My Indoor Champion Saga

A kit bash for small lot aerobatic flying by Greg Covey

Introduction:

The Akro Champion is a 4-channel "full-house" aerobatic streetflyer sold by Hobby Lobby. In stock form, the geared Speed 280 motor provides adequate power for simple aerobatic maneuvers. Once the power system has been upgraded, this 28" symmetrical wing plane becomes an awesome outdoor performer. I have used both 280BB and Astro Flight 801G brushless motors with great success in this model.

The basic design is not just a stick fuselage but rather a very light 1oz custom fiberglass fuselage and balsa wing covered with Litespan. The molded carbon fiber landing gear, foam wheels, spinner, and pilot all help to make this model really look like a plane with eye-catching details.

My initial design goal for this project was to reduce the weight enough to make it an aerobatic flyer for small parking lots and indoor meets. After choosing lighter power components like the GWS IPS DX-B motor, 10x4.7 Lite Stik prop, and 300mAh NiMH cells, I managed to reduce 2.6 ounces from my stock 10.6oz ship. Additionally, I eliminated the rudder servo and coupled it into the aileron linkage for ground steering and improved turning capability. It was important to me to have a steerable tail wheel for taxiing around the indoor gym for better takeoff and landing positioning.

At the conclusion of my quest to fly this design indoors over the Winter months, you will see that my final configuration ended up in a different direction to my initial goals.

3 HS-55 Servos (0.9oz)                             2 HS-55 Servos (0.6oz)                                  -0.3oz

To gain even more agility, I enabled the exponential function on my Futaba Super-8 transmitter. By using a negative exponential, I could make the ailerons and elevator more docile around the neutral stick center and still have wild throw range near the outside ends of the stick travel for loops and snap roles. It was like getting the dual rate function without flipping a switch. I initially set the aileron and elevator exponential settings to -50%.

Mounting the DX-B Motor:

To mount my GWS DX-B motor, I epoxied a balsa block to the inside of the fiberglass fuselage nose. I then mounted a balsa post through the balsa block and drilled a partial hole for the gearbox shaft. A final hole was drilled through the balsa block for the motor and wires. This made for a secure mount that would still break in the event of a direct hit on the nose.

A balsa firewall and post are added to mount the DX-B motor

The stock cowl fit perfectly over the mounted DX-B motor. Later on after the pictures were taken, I spray painted the distinctive orange GWS prop with black paint and masked off the tips for a nice "circle of orange" when the prop was spinning.

The stock cowl and DX-B spinner are fitted as normal

Where's the Third Servo?:

By coupling the rudder into the aileron linkage, I could still have (or so I thought) my desired ground steering and also increase my turning authority in small flying areas. I used a special slowflyer control horn CH902 from Hobby Lobby that is very light and meant for a pull-pull system. I then used carbon reinforced "Fireline" to tie the rudder into the aileron linkage. Small pushrods were used to create a custom tailgear and adjusting clevis. I used a tiny 1/2" Sullivan tailwheel. The control lines connected to the aileron servo were crossed to opposite sides before tying to the rudder. Small bends in the aileron linkage rods allowed me to fine tune the rudder alignment without re-tying the lines.

The tailgear fits through a hole drilled into the center of the horn

The steerable tail wheel and rudder are coupled to the aileron servo

As it turned out, I did have my desired ground steering for "touring" the gym floor but the coupled rudder played havoc with my slow speed turns indoors. The resultant coupling made the plane bank and head for the floor instead of spinning around like I had imagined. I ended up removing the coupling and installing a third servo for independent rudder control.

The Finished Model:

The finished "Indoor" Champion is 2oz lighter than stock

Initial Flight Testing:

My initial flight was with a GWS DX-A motor. I quickly realized that there was insufficient thrust for this design and after 3 flights, DX-A motor burned out. Clearly I was stressing it too much with that setup. I switched to a GWS DX-B motor and kept the 10x4.7 (Lite Stik) prop and the improvement was amazing! On 8-cells of 300mAh NiMH, the DX-B motor was drawing 1.8 amps on full throttle. This was near the upper limit of the GWS motor so I decided to add one of those GWS heatsinks for additional safety.

A revved-up Champion is ready for the challenge

The Champion circles a double full-court basketball gym

The forth flight started well but ended abruptly with a crash

Tuning the Design:

After about 4 flights in two different gyms, we managed to crash the Champion. At that point we decided that it was still flying a bit hot indoors and needed to be slowed down. After repairing my leading edge wing damage, I added another 2" bay onto each end of the wing. This would provide additional lift and drag to slow the ship a bit. My wingspan was now 32".

