Revolution.Aero Uplift: Certified battery-powered VTOLs to lift off ‘within a few years’
Certified battery-powered vertical take-off and landing aircraft will lift off within a few years, according to speakers at Revolution.Aero’s Town Hall online meeting. Progress is being powered by battery innovation dedicated to improving safety systems, performance and maintenance.
“We are within a couple of years off electrically powered [certified vertical take-off and landing] flight,” Joshua Stewart, BAE Systems, principal power systems engineer, told attendees. “Even with today’s battery technology, we are very close. So, it’s going to be in the next couple of years.
“People are flying electrical vehicles now – but they are just not (certified) vertical take-off and landing vehicles,” said Jesse Crispino, Jaunt Air Mobility, chief operations officer. “The future is as close as here,” he said. “When aircraft start getting certified, these vertical take-off and landing aircraft are going to become a reality.”
Innovations in battery technology, experience with aircraft certification, and proven technologies from the transit markets are facilitating progress, said Stewart from BAE Systems. The company is addressing three key battery safety challenges: ground fault and arc flash, thermal runaway and loss of critical power. It’s important to combat arc flash because this occurs when high voltage jumps through the air to the ground or another voltage; superheating the air and potentially causing an explosion. “This can produce extreme damage to the battery, other parts of the high voltage system and be very dangerous for personnel,” said Stewart.
Mitigations included understanding the potential for arc flash and designing battery fuses, contactors and other systems to minimise the risks. The arc flash potential of a battery system needs to be understood across the entire envelope of operating and storage conditions. For example, with increasing altitude, the voltage breakdown of air decreases making insulation and isolation more important.
Thermal runaway was another key challenge. Propagation occurs when one battery cell rapidly overheating causes adjacent cells to overheat, sparking a chain reaction throughout the system. Typical causes of thermal runaway included battery penetration or crushing, over-charging or over-discharging cells, and thermal abuse. BAE Systems is developing preventive measures such as safe cell design, advanced materials and innovative control strategies. Modelling and analysis, coupled with rigorous laboratory testing, under various thermal runaway scenarios are critical to understanding the physics of thermal runaway and the strategies that successfully mitigate safety concerns for safe continued flight. BAE is working on different strategies that focus on preventing propagation and containment. System-level analyses are required to understand which strategies should be implemented in a given design.
The third battery challenge identified by BAE Systems is mitigating loss of critical power. This relies on a redundant high voltage battery system architecture in which the aircraft can still fly and land safely despite the failure of one or more battery sub packs. “It’s important not to put all your eggs in one basket – or all your [battery] cells in one pack,” said Stewart. The probability of a catastrophic failure must be less than 10(-9). Understanding how many battery sub packs are required to fly safety and land the aircraft is critical.
A vital part of the architecture is a highly reliable and certified battery management system. This tracks voltage and current, state-of-charge and remaining battery life, plus detects faults and other factors. “An accurate state-of-charge is so important because that is your fuel gauge.” BAE uses a distributed battery management system.
Stewart concluded: “BAE is uniquely positioned” to help meet the safety challenges of aircraft electrification, drawing on its experience with high voltage energy storage systems, designing and building flight-critical electronics with the support of its highly qualified team.
BAE System’s partner Jaunt Air Mobility explained how BAE’s battery and other technology is helping to build its Jaunt Journey aircraft. “The aircraft is going to come with fly-by-wire triplex redundant systems designed by BAE Systems,” said Crispino.
Described as “a slow rotor compound aircraft”, it has a main rotor and wing-mounted propellors. The main rotor is always powered in flight and enables the aircraft to hover like a helicopter. “When the aircraft accelerates the main rotor slows, which gives tremendous aerodynamic efficiency, reducing drag and lowering the acoustic profile of the airplane,” said Crispino. The aircraft is like a twin-engine helicopter leading Jaunt to seek Part 29 standards certification.
The battery-powered aircraft will need the same level of fire prevention and cabin intrusion as legacy fuel models plus extra monitoring during battery recharges. A key difference is that fuel systems are linear while electric propulsion systems are not driving key monitoring requirements. For example, low volage can damage cells and potentially lead to the failure of a motor. Also, older batteries can lose capacity over time and heat more quickly – requiring monitoring to avoid damage or loss of propulsion.
Pilots needed to know not just the level of battery charge but also the manoeuvres that might be needed during the flight. Hover landings will require significantly more battery power than non-hover landings. Also, recharging batteries at the correct charging profile is important to avoid battery system damage or even ground fires.
Despite certifying the aircraft to a reliability level of 10(-9), a total or partial power loss is inevitable over the life of every urban air mobility aircraft configuration, said Crispino. Fortunately, the Journey’s high inertia blades and energy storage enable the aircraft to autorotate or glide to a precision landing from any attitude or speed, he said.
Returning to the problem of thermal runaway, Jon Harrison, Gamma Technologies, staff engineer – Business Development, explained how simulations could prevent battery damage and loss of power. “Thermal runaway is a significant risk to the safety of the system and catastrophic failure,” Harrison told attendees. “Simulations can be used to complement the testing when designing these [battery] packs to optimise for safety concerns.”
The Revolution.Aero Town Hall – Safety considerations for aircraft batteries – took place on Tuesday November 9th. Listen the online meeting, sponsored by BAE Systems, here.