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Electric machines and hybrid drives for vertical takeoff and landing (VTOL) tactical air vehicles



OBJECTIVE: Develop and demonstrate lightweight hybrid or electric-enhanced drive system technologies for novel vertical take-off and landing (VTOL) tactical aircraft with powerplant output in the range of 50-250 kW class. 

DESCRIPTION: Aircraft capable of vertical takeoff and landing are a critical enabler for many US Army operations. A large fleet of several thousand crewed helicopters provide much of this capability today, while future Army operations will increasingly employ uninhabited aerial systems. The smallest uninhabited VTOL aerial systems rely on fully electric propulsion and batteries, while large crewed rotorcraft are primarily powered by liquid-fueled turbine engines coupled to mechanical drive systems. In the intermediate Group III unmanned aerial systems (max. gross takeoff weight <1320 lbs), with equivalents up to crewed light helicopters and small personal aerial vehicles, the design range from “light” hybridization or electrical assistance through to full electric propulsion may enable new military-relevant vehicles for expeditionary maneuver and sustainment of theater operations. Recent rapid improvements in the specific power of aviation electric motors and electrical energy storage are enabling revolutionary VTOL aircraft configurations in this intermediate size class. However, conversion of energy stored in readily available and transportable liquid fuels remains an important capability for the Army for aerial maneuver where electrical infrastructure is inadequate for all-electric aircraft. This, coupled with a desire for long range and versatility, is likely to differentiate propulsion architectures from those envisioned for domestic logistics markets and urban aerial transportation. This topic seeks development of high power density drivetrain technologies needed for efficient and flexible distribution and transfer of propulsive energy in lightweight aerial vehicles capable of vertical takeoff and landing. These technologies will reduce mechanical interfaces and components through hybridization or electrification of VTOL conventional mechanical drive systems. While it is desirable that chosen technologies be relevant to more than a single vehicle architecture, proposers are encouraged to choose a general vehicle configuration such as lightweight helicopter, ducted fan personal vehicle, tiltwing/tiltrotor, etc. to allow engineering analysis and trade studies to quantify the impacts of the technology. The focus of the proposal should remain on the drivetrain technology offered; the overall vehicle configuration need not be treated in depth provided reasonable engineering assumptions are made. Aviation electric machines with high specific power (>5 kW/kg) and low inertia feature prominently in many current designs in this type of vehicle. Technologies specifically related to the propulsive devices (rotors, fans, propellers, etc.) are considered outside the scope and not sought within this topic. Powerplants such as heat engines and energy storage devices such as batteries are also not sought within this topic, but reasonable assumptions about technology improvements or advancements in those areas may be made by offerors if required to support the proposed power distribution technology. Any assumed technology improvements or advancements should be likely within 5-10 years with little additional investment given current technology trends and the state-of-the-art. Proposals should address any increases in weight relative to traditional technologies such as mechanical drive systems, and these weight penalties must be reasonable and offset by other improvements in performance, endurance, durability, flexibility or mission capability afforded by the new configuration. 

PHASE I: Establish feasibility of the proposed energy distribution/transfer technology. Develop a specific detailed design for this technology within the 50-250 kW class, and a concept of the propulsion distribution system in sufficient detail to support feasibility assessment. Clearly identify the vehicle configuration and flight profiles for which the propulsion system will be analyzed and provide a comparison to a conventional mechanical drivetrain in terms of propulsion system mass, propulsive efficiency, specific power, range and endurance. A minimum endurance of sixty minutes should be achievable by the proposed architecture/technology. Conduct simulation and/or breadboard evaluation of the technology for demonstration. Provide a detailed technical and commercialization plan demonstrating a credible path toward a commercial product. Identify technical risks in further development as well as requirements and assumptions about companion technologies needed to achieve system level performance. 

PHASE II: Further develop the technology concept including performing a detailed design and construction of an engineering prototype to validate performance through some form of physical test. This phase may also include screening tests required in advance of the prototype design, modeling and simulation efforts, or other supporting tasks required to demonstrate the proposed concept. Establish scale tolerance of the design, minimally across the 50-250 kW range described by this topic. Evaluate performance across the designed envelope of static conditions as well as transient and dynamic flight conditions. Refine estimates of propulsion system mass, propulsive efficiency, specific power, vehicle range and endurance. Specific power of individual electric machines should exceed a minimum of 5 kW/kg and overall propulsion system dry weight should be minimized. 

PHASE III: Transition the Phase II effort into commercial use. Proposals to this topic must articulate a feasible strategy to transition the successful Phase II effort into commercialization. This strategy should address whether technology will be patented and licensed, produced internally or through partnering, etc. Barriers to adoption in an aviation application should be identified and mitigations offered. Initial markets in the transition effort may be civil or military, and may be non-aviation, provided the offeror demonstrates a feasible path to a future Army-relevant aircraft. Technology developed herein has considerable potential to be integrated in a broad range of both military and civilian aircraft or personal vehicles where flexible adaptable and distributed propulsion may be employed. Military logistics represents a large and obvious future market to which this topic is targeted. An emerging market of civil personal and logistical air vehicles is also developing rapidly, enabled by advancements in vehicle autonomy. 



2:  Proceedings of the 3rd Joint Workshop on Enabling New Flight Concepts Through Novel Propulsion and Energy Architectures,

3:  Siemens, "Siemens develops world-record electric motor for aircraft," Press Release. March 24, 2015

4:  Fredericks, W. J., Moore, M. D., & Busan, R. C. (2013). Benefits of Hybrid-Electric Propulsion to Achieve 4x Cruise Efficiency for a VTOL UAV. In 2013 International Powered Lift Conference (p. 4324).

5:  Warwick, G. (2014). Electrifying Aviation: Light aircraft are early targets for the efficiency and safety benefits touted for electric propulsion. Aviation Week & Space Technology, 176(23).

6:  Brown, G. V., Kascak, A. F., Ebihara, B., Johnson, D., Choi, B., Siebert, M., & Buccieri, C. (2005). NASA Glenn Research Center program in high power density motors for aeropropulsion. NASA/TM-2005-213800

KEYWORDS: VTOL Aircraft, Drive System, Motors, Power Transmission, Propulsion 


Brian Dykas 

(410) 278-9545 

Mark Valco 

(216) 433-5742 

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