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SmartCockpit - Airline training guides, Aviation, Operations, Safety
The large vortex generator installed on the inboard side of the nacelle is commonly called a nacelle chine. 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.
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.
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.
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 delaying the wing influence to burst the vortices until a higher angle of attack. The result is that the lift loss is essentially regained.
Due to air condensation under certain atmospheric conditions, the vortex shed by the nacelle chine can be clearly viewed from the cabin. 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. The nacelle chine is a significant contributor to the short field performance of the 767.