Recent from talks
Blown flap
Knowledge base stats:
Talk channels stats:
Members stats:
Blown flap
Blown flaps, blown wing or jet flaps are powered aerodynamic high-lift devices used on the wings of certain aircraft to improve their low-speed flight characteristics. They use air blown through nozzles to shape the airflow over the trailing edge of the wing, directing the flow downward to increase the lift coefficient. There are a variety of methods to achieve this airflow, most of which use jet exhaust or high-pressure air bleed off of a jet engine's compressor and then redirected to follow the line of trailing-edge flaps.
Blown flaps may refer specifically to those systems that use internal ductwork within the wing to direct the airflow, or more broadly to systems like upper surface blowing or nozzle systems on conventional underwing engine that direct air through the flaps. Blown flaps are one solution among a broader category known as powered lift, which also includes various boundary layer control systems, systems using directed prop wash, and circulation control wings.
Internal blown flaps were used on some land and carrier-based fast jets in the 1960s, including the Lockheed F-104, Blackburn Buccaneer and certain versions of the Mikoyan-Gurevich MiG-21. They generally fell from favour because they imposed a significant maintenance overhead in keeping the ductwork clean and various valve systems working properly, along with the disadvantage that an engine failure reduced lift in precisely the situation where it is most desired. The concept reappeared in the form of upper and lower blowing in several transport aircraft, both turboprop and turbofan.
In a conventional blown flap, a small amount of the compressed air produced by the jet engine is "bled" off at the compressor stage and piped to channels running along the rear of the wing. There, it is forced through slots in the wing flaps of the aircraft when the flaps reach certain angles. Injecting high energy air into the boundary layer produces an increase in the stalling angle of attack and maximum lift coefficient by delaying boundary layer separation from the airfoil. Boundary layer control by mass injecting (blowing) prevents boundary layer separation by supplying additional energy to the particles of fluid which are being retarded in the boundary layer. Therefore, injecting a high velocity air mass into the air stream essentially tangent to the wall surface of the airfoil reverses the boundary layer friction deceleration; thus, the boundary layer separation is delayed.
The maximum lift coefficient of a wing can be greatly increased with blowing flow control. With mechanical slots, the natural boundary layer limits the boundary layer control pressure to the freestream total head. Blowing with a small proportion of engine airflow (internal blown flap) increases the lift. Using much higher quantities of gas from the engine exhaust, which increases the effective chord of the flap (the jet flap), produces supercirculation, or forced circulation up to the theoretical potential flow maximum. Surpassing this limit requires the addition of direct thrust.
Development of the general concept continued at NASA in the 1950s and 1960s, leading to simplified systems with similar performance. The externally blown flap arranges the engine to blow across the flaps at the rear of the wing. Some of the jet exhaust is deflected downward directly by the flap, while additional air travels through the slots in the flap and follows the outer edge due to the Coandă effect. The similar upper-surface blowing system arranges the engines over the wing and relies completely on the Coandă effect to redirect the airflow. Although not as effective as direct blowing, these "powered lift" systems are nevertheless quite powerful and much simpler to build and maintain.
A more recent and promising blow-type flow control concept is the counter-flow fluid injection which is able to exert high-authority control to global flows using low energy modifications to key flow regions. In this case, the air blow slit is located at the pressure side near the leading edge stagnation point location and the control air-flow is directed tangentially to the surface but with a forward direction. During the operation of such a flow control system two different effects are present. One effect, boundary layer enhancement, is caused by the increased turbulence levels away from the wall region thus transporting higher-energy outer flow into the wall region. In addition to that another effect, the virtual shaping effect, is utilized to aerodynamically thicken the airfoil at high angles of attack. Both these effects help to delay or eliminate flow separation.
In general, blown flaps can improve the maximum lift coefficient of a wing by two to three times. Whereas a complex triple-slotted flap system on a Boeing 747 produces a maximum lift coefficient of about 2.45, external blowing (upper surface blowing on a Boeing YC-14) improves this to about 7, and internal blowing (jet flap on Hunting H.126) to 9.
