Frank Piasecki is most frequently remembered as the father of the tandem-rotor helicopter. However, two new designs that borrow from his research vehicles of the 1950s and 1960s could soon see a new light in the emerging civil electric vertical takeoff and landing (eVTOL) and utility unmanned aerial systems military markets.
Piasecki designed and flew the second successful helicopter in the U.S., the PV-2, in 1943 and then formed and ran Piasecki Helicopter from 1946 to 1955. During that time, it produced iconic aircraft, including the tandem-rotor H-21 “flying banana,” used by militaries around the world. The mission of that company soldiers on today as Boeing Rotorcraft Systems, builder of the ubiquitous CH-47 “Chinook” series of helicopters.
Freed from the constraints of running an OEM, Piasecki returned to vertical vehicle research and development, launching Piasecki Aircraft and designing and building some of the most innovative vertical takeoff vehicles of the day. Those included the 16H “Pathfinder” series of shaft-driven, compound helicopters and the PA-59 series of ducted fan “Air Geeps.” More recently, in 2006, the company built and demonstrated the X-49A “Speed Hawk,” a specially modified compound variant of the Sikorsky UH-60 fitted with a vectored-thrust ducted propeller that replaced the tail rotor on a conventional helicopter and provided forward thrust in addition to anti-torque, vectored thrust, and yaw control. The system also mated a lifting wing with flaperons to the fuselage.
Like most of the company's work today, nearly all of these projects were one-offs funded by the U.S. military and/or large defense contractors. For his innovative work, Frank Piasecki received more than 20 patents and was awarded the National Medal of Technology and Innovation by President Reagan in 1986. He died in 2008, and Piasecki Aircraft is now led by his sons John, the company CEO, and Fred, its chairman and CTO. It operates from a 100,000-sq-ft facility in Essington, Pennsylvania, and has 75 employees (including full-time equivalents). “We’re a small, innovative company," John Piasecki told AIN. The company specializes in rapid prototyping and is involved in vehicle and vehicle-control projects as well as two relatively new efforts that build on Frank Piasecki’s work of more than 50 years ago: the PA-890 compound helicopter and the Aerial Reconfigurable Embedded System (ARES).
The PA-890 is an all-electric, hydrogen-powered, slowed-rotor, and winged compound helicopter designed to fly missions currently provided by conventional, FAR Part 27, single-pilot IFR-certified helicopters, including ones operated as air ambulances and for law enforcement. The design goals for the aircraft are a 50 percent reduction in operating costs compared with those of conventional turbine helicopters, zero emissions, a 200-nm range (with IFR reserves), and a very low external noise signature. The design utilizes a four-bladed main rotor, a variable-incidence wing that rotates up to 90 degrees to minimize download for efficient hovering; a swiveling tail rotor that produces anti-torque at hover and slow vehicle speeds and then rotates to maximize forward propulsion efficiency; digital motor controls; and conventional flight controls rather than a costlier fly-by-wire system. When it comes to systems design on the aircraft, John stated, “We’re trying to minimize the complexity wherever we can to simplify certification and lower costs."
The fuselage will be made using a variety of materials, including carbon fiber. John said that embracing traditional helicopter architecture substantially lowers the project’s risk. “There are a lot of novel [eVTOL] configurations proposed out there that will require the development of a new FAA certification basis. This represents a significant risk. We are able to meet our design objectives with a compound helicopter that can be certified under existing Part 27 conventional helicopter standards. So, given the lower risk of the certification process and the simplicity of the design, in our minds, it reduces costs both in terms of acquisition and operations” by using a design that more closely parallels that of a conventional helicopter, albeit a compound one.
That compound configuration maintains the aircraft’s efficient hover capabilities with a low disk loading rotor and a variable-incidence wing that minimizes rotor wash download in a hover. As the aircraft increases forward speed, more lift is produced by the variable-incidence wing, unloading the main rotor, allowing it to operate at a lower rpm, and decreasing drag. “A large, single rotor has low disk loading and low pounds per square foot of disk area and is a very efficient hovering device,” John explained, citing its advantage over “high disk loading solutions like small props, tiltrotors, or even jet engines, like in the case of the Harrier [military AV-8B Harrier II jump jet manufactured by Boeing]. So, that’s important because a lot of the customers that we're working with have requirements for the ability for sustained hover. However, helicopter rotors are not very efficient in forward flight due to asymmetric rotor flow with retreating blade stall on one side and compressibility effects on the advancing rotor tips. Compounding the helicopter by adding a variable incidence wing and thrusting tail rotor allows us to slow the main rotor significantly, improving forward flight efficiency and range, while reducing noise."
Electric propulsion makes it demonstrably easier to control main rotor speed and achieve noise reduction, John said, noting that “electric motors are very easily operated at different RPMs.” He added that the external noise profile of the PA-890 has been predicted to be a relatively quiet 68-73 decibels by researchers at Penn State and Continuum Dynamics which employed an integrated, high-fidelity model and the “WOPWOP” helicopter main rotor noise predictive tool. “The fundamentals of acoustics are really driven by some key aircraft parameters," he said. "One of them is [blade] tip speed and the other is blade loading. Across both those parameters, a compound helicopter does extremely well.”
The swivel tail rotor/thruster was selected for its lightweight and efficiency in hover and the PA-890's moderate forward design speed of 120-140 knots, comparable to other Part 27 turbine helicopters. With the PA-890, Piasecki is designing for maximum efficiency as opposed to forward speed and combat maneuverability, as was the case with its previous designs such as the 16H and the XP49-A, that used ducted propellers for thrust vectoring control and propulsion. John remarked that commercial customers are not clamoring for higher speed, but are interested in lower operating costs. However, the PA-890's design could be modified in the future to accommodate a "need for speed."
John said the company rejected a battery-only electric design based on performance and a hybrid propulsion system based on cost, given the low associated energy density and limited life cycles. However, hybrid diesel-electric and hydrogen fuel cells remain options. “Hybrid turned out to be excellent from a performance and cost point of view, but it still had a residual carbon footprint. Hydrogen fuel cells offered double the cost-saving and have zero carbon footprint," he said.
But traditional hydrogen fuel cells didn’t fit the bill, either. Last August, Piasecki signed an agreement with California-based HyPoint to collaborate on the development of turbo air-cooled, high-temperature hydrogen fuel cell systems for the eVTOL market. John pointed out that the “turbo” hydrogen fuel cells have five times the energy density of existing lithium-ion batteries and up to three times the specific power of existing hydrogen fuel cells. The deal with HyPoint calls for the development of five 650-kW hydrogen fuel cell systems for the PA-890. Piasecki and HyPoint intend to make the new system available to other eVTOL makers. HyPoint said its design yields 2,000 watts per kilogram of specific power, more than triple the power-to-weight ratio of traditional, low-temperature liquid-cooled, hydrogen fuel cells. On-aircraft certification testing could begin as early as 2024.
“The weight and the cost of managing the low-temperature fuel cell systems make those powerplants heavy and complex. The exciting thing about the technology that we're working on with HyPoint is that, by virtue of being a turbo air-cooled, high-temperature fuel cell system, operating temperature is managed by air and the resulting water is exhausted in vapor form, eliminating all the weight and complexity of low temp fuel cell water management systems. The resulting impact on performance is pretty significant, as is the specific power improvement, over a low-temperature fuel cell,” John said. The high-temperature cell will operate at around 300 degrees Fahrenheit and, other than the turbo, will have no moving parts.
A variety of hydrogen supply scenarios for the PA-890 are being explored, including on-site generation and/or storage. Producing hydrogen from electricity and water is well-established, but scaling it affordably needs to be addressed. Other approaches include extracting hydrogen from renewable methanol, with the latter being derived from biomass, biogas, or carbon dioxide. In February, the U.S. Department of Energy announced a $9.5 billion hydrogen infrastructure development program to create hydrogen production, storage, and transport networks designed to substantially reduce its unit cost.
