dick

An inexpensive twin electric that delivers maximum fun!

The first twin-engine model I owned was powered by a Fox .35 and a Fox .29; yes, it was a control-line stunter, and I flew it in the 1953 AMA Nationals in Willow Grove, PA. Since then, I've designed a number of twins with power ranging from a pair of .19 glow engines to a pair of 42cc gas burners. All of them were fun because of that unmistakable twin-engine sound.

Electric-powered aircraft have made it easier for everyone to build twin-motor projects; unlike glow engines, electric motors just keep on running as long as there is power available. I wanted a smaller, electric-powered profile foamie version of one of my favorite twins: the .40-powered Double Impact that I designed several years ago. The Minimum Impact is the result, and although it's smaller and a lot quieter than any other twin, it's just as much fun.

The construction technique used for this project is the quickest and easiest way to make an RC aircraft that I know of. Material costs are absolutely minimal, and the model is very light and flies very well. Profile models have been well accepted over the years and now have their place in modeling. It can take some abuse and can be repaired quickly. If you happen to total it, hey; it sure doesn't take long to build another one.

With a 36-inch wingspan and about 320 square inches of wing area, Minimum Impact is fairly small. For power, I used two GWS IPS twin-motor units-the IPSD-RXC-AS versions. These units each have two motors geared to one prop shaft. I fitted each motor with the optional GWS heat sink for extra protection. I'm a big fan of lithium-polymer batteries, as they provide so much power and are very light compared with any other battery type. The all-up weight of the prototype with a 3-cell, 1500mAh pack is only 16 ounces. I use APC 9x7 props, but I want to do more experimenting with prop sizes; for now, however, flight performance is good. The GWS brushed motors are inexpensive, and if there were a brushless equivalent in this size, two would make the plane a real barn-burner.

I built the model with Dow Bluecor extruded-polystyrene FanFold sheet. Available at home-improvement stores, this ¼-inch-thick, blue material is used for house insulation, and it has a thin, plastic-film layer on each side. It is available in 25-count bundles of 24x48-inch sheets for about $30. The materials cost for one model is about a buck; you can't beat that! As shown on the plans, I used hard, ¼-inch-square balsa strips for reinforcement, but I recommend the use of carbon-fiber tubes for increased stiffness. They cost more but make the airframe more rugged.

CONSTRUCTION

Start by cutting the plans to form paper patterns; you can trace around them on the foam or pin the patterns into place and cut around them. A single-edge razor blade or a very sharp hobby knife held perpendicular to the sheet easily cuts through the foam. Cut the wing to fit the carbon-fiber tube or hard-balsa reinforcement, and epoxy it into place. It is easier if you first epoxy the reinforcement to the top of the fuselage, epoxy it to the wing and then epoxy the lower fuselage into place. After the epoxy has cured, glue the horizontal stab into place, and then epoxy the motor nacelles to the wing.

Before you glue the wing and horizontal stab to the fuselage, hinge the ailerons and the elevator into place. I use clear, thin, low-cost, packing tape for the hinges. Attach the tape to the top surfaces first, and then fold the movable control surface over to apply the bottom piece of tape.

I made the control horns out of 1/16-inch plywood and epoxied them into slots cut in the control surfaces. To install the servos with small screws, I epoxied small pieces of 1/16-inch plywood to the foam on either side of the servo cutouts. For the lightest possible weight, some builders simply cut holes in the foam and hold the servos in place with Goop or hot glue. I installed the aileron servo in the wing next to the fuselage and cut a small slot through the fuselage to allow the servo arm and control linkage to reach the other aileron. To make the wire pushrods, I use 0.047-inch music wire and make Z-bends on each end. I use a V-bend near the servo for length adjustment. To support the middle of the two pushrods, I glued small plywood braces into slots cut in the fuselage.

POWER SYSTEMS

I got all of my GWS products from Balsa Products. The motor/gearbox assemblies slide into place on a strip of hard balsa or pine in each motor nacelle. Sand the strips to fit the gearbox frames. I drill a small hole through the side of the gearbox mounts and hold the motors in place with small screws. I wrap the ESC and the receiver with thin foam and clear tape and hold them in place with Velcro®. The battery pack fits through an opening cut in the fuselage just below the wing. The ESC, receiver and pack are held in place with rubber bands. Two pieces of 3/8-inch-square balsa, each fitted with two short lengths of 1/8-inch dowel, are epoxied to the wing to anchor the rubber bands.