Fences and Wing Engine Pylons
Wing fences are streamwise panels on the wing’s upper surfaces. They are intended to interrupt and shed the wing’s low-energy boundary layer outflow (toward the wing tips) that would otherwise accumulate and cause flow separation and tip stall. Fences are found
Figure 11.10 Stall patterns on a sweptback wing equipped with slat, leading-edge extension, and drooped-nose leading-edge flap. Initial tip stall is prevented in all three cases. (From Furlong and
McHugh, NACA Rept. 1339, 1957)
on some early swept-wing jets, such as the Comet 1, Sud Caravelle, Tupolev Tu-54M, and Gulfstream II.
The pylons for underwing jet engines can be a substitute for wing fences on high – aspect-ratio-swept wings. This was discovered by the Boeing Company, probably during the wind-tunnel test program for the B-47 airplane. Figure 11.11 shows how bound vorticity of a lifting wing induces sidewash at the nacelle-pylon combination, which in turn causes a sideload. The side-loaded pylon-nacelle combination creates a tip or edge vortex over the top of the wing, opposing the normal outward wing boundary layer flow, which tends to
Figure 11.11 Wing bound vorticity induces sidewash on jet engine pylons. The pylon load creates an upper wing surface vortex that opposes the normal outflow of wing boundary layer, reducing the tendency to flow separation at the wing tips.
follow isobars on the wing and so reduces the tendency toward wing tip stall and airplane pitchup. This same phenomenon is taken advantage of on the B-52 and 707 airplanes, neither of which ever required boundary layer fences on the upper surfaces of their wings.
The Boeing 707 pylon-nacelle arrangement was adopted by the Douglas Aircraft Corporation for the DC-8 airplane. On the DC-8, in addition to reducing spanwise wing boundary layer flow, the pylon-nacelle combination also caused early wing stall at the pylon locations. When fixed slots (opened only with full flaps) were put on the wing near the pylons to inhibit local stall, pilots complained of airplane pitch-up problems (Shevell, 1992). Pitchup was remedied by reducing slot size and later by cutting back the pylons to the location of the wing stagnation point at maximum lift coefficient. Cutting back the pylons also reduced local high-Mach-number “hot spots” on the wing’s upper surface at cruising speeds.
Douglas company aerodynamicists realized what a good thing pylon-nacelle combinations were during the wind-tunnel development for the DC-9 airplane, which had none. Fitting two pylon-nacelle combinations from the DC-8 model to the DC-9 model cured its tip stalling problems; removing the nacelles from the pylons worked, too. Finally, the pylons were reduced in size, streamlined, renamed “vortillons,” and patented. The spanwise vortillon location was chosen to produce desirable vortex flow at the tail, which up till then had been insufficient to recover from a deep stall.
The DC-10 airplane has large vortex generators, or strakes, on the sides of its nacelles to alter nacelle and wing stall behavior at high angles of attack. Nacelle strakes also are found on some Boeing airplanes, but on one side of the nacelle only. David A. Lednicer writes:
The story I have heard is that McDonnell Douglas [held] the patent for the use of strakes on both sides of the nacelle, so Boeing circumvented the patent by putting them on only one side.