I was looking around for a suitable 40 size electric conversion project and had heard good things about the Kyosho Quality Series of ARF's. A quick search of the E-zone discussion groups came up with a couple of previous successful conversions to give some good guidelines.|
The basic specs from the kit box are as follows:
The individual components where then weighed so an estimate could be made of the total weight.|
|Bolts & Fittings||90|
|Top Right Wing||115|
|Top Left Wing||110|
|Bottom Right Wing||135|
|Bottom Left Wing||135|
|Pushrods, Dowels, Horns Etc||65|
|Ply blocks, Motor Mount, Rubber Surround, Tail wheel||155|
The ailerons use separate servos, the opportunity was taken to save a few grams so I used Hitec HS-81's instead of standard servos. The supplied mounting blocks were cut down to size.
The real challenge in electric conversions is the motor mount. With outrunners you have to be careful as nothing can touch the rotating can, therefore they can only be mounted from there mounting plate at the front of the motor. This represents a challenge as the firewall is usually along way back from where you want the motor shaft to end up.
In the case of the PT-17 I used four stainless steel 4mm threaded rods, some 6mm carbon fibre tube spacers and some fibreglass circuit board to make the mount as shown. It attaches to the firewall using the supplied 4mm blind nuts. This has proved to be a very solid mount. The hole in the firewall was enlarged and an additional one cut for the speed controller wires and cooling air.
|Another area of concern with electric power is access to the batteries. Its a pain to have to remove wings to get to the battery pack, especially with a Biplane! Therefore a hatch was required to be cut somewhere. The front cockpit seemed the best place, good access without compromising strength. I cut through the top with a razor saw and knife to remove the cockpit. I then added a balsa floor and back to it. Neodymium magnets and washers were used to hold the hatch in place. Luckily I had a few scraps of covering film to cover up the cuts.|
|Rudder & Elevator servos where installed as per design behind the trailing edge of the lower wing however I used lighter GWS Mini L BB's instead of standard servo's. These servo's are light (~22g) but still fairly large and robust with the same torque as most standards.|
|The only task left that deviates from the manufacturers instructions was the fitting of the main battery pack. This required installing a light ply battery tray into the front of the fuselage with some velcro patches and straps. The former at the front of the cockpit had to be trimmed to allow the pack to fit. When balanced only about 10mm of the battery protrudes past this former, with most of the battery in the nose.|
After waiting weeks, finally got a break in the weather and managed to get two flights on the PT-17. Still hadn't quite finished the Dummy Radial and need to find a pilot.
C.G was pretty much spot on, seemed to be plenty of power and cruised around nicely at half stick. Decided to err on the side of caution and called for landing at 5min on the timer. After lining up the strip I basically just throttled back until she gradually sank onto the grass... very nice. A quick check of temps, the batteries where bordering on cold and the motor was barely warm. After a quick recharge it only put back in 1500mah into the 1950's so I should be able to get a few more minutes in the future.
Second time around things were a bit more orderly on takeoff, I'd heard so much about Biplanes being squirrelly but found the PT-17 tracks nicely. This time I tried a loop, roll, Stall turn and full power climb. It is defiantly a little more power than scale, like straight up for the first part of the pack, however can be throttled back and tootle around in a pleasant scale manner. This flight was about 6:30 power left at the end.
Since had a few more flights and completed the dummy radial. She really looks sweet in the air. Most flights are around the 7min mark.
An easy conversion project and a very smooth easy flyer and good looking too. With a bit more effort I'm sure it could be made more scale with the addition of some flying wires etc. Maybe big planes do fly better....