After my experiments with electric power this past season I’m going to back to pistons. The electrics, while they do work, are too inefficient and expensive to fly with relatively limited performance because of too much of the helicopter’s payload capacity being used in battery weight.
Not a lot of people flying combustion engine helicopters it seems. Electric gets most of the focus, I suppose because of its simplicity. And even the heli setup wiki says, for some reason
The system has not yet been proven to work on fuel powered helis, but it should be possible assuming adequate vibration damping is provided for the APM/Pixhawk.
A Trex 700 nitro was the first helicopter I ever flew with the ArduPilot system and it works fine. So that wiki really needs some updating.
For my newest UAV helicopter, rather than buy a hand-me-down piston conversion like the last two were, I decided to start with a fresh sheet of paper and build one the way I wanted it from the ground up
It is based on a Synergy 766 with the gas engine conversion kit supplied by Carey Shurley of Gas Powered Helicopters. I selected a TRM VX310SX engine for power. The engine has stock port timing so it has excellent low rpm range torque and fuel burn. The engine runs on 91 octane non-ethanol blended pump gas, or 94UL avgas. It is a two-stroke based on the proven Zenoah G290RC format short block with a 2mm stroker crank. It uses the heavy-duty Zenoah metallic-shoe industrial clutch instead of the cheaper lined clutch bells common with nitro/PUH format drives. The transmission is a belt primary with geared secondary
My original design called for installation of a Jewel generator. But in the end I instead went with a larger flight systems power battery - a 5000mAh 2S Pulse. The battery easily powers the flight electronics and servos for a 2 hour flight without a generator. And I would’ve lost my electric top-start system with a generator drive. So the generator idea was scrapped.
The flight electronics is a Pixhawk 2.1 with a Here GPS/compass. Standard MavLink and 5.8GHz FPV system. This is a big heli with lots of room inside so all the flight electronics, short of the GPS module, can be hidden inside and weather-proofed.
I designed the payload system for front-mounted gimbals and cameras. And while can’t be seen in the photo, belly mounted tethered or solid-mounted payloads. The belly payload bar wraps around the engine’s recoil starter, which I retained instead of putting the fan intake screen on it, in the event the electric starter would fail in the field. The belly payload bar also carries two saddle tanks that give it two hour flight time with 15 minutes reserve fuel.
The heli can easily lift and fly with 10kg payload. Maximum takeoff weight is 17.7kg. With full 2 1/4 hours fuel onboard useful payload is decreased to 7kg - but still about double what a comparable sized electric can carry for less than half the flight time. The mainshaft is equipped with fully serviceable axial thrust bearings with grease ports to carry heavy continuous loads. Normal headspeed is 1,400 rpm with 1,460 and 1,530 rpm idleups for heavier payloads. Best cruise speed is 38 kts, maximum speed is 85kts.
I am custom building these helicopters for operators that need a high-end UAV for commercial purposes. This is the second one I have built. Entry price is $11,000 USD.