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 theBoeing 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
Each of the above options resulted in a significantly less efficient airplane. A better solution was desired, and vortex generators provided that solution. A 1/10 scale model of the 767 airplane was built and tested at a high Reynolds number wind tunnel in order to obtain data simulating full scale 767 conditions. Vortex generators were evaluated in detail. The test results were very encouraging
because it was determined that only a few small vortex generators located on the wing just outboard of the nacelle were very effective in improving the wing stall pattern and hence the stick force
characteristics. It remained to be proven on the flight vehicle.
The early flight tests on the 767 airplane without the vortex generators confirmed the initial wind tunnel test results. When vortex generators were added, the stick force characteristics beyond initial buffet met the new Boeing design requirements. The vortex generators also provided increased buffet intensity with increasing load factor thereby contributing additional deterrence to a pilot as he pulled into these conditions. The production vortex generator configuration required only seven 3/4 inch high
vortex generators per wing shown in Figure 5. The effect on weight and drag were negligible.
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