Blown flaps are a powered aerodynamic high-lift device invented by the British and used on the wings of certain aircraft to improve low-speed lift during takeoff and landing. The process is sometimes called a boundary layer control system (BLCS). They were a popular design feature in the 1960s, but fell from use due to their complex maintenance needs. Today a simpler version can be found on military transport aircraft, although the term is not widely used. Additionally, the early concepts have been improved by modern engineers to create the circulation control wing, a far more effective device with applications in the modern aviation industry.
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 effectiveness of wings can be greatly improved by using blow-type flow control, while if the intensity of the blown jet is high enough, even the lift predicted by potential flow theory can be surpassed (i.e. the jet flap effect) due to the initiation of supercirculation. Streamwise blowing however can require large amounts of air and energy thus reducing the overall benefits of the flow control solution itself. At low speeds, the amount of air being delivered by this system can be a significant fraction of the overall airflow, generating as much lift as if the plane were traveling at much higher speeds. This costs little, during landing at least, as the engine power is significantly reduced anyway. During takeoff the trade-off is not so obvious, particularly in conditions of low air density.
Development of the general concept continued at NASA in the 1950s and 60s, 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 lift of a wing by two to three times. Whereas a complex triple-slotted flap system on a Boeing 747 delivers a coefficient of lift of about 2.8, external blowing improves this to about 7, and internal blowing to 9.
During the 1950s and 60s, fighter aircraft generally evolved towards smaller and smaller wing planforms in order to have low drag at high speeds. Compared to the fighters of a generation earlier, they had wing loadings about four times as high; for instance the Supermarine Spitfire had a wing loading of 24 lb/ft2 (117 kg/m2) and the Messerschmitt Bf 109 had the "very high" loading of 30 lb/ft2 (146 kg/m2), whereas the 1950s-era F-104 Starfighter had 111 lb/ft2 (542 kg/m2).
One serious downside to these higher wing loadings is at low speed, when there simply isn't enough wing left to provide lift to keep the plane flying. Even huge flaps could not offset this to any large degree, and as a result many aircraft landed at fairly high speeds, and were noted for accidents as a result.
The major reason flaps were not effective is that the airflow over the wing could only be "bent so much" before it stopped following the wing profile, a condition known as flow separation. Effectively, there is a limit to how much air the flaps can deflect overall. There are ways to improve this, through better flap design; modern airliners use complex multi-part flaps for instance. However, large flaps tend to add considerable complexity, and take up room on the outside of the wing, which makes them unsuitable for use on a fighter.
The principle of the jet flap had been proposed and patented in 1952 by the British National Gas Turbine Establishment (NGTE) and thereafter investigated by the NGTE and the Royal Aircraft Establishment. The concept was first tested at full-scale on the experimental Hunting H.126. It reduced the stall speed to only 32 mph (51 km/h), a number most light aircraft cannot match. The first production aircraft with BLCS was the Lockheed F-104 Starfighter, where after prolonged development problems, it proved to be enormously useful in compensating for the Starfighter's tiny wing surface. It was shortly adopted for North American Aviation's A-5 Vigilante, the McDonnell Douglas F-4 Phantom II, the Blackburn Buccaneer carrier aircraft and the ill-fated BAC TSR-2. On the TSR-2 it reduced the takeoff distance for this large and highly loaded aircraft from 6,000 ft (1,800 m) without the blowers, to about 1,600 ft (490 m) with them turned on.
In production aircraft, blown-flap systems were found to be a maintenance nightmare. They were continually breaking down due to clogging with dirt, and were generally unreliable. This made blown flaps practically useless as a landing aid on many aircraft. They were removed from later production runs of some aircraft.
Starting in the 1970s the lessons of air combat over Vietnam changed thinking considerably. Instead of aircraft designed for outright speed, general maneuverability and load capacity became more important in most designs. The result is an evolution back to larger planforms to provide more lift. For instance the F-16 has a wing loading of 78.5 lb/ft2 (383 kg/m2), and uses leading edge extensions to provide considerably more lift at higher angles of attack, including approach and landing. Given the problems in service and the better lift from the larger wings, blown flaps have generally disappeared. More recently designed fighter aircraft achieve the same improved low-speed characteristics using the technically more complex swing-wing design.
In the 1970s new methods of constructing blown flaps were designed, with the original system becoming known as internal blowing. Two systems of externally blown flaps were developed, both using the direct exhaust of wing-mounted engines on otherwise simple flaps. Typical flap designs are split near the engine such that they don't deflect the thrust; however, with sufficiently powered engines, the effect of the flaps being in the path of the exhaust can be tremendous. The Airbus A380, because of its massive size, is one of the few major commercial airliners to use externally blown flaps, which continue behind its engines.
- Carpenter, Chris (1996). FlightWise. UK: Airlife Publishing Ltd.
- Aerodynamics for Engineering Students, E.L. Houghton & P.W. Carpenter, Elsevier
- Boundary Layer Theory, H. Schlichting & K. Gersten, Springer Verlag
- Control of High-Reynolds-Number Turbulent Boundary Layer Separation Using Counter-Flow Fluid Injection, B.E. Wake, G. Tillman, S.S. Ochs, J.S. Kearney, 3rd AIAA Flow Control Conference, 2006
- Flight International 1963 p454
- Filippone, A. "Powered Lift Systems, Propulsion Systems." Advanced Topics in Aerodynamics. 26 Mar. 2008 
- "Blow, blow thou BLC wind", Flight International, 1971