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British Aircraft Corporation BAC 1-11

The British Aircraft Corporation One-Eleven, also known as the BAC 1-11, the BAC-111 or the BAC-1-11, was a British short-range jet airliner of the 1960s and 1970s. Conceived by Hunting, it was developed and produced by the British Aircraft Corporation when Hunting merged into BAC along with other British aircraft makers in 1960. Like Vickers' successful Viscount, the BAC-111 also cracked the American domestic market-in itself no mean feat. But by dividing its effort between the VC-10 and the "One Eleven", the company lost its margin over Douglas's DC-9. The handwriting was on the Hangar wall by the mid-fifties-the Americans were going to get the big jet market.

The One-Eleven was stretched for a new series 500 version which BEA purchased. Many charter airlines in Europe also liked the new higher capacity jet, one of which was Luton-based Court Line which flew a fleet of the jets in pretty pastel liveries.

The T-tail empennage configuration offers significant aerodynamic advantages over conventional designs. The relatively high location of the horizontal tail places the tail in a relatively undisturbed airflow at normal cruise conditions, thereby maximizing the contribution of the tail to stability and control. Many military and civil design teams have adopted this tail configuration very successfully. However, the application of the T-tail requires consideration of critical aerodynamic factors-especially an analysis in wind-tunnel tests to ensure satisfactory handling characteristics at extreme pitch attitudes. At high angles of attack associated with wing stall, the low-energy wake of the stalled wing can impinge on the horizontal tail and result in a loss of longitudinal stability (pitch up) and markedly reduced longitudinal control effectiveness. As a result of these flow phenomena, the angle of attack can increase to a deep-stall condition, in which the aircraft enters a stable but uncontrollable trim point with a very high rate of descent.

The BAC 111 suffered a major setback during early flight testing of the prototype on October 22, 1963; the aircraft was destroyed and crew members lost their lives in a fatal accident during tests to evaluate the stall characteristics of the configuration. The aircraft entered a deep stall and descended in an uncontrollable condition at a very high rate of descent with an almost horizontal fuselage attitude until impact. The accident investigation board conducted an exhaustive investigation of the aerodynamic, stability, and control characteristics of the BAC 111 and concluded that the cause of the accident was an unrecoverable deep-stall phenomenon, which was precipitated by the geometric and aerodynamic characteristics of the configuration. In particular, wind-tunnel tests indicated that at high angles of attack near and above those associated with wing stall, the low-energy wakes of the stalled wing and fuselage-mounted engine nacelles impinged on the horizontal tail and significantly reduced its stabilizing effect.

The BAC 111 design was subsequently modified to incorporate a stick pusher, modified leading-edge camber, and powered elevator controls to prevent the aircraft from entering high-angle-of-attack conditions. As news of the BAC 111 accident and its causal factor spread throughout the technical community, other design teams raced to examine the characteristics of their own T-tail aircraft.







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