Electric helicopter and drone developer Luminati Aerospace has urged eVTOL aircraft developers to take a fresh look at its approach to batteries, warning that the risk of fire has not been adequately addressed and offering a solution in the shape of its own technology. “Automotive battery technology is completely unsuitable for aerospace,” the company's CEO, Daniel Preston, maintained during a July 27 briefing during the EAA AirVenture show in Oshkosh, Wisconsin.
“Power density-wise, one ton of avgas [fuel] equals 200 tons of the best lithium polymer batteries in the world,” he said. “Lithium polymer [lipo] batteries are not just worse than petroleum, they are monumentally worse than petroleum. The only reason people are interested in electric aviation is that the motors have incredible power densities. A piston engine is half a horsepower per pound and with a turbine, you can be one to two horsepower per pound. On our electric motors, we are at 12 horsepower per pound. So, the first rule in aerospace is that even bricks fly with enough power. Electric motors can make anything fly."
Preston claimed that almost every major eVTOL aircraft developer has had a potentially catastrophic fire based on battery-pack failures. That includes his own company, where a single cell anomaly in a 4,000-cell battery pack produced a chain reaction, and a nine-alarm fire in December 2019 did $2.4 million worth of damage. The conflagration would have been far worse had it not been for the proprietary design of Luminati’s battery pack, which incorporates active and passive cooling as opposed to trying to shroud the battery pack in heavy, fire-resistant materials, Preston said.
Luminati has applied for a patent on that design and hopes to sell it to other eVTOL aircraft manufacturers. He predicted that the company could make far more money on its battery pack design than it would by selling eVTOL aircraft.
The design, which incorporates gas channels within the battery pack, directs any battery fire that may erupt into a finger-thin single beam and away from the aircraft, pilot, and passengers, and any collateral persons or property on the sides of the aircraft. It was the design of the pack aboard Luminati’s electric helicopter that caught fire, and the adaptation of the Gyrodyne QH-50 unmanned aircraft being developed for the U.S. Navy. Preston described the portion of the company’s heavy metal building hit by the fire beam from the battery pack as “vaporized” and said that overall damage to the building was exacerbated because the beam made a direct hit on multiple barrels of magnesium and titanium shavings from machined helicopter components.
Immediately before the fire, Luminati, which is based in Little Falls, New York, had halted flight testing of the QH-50 when flight engineers noticed that the battery pack was not making full power. The pack was then mistakenly plugged into a charger, and the chain-reaction fire soon resulted.
Preston said the company’s unshrouded battery pack is preferable because of enormous weight savings that accrue from eliminating any protective encasement, which can increase the battery pack's weight by almost 66 percent. Luminati’s 4,000-cell battery pack weighs just 400 pounds, compared with a conventional design, which would tip the scales at 1,200 pounds, Preston said. The weight savings justify any risk from the directed fire emission, he said. “Everyone is focused on containing the fire while we are focused on directing it. Directing it does not require additional weight. Electric air taxis have almost no weight allowance to protect the pilots and the passengers from the batteries. And that is why we are really trying to promote this and make sure no one gets killed.”
Preston acknowledged that “directed” battery fires still carry risks. “If you have a problem, land the aircraft, and point the aft toward something combustible, you are going to have a problem,” he acknowledged. Preston said it might be possible to affix a device similar to a jet engine thrust reverser can to direct the fire beam straight down but warned that any battery fire was “incredibly hot.”
Longer-term, Preston said, battery-monitoring technology must be improved so that the condition of every cell is monitored, not just banks of cells as is done with current system designs. He said the company has a proposal pending with the Defense Advanced Research Projects Agency to use artificial intelligence computers to “monitor the variables on every cell on a multipack of thousands of cells.”
Preston said that better education is needed with regard to how to best handle what he sees as continuing and inevitable battery vehicle fires. “It takes only one cell out of 4,000 to be slightly loose from the manufacturing spec and you are going to have a runaway fire.”
Preston claimed that federal recommendations on fire extinguishers and fire extinguishing chemicals for battery fires are not valid. “We’ve replicated those tests. Fire extinguishers are not putting out lithium [battery] fires,” he said.
Meanwhile, Luminati is continuing to work on developing a revised QH-50 but for the time being with hybrid-electric power.