Blown flaps are a powered aerodynamic high-lift device invented by the British 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 built upon 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 of 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. This air follows the flap profile, aimed downward to provide more lift. The bleed air prevents the boundary layer (slow-moving air that accumulates on the airframe surface) on the upper surface of the flap from stagnating, further improving lift. 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 "hot and high" conditions.
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.
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, where 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 concept was first tested on the experimental Hunting H.126. It reduced the stall speed to only , 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 F-4 Phantom, the Blackburn Buccaneer 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 feet (2800 m) without the blowers, to about 1,600 feet (750 m) with them turned on.
When they reached operation, blown flap systems were found to be a maintenance nightmare. They were continually breaking down due to clogging with dirt, and were generally unreliable. As a landing aid then, the system was practically useless on many designs due to poor reliability. On many designs the system was removed from later production runs.
Starting in the 1970s the lessons of air combat over Vietnam changed the 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. The amount of thrust from a modern engine means that typical flap designs are "split" near the engine such that they don't deflect the exhaust; however, with sufficiently powered engines, the effects the flaps being in the path of the thrust can be tremendous. The Airbus 380, because of its massive size, is one of the few major commercial airliners to use externally blown flaps, as evidenced by the lack of a "split" in its flaps behind its engines. Another modification, over-the-wing blowing, relies on the Coandă effect to make the exhaust of an over-wing engine follow the flaps to be deflected downwards. Experimental designs using both of these systems were tested in the 1970s, but have only entered production on the Antonov An-72, and later the C-17 Globemaster III.