Sig Hacker Cub Freeflight-turned-R/C Conversion ala Astro Firefly Power

I saw the new Astro Flight Firefly motor and 4:1 gear drive at the local hobby stor eand a tiny 1 amp speed control to match. I wanted to buy this power system so I did some research and found that to fly this motor system, I would have to produce a model, with all components, weighing no more than about 4 ounces (about 112 grams)! Now there was a challenge!

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Wing Area:114 sq. in.
Weight:100g (3.5 oz.)
Wing Loading:4-4.4 oz/sq. ft.
Servos:HS50 submicro
Receiver:GWS pico
Battery:6-c 50mAh
Motor:Astro Firefly w/ gearbox
ESC:Astro Coreless 1A
Available From:Sig Manufacturing

I saw the new Astro Flight Firefly motor and 4:1 gear drive at the local hobby store and was impressed by its small size. There was also a tiny 1 amp speed control to match. This had caught my interest. I wanted to buy this power system so I did some research on the internet. I found that to fly this motor system in an RC airplane, I would have to produce a model, with all components, weighing no more than about 4 ounces (about 112 grams)! Now there was a challenge! As if that wasn't enough, I wanted this plane to be a well detailed scale model.

I purchased a Sig-distributed Hacker rubber power free flight balsa Piper Cub kit that seemed to be made up of very light wood and materials. This was a scale model kit with a wingspan of 28 inches. There were full size plans, pre-cut balsa parts, balsa stripwood and sheet, a light styrene cowl, wheels, gear for flying with rubber, silkspan covering, and just about everything needed but glue and paint to finish the kit. I was especially impressed with the laminated curved balsa for tail and wing sections. As well, the plan suggested RC was possible. I knew I would have to build light.

I purchased the kit and the motor system, as well as a 6 cell pack of 50mah batteries for powering the motor and radio. I borrowed a scale that could weigh everything. I would be using a GWS Pico receiver, plus two Hitec HS 50 sub micro servos; at 5.8 grams each that was as light as I could get without purchasing any really expensive or hard-to-get components. All the wood in the kit, the covering, plastic parts, power system and radio all weighed less than 100 grams. It looked like I might be able to complete this model near the projected weight goal. I also tested the motor system with a Gunther (Zagi) plastic prop; it only needed to be pushed on the motor shaft. It seemed to pull quite well and only drew 0.7 amps at full throttle. Wattage was about 4.2. This amazing little motor system was extremely quiet at full throttle.


I built the fuselage almost as per the instructions, just leaving out a few pieces of wood that I felt would not be necessary. I hoped this might make up for other wood that I might use to mount the radio and batteries. Using some scrap, I also made a motor mount that held the motor with a small elastic.

Next was the tail and wings. The laminated and shaped curved outlines were nice; they just had to be trimmed to shape. The wing leading edge was preshaped -- that saved some delicate work. The ribs were not all accurately cut so I pinned them together on a spar and then sanded them to match each other. I used every second rib only and left out a lower rear spar to save weight.

I cut very small CA-absorbing hinges for the tail and I cut extra servo arms to be control horns. I attached the wire for the tail wheel to the rudder instead of the fuselage in order to have steering control. I installed the servos with double sided tape onto small pieces of foam, cut from a picnic plate, which were very lightly and carefully epoxied to the inside fuselage sides. I also cut the fairings for the landing gear from the same foam. These would be lighter than the balsa supplied in the kit.

Covering and Assembly

The next step was to carefully sand all the structures. The kit included white silkspan which was intended to be used for covering and then painted, but I just used yellow tissue for covering to save the weight of the paint. I brushed onto the balsa surfaces a light amount of water and white glue, mixed about 3 to 1, and then applied tissue on top. After the surfaces dried I wet the tissue and it shrunk nice and tight. During this step I pinned the wing to a piece of stryrofoam so it wouldnít warp. Everything was covered except the bottom of the fuselage so I could later arrange the receiver and battery to balance the plane.

I then assembled the cowl and various details. These were made from a combination of what was supplied in the kit and scrap pieces of foam. I used whatever I thought would be lighter at every step. The wing was then glued in place and all final details were glued or put in place. At this point I put the receiver and battery temporarily into the plane and taxied the plane around my basement to test the steering. That was fun and it taxied well. I also weighed the plane; it weighed 95 grams. That meant that with the wing struts and decals, the plane would likely weigh under the 112 grams I was targeting.

I carefully tried balancing the plane, as indicated on the plans, with the battery in the fuselage at various locations. The plane balanced when the battery was inside the fuselage just ahead of the landing gear. I built a battery box out of the picnic-plate foam with thin pieces of soft foam to hold the battery in place with just light pressure. The bungee-style landing gear was made up of black elastic and black heat shrink tubing.

I sanded the balsa wing struts to an oval cross section and then glued them on. Very light clear plastic was used for the windows and glued on with just a touch of white glue. The decals were put on except for the large numbers for the wings; this was to save weight.

Finally I weighed the plane. It was exactly 100 grams, or just over 3Ĺ ounces; below the 4 ounces or 112 target grams weight.


The first flight for this plane was a challenge. Our club's indoor flying site had been closed and I couldnít wait to fly the plane. I finally worked up the courage to fly the model outside, on a windy day, in Toronto (Canada) in the middle of January. (Yes, Iím crazy.) Well, the flight was and wasnít a success. I hand-launched the model with full throttle in a large field, behind a small hill as protection from the wind. The model climbed nicely and the model moved out well; it was flying properly from the start. I then trimmed the model (slight changes for the elevator and rudder) so the plane would fly straight. It continued nicely.

As the plane climbed over the hill, there was no more protection from the wind. The plane just blew right over and down to the ground on its back. I picked up the plane and realized that there was only one small tear in some of the tissue and the prop came off. I guess that when a 3Ĺ ounce model airplane falls, there isnít much of an impact.

I was satisfied that the model was a successful project. The tissue repair was only a matter of a few minutes work. I pushed the prop back on the motor shaft, but this time I put a touch of white glue on the shaft to secure it a little better. I decided to wait for a windless spring day or a large indoor area to fly.

As soon as I had a chance, I tried the plane at an indoor funfly in Oakville, Ontario, a suburb of Toronto, sponsored by the Electric Model Flyers of Southern Ontario. This took place in a mid-size gymnasium. I put the model on the ground and slowly applied full throttle. The model went straight ahead and took off in less than 10 feet. Although the plane flew slightly quickly, it was extremely stable and easy to control. I soon landed it, satisfied that it flew well.

I have since flown the model outdoors at local clubs and various funflies. It is a superb outdoor flyer; itís as if it were designed for RC. I have since sent the motor to Astro Flight for an upgrade to a metal motor box and a speed control that can handle more than 1 amp if need be. With this upgraded motor system I can now use an 8-cell pack of 50mah nicads. Climb is stronger and with the added weight it flies better in a slight wind.


If you have experience building models, donít be afraid to try to experiment and do something that hasn't been done before, or simply that YOU haven't done before! This was a great learning project for me, and I was rewarded with a fabulous flying model. This summer I plan to try lighter lithium batteries for even slower and longer flights.

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