A Step-by-Step Guide

 for Converting the electric free flight model

Sky Runner

to R/C using the RFFS100 Module

(by Greg Covey)

This step-by-step set of instructions includes many pictures that will help you to convert your own free flight Sky Runner kit into a 3-channel R/C airplane using the combo package (P51RFFSLP) from Dynamic Web Enterprises. The converted model will have fully proportional elevator, ailerons, and throttle controls. You can click on any picture for a larger view.

Harbor Freight "Sky Runner" Electric Free-flight kit

Converted Sky Runner using the DWE Combo package

The P51RFFSLP Combo package includes:

Introduction:

In my search to find the perfect model for an entry level micro-flight enthusiast, the Sky Runner had many of the qualities that I was looking for:

Since the stock weight of the free flight electric powered model was only 1.9oz, I knew that to have a successful conversion to R/C, I would need the latest technology. The RFFS100 module, KP-00 motor, and a single 230mAh Lithium Polymer (LiPoly) cell from Dynamic Web Enterprises (DWE) provided me with the technology I needed to convert my model.

RFFS100 module, DWE Lithium Polymer cell, KP-00 motor

Additionally, I was further aided by the use of fixed magnet actuators from Dynamic Web Enterprises and the so-called "Azarr" M-72-U ultra micro antenna from Eclectic Electric Essentials (or E-cubed). The antenna and actuators are available from DWE. The actuators have a tiny circuit board for easy mounting and come "terminated" with pins that mate with the RFFS100 module.

DWE Magnetic Actuator "servo" and Azarr Ultra Micro antenna

Additional Items Needed:

In addition to the Sky Runner Combo kit from DWE, you should have these additional items for a successful conversion.

The Sky Runner conversion is a great deal of fun to build and fly, so let's get started!

Step 1: Battery Removal

- Remove the screw holding the battery hatch closed. Also, remove the "China" sticker just behind the nose skid.

- Remove the 270mAh 2/3AAA NiMH cell. It weighs  0.2oz. We will replace it later with a 230mAh LiPoly cell that weighs 1/2 the weight (0.1oz) and has 3 times the voltage!

Step 2: Motor Removal

- Remove the prop and spinner from the stock motor shaft. It pries off easy with a flat blade screwdriver. It is similar in size to our U-80 (3x1) prop so you can re-use the spinner, if desired.

- The foam pod that covers the motor is only held on by some double-side tape. It pries off easily with a flat blade screwdriver. Likewise, the motor is also held with tape and easily pulls off by hand.

- Unsolder the wires to the motor and remove the motor.

Step 3: Battery Tray Removal

- Trace the long sides of the plastic battery tray housing with a sharp razor knife. The housing also contains the On/Off switch and charging plug. This plastic assembly will be known as our "Electronics Center". After tracing the sides to cut into the foam, use small pliers to gently pry under one end and lift the plastic assembly up. Repeat the lifting from the opposite end until the assembly lifts out of the foam.

- Remove it from the plane pulling the motor wires out that were freed from the previous step.

Step 4: KP-00 Motor Installation

- Install the U-80 prop onto the KP-00 motor geardrive shaft. Since this motor will operate as a pusher, the "U-80" raised lettering must face the motor. The prop will rotate clockwise when viewed from the rear of the plane.

- After the prop has been mounted, sand the exposed flat side of the motor. Route the KP-00 wires through the motor pod access hole out through the bottom of the fuselage. Test fit the motor against the pod bottom and note that a small slot needs to be cut in the foam for the plastic motor mount eyelet.

- Use a small drop of odor-less (foam safe) CA or 5-minute epoxy to glue the motor to the foam pod base. Ensure proper alignment of the motor within the pod. It should point straight back (aft) and follow the incline in the foam pod base for proper upthrust.

Note: Upthrust is built into the stock design because the motor sits above the wing. No right or left thrust is needed, only upthrust.

- After the glue dries, secure the foam pod cap using the original sticky-back tape. Optionally, a few knife cuts and a few drops of CA provide a more perfect and secure fit.

Step 5: Electronic Center Preparation

- Unsolder the free thick yellow wire (that was routed to the stock motor) from the "On" side of the switch. The switch pin will be routed later to the RFFS100 module (+) pin side. For now, solder a 2" thin blue wire from the kit onto the free switch pin.

- Unsolder the yellow and red wire ends from the battery terminal lugs. Leave the other ends soldered in place.

- Remove the 2 metal terminal lugs from the battery compartment. This is easily done by first cutting off the bottom soldered ends with wire cutters (or diagonals) and simply pulling the terminal out through the top of the battery compartment.

