While much of the aviation industry looks towards ducted fans and vertical take-off and landing (VTOL) as the next stage of electric aviation, US-based engineering services firm Tier 1 Engineering is giving the tried-and-tested helicopter an electric shock.
The company’s retrofitted Robinson R44 made its first hover taxi on September 14, followed by a five-minute cruise flight on September 21. The latter saw the R44 reach a maximum altitude of 400 feet and a top speed of 80 knots – all powered by a set of lithium-polymer batteries and electric motors.
If that doesn’t intrigue you enough, the plan is that the helicopter will be flown semi-autonomously in the future.
The weight is over
It took the 9-person Tier 1 team around six months to design, build and install the electric propulsion system in the R44, starting in January 2016. Tier 1 designed and integrated all of the helicopter sub-systems, which include 1,100lbs (500 kg) of Brammo Lithium Polymer batteries, twin electric motors and a control system from Rinehart Motion Systems.
Although some complex engineering was involved, the actual retrofitting process seems to have been relatively straightforward – the team simply purchased a used R44 and set to work (manufacturer Robinson Helicopters has had no further involvement in the process).
Crucially, the company has said that no changes were made to the drive train or flight control system, although a digital display was added to cockpit to provide torque and power readings, and log data.
In a more detailed statement, Tier 1 explained that: “We removed [the] Lycoming IO-540 internal combustion engine, installed a custom mount for the electric motors and a reduction gearbox to interface with the existing drivetrain… The engine bay contains the motor controllers and cooling system for the electric drive components. The battery modules are supported by a lightweight composite panel and attached to the landing gear under the belly.”
The R44 was chosen largely because it combined a heavy engine with a forgiving certified takeoff weight of 2500lbs (1,130 kg). By removing the existing 500-lb (225 kg) engine and replacing it with 100-lb (45 kg) set of electric motors, 400lbs (180 kg) of additional payload became available for batteries. In this case, these are 700V, 100 amp-hour Brammo lithium polymer packs. In total, 11 battery modules were used and enabled the team to stay within the 2,500lb limit.
The modified R44 requires a large payload of batteries.
The modules power two three-phase permanent magnet synchronous motors. Tier 1 notes that: “The motors are stacked together and provide redundancy in the event of a motor failure. The motor assembly, which has a very a high power-to-weight ratio, was installed by one person by hand.”
In its present state however, these specifications will not carry the craft too far. The range of the proof-of-concept helicopter flown in September is estimated at “20 minutes or approximately 30 nautical miles.” Greater battery density (and the lack of a human payload) will be the key to achieving longer flights over greater distances.
The next Tier
The project is also backed by an interesting sponsor. Lung Biotechnology PBC contracted Tier 1 to develop the craft under a project dubbed EPSAROD– Electrically-Powered Semi-Autonomous Rotorcraft for Organ Delivery. The intention is that future versions of the helicopter would be fitted to fly autonomously and transport lab-manufactured organs to hospitals for transplantation. Under the terms of the project, EPSAROD aircraft must be able to carry two people and three manufactured organs, with a total payload weight of 600lbs (270 kg) for not less than 150 minutes, including a 30-minute reserve.
Although the plan is to include semi-autonomous controls, flights so far have been manned. The recent tests were conducted at the Los Alamitos Army Airfield under a special airworthiness certificate in the experimental category issued by FAA’s Los Angeles MIDO. Certainly, further qualification will be needed if these choppers are to fly themselves.
Nevertheless, it’s an important step for electric aviation. In a statement, Tier 1 Engineering president Glen Dromgoole commented that: “I’m very pleased to achieve this historic breakthrough in aviation… Never before has a manned helicopter performed a vertical takeoff, cruise and landing solely on battery power, and we are thrilled to have further achieved 400 feet altitude and 80 knots during our first full test flight.”
While fixed-wing electric aircraft have made numerous flights, the Tier 1 “[does] not believe there is another electric helicopter in this weight class.”
Over the next year or so, the team’s goal appears relatively clear. “We expect to improve the endurance using higher energy density batteries, a more efficient electrical drive system, and ultimately a more aerodynamic airframe.” Its ongoing EPSAROD test program will extend at least through 2017, during which time “a more advanced prototype aircraft will be designed and built,” Tier 1 said.