Hybrid-Electric Distributed Propulsion Explored

April 4, 2014

Benjamin Schiltgen,

A generic TeDP system architecture featuring eight fan/motor assemblies powered by one turboshaft engine/electrical generator combination located in each wing half. The turboshaft engine spins a generator to produce electrical power but is assumed to not provide propulsive thrust on its own. This electrical power is bussed through the wing to each motor/fan assembly and to the onboard subsystems, if necessary. A detailed electrical system layout was beyond the scope of this work; however, it did become apparent during this study that transformers would be too heavy for this type of vehicle.

A turboelectric distributed propulsion (TeDP) system is a powertrain consisting of a turboshaft engine used solely to provide electrical power through a generator to electric motors driving multiple propulsive fans that are distributed above, below, or inside a wing.

Earlier studies hinted at the possibility that cryogenic cooling may not be necessary for certain classes of aircraft to achieve marked improvements in fuel burn and airport area noise. A more recent study, for NASA Ames Research Center under the Environmentally Responsible Aviation Project, applies non-cryogenically cooled electrical components suitable for the 2025 time frame to both a dual-use transport (shown) and a regional airliner concept.

Empirical Systems Aerospace, Inc. (ESAero) has studied the application of TeDP in several forms to a wide variety of manned and unmanned air vehicles since its inception in 2003 with several approaches to distributed propulsion. In the studies, it developed a distributed propulsion system architecture and used it to estimate vehicle performance including details on the propulsion components, battery sizing and augmentation, and system performance using in-house design tools. It also identified tools that could be developed during future work.

Its most recent project (certain aspects of which are discussed here) was done under NASA’s Environmentally Responsible Aviation Project, part of the Integrated Systems Research Program, to provide configuration alternatives to those being studied by the major airframe manufacturers and two universities.

Decoupled energy management

Participants in an FY11 NASA-sponsored TeDP workshop collectively agreed that TeDP might be more accurately considered decoupled energy management (DEM) as the overarching concept that enables hybrid-electric propulsion. The major benefit that DEM brings is an abundance of options pertaining to the aircraft's configuration and operation.

Decoupling the gas generator from the thrust producer allows for several unique advantages that could very well improve aircraft performance and efficiency. Turbofan engines are designed with a mechanical linkage between fan and gas generator that forces all components—fan, low- and high-pressure compressor, and turbine—to compromise on their individual peak operating points. Removing the mechanical linkage, or shaft, and replacing it with an electrical system allows components to be optimized with less concern for each other especially in regards to rotational speed.

ESAero's ECO-150-16 is a cryogenically cooled TeDP airliner concept for the 2035 time frame.

Not only does this yield the possibility for reduced component weight and fuel burn, but other advantages can be garnered, such as:

Use of supplemental power (batteries, super capacitors, fuel cells) for failure modes or downsizing of gas generators
Effective bypass ratio increases without excessively large diameter fans
Finer control over power distribution during any flight segment.

Introducing this method of DEM does bring with it high transmission losses and an overall increase in propulsion group weight. From an efficiency standpoint, propulsive efficiency can be increased. But until technology of conventional machines is improved, the increase cannot offset the weight penalty for regional missions. Savings in fuel burn must be much higher than the increase in propulsion system weight for a fixed takeoff gross weight if TeDP is to be economical.

Conventional electric component sizing

Electric components to power large commercial aircraft do not currently exist and thus it is difficult to estimate their performance and weight without a focused motor, generator, and controller design effort. With hybrid electric system research in full stride, it may require many years to establish a flight-worthy component database for use in modern conceptual and preliminary aircraft design.

In the meantime, off-the-shelf electric components must be used for scaling and performance curve extrapolation. Using this method has potential side effects since power requirements of airliner components is exceptionally larger than what current off-the-shelf components can provide.

ESAero maintains a database that now includes nearly 100 state-of-the-art machines, the most powerful of which delivers up to 885 hp and weighs roughly 7300 lb. Meanwhile, the commercial and military dual-use variants require 1500- to 2000-hp motors and 10,000- to 16,000-hp generators. Without an electric machine design effort with an emphasis on flight-ready components, extrapolating motor and generator size has a large potential for error.

Summary and conclusions

Aircraft integration, terminal-area operations, and climb and approach angles are among the other aspects of the researchers’ work, and are detailed in the technical paper on which this is article based (paper number at end).

Building off of earlier work involving cryogenically cooled, superconducting technology, more recent effort succeeded in preliminarily closing the ECO-150 and dual-use aircraft using conventional, state-of-the-art electric machines and components. While several benefits appear to show promise of improved mission performance and efficiency, concerns of integration, current state of high-performance electric machines, and the ever growing propulsion system weight of TeDP casts a dark shadow on future feasibility. As technology improves in the area of electric components so will their use and application into regional aircraft. In the meantime, it may behoove the aerospace industry to invest time in research and development tools to aid in the analysis of hybrid transport aircraft.

The article is based on SAE International technical paper 2013-01-2306 by Benjamin Schiltgen, Andrew R. Gibson, Michael Green, and Jeffrey Freeman, Empirical Systems Aerospace Inc.