- Cut away the side of the battery compartment as shown above. Also, cut away the plastic rails inside the compartment that were used to hold the metal terminals. Be careful and think safely when cutting plastic with your razor knife.

- The battery compartment is now the RFFS100 compartment. I applied two layers of servo tape to the bottom of the RFFS100 module (to make up the space equal to the crystal pin bottoms) and slid it into place in the compartment. A slight press keeps it secured in place.

- Using another piece of servo tape along the outside of the remaining wall, mount the LiPoly cell sideways with the terminals facing the switch.

- Solder the free end of the thick red wire from the center switch terminal to the LiPoly cell "+" terminal.

- Cut the free yellow wire end to proper length and solder it to the LiPoly cell "-" terminal. The other end should still be soldered to the charger jack.

- Route the thin blue wire that we soldered to the switch "On" side into the RFFS100 module "+" pin that is marked by a red dot.

- Lastly, solder a thin yellow wire from the kit to the thick yellow wire on the charger jack. Yellow is our minus (-) or ground side so connect the other end of the thin yellow wire to the "-" pin of the RFFS100 module. A "-" sign is marked on the circuit board.

Our Electronics Center is now powered and complete. It weighs 0.3oz. You may charge the LiPoly cell now with the switch set to "Off". I wired my plug, after cutting it from the supplied 2-cell AA pack, to a Bob Selman BSLipC1 Lithium Charger. Simply connect (solder) the red and black wires from the cutoff plug to the red and black wires on the BUC (Battery Under Charge) side of the BSLipC1 charger.

Step 6: Electronic Center Installation

- Connect the Azarr M-72U antenna to the RFFS100 module antenna pin.

- Cut two slots in the fuselage foam to allow a proper fit for the LiPoly cell on one side and the edge of the RFFS100 module on the other side.

- Feed the antenna towards the front of the plane on the opposite side from the LiPoly cell. Hold the antenna in place with a small piece of servo tape and press the electronic center in place. Secure it with several pieces of tape.

Step 7: Elevator Assembly

- Cut the elevator free from the stabilizer using a sharp razor knife. Note that my plane was already assembled for free-flight testing but yours may still be unassembled. Either way works fine.

- Sand a 45° bevel into the leading edge of the elevator as shown in the diagram above.

- The new RFFS100 module software provides a stronger magnetic actuator force. Additionally, both the new DWE actuator and the Selman actuator provides a self-centering action for the magnet so I feel that using rubber hinges is no longer required and that a few pieces of tape works just fine as a hinge and it is much easier to install. It is your choice, but, do not mount the elevator yet.

I recommend using either the new DWE or the Selman self-centering magnetic actuators because they are easier to work with and maintain. The other magnetic actuators with the separate magnet will, however, work also.

Note: Typically the actuators have rather short wires so the one used for elevator control will need to be extended long enough to plug into the RFFS100 module in the Electronics Center. There should be some additional magnet wire in your kit to do so. On my conversion, I used an old DWE actuator set with the coil removed from the tiny circuit board and combined it with my Selman actuator to extend the length of the wires. Your actuator in the kit may already have the proper length wires.

- Cut a 1/4" square slot into the stabilizer and mount the Selman actuator as shown using a small amount of CA. I also applied a small drop of CA on the coil bottom to secure it to the plastic actuator base. For the DWE actuator, cut a 1/2 circle into the stabilizer instead.

- Tape the elevator into position using 3 or 4 thin (1/8") wide pieces of tape. Ensure that the elevator can swing up by 45° and down by 45° from the neutral (or center) position. I have found that the 3M Blenderm brand transparent tape works very well for hinges. It has a stretchable property to it and also has plenty of adhesion. Optionally, the elevator can be attached using 2 small rubber hinges, as show below.

Either tape strips or rubber bands can be used for hinging the elevator

- Strip a small 1" long piece of the solid kit wire bare and make a "Z" bend in one end for the actuator swivel arm. Bend a right angle into the wire so that it penetrates the control surface of the elevator just beyond the actuator. Insert the "Z" bend into the actuator and glue the other end of the wire in place on the foam. I used a small amount of CA.

Step 8: Aileron Assembly

Initially, a full aileron was cut from each wing half

It was later determined that half-ailerons worked best with only 2 hinges per side

The tiny circuit board allows for easy mounting

- Cut out the aileron along the molded lines the entire length of wing half.

- Sand a 45° bevel into the leading edge of the aileron.

