For the past century, airplane control mechanisms have relied on hinged surfaces such as ailerons and rudders. Shifting their positions alters the shape of the wings or tail, changing the surrounding airflow—and thus air pressure. This adjustment pushes the aircraft to maneuver in predictable ways. But traditional control surfaces require external seams that radar can detect with relative ease. A seamless airplane would have greater stealth capabilities and performance. It could also have lower weight, size, complexity and cost, compared with planes that use traditional steering methods.
These traditional control mechanisms create challenges for designing extremely low-observable aircraft. Moving control surfaces cannot be incorporated in the design of such aircraft without the introduction of gaps, edges, actuators, and support structures, all of which can introduce discontinuities, edges, and gaps, increasing the signature, even when the air vehicle is flying straight and level and control effectors are not being used. The deflection of control surfaces required to maneuver stealthy air vehicles can be quite large and can generate undesirable and detectable radar returns.
Aerospace companies have been researching stealthy control mechanisms for Aircrafts and drones. The international aerospace company Airbus unveiled a model of it’s new drone called the Low Observable UAV Testbed (LOUT), which reportedly combines several undisclosed stealth technologies. Hints in the aircraft’s description led some aviation experts to speculate that one of LOUT’s radar-evading powers could come from a lack of conventional moving control surfaces.
In August 2019, the U.S. government’s Defense Research Projects Agency’s (DARPA’s) established Control of Revolutionary Aircraft with Novel Effectors (CRANE) program asking innovators to design and build an airplane that can be maneuvered without movable surfaces—and to produce this functional, full-scale craft by 2024.
Aurora Flight Sciences has completed wind tunnel testing its design for DARPA’s Active Flow Control demonstrator X-Plane in the USA, reported in May 2022.
Aurora Flight Sciences received a design contract(opens in new tab) from DARPA Tuesday (Jan. 2023) under the Control of Revolutionary Aircraft with Novel Effectors (CRANE) program, which seeks fly an experimental aircraft without moving joints.
“Over the past several decades, the active flow control community has made significant advancements that enable the integration of active flow control technologies into advanced aircraft,” CRANE program manager Richard Wlezien said in a statement. “We are confident about completing the design and flight test of a demonstration aircraft with AFC as the primary design consideration. With a modular wing section and modular AFC effectors, the CRANE X-plane has the potential to live on as a national test asset long after the CRANE program has concluded.”
DARPA CRANE Program
The Control of Revolutionary Aircraft with Novel Effectors (CRANE) program aims to design, build, and flight test a novel X-plane that incorporates Active Flow Control (AFC) as a primary design consideration. Crane seeks to optimize the benefits of active flow control by maturing technologies and design tools, and incorporating them early in the design process. Active flow control could improve aircraft performance by removing jointed surfaces, which currently drive design configurations that increase weight and mechanical complexity. Demonstrating AFC for stability and control in-flight would help open the design trade space for future military and commercial applications.
Effectors and actuators are usually the enabling technologies of active flow control. Effectors and actuators are typically light weight, have no moving parts, and are energy-efficient. The CRANE project excludes large external moving surfaces like ailerons, rudders, flaps, elevators, and trim surfaces; mechanical vectoring of engine jet exhaust, or other traditional moving aerodynamic control devices. CRANE’s goal is to demonstrate in flight that active flow-control actuator technologies can maintain flight safely, and provide quantifiable aircraft capabilities.
The program is a four-phase developmental cycle with each phase building on the previous phase.The first phase (Phase 0) will examine mission applications in which AFC technologies would be useful. Phase 0 focuses on the design process and maturing understanding through configuration agnostic-designs, geometric and technology trade studies, and process documentation.
The programme will then go on to develop, in Phase 1, a design for an X-plane demonstrator exploiting AFC, and presumably able to demonstrate the benefits of such a technology in relevant mission scenarios, informed by the Phase 0 studies. Phase 1 will validate analytical predictions, control loop analyses, and modeling verification. Phase 2 would test integrated subsystem component, and phase 3 would include fabrication, assembly, ground test, and flight demonstration.
The US Defense Advanced Research Projects Agency (DARPA) has issued contracts to three industry teams to develop experimental aircraft (X-plane) based on active flow control, an area relatively little explored compared with traditional flight controls. Aurora Flight Sciences, Lockheed Martin, and Georgia Tech Research Corporation were selected to work on the Control of Revolutionary Aircraft with Novel Effectors (CRANE) programme, according to an agency statement. The goal is to demonstrate significant efficiency benefits of active flow control, as well as improvements in aircraft cost, weight, performance, and reliability.
