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26 ended pitch links that fit into sockets. Alpha uses four of them, but many other designs use three for simplicity. As Escarpenter explains, “Three is a very simple configuration, as it is not hyper-static, meaning that the servos do not fight each other. If though you have no back-up at all then the failure of one servo means total loss of control. “With our four-servo swash plate, if one servo fails you lose a bit of precision on that axis [pitch or roll] but you can still land safely. “On the other hand, configuration is more delicate, and if it is not done properly the servos will fight each other, reducing their service life and increasing power consumption.” A single Futaba servo controls the pitch of the tail rotor blades. Alpha is preparing to register a patent related to the control of the tail rotor. Engine and ECU The engine is a Japanese Zenoah G29, a single-cylinder, 29 cc air-cooled, carburettor-fed two-stroke producing up to 2 kW and 1.8 Nm (1.4 lb-ft) of torque. Regular unleaded petrol is mixed with 2.5% of two-stroke oil, and is contained in a pair of externally mounted polymer tanks, from which it is drawn by the natural suction of the carburettor. Escarpenter explains that in a helicopter the main rotor speed is governed within a very narrow range, so the engine operates at almost constant rpm. In these circumstances the fuel efficiency benefit of electronic fuel injection is limited, so the cost and complexity of an injection system and its associated fuel pump is not worthwhile. Furthermore, the lack of large changes in engine speed means there’s no need to alter the timing in the (ultra-simple) magneto ignition system. However, the ECU still has to compensate for air density changes with altitude. To do that, it uses a set of sensors to monitor the cylinder head temperature, exhaust gas temperature, air pressure and ambient temperature. “Our approach, which is patent pending, uses the sensors to determine the optimal mixture in all conditions,” Escarpenter says. The output of all these calculations is a height setting for the metering needle in the carburettor’s main jet, controlled independently of the throttle setting. Depending on the configuration, Alpha uses two or three Futaba servos – one of them for the throttle, and either one of them to handle mixture control for altitude compensation or ventilation control in very cold environments, or both. With rpm governed within a narrow range, power changes are made by increasing the collective pitch on the main and tail rotor blades to increase the blades’ angle of attack, while the throttle is opened in coordination with the pitch change. This contrasts with the way most multi-copters work, which is by changing the speed of their fixed-pitch rotors. Alpha developed its own June/July 2018 | Unmanned Systems Technology Dossier | Alpha Unmanned Systems Alpha 800 The Alpha 800 is powered by a Zenoah G29 two-stroke single fed with fuel and air by a carburettor (Author’s image) The power and comms module contains the ECU and manages power distribution for the core electrical loads and payloads (Courtesy of Alpha)