VORTEX GENERATORS Original idea from Donald E. Stein Vortex generators have been utilized on most of commercial aircraft. If observed carefully, one can see vortex generators installed at specific locations on Boeing aircraft: for instance, on wing upper surface & on the engine nacelles for the B737 & the B767. (see pictures, page 5) Some specific examples of vortex generator applications are shown in the accompanying photographs. These devices have been installed in each case to enhance flying qualities, but each application has its own story. A brief discussion of what vortex generators do is necessary before the design decision process for specific applications can be discussed. As can be observed from the previous photographs, these devices are used in assorted sizes and combinations, and can be mounted in various locations on an airplane. What they all have in common, however, is that they all act like miniature wings, each creating lift perpendicular to its own surface. By creating lift, they each shed a downstream vortex which can influence airflow in two distinct ways: The vortex interacts with the boundary layer air on the aircraft surface behind the device by inducing high energy air from outside the boundary layer down to the surface displacing low energy air in the process as shown in Figure 1. The air adjacent to the surface is reenergized, and by suitable tailoring of the configuration, the vortex generators can be used to delay, control, or sometimes prevent separation of the boundary layer from the surface. The wing vortex generators installed on the 737, 757 and 767 are applications which take advantage of this mechanism. Figure 1. A vortex generator reduces the boundary layer height & reduces boundary layer separation The vortex is oriented by appropriate placement of the vortex generator in order to redirect airflow in the flow field so that adverse interactions are prevented or delayed. With this mechanism, the generators act as a flow deflector. The large vortex generators installed on the 767 and 737-300 nacelles are examples of applications which take advantage of this mechanism. Vortex generators have been used to increase aircraft speeds, improve initial buffet boundaries, improve control authority, and reduce vibrations induced by boundary layer separation on some aircraft. However, these reasons do not explain why vortex generators have been used on the Boeing 737, 757, and 767 aircraft. Vortex generators are used on these aircraft to improve high Mach pitch characteristics beyond initial buffet and to lower stall speeds in the landing configuration. Editor : Copyright Smartcockpit.com / Ludovic ANDRE PAGE 1 version 01
To further indicate why vortex generators were used on these latest aircraft, the case history for the 767 is presented to explain why the airplane was configured with wing vortex generators and with the large vortex generator installed on each engine fan cowl. 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 2. This loading minimizes lift induced drag and thereby maximizes lift Figure 3. Sectional lift coefficient variation relative to span Figure 2. A wing is designed to achieve an approximately elliptical spanwise lift distribution at cruise angle of attack Figure 4. On swept wing airplanes, the greatest increase in section lift coefficient with increasing angle of attack occurs on the outboard region of the span 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 3. On a swept wing planform, the largest increase in section lift coefficient with increasing angle of attack occurs on the outboard region of the span as seen in Figure 4. 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. Editor : Copyright Smartcockpit.com / Ludovic ANDRE PAGE 2 version 01
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 Figure 5. Model 767 wing vortex generators Figure 6. Relationship of landing gear length to nacelle lip height on high-bypass ratio engines. 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. NACELLE CHINE The large vortex generator installed on the inboard side of the nacelle is commonly called a nacelle chine as shown in the 767 photograph at the end of the article. These devices are used on both the 767 and 737 airplanes. Modem efficient aircraft utilize high bypass ratio engines mounted from pylons off the wing. In order to minimize landing gear length (minimize weight) and to maintain adequate runway clearance (minimize foreign object ingestion), the engines are installed in relative close proximity to the wing as shown in Figure 6. Editor : Copyright Smartcockpit.com / Ludovic ANDRE PAGE 3 version 01
This close coupling of the large engines with the wing results in increased flowfield interaction between the engines and the wing at high angles of attack, and can result in reduced airplane performance unless special consideration is given to counteracting these effects. One such adverse interaction is a loss in maximum wing lift capability in the landing configuration. At the high angles of attack required at low airspeeds, vortices are shed from the fan cowl. For engine installations where the nacelle is located further below the wing, such as JT9D installations on the 747, these vortices pass underneath the wing. For more close coupled nacelle configurations, these vortices flow over the top of the wing and interact with the wing flowfield. The effect of these vortices is generally favorable as long as they remain intact. Unfortunately the wing, at high angle of attack, will impose large adverse pressure fields on these vortices as they flow rearward along the wing surface as shown in Figure 7. Figure 7. Wing vorticies resulting from close coupling of large engines to the wing These vortices will break up and burst, causing the boundary layer air over the wing behind the engine to separate. This results in lower maximum lift levels than would be the case with less closely coupled nacelles as shown in Figure 8. Figure 8. Effect of nacelle chines on lift The solution was the development of a large vortex generator installed on the inboard side of the engine nacelle which was sufficient to delay the nacelle vortex bursting phenomenon. The Boeing invention disclosure identifies this as a vortex control device (VCD), but it is more commonly known as a nacelle chine. The nacelle chine was sized and positioned on the inboard side of the nacelle to control where the nacelle vortex is shed so that it will not attach to the wing. The strong vortex shed by the nacelle chine will cause the nacelle vortex to flow over the wing as shown in Figure 9 delaying the wing influence to burst the vortices until a higher angle of attack. The result is that Figure 9. Effect of the nacelle chine on the nacelle vortex the lift loss shown in Figure 9 is essentially regained as shown in Figure 10. Figure 10. Regained lift loss due to the use of chines Due to air condensation under certain atmospheric conditions, the vortex shed by the nacelle chine can be clearly viewed from the cabin as shown in Figure 11. In terms of airplane performance, the nacelle chine reduced approach speeds by 5 knots and landing field lengths by approximately 250 feet for the 767-200 as shown in Figure 12. The nacelle chine is a significant contributor to the superior short field performance of the 767. Editor : Copyright Smartcockpit.com / Ludovic ANDRE PAGE 4 version 01
OPERATIONAL CONSIDERATIONS Each airplane in the fleet should be periodically inspected to determine if all the vortex generators are installed. On most models, dispatch is allowed with a limited number of missing vortex generators. The Configuration Deviation List (CDL) in the Airplane Flight Manual should be consulted to determine the minimum number required for dispatch and whether operational limitations are to be imposed. Repair and replacement of vortex generators is explained in the appropriate Maintenance Manual for each model. CONCLUSIONS Vortex generators are a valuable aerodynamic tool which can be used by aircraft designers to enhance airplane flying qualities. Judicious use of vortex generators results in optimum aerodynamic characteristics over a wide range of flight conditions (e.g. from cruise flight to Figure 12. Approach speed & landing field length effect of nacelle chines high g and/or high angle of attack maneuvers into heavy buffet). The use of these devices on the new Boeing aircraft have contributed to: more efficient aerodynamic designs with low fuel bum performance aircraft with lower initial cost and maintenance expense Editor : Copyright Smartcockpit.com / Ludovic ANDRE PAGE 5 version 01