Alexander Walan, CRANE programme manager, was quoted by DARPA on 20 July as saying that the contract recipients are looking at using active flow control early in the design scope, which he said has not been done before. Active flow control, he said, has been explored at a component level but not as an integral piece of aircraft design. By altering the design approach, CRANE seeks to maximise the chance of a successful X-plane development while also integrating active flow control into the aircraft’s stability and control. CRANE programme is organised into four phases.
The company developed two X-plane concept designs, including tools and technologies to incorporate AFC technologies in the aircraft design, in phase 0. The subsequent phase saw the aircraft’s preliminary design and wind tunnel testing, providing data for “developing flight control laws using AFC as a primary control effector.”
Aurora Flight Sciences to develop active flow control to change aerodynamic qualities of electric aircraft
U.S. military researchers needed a company to push the bounds of future electric aircraft by eliminating control surfaces like ailerons, rudders, and flaps. They found their solution from Aurora Flight Sciences Corp. in Manassas, Va. Officials of the U.S. Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., announced a $7.1 million contract to Aurora in June 2020 for the Control of Revolutionary Aircraft with Novel Effectors (CRANE) project.
Instead of using ailerons, rudders, and flaps for control surfaces on future electric aircraft, the CRANE project seeks to use actuators or effectors to add energy or momentum to the flow of air over the aircraft. Aurora engineers will seek to inject a disruptive technology early in aircraft design with new flow-control technologies and design tools. The idea is to configure and optimize an aircraft with active flow control to enhance efficiency and effectiveness of new commercial and military aircraft.
Effectors and actuators typically are the enabling technologies of active flow control, yet have been the weakest link in developing active flow-control technology. Despite their relatively high costs, effectors and actuators typically are light weight, have no moving parts, and are energy-efficient.
DARPA officials say they would like Aurora experts to demonstrate new active flow control on an X-plane, yet are encouraging the company to focus on the best ways to develop and flight demonstrate their flow-control technologies on a clean-sheet design or modification of an existing aircraft. DARPA expects Aurora to make substantial use of off-the-shelf components, especially for items unrelated to the active flow control subsystems.
The exemplar used in the Aurora press release to illustrate the concept is a wind tunnel model of a joined wing concept, with blended body and a diamond planform. The model has no vertical fin, and no conventional control surfaces are visible. According to experts, Attractions of joined wing concepts include an inherently strong structure with both forward and aft swept lifting surfaces which, being joined at the tips, results in low structure weight for a given stiffness. Other advantages include a wide range of centre of gravity; predictable behaviour at the stall; and low rolling inertia.
A range of active flow control technologies might be used, including circulation control exploiting the Coanda effect, or by the use of air jets or suction to manipulate the boundary layer. The Coanda effect has been described by its inventor, Henri Coanda, as “the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surroundings so that a region of lower pressure develops”.
This technique has been investigated (by Dr R V (Ron) Smith, among others), and shown to be capable of generating large, and controllable, forces which might be used for control and lift augmentation purposes. The diagram below shows the flow around an aerofoil with an air blowing system providing very high lift. The principle has been successfully applied in the development of the MD900 helicopter, where the tail boom is used as a Coanda device, replacing the tail rotor.
Aurora Flight Sciences has been working on design and development of an eVTOL prototype for longer than most newer eVTOL outfits, and was acquired in 2017 by Boeing. Since the acquisition, the company debuted the first flight of the AFS/Boeing Personal Air Vehicle (PAV) and has continued with their flight test campaign. While effectors will likely serve a larger benefit for military R&D projects, they could be applied in simplistic forms to aerial mobility designs sooner than expected. The contract is set to complete in the summer of 2021.
CRANE Completes Phase 1
In August 2021, DARPA selected Aurora Flight Sciences and Lockheed Martin to enter Phase 1, which includes system requirements development, initial design work, software development, and initial airworthiness activities that culminate in a preliminary design review.
“The Phase 1 researchers have completed conceptual designs of novel flight demonstration configurations with quantifiable performance benefits enabled by active flow control (AFC),” said Alexander Walan, programme manager for CRANE in DARPA’s Tactical Technology Office. “Multiple AFC technologies will continue to be matured through advanced analytical and testing activities for incorporation in relevant demonstrator designs.”
One of the primary objectives of the previous phase, named Phase 0, was the development and maturation of AFC design software and databases for inclusion in future aircraft development activities.
Georgia Tech Research Corporation’s Phase 0 effort has been extended to allow further refinement of these tools for transition to relevant military and government partners. “In addition to its role in upcoming flight test activities, AFC design software is a critical piece for the inclusion of AFC technologies in future defense and commercial aircraft designs,” said Walan. “The CRANE programme is in a unique position to provide a comprehensive AFC database and the associated tools to future aircraft designers. The continuation of Georgia Tech Research Corporation’s work in this area will ensure this valuable capability is successfully transitioned to the aircraft design community.”
