Blown flap
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Blown flaps are a powered aerodynamic high-lift device the wings of certain aircraft to improve the 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.
Mechanism
The blown flap system provides extra downwards airflow not by improving the flap, but by improving the airflow. 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, improving lift. At low speeds the amount of air being delivered by this system can be a significant fraction of the overall airflow, effectively "fooling" the plane into thinking it is flying at a higher speed. This costs little, during landing at least the engine power is not being used anyway.
Boundary layer control systems usefully lower the stall speed of an aircraft, making them useful for STOL aircraft (like cargo transports intended for use on short fields) and high-performance fighter aircraft with poor low-speed characteristics. Their disadvantage is that they rob the engine of some thrust while in use, which can harm take-off performance, particularly in "hot and high" conditions.
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 767 deliveres a coefficient of lift of about 2.8, external blowing improves this to about 7, an internal blowing to 9.
History
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 just over 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 enough 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. The flaps had a sort of built-in limit to how much air they could deflect overall. There are ways to improve this through better flap design, modern airliners use complex multi-part flaps for instance, but they tend to add considerable complexity and take up room on the outside of the wing, making them too difficult to use on a fighter.
Many designs of the era used the system, including the F-4 Phantom, some versions of the F-104, and most other aircraft designed in the later half of the 1950s.
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 service. It was shortly adopted for North American Aviation's A-5 Vigilante and the Blackburn Buccaneer. It later found use on civilian airliners.
The power of the blown flap system can be impressive. On the BAC TSR-2 it allowed the takeoff distance for this large and highly loaded aircraft to fall from 6,000 feet (2800 m) without the blowers, to about 1,600 feet (750 m) with them turned on. However, the BAC TSR-2 project was cancelled in 1965.
When they reached operation, the 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 Viet Nam changed the thinking considerably. Instead of aircraft designed for outright speed, general manuverablity 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 a higher angles of attack, including landing. Given the problems with in service and the better lift from the larger wings, blown flaps have generally disappeared.
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 so they don't have to deflect the exhaust, but if you make the flaps strong enough the effects can be tremendous. Another modification, over-the-wing blowing, relies on the Coanda effect to make the exhaust of an over-wing engine follow the flaps to be deflected downwards.
More recent designed fighter aircraft achieve the same improved low-speed characteristics using the technically more complex swing-wing design.