The "Trinity" Tricopter with petrol supplement drive.
At CanberraUAV we have recently been mucking about with all sorts of VTOL aircraft with the general aim of finding something for the UAV Outback Challenge that is perhaps a little more elegant than the quadplane we used in 2016.
Jack had often mentioned the idea of putting a petrol engine in the centre of some form of multirotor. This concept is not new, it has been done before:
The concept is fairly simple, build a multirotor with a fixed internal combustion engine to provide the majority of the thrust, the electric motors provide mainly control and stability. All in the hope the endurance of the combination is greater than electric alone.
So I thought I would have a crack at it.
The obvious problem of the internal combustion engine is the torque produced by it needs to be counteracted. I chose to use a tricopter as the tilting tail out of the box provides the flexibility of providing the stabilisation in all situations whether the engine is running or not. So I started hunting for a tricopter/engine/motor combination.
I wanted the electric motors to be powerful enough to fly the machine in the case of an engine outage. I also did not want to go overboard and kept it modest in size. In the end I came up with the following set of main hardware components:
Turnigy Talon (350g) https://hobbyking.com/en_us/turnigy-talon-tricopter-v1-0-carbon-fiber-frame.html
An RCTimer combo (300g) http://rctimer.com/product-1298.html
RCGF 10cc engine (623g) http://www.zjrcgf.com/product663.html
Batteries, fuel, tanks and other hardware it all came up a little over 2kg. The 3 electric motors @ 4S suggest about 4Kg of thrust, so I felt my goal of the electric safety net was possible. The electric motors I eventually got enough to run all clockwise opposite to the engine so as to naturally counteract as much engine torque as possible.
For the flight controller I chose a PixRacer as the 3 Rotors, tail and throttle tail servo and ignition kill switch added up to 6 channels. I really did not have the space for anything larger.
The build came together fairly quickly over a few evenings the tricopter was largely unmodified with some aluminium plate and some polyethylene blocks improving rigidity due to the extra weight. It needed boots on its legs to accommodate the belly fuel tank. Add some anti vibe mounts for the GPS and flight controller.
CanberraUAV had a build day and the extra help allowed the build to be completed and some electric only flights performed:
The next day we fired up the engine, with the throttle servo being directly mapped to throttle on the transmitter sticks. In this sense it was pilot controlled and the autopilot had no say in the throttle position.
It quickly became apparent after these flights we needed a better way to control the ratio of electric to petrol thrust and let the autopilot have some say in proceedings. Tridge quickly made some changes:
And a fortnight later we tried out these changes for real:
As you can hear in the dialogue, for stable flight the electric motors are running at about 6A with the engine running as opposed to about 30A when on electric only with the copter just barely staying in the air. The small 2.4Ah battery was not able to deliver sufficient current nowhere near the 17A/motor the spec sheet suggests.
All in all things are looking promising thus far.
- Tune the engine. This is proving to be troublesome as is often the case on small engines.
- Move to a 5Ah main battery to improve the peak electric thrust and overall endurance.
- Experiment with prop sizes to provide a more responsive thrust profile from the engine. The flights shown had a 13x6, we will try 13x5 and 13x4 next.
- Is the gyroscopic precession effect of the heavier engine/propeller combination require consideration in the controller ?
- Optimise the fuel tank and battery size for endurance and get some real numbers.
- If the numbers stack up, build a bigger one!
I have to say this build has been a lot of fun, and came together very quickly when compared to helicopters of similar size. The ease at which Arducopter was adapted to this application is testament to the hard work of many.
Many thanks go to Tridge and Greg for wiring and footage and to the test pilots Stephen and Justin. As well as everyone else who helped even those who held it down while we attempted to tune the engine.