Hub AI
Blown flap AI simulator
(@Blown flap_simulator)
Blown flap
Blown flaps, blown wing or jet flaps are powered aerodynamic high-lift devices used on the wings of certain aircraft to improve their low-speed flight characteristics. They use air blown through nozzles to shape the airflow over the trailing edge of the wing, directing the flow downward to increase the lift coefficient. There are a variety of methods to achieve this airflow, most of which use jet exhaust or high-pressure air bleed off of a jet engine's compressor and then redirected to follow the line of trailing-edge flaps.
Blown flaps may refer specifically to those systems that use internal ductwork within the wing to direct the airflow, or more broadly to systems like upper surface blowing or nozzle systems on conventional underwing engine that direct air through the flaps. Blown flaps are one solution among a broader category known as powered lift, which also includes various boundary layer control systems, systems using directed prop wash, and circulation control wings.
Internal blown flaps were used on some land and carrier-based fast jets in the 1960s, including the Lockheed F-104, Blackburn Buccaneer and certain versions of the Mikoyan-Gurevich MiG-21. They generally fell from favour because they imposed a significant maintenance overhead in keeping the ductwork clean and various valve systems working properly, along with the disadvantage that an engine failure reduced lift in precisely the situation where it is most desired. The concept reappeared in the form of upper and lower blowing in several transport aircraft, both turboprop and turbofan.
In a conventional blown flap, a small amount of the compressed air produced by the jet engine is "bled" off at the compressor stage and piped to channels running along the rear of the wing. There, it is forced through slots in the wing flaps of the aircraft when the flaps reach certain angles. Injecting high energy air into the boundary layer produces an increase in the stalling angle of attack and maximum lift coefficient by delaying boundary layer separation from the airfoil. Boundary layer control by mass injecting (blowing) prevents boundary layer separation by supplying additional energy to the particles of fluid which are being retarded in the boundary layer. Therefore, injecting a high velocity air mass into the air stream essentially tangent to the wall surface of the airfoil reverses the boundary layer friction deceleration; thus, the boundary layer separation is delayed.
The maximum lift coefficient of a wing can be greatly increased with blowing flow control. With mechanical slots, the natural boundary layer limits the boundary layer control pressure to the freestream total head. Blowing with a small proportion of engine airflow (internal blown flap) increases the lift. Using much higher quantities of gas from the engine exhaust, which increases the effective chord of the flap (the jet flap), produces supercirculation, or forced circulation up to the theoretical potential flow maximum. Surpassing this limit requires the addition of direct thrust.
Development of the general concept continued at NASA in the 1950s and 1960s, leading to simplified systems with similar performance. The externally blown flap arranges the engine to blow across the flaps at the rear of the wing. Some of the jet exhaust is deflected downward directly by the flap, while additional air travels through the slots in the flap and follows the outer edge due to the Coandă effect. The similar upper-surface blowing system arranges the engines over the wing and relies completely on the Coandă effect to redirect the airflow. Although not as effective as direct blowing, these "powered lift" systems are nevertheless quite powerful and much simpler to build and maintain.
A more recent and promising blow-type flow control concept is the counter-flow fluid injection which is able to exert high-authority control to global flows using low energy modifications to key flow regions. In this case, the air blow slit is located at the pressure side near the leading edge stagnation point location and the control air-flow is directed tangentially to the surface but with a forward direction. During the operation of such a flow control system two different effects are present. One effect, boundary layer enhancement, is caused by the increased turbulence levels away from the wall region thus transporting higher-energy outer flow into the wall region. In addition to that another effect, the virtual shaping effect, is utilized to aerodynamically thicken the airfoil at high angles of attack. Both these effects help to delay or eliminate flow separation.
In general, blown flaps can improve the maximum lift coefficient of a wing by two to three times. Whereas a complex triple-slotted flap system on a Boeing 747 produces a maximum lift coefficient of about 2.45, external blowing (upper surface blowing on a Boeing YC-14) improves this to about 7, and internal blowing (jet flap on Hunting H.126) to 9.
Recent media