Single-engine cruise flight on twin-engine helicopters, to save fuel, could be possible, according to researchers at the Technische Universität of Munich, Germany. Their work focuses on how to cope with failure of the operating engine when the other has already been shut down. Two elements are key: quick automated restart; and limiting to safe levels the rate at which the engine builds torque during that quick restart.
During most steady flight phases, a twin’s engines are not heavily taxed and one engine could deliver the power required, Professor Manfred Hajek pointed out. Specific fuel consumption falls as the load on the engine rises, said Hajek, so cruising on one engine at a high power setting in suitable areas of the flight envelope would cut down fuel consumption “significantly.” For a given power requirement, he noted, one turboshaft operating at a higher output is approximately 30 percent more fuel-efficient than two turboshafts running at a lower output.
Intentional single-engine operation of a twin would be worth investigating only if the inactive engine could restart and produce power quickly enough to minimize altitude loss after failure of the operating engine, Hajek conceded. As part of the study, his team used an engine test rig and a BO105 flight simulator. One engine (the one that had to restart swiftly) was a Rolls-Royce 250-C20B fitted with a quick-start system designed in previous research work and using a high-pressure air supply. The modified engine can reach flight idle in two seconds, rather than the usual 27 seconds, according to Hajek. The second engine (the one that had to “fail”) was simulated.
The first priority was to limit the rate of torque increase to 65 Nm per second, beyond which the engine surged.
The researchers then compared how pilots and automation handled the restart. A “quick-start controller” with an altitude-hold mode lost 330 feet of altitude; pilots lost about 560 feet. Pilots tended to regain 100-percent rotor rpm more quickly, while the automated system kept collective pitch higher, helping to explain the different altitude losses.
More work needs to be done, Hajek said, especially in assessing the effects on the main gearbox. He emphasized that single-engine cruising would not be suitable for offshore twin operations or for brief EMS flights. He also noted that rolling an engine to idle rather than shutting it down would likely make only a small dent in fuel consumption.