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WING VORTEX GENERATORS


 WING VORTEX GENERATORS

During the early design phase of the 767, more stringent high angle of attack stability requirements were established. Specifically, the new requirements established criteria for acceptable stick force vs. g (load factor) characteristics for pitch maneuvers above the angle of attack for initial buffet. Reduced stick force gradients at angles of attack beyond initial buffet are typical for low tail, swept wing aircraft due to the tendency for the boundary layer air on the outboard wing panel to separate prior to the inboard wing. Although the probability of encountering these characteristics in normal service is very small, history has shown that high speed upsets followed by high load factor recoveries do occur. It was the
Boeing Company's desire that this new requirement be met by aerodynamic means,although a solution by means of a pitch augmentation control system was carefully considered.

This alternative was not desired since it would add cost and complexity to the airplane.Early 767 wind tunnel test results showed, as expected, that the configuration with the best
aerodynamic cruise efficiency displayed predicted stick force per g characteristics similar to previous low tail, swept wing transports and did not meet the more stringent design criteria beyond initial buffet.For optimum efficiency, a wing is designed to achieve an approximate elliptic spanwise lift distribution
at cruise angle of attack as shown in Figure 

.
 

This loading minimizes lift induced drag and thereby maximizes lift to drag ratio. The elliptical lift distribution
is accomplished by proper selection of airfoil camber and twist along the wing span. The resulting sectional lift coefficient variation with span is presented in Figure . On a swept wing planform, the largest increase in section lift coefficient with increasing angle of attack occurs on the utboard region of the span as seen in Figure . This is because wing sweepback causes the outboard wing to operate in a local upwash field created by the inboard wing; therefore, the outboard wing effectively operates at a higher angle of attack than the inboard wing.


 Another important factor is the spanwise flow of the low energy boundary layer air which makes the outboard wing more susceptible to initial flow separation. The loss of outboard wing lift at high angles of attack is the direct cause of the reduction in stick force per g for swept wing airplanes.Several candidate design modifications were studied as configuration options for improving the stick force per g characteristics to meet the new design requirements:
 

·  Several T-tail configurations were studied in order to separate the wing flow field from the
horizontal tail
 

·  The wing span loading was modified by retwisting the wing to unload the outboard wing
 

·  The inboard wing airfoils were modified in order to promote initial separation on the inboard wing