DARPA CRANE: Elevating Stealth with X-Planes and Active Flow Control

Rajesh Uppal August 11, 2024 Air Force, Manufacturing Comments Off on DARPA CRANE: Elevating Stealth with X-Planes and Active Flow Control 817 Views

Introduction:

In the ever-evolving landscape of aerospace technology, advancements in stealth capabilities remain a top priority for military agencies worldwide. DARPA, the Defense Advanced Research Projects Agency, has been at the forefront of fostering innovation in this domain. One of their latest endeavors, the Control of Revolutionary Aircraft with Novel Effectors (CRANE) program, aims to push the boundaries of stealth technology even further. In this article, we delve into the fascinating world of CRANE and its development of X-planes equipped with Active Flow Control (AFC) to enhance stealth capabilities.

Challenges of Traditional Control Mechanisms:

For decades, the aviation industry has relied on conventional control mechanisms like ailerons and rudders to maneuver aircraft. However, these traditional surfaces come with limitations, particularly in stealth aircraft design, where seams and gaps can compromise radar evasion. Traditional control surfaces, while effective for maneuvering aircraft, pose challenges in stealth design due to their inherent structural features. Moving control surfaces introduce seams, edges, and actuators that can increase radar cross-section, undermining the aircraft’s stealth characteristics. Moreover, the deflection of these surfaces during maneuvering generates detectable radar returns, further compromising stealth.

Understanding CRANE: The Rise of Active Flow Control

In response, DARPA’s Control of Revolutionary Aircraft with Novel Effectors (CRANE) program has set out to revolutionize aircraft control using Active Flow Control (AFC) technology. AFC involves manipulating the airflow over an aircraft’s surface using pneumatic or synthetic jet actuators to optimize aerodynamic performance and reduce radar cross-section.

At the heart of the X-65’s design lies active flow control (AFC), a cutting-edge technology that uses directed jets of pressurized air to steer the aircraft, eliminating the need for conventional flaps and rudders. This revolutionary approach not only enhances maneuverability but also offers potential applications in reducing aircraft weight, improving aerodynamics, and streamlining aircraft design.

By smoothing airflow and reducing surface irregularities, AFC enhances stealth capabilities while maintaining maneuverability and agility.

X-Planes and Stealth Enhancement:

In August 2019, DARPA launched the CRANE program, challenging innovators to design and build an aircraft capable of maneuvering without movable surfaces. The goal was to produce a functional, full-scale aircraft by 2024, incorporating AFC as a primary design consideration. Aurora Flight Sciences, Lockheed Martin, and Georgia Tech Research Corporation were among the industry leaders selected to participate in the program.

Central to the CRANE program is the development of experimental aircraft, or X-planes, equipped with cutting-edge AFC systems. These X-planes serve as testbeds for evaluating the effectiveness of AFC in enhancing stealth characteristics. By actively controlling airflow around the aircraft’s surfaces, AFC can mitigate turbulence and reduce the presence of sharp edges that typically contribute to radar reflections, thus improving overall stealth.

Key Benefits of AFC in Stealth Enhancement:

Reduced Radar Signature: AFC allows for precise control of airflow, enabling aircraft designers to minimize radar cross-section by smoothing out surface irregularities and disrupting the formation of detectable radar reflections.
Enhanced Maneuverability: AFC systems can also improve aircraft maneuverability by providing finer control over airflow during high-speed maneuvers, resulting in increased agility while maintaining stealth.
Flexibility in Design: Unlike traditional stealth designs that rely heavily on fixed geometries, AFC offers greater flexibility in aircraft design, allowing for more innovative and adaptable configurations without compromising stealth performance.

Progress and Challenges:

While the CRANE program shows promising advancements in stealth technology, several challenges remain. Developing AFC systems that are lightweight, reliable, and compatible with existing aircraft structures is a significant engineering feat. Additionally, integrating AFC into operational aircraft platforms requires thorough testing and validation to ensure seamless integration and performance under real-world conditions.

