Because the CityAirbus is designed to operate in sensitive urban areas, engineers are studying how to keep noise levels low. Electric motors help, but using contra-rotating propellers in the ducts can result in unwanted noise from blade interactions, so tip speed is reduced to 395 fps (120 m/s) compared to the 690 fps in a traditional helicopter.
Behind the cabin, Airbus is suspending the four 100-kWh lithium-ion batteries on a rail that allows them to be moved to adjust the center of gravity during flight testing. Suspending them high in the fuselage also improves handling and flight characteristics, Bebesel says.
Airbus powered up a ground-test rig for the CityAirbus propulsion and flight control systems at the end of 2017 and is aiming to fly the demonstrator, unmanned, at the end of 2018. The first hop will take place in Donauworth, Germany, before testing is transferred to the nearby military airfield at Manching.
Once early testing is complete, the aircraft will be converted for manned flights. While initial commercial flights could be piloted, ultimately the CityAirbus would fly autonomously between preassigned stations. Passengers would walk under the ducted fans to enter the cabin, but as they are 6.4 ft. off the ground, only a handful would have to duck.
Props can be stopped within 2 sec., which should speed up turnaround times, allowing passengers to leave the vehicle moments after landing, unlike on a helicopter, where they might wait for rotor blades to stop turning before exiting.
Because the CityAirbus is carrying its 1,100-lb. (500-kg) energy source at all times, a key driver has been to keep weight low. The airframe itself is only about 10% of the vehicle’s empty weight. Motors and inverters feature separate liquid cooling systems, although Bebesel says production systems likely would have a more simplified approach. Artificial intelligence also would play a role, with the electrical systems calculating the temperature and charge status so it can be optimized for the upcoming flight.
Another lesson from Voom’s first 10 months of operations in Sao Paulo? “Weather can be a real problem. In January, we had to cancel 21 trips in 1.5 hr.,” Subramanian says. “We need to be thinking how weather factors in, and we need to design the vehicle to deliver a reliable product, even in inclement weather.”
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