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Aviation History
1978
1978 - 0791.PDF
XINGU in the air By HUGH FIELD in Sao Jose dos Campos, Brazil H ow many countries have successfully designed, developed and delivered an executive pressurised turboprop tipping the scales at less than 12,5001b? The United States, inevitably, as befits the home of business aviation; Japan, though the Mitsubishi MU-2 might be held to be as American as most of its competi tors; the United Kingdom, but the Jetstream became a political animal before its true potential could be demon strated. Now, like a young David flexing its muscles before the American Goliath, comes the Brazilian aerospace industry, thrusting, determined and by no means lacking in design expertise. No 313 in the series "Flight" photography by TOM HAMILL It is still less than ten years since the prototype Bandeirante lifted off from Sao Jose dos Campos for the first time, and yet Embraer has already handed over the first major derivative, the EMB-121 Xingu, to the Brazilian Air Force. The step forward is a major one, for although the Xingu shares the wings and some systems of the Bandeirante, the fuselage, tail unit and engine installa tion are all new. Embraer has set out to launch a family of pressurised models (the next will be the EMB-120 pressurised commuter), deciding from the outset that a T-tail was to be a family trademark. In the case of the short-coupled Xingu, Embraer may have regretted the choice of a T-tail, which led to some headaches, but later designs will benefit from the effort that has gone into flight development. Europe first saw the Xingu last summer, when it put in more than lOOhr of flying, initially as a Paris Show exhibit, then as the personal transport of Brazilian racing driver Emerson Fittipaldi, and finally, sponsored by Embraer distributor CSE Aviation, at Flight's Cranfield Show. Embraer admits that the interior was rushed and not up to the standard expected of production models, and it now looks as though the demonstration tour was also premature from both the handling and performance standpoints. Certainly Fittipaldi made the intended use of the aircraft, thus ensuring that it was seen about its business, but few people were invited to fly it. After my pre-flight and post-flight discussions with chief test pilot Col Luiz Cabral and engineering director Guido Pessotti I can understand why. Two aerodynamic areas needed detailed investigation. The T-tail, allied as it is to such a short-coupled fuselage, predictably lost effectiveness at high angles of attack. Pessotti told me that the aircraft had reached 25° angle of attack, though when taken to this figure at high weight and with the e.g. on the aft limit it showed a marked lack of elevator response and began to pitch up. Cabral was forced to pull the anti-spin parachute to re cover from this particular nasty moment. Minor aero dynamic fixes, including a small "biplane" tail, were tried. But there was not much doubt that the aircraft would have to have a stick-shaker, and probably a pusher as well, if civil certification requirements were to be met. The precise extent to which the stall would have to be artificially restrained could not be decided at first because work was still going on in the other area—control response. Cabral describes the prototype as a "real little fighter," and while he probably enjoyed the control characteristics, he is the first to admit that they were just too lively for an executive aircraft. The rate of roll was very rapid and more roll damping was needed, while the elevator forces made the flare and touchdown sensi tive in the extreme. More than 250hr of development flying has been devoted to refining the behaviour, and it was a fully modified aircraft—actually the first for the Brazilian Air Force— that I flew. Visible changes include extended wingtips, about one foot having been added to each to help the roll damping. Less apparent is the change of aileron gearing that has produced a heavier and more acceptable control force. At the back, the relative positions of the elevators and their tabs betoken the use of anti-balance tabs to produce the same effect in pitch, while an external refine ment affecting yaw is the use of anti-snaking strips on the rudder trailing edge. Closer inspection reveals that the rudder-shroud gap is as narrow as possible. External attention to yaw is matched internally by the use of a yaw damper, all made necessary because the aircraft was originally prone to severe Dutch roll. This type of behaviour—movement around all three axes fol lowing a disturbance in yaw—is characteristic of short- coupled designs with a lot of dihedral. I was to find another change to the flying controls when after cutaway *- •-
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