Advancements and Achievements:

Since its inception, the CRANE program has made significant strides in advancing AFC technology for aircraft design. Aurora Flight Sciences, in collaboration with DARPA, has conducted wind tunnel testing and developed design concepts for AFC-equipped X-planes. These X-planes serve as testbeds for evaluating the effectiveness of AFC in enhancing aircraft performance and stealth capabilities.

Phases of Development:

Effectors and actuators serve as the linchpin technologies driving active flow control, offering lightweight, energy-efficient solutions devoid of moving parts. In essence, CRANE seeks to eliminate large external moving surfaces like ailerons and flaps, aiming to demonstrate the safety and efficacy of active flow-control actuator technologies in maintaining flight while enhancing aircraft capabilities.

The CRANE program is structured into four developmental phases, each building upon the previous phase’s achievements. Phase 0 focused on mission applications and AFC technology maturation, while subsequent phases involved design, testing, and flight demonstration of AFC-enabled X-planes. Through rigorous testing and validation, the program aims to prove the viability of AFC for future military and commercial aircraft applications.

The program unfolds in a structured four-phase developmental cycle, where each phase builds upon the preceding one. Phase 0 serves as the foundational stage, exploring mission applications conducive to AFC technologies and refining design processes through rigorous studies and documentation. Subsequent phases delve deeper into design development, validation, and testing, culminating in the fabrication, assembly, ground testing, and flight demonstration of an X-plane demonstrator equipped with AFC technologies. Through this meticulous approach, CRANE aims to realize the full potential of active flow control, paving the way for transformative advancements in aerospace engineering.

Awards

The US Defense Advanced Research Projects Agency (DARPA) has embarked on an ambitious initiative, awarding contracts to three industry-leading teams to develop experimental aircraft (X-plane) leveraging active flow control—a realm relatively unexplored compared to conventional flight controls. Aurora Flight Sciences, Lockheed Martin, and Georgia Tech Research Corporation have been tasked with spearheading the Control of Revolutionary Aircraft with Novel Effectors (CRANE) program, as outlined in an agency statement. The overarching objective is to showcase substantial efficiency gains facilitated by active flow control, alongside enhancements in aircraft cost, weight, performance, and reliability.

Alexander Walan, CRANE program manager, underscored the groundbreaking nature of the endeavor, emphasizing the novel approach of integrating active flow control early in the design process—a departure from prior practices. While active flow control has been previously explored at a component level, its integration into the fundamental architecture of aircraft design represents a paradigm shift. By recalibrating the design approach, CRANE aims to optimize the prospects for successful X-plane development while seamlessly embedding active flow control into the aircraft’s stability and control framework. The CRANE program unfolds in four meticulously planned phases, each building upon the insights gained from its predecessor.

During the initial phase, the participating company devised two X-plane concept designs, equipped with tools and technologies tailored to integrate active flow control into the aircraft’s blueprint. Subsequent phases focused on refining the aircraft’s preliminary design and subjecting it to rigorous wind tunnel testing, generating crucial data pivotal for formulating flight control laws where active flow control serves as the primary control effector. Through these concerted efforts, the CRANE program aims to propel aerospace innovation to new heights, paving the way for transformative advancements in aircraft design and performance.

Aurora Flight Sciences,

Aurora Flight Sciences, a Boeing company, is advancing through Phase 1B of DARPA’s Liberty Lifter program, which aims to design, build, float, and fly an affordable X-plane demonstrating heavy airlift capability from the sea. The vehicle will operate efficiently in ground effect at high sea states and showcase affordability through low-cost manufacturing and innovative design.

DARPA envisions Aurora showcasing these advancements on an X-plane, encouraging the company to explore innovative design approaches and utilize off-the-shelf components wherever possible. Among the technologies being explored is circulation control leveraging the Coanda effect, which has demonstrated promising capabilities in generating controllable forces for control and lift augmentation. Aurora Flight Sciences’ extensive experience in electric Vertical Takeoff and Landing (eVTOL) aircraft development positions them well to spearhead this initiative, with their ongoing flight test campaigns further validating the potential of active flow control technology. The CRANE program, now progressing into Phase 1, has already seen significant advancements, with researchers completing conceptual designs of flight demonstration configurations showcasing quantifiable performance benefits enabled by active flow control. As the program advances, the focus remains on refining design tools and technologies to ensure seamless integration into future aircraft designs, thereby revolutionizing the aerospace industry landscape.

