Piasecki Aircraft Corporation says it is on track to start flight testing a full-scale prototype of its hydrogen-powered PA-890 helicopter in 2023 as it aims for type certification by the end of 2024. During last week’s Vertical Flight Society H-2 Aero Symposium & Workshop in California, the company’s president and CEO, John Piasecki, said it is working on a subscale demonstrator model as well as plans to demonstrate the fuel cell technology being developed for the program by HyPoint and a full-scale operational ground test stand.
The planned flight demonstration of a subscale version of Piasecki’s hydrogen powertrain will involve fitting an 80-kW high-temperature proton exchange membrane (PEM) fuel cell provided by its partner HyPoint. These will be integrated into an existing coaxial helicopter in what the company says will be the first manned VTOL flight using hydrogen fuel cells.
The technology maturation process will also involve the TAPU-660 ground test stand, which will focus on evaluating thermal management, power electronics, software controls, seeded faults, component durability, and cyclic behaviors. The PA-890 flying testbed itself will be used for work on control laws, certification standards, vehicle performance, and weight and balance.
“Our mission is to integrate the [HyPoint] technology and test the hell out of it," Piasecki told the H-2 Aero audience. "It is going to go through a very robust technology demonstration and validation program.”
Piasecki is adamant that the future of vertical lift rests on hydrogen propulsion. “I’m very excited about the opportunity that hydrogen presents to the sector,” he said in a presentation at the Vertical Flight Society’s Transformative Vertical Flight event in late January. “It has the potential to break down the barriers of power and energy, allowing us to achieve truly sustainable vertical flight.”
This enthusiasm, however, is rather new. In fact, early designs for the PA-890 slow-rotor winged compound helicopter called for battery power. But as the eVTOL advanced, it soon became obvious that battery power was more hype than substance.
“While batteries have a lot of potential, at least as of today, they simply don’t have the energy density needed to meet the long-range missions we’re looking to address,” explained Piasecki. “As much as I love our objective to have sustainable aviation, I also know the importance of the dollar [value] to operators that fly vertical lift aircraft. And unless you can provide a compelling economic case for the adoption of the new technology, you put your program at risk.”
So, it was back to the drawing board. Ultimately, the company decided to conduct direct, apples-to-apples comparisons between battery-electric, diesel-electric hybrid, and hydrogen fuel cell configurations using the same aircraft flying the same 200-nautical-mile mission.
According to Piasecki, out of the three options, the hybrid configuration offered the best performance, but this came with a carbon footprint. “Depending on whether you use jet-A or SAF, a hybrid configuration can get you a 60 to 80 percent reduction in CO2 emissions, which is pretty good,” he said. “But the battery and hydrogen options have the potential to get you to zero, and you can’t get any better than that.”
Although batteries and hydrogen may be on par in terms of direct emissions, hydrogen has the upper hand when it comes to operating costs. In fact, tests showed that batteries subjected to the rapid discharge/charge rates required for VTOL operations caused significant degradation in battery cycle life. As a result, the battery-electric configuration offers only a slight cost improvement over turbine engines.
And while the hybrid configuration reduced costs by about 25 percent, the hydrogen fuel cell reduced direct operating costs by up to 50 percent (relative to the turbine helicopter). “This is because hydrogen fuel cells have substantially fewer moving parts and are not subject to the extreme thermodynamic stress of a turbine, meaning they offer significant opportunities for reducing direct operating costs,” explained Piasecki.
Furthermore, because hydrogen fuel cells offer far better energy density, hydrogen-powered aircraft benefit from a greater range. Unlike battery-electric aircraft, which must be charged, hydrogen aircraft are refueled, giving them a potential turnaround advantage, too.
Piasecki also noted that the higher energy density of hydrogen has the edge when it comes to hovering. “Because hovering requires significant energy, it can zap most of a battery’s stored energy before the journey really begins, which also reduces the aircraft’s range,” he said.
Even hybrid configurations struggle with the hover stage. “To optimize the propulsion system for efficiencies, we sized the system’s diesel component for cruise and used the battery to augment for peak power demands like hover,” added Piasecki.
However, as he explained, this means battery-enabled hover capability is time-limited, and the aircraft must have a long enough recharge cruise segment during the rest of the journey to recover the battery’s capabilities for the end-of-flight hover. “This may be acceptable for many operations, but it introduces limits that need to be managed,” he said.
Although both the hybrid and hydrogen configurations satisfied Piasecki’s design requirements, ultimately, the company sees the most opportunity with hydrogen. But this isn’t to say hydrogen doesn’t come with its own set of unique challenges.
“It’s not just a question of performance," Piasecki maintained. "There are also some very practical challenges with hydrogen in terms of integrating it into the aircraft.”
According to Piasecki, the low-temperature fuel cells that would be the alternative to HyPoint’s equipment use complex and heavy mechanisms to manage water and cool the system. Each PA-890 aircraft will employ five of HyPoint's 650 kW fuel cells operating at a high temperature with the water in vapor form and exhausted and use air to regulate system temperature, thus eliminating the weight and complexities of low-temperature fuel cells.
“What we are designing is a very simple and elegant electric generator that is solid-state, easy to manage and control, significantly lighter than batteries or turbines, and void of any combustion, and that produces water vapor as a by-product,” said Piasecki. “The benefit is you get a much higher specific power than traditional low-temperature fuel cell systems, which suffer from the weight of their cooling apparatus.”
The company has aggressive targets and high expectations for its PA-890, which is targeting longer-range missions (+200 nm) with a payload of a pilot and up to seven passengers to replace existing helicopters. “Within the next few years, we are confident that we will be able to demonstrate a viable, practical method for applying hydrogen fuel cell technology to vertical lift solutions,” concluded Piasecki. “In doing so, we will significantly reduce the cost and carbon footprint of vertical mobility.”