DARPA has also selected another performer, BAE Systems, to initiate a Phase 0 conceptual design activity. Phase 0 is focused on AFC trade space exploration and risk reduction activities to inform this work. Under the recent Phase 0 award, BAE Systems will evaluate the benefits of using AFC integrated into different air vehicle concepts leading to a conceptual design review. “All of the CRANE performers are exploring unique configurations and performance objectives; this additional performer adds to the diverse concepts and technologies being matured by the CRANE program,” said Walan.
The award, announced in September 2021 , covers the design of a full-scale demonstrator concept with active flow control (AFC) at its core. “The aircraft’s ability to manoeuvre in flight without conventional flight control surfaces will enable improved performance, maintainability, and survivability,” BAE Systems said. While no value for the award was disclosed, BAE Systems said that it will perform the design, integration, and de-risking activities, including wind tunnel testing, at its facilities in the northwest England in 2022.
“As military aircraft confront increasingly contested and sophisticated threat environments, active flow control offers potential military benefits that could deliver operational advantage in the battlespace. Active flow control technologies can supplement or replace conventional moveable control surfaces to improve the performance of an aircraft at various points in the flight regime, as well as reduce mass and volume compared to aircraft with conventional controls to enable greater payloads and greater flexibility to the operator,” BAE Systems noted.
25% scale model of the Active Flow Control X-Plane being developed as part of DARPA’s Control of Revolutionary Aircraft with Novel Effectors program reported in May 2022
Aurora Flight Sciences has completed wind tunnel testing its design for DARPA’s Active Flow Control demonstrator X-Plane in the USA, reported in May 2022.
Aurora engineers conducted the tests using a 25% scale model over four weeks at a wind tunnel facility in San Diego, California. In addition to 11 movable conventional control surfaces, the model featured 14 AFC banks with eight fully independent controllable AFC air supply channels.
Over 14,000 data points were collected, including 8,860 AFC control power points, forming the foundation for a flight-quality aerodynamic database to use in future program phases.
Per Beith, president and CEO of Aurora Flight Sciences said, “Aurora’s work on CRANE continues our history of proving ground-breaking technologies from concept to flight test. Through the DARPA CRANE program, Aurora is advancing AFC technology for application to next generation aircraft.”
DARPA Funds Next Phase of Active Flow Control X-Plane
The US Defense Advanced Research Projects Agency (DARPA) has announced it is funding Aurora Flight Sciences’ active flow control (AFC)-based aircraft development phases 2 and 3.
Aurora will perform “detailed engineering design work” for a full-scale version of its experimental aircraft (X-plane) under the Control of Revolutionary Aircraft with Novel Effectors (CRANE) program phase 2.
The AFC technology uses actuators or effectors to enhance airflow over the aircraft, eliminating control surfaces like ailerons, rudders, and flaps. The wind tunnel test data has been incorporated into vehicle models to characterize AFC performance across the operational envelope and continue progressing design development and is being used as a foundation for developing flight control laws using AFC as a primary control effector, Aurora said.
The 30-feet (9 meters) wide, 7,000-pound (3,175 kilograms) uncrewed aircraft will have pressurized air supplied to AFC effectors embedded in the aircraft’s body for “flight control at tactical speeds and performance enhancement,” Aurora explained. The aircraft’s exchangeable outboard wings and AFC effectors will enable it to perform as a modular testbed for Aurora and other designers’ AFC effectors.
Aurora is a subsidiary of Boeing which conducts aerospace R&D. The test team consisted of Aurora and Boeing engineers with expertise in aerodynamics, conceptual design engineering, and test and evaluation. Laurette Lahey, senior director of Boeing Research and Technology, Flight and Vehicle Technology said, “Leveraging Boeing’s targeted investments in active flow control, our advancements on the CRANE program aim to further validate the technology’s potential benefits to improve efficiency and performance for both commercial and military aircraft.”
Flight Demonstration Expected in 2025
Phase 3 will see Boeing build and fly the aircraft. The aircraft’s AFC validation and demonstration at flight speed up to Mach 0.7 are expected in 2025. “Given all that we have learned about AFC and its application to tactical aircraft in prior phases of CRANE, the next step is to prove out these learnings in flight,” said the vice president of government programs at Aurora Graham Drozeski. “The CRANE X-plane is designed specifically to explore the effectiveness of AFC technologies at mission-relevant scale and Mach numbers.”