Flow control is a field of fluid dynamics. It involves a small configuration change to serve an ideally large engineering benefit, like drag reduction, lift increase, mixing enhancement or noise reduction. This change may be accomplished by passive or active devices.
Passive devices by definition require no energy. Passive techniques include turbulators or roughness elements geometric shaping, the use of vortex generators, and the placement of longitudinal grooves or riblets on airfoil surfaces.
Active control requires actuators that require energy and may operate in a time-dependent manner. [1] Active flow control includes steady or unsteady suction or blowing, [2] the use of synthetic jets, valves and plasma actuators. Actuation may be pre-determined (open-loop control) or be dependent on monitoring sensors (closed-loop control).
Airplane wing performance has a substantial effect on not only runway length, approach speed, climb rate, cargo capacity, and operation range but also noise and emissions. Wing performance can be degraded by flow separation, which depends on the aerodynamic characteristics of the airfoil. Aerodynamic and non-aerodynamic constraints often conflict. Flow control is required to overcome such difficulties. Techniques developed to manipulate the boundary layer, either to increase lift or decrease drag, and separation delay come under the general heading of flow control.
Aurora Flight Sciences is a DARPA CRANE (Control of Revolutionary Aircraft with Novel Effectors) grantee. It initially involved testing a small-scale plane that uses compressed air bursts instead of external moving parts such as flaps. The program seeks to eliminate the weight, drag, and mechanical complexity involved in moving control surfaces. The air bursts modify the air pressure and flow, and change the boundaries between streams of air moving at different speeds. The company built a 25% scale prototype with 11 conventional control surfaces, as well as 14 banks fed by eight air channels. [3] In 2023, the aircraft received its official designation as X-65. [4] In January 2024, DARPA and Aurora started CRANE Phase 3, building the first full-scale X-65 aircraft using active flow control actuators for primary flight control. [5] [6] The 7,000-pound X-65 will be rolled out in early 2025 with the first flight planned for summer of 2025. [7]
In fluid dynamics, angle of attack is the angle between a reference line on a body and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack is the angle between the body's reference line and the oncoming flow. This article focuses on the most common application, the angle of attack of a wing or airfoil moving through air.
Transonic flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transonic flow is seen at flight speeds close to the speed of sound, typically between Mach 0.8 and 1.2.
An airfoil or aerofoil is a streamlined body that is capable of generating significantly more lift than drag. Wings, sails and propeller blades are examples of airfoils. Foils of similar function designed with water as the working fluid are called hydrofoils.
Parasitic drag, also known as profile drag, is a type of aerodynamic drag that acts on any object when the object is moving through a fluid. Parasitic drag is a combination of form drag and skin friction drag. It affects all objects regardless of whether they are capable of generating lift.
The Grumman X-29 was an American experimental aircraft that tested a forward-swept wing, canard control surfaces, and other novel aircraft technologies. The X-29 was developed by Grumman, and the two built were flown by NASA and the United States Air Force. The aerodynamic instability of the X-29's airframe required the use of computerized fly-by-wire control. Composite materials were used to control the aeroelastic divergent twisting experienced by forward-swept wings, and to reduce weight. The aircraft first flew in 1984, and two X-29s were flight tested through 1991.
A conventional fixed-wing aircraft flight control system (AFCS) consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft engine controls are also considered flight controls as they change speed.
A flap is a high-lift device used to reduce the stalling speed of an aircraft wing at a given weight. Flaps are usually mounted on the wing trailing edges of a fixed-wing aircraft. Flaps are used to reduce the take-off distance and the landing distance. Flaps also cause an increase in drag so they are retracted when not needed.
A leading-edge cuff is a fixed aerodynamic wing device employed on fixed-wing aircraft to improve the stall and spin characteristics. Cuffs may be either factory-designed or an after-market add-on modification.
A foil is a solid object with a shape such that when placed in a moving fluid at a suitable angle of attack the lift is substantially larger than the drag. If the fluid is a gas, the foil is called an airfoil or aerofoil, and if the fluid is water the foil is called a hydrofoil.
The NACA cowling is a type of aerodynamic fairing used to streamline radial engines installed on airplanes. It was developed by Fred Weick of the National Advisory Committee for Aeronautics (NACA) in 1927. It was a major advance in aerodynamic drag reduction, and paid for its development and installation costs many times over due to the gains in fuel efficiency that it enabled.
The Gurney flap is a small tab projecting from the trailing edge of a wing. Typically it is set at a right angle to the pressure-side surface of the airfoil and projects 1% to 2% of the wing chord. This trailing edge device can improve the performance of a simple airfoil to nearly the same level as a complex high-performance design.
In aeronautics and aeronautical engineering, camber is the asymmetry between the two acting surfaces of an airfoil, with the top surface of a wing commonly being more convex. An airfoil that is not cambered is called a symmetric airfoil. The benefits of cambering were discovered and first utilized by George Cayley in the early 19th century.
Boundary layer control refers to methods of controlling the behaviour of fluid flow boundary layers.
Boundary layer suction is a boundary layer control technique in which an air pump is used to extract the boundary layer at the wing or the inlet of an aircraft. Improving the air flow can reduce drag. Improvements in fuel efficiency have been estimated as high as 30%.
An adaptive compliant wing is a wing which is flexible enough for aspects of its shape to be changed in flight. Flexible wings have a number of benefits. Conventional flight control mechanisms operate using hinges, resulting in disruptions to the airflow, vortices, and in some cases, separation of the airflow. These effects contribute to the drag of the aircraft, resulting in less efficiency and higher fuel costs. Flexible aerofoils can manipulate aerodynamic forces with less disruptions to the flow, resulting in less aerodynamic drag and improved fuel economy.
Aurora Flight Sciences is an American aviation and aeronautics research subsidiary of Boeing which primarily specializes in the design and construction of special-purpose Unmanned aerial vehicles. Aurora has been established for 20+ years and their headquarters is at the Manassas Regional Airport in Manassas, Virginia.
Plasma actuators are a type of actuator currently being developed for aerodynamic flow control. Plasma actuators impart force in a similar way to ionocraft. Plasma flows control has drawn considerable attention and been used in boundary layer acceleration, airfoil separation control, forebody separation control, turbine blade separation control, axial compressor stability extension, heat transfer and high-speed jet control.
The Kline–Fogleman airfoil or KF airfoil is a simple airfoil design with single or multiple steps along the length of the wing. It was originally devised in the 1960s for paper airplanes.
Sweeping jet actuators are a type of active flow control technology based on fluidic oscillators used to produce sweeping jets. The first use of fluidic oscillators in the form of sweeping jets for flow control was demonstrated by Raman et al., 1999.<Cavity Resonance Suppression Using Miniature Fluidic Oscillators, G. Raman, S. Raghu and T.J. Bencic' AIAA-99-1900, 5th AIAA/CEAS Aeroacoustics Conference, Seattle, WA, May 10–12, 1999> and later by several authors working in the area of flow control. Many organizations have been working on the use of such actuators for flow control. Boeing, NASA and the University of Arizona Department of Aerospace and Mechanical Engineering, Illinois Institute of Technology, [Advanced Fluidics], Technical University of Berlin are a few of them. They are slots built into the control surface of an airfoil that build on the same principles as that of blown flaps; that by actively blowing air over the surface of an airfoil the effective lift produced by it is increased.
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