- Cut a 1/2 circle opening into the wing and mount the DWE actuator using a small amount of foam-safe CA. I also applied a small drop of CA on the coil bottom to secure it to the plastic actuator base. It should be mounted near the fuselage about 5/8" from the end of the aileron.

- Tape the aileron into position using 4 to 6 thin (1/8") wide pieces of tape. Ensure that the aileron can swing up by 45° and down by 45° from the neutral (or center) position. Optionally, 3 small rubber band hinges can also be used to attach the aileron as shown below.

Small rubber band hinges can also be used to attach the aileron

- Strip a small 1" long piece of the solid kit wire bare and make a "Z" bend in one end for the actuator swivel arm. Bend a right angle into the wire so that it penetrates the control surface of the aileron just beyond the actuator. Insert the "Z" bend into the actuator and glue the other end of the wire in place on the foam. I used a small amount of CA but 5-minute epoxy works well also.

Six washers are used to gently press the flat carbon rod against the wing bottom while drying

Although the aileron seemed sufficiently strong with the stock taped trailing edge, the wing itself was too flimsy. I decided to add a length of flat carbon spar to each wing half for strength.

The 10" piece of .120" width by .020" thick carbon rod weighed less than 1 gram. The rod can be purchased from Air Dynamics in 36" lengths.

I used 5-minute epoxy to glue the carbon spar against the foam wing and gently held it in place with 6 metal washers until it dried. The resultant wing was significantly stronger with very little weight added.

The finished wing half is ready to assemble onto the fuselage and plug into the RFFS100 module. Since there are two actuator pin sets, I first tested them for opposite direction of movement and then soldered one set of pins to the top of the other set before plugging into the RFFS100 module.

The aileron droop was fixed using a #47 Wonder Magnet

To eliminate the droop from the weight of the aileron (or elevator), a tiny 1/16" diameter X 1/32" thick WonderMagnet was used. These tiny magnets can be CAed directly to the actuator coil to offset the weight of any control surface. A great find for only about 13 cents per magnet!

Step 9: Balancing the Model

The CG is 1 7/8" back from the leading edge near the fuselage

The stock CG of the Sky Runner was 1 7/8" back from the leading edge of the wing bottom where it meets the fuselage. To achieve this balance, I buried a penny in the foam under the nose next to the plastic skid. Optionally, I later replaced the 0.1oz penny with a second 230mAh LiPoly cell (also 0.1oz) in parallel with the first cell to double the flight duration! It worked great!

A penny was used to achieve proper balance

My Sky Runner was RTF at 1.9oz which is equal to the stock free flight weight.

If your model does not balance correctly, if should not be far off. If you are a bit tail heavy, I would recommend adding a small screw (or penny) into the foam fuselage bottom just behind the rubber nose. You could also shorten the wooden fuselage by 1/4".
 

Step 10: Strengthening the Wing

After my maiden voyage with the Sky Runner, I discovered that the wing was still a bit flimsy. In a slight breeze, the wing would warp under stress causing the aileron to distort upwards as the dihedral changed. While this may not happen during indoor flight, I decided it would be best to strengthen the wing.

A fishing line is used to strengthen the wing and maintain dihedral

To strengthen the wing, I glued 3 small pieces of white snake tubing to the plane. One on each wing tip and one under the belly of the fuselage into a carved channel. The little tubes were sanded before gluing with CA. A fishing line (or carbon Fireline) was then tied from one tip through the center tube to the other tip so that it held a little less than normal dihedral.

I also tied a Fireline across the top side of the wing to help hold the existing dihedral in place, much like the well-known Lite Stik modification.

My wing was now much stiffer and could handle a slight breeze outdoors.

Step 11: Testing the Finished model

Be sure to test the Sky Runner at home before you take it out to fly. A transmitter with dual rates works best with the RFFS100 module so you can set the reduced rate settings just beyond the full deflection range.

Remember that up stick makes the elevator move down and right stick makes the right aileron move up and the left aileron move down. I use 70% reduced rates on my elevator and full rates on my ailerons.

Flying the model:

The Sky Runner seemed almost overpowered with the tiny KP-00 motor. I could fly around at 1/3 throttle and made good use of the improved RFFS100 software throttle control.

I can loop my Sky Runner in small 15' diameter circles and it flies very slow and docile.

After suffering no damage from several nose hits and hard surface landings, I did discover that my crystal popped out of the socket on the RFFS100 module. I re-installed the crystal with a tiny piece of servo tape between the bottom side of the crystal and the top of the IC chip that it normally rests on.

Sky Runner Weight Chart