Unmanned Systems Technology 002 | Scion SA-400 | Commercial UAV Show report | Vision sensors | Danielson Trident I Security and safety systems | MIRA MACE | Additive manufacturing | Marine UUVs

22 eases installation and improves overall craft performance. It is no coincidence that these days almost all manned military and commercial helicopters are turbine powered. Turbines are not well suited for applications that involve a lot of stopping and starting, such as cars, but they are very well suited to high-power constant-load applications. “Helicopters operate at high power all of the time,” observes Sampson. “You are hanging on your horsepower – helicopters don’t glide well at all! “An equivalent gas engine would be far heavier,” he emphasises. “Put a diesel in there and you wouldn’t lift it off the ground.” Eagle R&D designed the Helicycle around the Solar Turbines T-62 Titan gas turbine, as used as an auxiliary power plant in the Sikorsky Super Stallion and Chinook military helicopters; these days Solar Turbines is a wholly owned subsidiary of Caterpillar. Scion UAS uses reconditioned T-62s since they are perfectly acceptable for its needs, making them cost-effective. The T-62 uses a centrifugal charge air compressor, while its power section – having introduced the fuel supply and then ignited the mixture – uses a radial inflow impeller (echoing a typical automotive turbocharger with the addition of a fuelling stage). This is a so-called single-spool turbine, meaning the compressor and turbine wheel are directly connected to the output shaft. The assembly rotates at 61,000 rpm and the bearings are lubricated by a very fine oil mist. This is a dry-sump oil system; there is a gear-type oil pump driven off the gearbox. The T-62 comes complete with a gearbox, but Scion UAS replaces that with a lighter production by Eagle R&D, made possible since the power output is de-rated from 160 to 90 bhp. “A turbine operates like a naturally aspirated engine: if you want to have power at altitude on a hot day, you can’t get much more than 100 bhp out of it due to the altitude and the heat,” explains Sampson. “In view of that, we de-rate it to where we are going to be operating, which is an 8000 ft maximum [beyond which the air is too thin]. That is standard practice for turbines. You can’t supercharge a turbine; you have to oversize it and then de-rate it.” In the SA-400, the T62 is fitted with a digital governor and a number of sensors for temperature, pressure and fuel flow. There is no throttling of a turbine’s charge air. “Turbines run ultra-lean,” notes Sampson. “There is always more air going through a turbine than is needed for a stoichiometric combustion. There is constant combustion and you control a turbine by how much fuel you give it.” The fuel supply to the T-62 incorporates an engine-driven gear pump that creates constant pressure, beyond which is a metering valve: a needle valve that in the case of the SA- 400 is operated by a servomotor, which in turn is controlled by the governor. Sampson notes that while the engine is running, the needle valve is always moving, feeding half-a-dozen constant flow (fixed orifice) injectors via a common fuel rail, so it is the only control over the amount of fuel supplied to the engine. “We run at optimum engine speed all of the time: 61,000 rpm,” remarks Sampson. “We have three speed settings – zero, idle and then, once we have engaged the rotors, flight speed. Once we have engaged the rotors it stays at the 61,000 rpm flight speed. If a rotor is pitched, that loads the engine, and with that the servomotor moves a lever so that the needle supplies more fuel. We call that throttle-by-wire.” Flight speed is adjusted by manipulating the rotors, so it follows that the turbine can be kept at constant revs at all times. Spring 2015 | Unmanned Systems Technology The SA-400’s Solar T62-32 engine awaiting installation If I fly manually and aggressively, it still only changes speed momentarily by about 100 rpm out of 61,000. That is remarkable!