In May 2022, Aurora Flight Sciences reached a significant milestone in developing DARPA’s Active Flow Control (AFC) demonstrator X-Plane by completing wind tunnel testing of its design.

The tests, conducted over four weeks at a wind tunnel facility in San Diego, California, utilized a 25% scale model equipped with 11 movable conventional control surfaces and 14 AFC banks, each comprising eight fully independent controllable AFC air supply channels. This rigorous testing regimen yielded over 14,000 data points, including 8,860 AFC control power points, establishing a robust aerodynamic database essential for informing future program phases. Per Beith, president and CEO of Aurora Flight Sciences, emphasized the company’s commitment to innovation, stating that their work on the DARPA CRANE program represents a continuation of their track record in pioneering groundbreaking technologies from conceptualization to flight testing, thus advancing AFC technology for application in next-generation aircraft.

In January 2024, DARPA selected Aurora Flight Sciences to construct a full-scale X-plane for Phase 3 of the CRANE program, which will demonstrate the effectiveness of AFC actuators as the primary means of flight control. This phase involves creating an X-plane without conventional moving control surfaces

The primary goal of Phase 1B is preliminary design, focusing heavily on risk reduction through testing. ReconCraft, an Oregon-based shipyard, is building full-scale structural test articles, including part of the fuselage. This testing ensures quality and reduces risk as the team navigates novel materials and unique requirements. Additionally, a scale model of the hull was tested at Virginia Tech’s tow tank to study craft slamming during landing. Upcoming tests will involve sensors and software for wave detection and prediction, crucial for the X-plane’s advanced control system.

Aurora’s design has evolved to include a pi-tail for structural efficiency and an aft cargo door, along with relocating floats from side sponsons to the wing tips, balancing affordability and performance in ground effect.

The team includes naval architecture firm Gibbs & Cox, a Leidos company, and advisors from Boeing, leveraging Boeing’s extensive seaplane and flying boat R&D. Dan Campbell, Aurora program manager, highlighted the innovative intersection of maritime and aerospace expertise in the project.

Phase 1B will conclude with a preliminary design review in January 2025. If successful, flight testing is projected for 2028

BAE Systems, a leading defense contractor, has been tapped by DARPA to embark on a Phase 0 conceptual design endeavor within the CRANE program.

This initial phase is dedicated to exploring the trade space of active flow control (AFC) and undertaking risk reduction activities to lay the groundwork for subsequent efforts. Through the Phase 0 award, BAE Systems will assess the advantages of integrating AFC into various air vehicle concepts, culminating in a comprehensive conceptual design review. The recent award, unveiled in September 2021, encompasses the development of a full-scale demonstrator concept centered around AFC technology. Highlighting the significance of this advancement, BAE Systems emphasized that the aircraft’s ability to maneuver in flight without conventional control surfaces will enhance its performance, maintainability, and survivability. While the exact value of the award remains undisclosed, BAE Systems has committed to executing design, integration, and de-risking activities, including wind tunnel testing, at its facilities in northwest England throughout 2022. Recognizing the evolving complexities of modern military operations, BAE Systems underscored the potential military benefits of active flow control in addressing increasingly contested and sophisticated threat environments. By supplementing or replacing traditional movable control surfaces, AFC technologies have the capacity to enhance aircraft performance across different flight regimes while simultaneously reducing mass and volume, thereby enabling greater payloads and operational flexibility for military operators.

DARPA has greenlit the next phases of development for Aurora Flight Sciences’ active flow control (AFC)-based aircraft as part of the Control of Revolutionary Aircraft with Novel Effectors (CRANE) program.