The new wing modifications are ready for covering with Litespan

I re-covered the wing parts using white Litespan and Balsaloc. It was my first time using these covering materials and a good learning experience. Litespan shrinks very little and the Balsaloc needs to sit for 15 minutes before using the Litespan covering and sealing iron. The trick here is to make it taught when applying the edges of the covering. In this manner, only a slight shrinkage is then needed for that tight "drum" effect.

The Indoor Champion now spans 32" across

For extending the ailerons out to the added bays, I cut scrap pieces of foam left over from my Ultra-Lite Stik project. The color and thickness of the Lite Stick foam was almost an exact match for the ailerons. The little flare at the ends gave my Indoor Champion some added character.

Bottom view showing increased throws and cross-coupling to rudder

I decided it was time to also increase the elevator and rudder throws. Both linkages were moved out one hole so that the ailerons used the outer most servo arm hole and the rudder used the middle holes. I re-programmed my exponential aileron setting from -50% to -75% but left the elevator setting at -50%. This would help compensate for the increased aileron throws and give me wild swings at full stick offset.

Final Flight Testing:

The longer wings had indeed given me more lift and allowed me to fly the Champion slower. Once I realized that the coupled rudder was actually hindering my turning performance, I removed it and installed a third servo for the rudder control. It then flew much better and had predictable turns.

I could fly my Champion indoors using an 8-cell, 300mAh, NiMH pack but it was not aerobatic like I had wished.

For my last indoor meet of the season, I decided to switch to the stock geared 280 motor using the same 300mAh NiMH pack. Since this pack would sustain short current demands up to 3 amps, it should be fine for a brief indoor flight. This result worked fairly well, but, in the end, it was not comfortable indoor flying in a double sized basketball court.

Conclusion:

Although I had not achieved my initial goal of flying the Champion indoors with aerobatic authority, I had managed to fly it and learned much along the way. When the indoor season had ended and Winter turned into Spring, my goals for the Champion had also changed.

The Champion now fitted with a geared 370 motor

I decided that my larger wing Champion still looked great and I received many compliments on it with the hand painted pilot figure. I have since changed the power system to a geared (3.67:1) Speed 370 using an 8-cell, 800mAh, 5/4AAA, NiMH pack like that used on the Wattage Crazy Max. This power system upgrade provides both great power and duration on my now 12oz. Champion. I can easily fly this model in a small lot or field and literally float to a landing. I was very happy with the final outcome of this project even though it started out in another direction. An equivalent GWS power system with ball bearings is the EPS-300C (AS) and can be used with a GWS 8x6 prop or APC 8x6 prop.

Sometimes your projects don't always work out and sometimes goals change along the way. The important things to remember are that you can always learn from any experience and don't shy away from trying new techniques or materials. When you have fun with your R/C projects and keep an open mind, the hobby will always be a rewarding experience.

Update:

Since initially writing and submitting this article, I had an opportunity to try yet another power system on my Champion. Although I was pleased with my geared 370 setup, a new Speed 280 sized brushless motor had become available so I decided to give it a try.

The Model Motors' Aerix Mini 7 brushless motor (AC 1215/16) is the size of a Speed 280 motor but with the power of a Speed 480 motor! A Speed 280 motor has a 24mm diameter and the Aerix Mini 7 has a 22mm diameter. At 1.7oz in total weight, the Mini 7 brushless is only 0.2oz heavier than a Speed 280 motor. The shaft is Speed 400 size at 2.3mm diameter.

I replaced my Wattage IC-15 ESC with a Jeti 18-3P (JEC183) brushless motor controller but used the same 5/4AAA 800mAh NiMH pack since my maximum current draw would be about 9amps. These cells have proven to hold up better at this current level than the more popular 500-720mAh NiMH AAA cells used in Speed 280 applications.

I used an MJ MJ8009 3.46:1 ball-bearing gearbox for Speed 280 motors from Hobby Lobby. It wasn't a perfect match but it was close. I needed to drill out the 2mm hole in the pinion gear to match the 2.3mm diameter shaft on the motor. I dabbed some medium CA onto the end of the shaft and installed the pinion gear.

The motor case was a bit smaller than the Speed 280 motor that the gearbox was designed for so I wrapped my brushless motor with a few winds of masking tape until I had a snug fit.

My APC 9x6 slowflyer prop fit onto the 3mm gearbox shaft using an MJ8030 short prop adapter. At 9.5amps, I was getting 5200 RPMs on my new power system. My RTF weight using the 4oz 8-cell 800mAh NiMH pack was 13.0oz. I became excited when I realized that my thrust was exceeding my model weight!

The Champion now had incredible power to climb straight up and stop like it was sitting on a shelf of air. The combination of power, light frame, and lift from the 32" wingspan has resulted in a truly great flyer!

Final Specifications with Mini-7 Brushless Motor: