UTC Reveals Hybrid-electric Aircraft Demonstrator

The newly formed United Technologies’ Advanced Projects (UTAP) division (see article on page 40) unveiled its initial hybrid-electric demonstration aircraft last month, which it said will yield 30 percent fuel savings on a typical one-hour mission. Dubbed “Project 804” (P804), the aircraft is expected to fly within three years. It is a reworked Bombardier Dash 8-100 twin turboprop, re-engined on one side with a two megawatt-class propulsion system and an engine optimized for cruise efficiency augmented by a battery-powered electric motor that assists during the 20-minute takeoff and climb sequence.

The test aircraft will have the hybrid system installed on one side of the aircraft for test flight, according to UTAP executive director Jason Chua, and the 30 percent fuel savings achieved over the course of a one-hour flight assumes the system is installed on both sides. So installed, the maximum range of the aircraft drops to 600 nm from the typical range of 1,000 nm due to the system’s weight and its impact on fuel load. UTAP claims the typical commuter mission is 250 nm and that 99 percent are under 500 nm.

According to UTAP, the engine and electric motor will each generate one megawatt of power in parallel hybrid configuration. The system uses off-the-shelf battery cells with a custom-designed packaging and battery management system. The battery, its power management system, and the power electronics will be installed in the aircraft cabin, while the hybrid-electric propulsion system will be mounted on a modified nacelle.

UTAP attributes the fuel savings to new engine efficiencies and those derived from electric power and that the system is suitable both for retrofit on existing aircraft and clean-sheet designs.  

United Technologies’ Collins Aerospace unit already has substantial experience with aircraft electrical power systems on “more electric” aircraft as it provided the six generators on the Boeing 787. The airplane’s electrical system produces nearly 1.5 megawatts (MW) of onboard electrical power and replaces many of the aircraft’s power systems historically powered by hydraulics or pneumatics.

Battery Modification

UTAP points out that current battery-only electric-powered flight has inherent limitations due to battery weights and capacities, making it only suitable for small 1-4 seat general aviation aircraft flying missions of no more than 108 nm. In a P804 white paper issued in March, UTAP points out, “ Hybrid-electric propulsion is a more interesting proposition for business, regional, and large commercial aviation domains.” Of the two existing hybrid options, serial, which uses electrical energy from hydrocarbon engine generators augmented by a battery system, and parallel, which augments hydrocarbon engine propulsion with electric energy, UTAP opted for parallel. The reasoning is that a parallel system enables the hydrocarbon engine to be downsized by augmenting it with stored electrical energy during peak demand flight phases such as takeoff. This, in turn, reduces stress on the hydrocarbon engine, allowing it to be operated at lower combustor temperatures and increases engine parts life. 

The UTAP white paper illustrates why solutions other than a hybrid system simply will not work with today’s battery technology, noting that to power a typical regional turboprop flight for one hour would require a battery system that would exceed the aircraft’s maximum takeoff weight. 

To meet anticipated power level demands, UTAP intends to develop higher power density motors, generators, and power electronics capable of reliably handling associated loads. This will require the use of advanced ceramic-based semiconductors, packaging methods, thermal management, and manufacturing techniques. UTAP said the Project 804 engineering team will develop products that approach, and sometimes even exceed, 20 kW/kg—surpassing the power densities available today. The engine and electric motor will each generate about 1 megawatt of power in a parallel hybrid configuration on the P804 aircraft. UTAP plans to initially use lithium-ion batteries for the system. “We want to use technologies that are commercially available and off-the-shelf,” Chua said. UTAP said the system has other airframe applications including category “A” helicopters, APUs and some turbofans. Chua said the system could be scaled “up and down,” suggesting that it could be applied to business turboprops.

Chua said that UTAP currently has 15 full-time employees working on the project and is adding more to the team. He emphasized that “dozens” of other employees from across UTC’s other units including Pratt & Whitney and Collins are contributing to the effort part-time as well as employees from project partner companies. 

Chua said UTAP’s goal is to make the system available at the same price as a typical engine replacement. “That’s the target we want to hit.” For now, the project is focusing on bringing aboard more manpower and partners with specialized expertise in areas such as battery systems and aircraft modifications. He said the project should be ready for an aircraft preliminary design review within the next few months. The test aircraft is currently housed at the Pratt & Whitney Canada flight test facility in Mirabel. “I think this is a pretty exciting project and we are looking to move really fast,” Chua said.