Multicopters would be able to lift heavier loads and stay in the air longer if they could use a high-density power source like gasoline instead of low-density lithium batteries. But gasoline engines, with their weird, peaky torque curves, aren't nearly responsive enough to keep a multicopter balanced against rapidly changing winds.
We've seen a number of different ways of addressing this – one memorable idea that springs to mind from many years ago proposed running both electric and combustion motors together, directly on each propeller shaft, with the gas engine supplying more or less constant torque, and the electric motors kicking in when high-speed adjustments were needed.
Edinburgh's Flowcopter has a different solution entirely. Its heavy-lift cargo drones will run aviation-certified combustion engines, and these engines will drive Digital Displacement pumps repurposed from the off-road and industrial vehicle markets, to run hydraulic motors at the props.
These pumps are able to distribute and regulate hydraulic flow between a number of different outputs, under digital control, with the kinds of near-instant response times you need to balance a drone in flight. Each hydraulic motor will deliver up to an enormous 96 kW (129 hp) of power, while weighing just 5.5 kg (12 lb) and costing less than US$1,000 apiece. Flowcopter says "nothing electric comes even close."
The weight of the Digital Displacement pump, combustion engine and fuel system might be significant, but gasoline offers so much more usable energy per kilogram than lithium that the benefits will more than outweigh the drawbacks in Flowcopter's estimations. The company promises endurance up to 6 hours on a tank of gas, and range figures up to 900 km (560 miles), from a straight-up multicopter with no efficient winged flight mode. It'll be holding itself up on propeller power alone for all six of those hours. Payloads will be up to 150 kg for shorter missions.
Flowcopter has built a fairly raw-looking prototype – none of your fancy carbon fiber here, folks, it's a welded metal frame – and has been doing some tethered flight testing. It's a little on the wobbly side, as you'll see below, but it flies, and as the world's first hydraulic hybrid multicopter, that's an impressive achievement. It'll be interesting to see if this technology pans out into widespread use.
Source: Flowcopter
