In 2015, NASA tested a Distributed Electrical Propulsion (DEP) system on a unique-looking aircraft called “Greased Lighting.” The aircraft never made it past the unmanned prototype scale model stage, but it did demonstrate the technology’s potential. Other companies are also researching DEP.
NASA tests Distributed Electrical Propulsion on Prototype
NASA classified Greased Lightning, officially known as the GL-10, as an uncrewed subscale hybrid-electric VTOL technology demonstrator. It had a carbon-fiber frame, a swept-back tilt-wing, and ten electric motors. NASA built ten prototypes of the aircraft and conducted several test flights.
Greased Lightning was a vertical takeoff and landing (VTOL) aircraft. It could also transition to a fixed-wing aircraft, relying on its wing for lift. NASA wanted it to have “the best aspects of an airplane and helicopter and fly with superb reliability, affordably, and excellent stability.”
DEP was perhaps the most significant technology NASA tested on Greased Lightning. Aircraft designers developed it based on goals to reduce fuel consumption, improve performance, and shorten takeoff and landing distances.
Electric Propulsors Provide Thrust for Aircraft With DEP Systems
There are multiple ways to define DEP, but it is a propulsion system in which the thrust generation comes from three or more electrically powered propulsors—fans or propellers. In many DEP designs, the propulsors are distributed parallel along an aerodynamic surface like an aircraft’s wing.
DEP systems may be fully electric, with electric motors receiving power from batteries, or hybrid, with electric motors receiving power from generators operating with gas turbines or electricity.
Propulsors and Power Sources Not Directly Connected
One of the main differences between DEP systems and traditional power plants is that with Distributed Electrical Propulsion, the propulsors are not connected mechanically, like with a driveshaft, to the power-producing components of the aircraft. The power systems can be multiple combinations of devices like turbines, electric generators, and fuel cells. They also require storage devices like batteries. The propulsors can be propellors or fans of various types and sizes.
Since the power sources are not directly connected to the propulsors, the gas turbines or electric generators can spin at different speeds from the propellors or fans. This feature allows both parts to operate at optimum speeds. Power inverters link the power sources to the propulsors.
DEP Systems Can Function As Control Surfaces
Another advantage of DEP systems is that multiple electric motors are often positioned close to each other along wing surfaces. The motors can then function as control surfaces and control how the aircraft maneuvers, as the thrust of each motor can be adjusted independently.
Another interesting feature of aircraft with DEP, like Greased Lightning, is their ability to shift from vertical to horizontal flight. Following takeoffs, pilots can shut down some of the motors and have the propellors fold back, increasing their aerodynamic efficiency.
DEP systems also promise to reduce operating costs. The cost reduction is partly because small electric motors and propellors are cheaper and easier to manufacture and replace than larger engines on other aircraft.
Safety From Redundancy of Multiple Motors
Having multiple motors also improves the safety of the aircraft. They provide redundancy if, for example, some are damaged since no single motor is responsible for keeping the aircraft in the air. In a system with 8 or 16 propulsors, the aircraft could continue flying if even a few motors failed.
NASA and several companies are developing other aircraft similar to Greased Lightning. These include NASA’s N3X with 16 propulsors, Electra’s EL-2 Goldfinch with 8, and Archer’s Midnight with 12.
Electra EL-2 Goldfinch Can Takeoff and Land at Very Slow Speeds
Electra’s EL-2 Goldfinch is an electric short takeoff and landing (eSTOL) aircraft that uses distributed electric propulsion and a hybrid electric power system. On 20 November 2023, the EL-2 Goldfinch completed its first crewed flight. It also has a blown lift system that uses electric motors to blow air over the wing and flaps, which increases the lift at slow speeds. This unique system enables the Goldfinch to take off and land at speeds as slow as 35 knots.
Archer’s Midnight has 12 Motors
Archer Aviation is developing its Midnight aircraft for urban transportation/taxi use. The Midnight will have 12 electric motors and six independent battery packs, each supporting a pair of motors.
Yet another advantage of these systems is reduced noise. For example, Archer reports that its DEP aircraft have smaller rotors and are up to 100 times quieter than helicopters.
Boeing and Wisk Collaborating on Air Taxi with DEP and No Pilot
Boeing is also working on a DEP project with Wisk, a company focused on “urban air mobility.” Wisk is developing an electric self-flying air taxi vehicle. This VTOL aircraft, named “Cora,” will carry two passengers. It has 12 independent lift fans and can perform level flight on only one motor.
NASA appears to have a very positive view on DEP’s potential for aircraft in the future. A NASA Technical Reports Server report on progress in DEP refers to it as a “disruptive” technology, comparing its potential for aviation to the impact of jet engines.
