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X-65

The Control of Revolutionary Aircraft with Novel Effectors (CRANE) aircraft received its official designation as X-65 in 2023. The 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.

Aircraft employ movable control surfaces to affect the aerodynamic lift of the aircraft. Control surfaces may include, for example, flaps, slats, ailerons, etc. When a control surface such as a flap is deployed, the airflow over the top of the wing separates from the airflow along the bottom of the wing and reattaches downstream of the wing. However, the airflow over the top of the wing does not follow the entire upper surface of the wing and control surface. Instead, the airflow detaches or separates from the upper surface of the wing and control surface and a separation pocket or deadzone is created behind the control surface. This separation pocket produces drag and decreases the lift generated by the wing.

Airfoil circulation control typically uses fluid injection in the form of a secondary fluid flow to create a steady wall-jet at the proximity of a rounded surface in a blade to leverage the Coandã effect. The Coandã effect can be defined as the effect by which a fluid jet attaches itself to an adjacent surface, such as an airfoil, and remains attached. Circulation control may result in increased lift and systems using this principle have been conceptualized for a wide variety of applications from aircraft wings to wind turbines.

In aircraft wings applications, the circulation control may work by increasing the velocity of the airflow over the leading edge and trailing edge of a specially designed aircraft wing using a series of blowing slots that eject high pressure jet air tangentially as the secondary fluid flow, in a substantially downstream direction as relates to the incoming primary fluid flow. The wing has a rounded trailing edge to tangentially eject the air through the Coandã effect, thus causing lift. The increase in velocity of the airflow over the wing may also add to the lift force through conventional airfoil lift production.

In other systems, the injection of the secondary fluid flow creates or enhances separation over the aerodynamic surface for lift destruction by creating a flow disturbance on or near the aerodynamic surface. As described, a method that can accomplish both lift destruction and lift enhancement in a single active system does not presently exist.

Since their conception, airfoils have suffered the risk of stall, or loss of lift, due to flow separation over the surface. In particular, it is known that airfoils at high angles of attack are at risk of the incoming primary flow separating from the surface of the airfoil, causing loss of lift.

An example active flow control system includes a plurality of nozzles arranged in an array across a surface of an aircraft. The nozzles are oriented to eject air across the surface to reduce airflow separation. The active flow control system includes an air source coupled to the nozzles and a controller to activate the nozzles to eject air from the air source in sequence from outboard to inboard and then from inboard to outboard to create a wave of air moving from outboard to inboard and then from inboard to outboard across the surface.

DARPA selected Aurora Flight Sciences, a Boeing Company, 17 January 2023 to move into the detailed design phase of the Control of Revolutionary Aircraft with Novel Effectors (CRANE) program. Aurora designs, builds, and flies advanced aircraft and enabling technologies. Aurora’s agility and innovation, combined with Boeing’s scale and strength, creates an unprecedented opportunity to help customers advance the future of flight. This award followed successful completion of the project’s Phase 1 preliminary design, which resulted in an innovative testbed aircraft that used active flow control (AFC) to generate control forces in a wind tunnel test. Phase 2 will focus on detailed design and development of flight software and controls, culminating in a critical design review of an X-plane demonstrator that can fly without traditional moving flight controls on the exterior of the wings and tail.

The contract includes a Phase 3 option in which DARPA intends to fly a 7,000-pound X-plane that addresses the two primary technical hurdles of incorporation of AFC into a full-scale aircraft and reliance on it for controlled flight. Unique features of the demonstrator aircraft will include modular wing configurations that enable future integration of advanced technologies for flight testing either by DARPA or potential transition partners.

“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. We are confident about completing the design and flight test of a demonstration aircraft with AFC as the primary design consideration,” said the CRANE Program Manager Richard Wlezien. “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.”

The AFC suite of technologies enables multiple opportunities for aircraft performance improvements, such as elimination of moving control surfaces, drag reduction and high angle of attack flight, thicker wings for structural efficiency and increased fuel capacity, and simplified high-lift systems.

“Thanks to a variety of innovative participants, the CRANE program has significantly advanced the state of the art of multiple active flow control technologies,” said Wlezien. “We are uniquely positioned to build on those achievements by evaluating a wide range of relevant technologies during our planned X-plane flight tests.”

Because of the complexity of modern aircraft, and the number of control surfaces, they are becoming increasingly difficult to fly manually. Accordingly, complex computerized "stability augmentation" and automatic pilot systems have been designed to automatically adjust control surfaces and reduce pilot work load. The space available for the multitude of control surfaces required for such redundancy is inherently limited, and only one region of the aircraft is available in some cases to achieve the desired degree of control (e.g., stabilizer for pitch, and fin for yaw control).

Virtually all modern jet aircraft in use today utilize a single wing extending laterally in both directions from a central portion of the fuselage. Such aircraft are designed and loaded so that the overall aircraft center of gravity will be located just forward of the aerodynamic center of lift of the wing; as required by stability considerations. In such conventional single wing aircraft, the fuselage structure is effectively two cantilever beams; one extending forward and the other aft of the wing. Maximum fuselage bending moments therefore occur near the wing in the central region of the fuselage.

The joined wing is a new type of aircraft configuration which employs tandem wings having positive and negative sweep arranged to form diamond shapes in plan view and front view. Wind-tunnel tests and finite-element structural analyses have shown that the joined wing provides the following advantages over a comparable wing-plus-tail system; lighter weight and higher stiffness, higher span-efficiency factor, higher trimmed maximum lift coefficient, lower wave drag, plus built-in direct lift and direct sideforce control capability.

A box wing may be defined as a wing that effectively has two main planes which merge at their ends so that there are no conventional wingtips. The box wings are the main lifting surface for such systems. Box wings have a potential of generating lift with considerably less induced drag and delayed stall angles than monoplane wings. In a boxplane wing structure two pairs of wings are provided, a first pair being rearwardly swept and the second pair being forwardly swept. These wing pairs are joined at their tips to provide structural rigidity and dynamic control surfaces. The wing pairs are staggered from one another both longitudinally and vertically for optimum aerodynamic efficiency and minimal aerodynamic interference with one another. The boxplane concept reduces the induced drag for a given span and lift by modulating the trailing vorticity distribution.

Max Munk and Ludwig Prandtl originally developed the theory in the late 19l0s that the closed rectangular box plane was the optimum wing system for minimizing induced drag in an airplane configuration of given span and height dimensions and operating at a given overall lift. Their theory with respect to box wing type construction was stated, for example, in a National Advisory Committee for Aeronautics publication entitled, The Minimum Induced Drag of Airfoils, Report No. 121, published by the Government Printing Office in 1921. Their studies, however, related only to straight line shaped airfoils, not to stagger or sweep with respect thereto. Their theories, although long available, to this date have not been successfully incorporated into a practicable aircraft configuration.