The SR-X1.

The SR-X1

This is a modified Sky Rider. The original model (henceforth ‘prototype’) was made entirely of EPS foam, with only one additional plastic piece used to clip the two wings to the body. It was very durable and had stable flight characteristics. The goal of the experiment here is to provide longer flight time by adding 2 additional features to the aircraft: in-flight control and propulsion. Further, we add the constraint of minimal modification the prototype so as to maintain the glider’s pre-existing capabilities.

Even though the flight time of the glider is more than satisfactory, with control and propulsion the flight would be extended to as long as the power pack had a charge. The hope is that, considering the protoype’s existing ability as a glider, the propulsion system would only be required in order to gain altitude, thus reducing the demand from the batteries. There are many glider kits in existence that already use this principle. Here we will attempt it with a $5 toy.

Control requires 4 elements: control surfaces, mechanical force, communication, and power. The control surfaces will be created by modifying the body of the existing aircraft. The mechanical force to move the control surfaces will be provided by a set of electric servo motors. Communication to command the servo motors will be provided by a radio transmitter and receiver. The power will come from an on-board battery pack. This is all standard.

The Control Surfaces

The prototype actually had grooves in its surface representing where control surfaces were located. This included elevators, flaps, ailerons, and a rudder. The pieces were easily cut from the aircraft with a hobby knife.

The vertical stabilizer of the prototype is very thick so the edges of where the rudder were cut was trimmed down to present a rounded surface to airflow. This may not have been the best choice.

The elevator was cut in one piece, even though there were two pieces indicated by the grooves on the prototype. The horizontal stabilizer was dihedral, so that if the two separate pieces were cut then their axes of rotation would be different. This would present possible binding, and a more complicated tail section that would normally be desired. The compromise was to only attach the single elevator piece with tape and the outside tips, thus (hopefully) allowing the foam to bend in the middle rather than bind. This may not have been the best choice.

Since the main wing configuration of the prototype is dihedral, turning in flight only requires a rudder, but we chose here to experiment with all three of the control axes - yaw, pitch and roll. The flaps are really only used for much larger aircraft, but in this case they were more convenient to use than the ailerons. Their positions were close to the fuselage and so connecting them to the servo motor would be easier. The actual ailerons, closer to the wing tips, were not touched, but might be used to extend the control surfaces if needed.

The Servos

Three servos were needed to move the control surfaces; one each for the rudder and elevator, plus one for the ailerons. The first two would be straightforward installations, with the control rods extending directly from the servos to the control horns.This is standard.

The aileron servo required quite a bit more planning. An aileron servo works by having two control rods attached, each going in a different direction. Moving the servo one way moves one aileron up and the other down, and vice versa. An ‘L’ shaped control rod and hinges for each aileron translates the servo movement into rotation. This is standard.

The difficulty here was that with the constraint of ‘minimal modification’, mounting the aileron servo was problematic at best. Part of the advantages of the protoype is its flexibility. The ease of disassembly comes from its simple construction, but it is actually designed to be flexible right at the junction of the wings and fuselage. The plastic clip is also flexible and will bend quite a bit. To maintain these advantages, the flexibility and removability of the wings should be preserved. The servo was therefore measured and a cavity was carved into the interior of the fuselage where the wings joined. This required a lot of careful experimenting to carve just the right grooves for the movement of the servo and all the accompanying control rods. This may not have been the best choice, but the final result was smoothly operating ailerons and flexible, removable wings.

Detail of the motors.

Propulsion

For the sake of appearance, it was decided to use twin ducted fan engines. They are not especially powerful or efficient, but they look good on the prototype. The difficult operations were mounting and balancing. Mounting was done by carving nacelles from scrap foam. Plastic cups were used to extend the ducts to the rear. The balancing was a guess, but they are attached with double-sided tape in anticipation of adjustments.

Communication and Power

Communication is handled by a standard radio receiver and ESC(Electronic Speed Control). Power requirements are still uncertain, but a 5-cell pack is being used for initial experiments.

Configuration

Detail of the electronics.

To install the servos, power pack, receiver, and ESC, a cavity was carved into the aft fuselage. Before any modifications began, a measurement of the center of gravity was taken and found to be just at the trailing edge of the wings where they meet the fuselage. The addition of servos, engines, and electronics would not only increase the overall weight of the aircraft, but influence its center of gravity. Care was taken to ensure that the positions of the engines and the location of the electronics would be balanced. Initial measurements show that the engines are making the aircraft nose-heavy. This will cause the plane to tend towards nose-diving. A larger battery pack in the aft might counter this.

The two aft servos were installed near the rear, and the control rods were run through the foam. The elctronics fit snuggly near them, and the battery pack on top of those. Aditional wiring to the aileron servo and engines are run through the fuselage, between the two wings. The cover to the electronics housing is held in place by small magnets. The wings are held together by the prototype’s clip, plus an additional clip at their trailing edge, inside the electronics housing. This was to stabilize the operation of the ailerons, which were under stress because the wings were flexing in and out of their mounts. This was a necessary compromise and seems to give the wings a rigidity that will be needed for controlled flight.

Even with all the wiring and control rods, the aircraft is still easily disassembled.