Piasecki entered an MOU with air ambulance provider and helicopter services company Metro Aviation earlier this year to collaborate on the design of the aircraft and is working with other end-users as well. Although yet undisclosed, Piasecki characterized these end-users as “mostly fleet operators.” The company hopes to have a prototype flying in the 2024-2025 timeframe and is targeting certification for 2027. Initially, John said, he expects acquisition prices to be on par with those for comparable Part 27 conventional helicopters but believes the model will best them on price once production increases and economies of scale can be achieved. A large factor in the price reduction will be the cost of the fuel cell.
“Once we get economies of scale, the fuel cell cost is going to come way down,” John remarked. The target for overhauls of the PA-890's fuel system is up to 20,000 hours.
Specific industrialization strategies for the PA-890 remain under discussion. “The transition of the product into production is still being thought out as to whether we do that internally or create a spin-off [company],” John said. “We are working closely with key suppliers and fleshing out three different production strategies which balance affordability with product introduction demands, quality assurance, and scaling."
Piasecki ARES: Return of the “Air Geep.”
Piasecki developed its first ducted fan “Air Geep,” the PA-59K, in 1958. The ducted fan vehicle was originally designed “to provide ground combat elements with an aviation capability,” said John Piasecki. “When troops were driving along and got to a blown-up bridge or a hill and needed to get to the other side, they could fly. It was a short-range mission.”
Modern warfare needs are different, but a similar vehicle could still be the solution. That is what is driving the Piasecki Aerial Reconfigurable Embedded System (ARES) ducted tiltrotor. “Requirements are emerging for distributed operations,” John said. “Look at what is going on in Ukraine right now. The military needs to be able to sustain small, disaggregated, highly mobile combat units on the ground. Large, concentrated formations of troops and equipment are at extreme risk. Both the [U.S.] Marines and the Army are moving toward small disaggregated unit operations over extended areas. This is going to require a huge increase in vertical lift logistics capability suitable for direct resupply of small units independent of ground-based logistics. Small units can only handle about 3,000 pounds per delivery without materials handling equipment and still retain the mobility required for survival in a high-threat environment.”
ARES is a scalable, modular system designed to sustain those operations with remotely piloted and/or autonomous troop, casualty, and supply transport with mission module payloads as well as those for intelligence, surveillance, reconnaissance, and weapons platforms. Unlike the Air Geep's fixed ducted fan system, the ducts on ARES tilt from vertical to horizontal as the aircraft goes from hover to wingborne forward flight. The ARES is sized for payloads up to 3,000 pounds, mission ranges that exceed 300 miles, and will have the ability to operate off small deck ships and deliver cargo directly to small units in a city street. Because the vehicle’s rotors have both collective and cyclic controls, the ducts can generate a pitching moment “that facilitates maintaining a small aircraft footprint,” John said.
For now, powerplants on the ARES will be conventional—the current test aircraft uses a pair of Honeywell HTS900 turboshafts—but John Piasecki said there is the potential for a hybrid fuel cell-powered solution in the future. The program has received funding from the Army, the Defense Advanced Research and Projects Agency (DARPA), the Marine Corps, and the Air Force and has had partners including Lockheed Martin. Piasecki is currently focused on the development of the aircraft's software and the configuration of the triplex flight control system with Honeywell. The first flight with a mission module should occur in 2024 if not before, according to John.
Piasecki is working on a variety of other projects. Some, such as adaptive flight controls, it can mention. Others it cannot. “Rapid prototyping is really become a lost art within DoD [the Department of Defense], and in many cases, the [military] services can't afford to meet a good idea because the cost of demonstrating its core capabilities is too expensive. So, by focusing on the front end and affordably proving new technologies at scale, we think we're adding a lot of value to our customers,” John said.