In this upcoming phase, Aurora will undertake detailed engineering design work for a full-scale iteration of its experimental X-plane, leveraging the insights gained from extensive wind tunnel testing. The AFC technology, which relies on actuators or effectors to optimize airflow over the aircraft, represents a paradigm shift in aircraft design by eliminating conventional control surfaces such as ailerons, rudders, and flaps. By integrating pressurized air supply to AFC effectors embedded within the aircraft’s body, the 30-feet wide, 7,000-pound unmanned aircraft is poised to achieve enhanced flight control and performance at tactical speeds, setting the stage for groundbreaking advancements in both commercial and military aviation.

Aurora’s collaboration with Boeing, its parent company, underscores the strategic alliance between industry leaders in aerospace research and development. With expertise spanning aerodynamics, conceptual design engineering, and test and evaluation, the joint effort aims to capitalize on Boeing’s targeted investments in active flow control technology to drive efficiency and performance gains in future aircraft designs. Looking ahead, Phase 3 of the CRANE program will witness Boeing taking the reins to build and conduct flight demonstrations of the AFC-equipped X-plane, with validation and demonstration expected at flight speeds up to Mach 0.7 by 2025. This pivotal phase represents a critical milestone in validating the efficacy of AFC technologies at a mission-relevant scale and Mach numbers, signaling a transformative leap forward in the evolution of aerospace engineering.

Transitioning to Production:

Boeing subsidiary Aurora Flight Sciences, the contractor behind the X-65, has received the green light to transition the aircraft into the manufacturing phase. Under the guidance of the Defense Department, Aurora aims to validate AFC technology through rigorous testing and flight demonstrations, with the ultimate goal of advancing aircraft design and performance.

Kevin Uleck, program director for Aurora CRANE, expressed enthusiasm for the X-65’s potential to reshape the future of aviation. With the manufacturing phase underway, the team is one step closer to realizing the vision of AFC-enabled aircraft and unlocking new possibilities in aircraft design.

Flight Testing and Validation:

The production phase of the X-65 will culminate in a comprehensive flight test, where the aircraft’s performance will be evaluated at air speeds up to Mach 0.7. Equipped with a set of 14 AFC banks, each with independently-controllable air supply channels, the X-65 represents a versatile platform for testing and validation.

To assess the efficacy of AFC effectors compared to traditional control surfaces, the X-65 will be equipped with both technologies. Traditional flight controls will establish a baseline performance, while AFCs will undergo extensive testing to evaluate their effectiveness in enhancing maneuverability and efficiency.

The CRANE program has fostered collaborations between industry leaders and government agencies to drive innovation in aerospace technology. BAE Systems, Boeing, and other industry partners have joined forces to explore the potential of AFC in enhancing aircraft performance and survivability. With flight demonstrations expected in 2025, the CRANE program promises to deliver groundbreaking advancements in aircraft design and stealth technology.

Real-World Applications:

DARPA emphasizes the immediate applicability of data obtained from the X-65 flight test to real-world scenarios. With its size, weight, and speed comparable to military trainer aircraft, the X-65 serves as a valuable testbed for exploring the practical implications of AFC technology in military and civilian contexts.

Future Implications:

The successful implementation of AFC technology in X-planes under the CRANE program could have far-reaching implications for future military aircraft design and operations. By enhancing stealth capabilities while maintaining maneuverability and flexibility, AFC-equipped aircraft may usher in a new era of stealth aviation, where next-generation fighters and unmanned aerial vehicles (UAVs) possess unprecedented levels of survivability and mission effectiveness.

Conclusion:

As DARPA’s CRANE program continues to push the boundaries of aerospace innovation, the development of X-planes with Active Flow Control represents a significant milestone in the pursuit of enhanced stealth capabilities. By harnessing the power of AFC to manipulate airflow around aircraft surfaces, researchers aim to redefine the limits of stealth technology, paving the way for the next generation of advanced military aircraft with unparalleled performance and survivability. With continued investment and collaboration, the CRANE program holds the potential to revolutionize aircraft capabilities and ensure a safer, more secure future for military and commercial aviation.