Aerodynamics of the Leading Edge Junction

The junction between the wing root and fuselage side of a low-wing airplane is one of the most aerodynamically tricky spots on the airframe. Historically, most of the attention has been focused on fairing and filleting the aft portion of this junction and ensuring that the air flows smoothly aft off of the wing trailing edge at the root. The leading-edge junction typically gets less attention unless a problem appears in testing, but it is aerodynamically just as significant.

The flow near the leading edge of the wing root junction is significantly different from the flow over the wing root and fuselage side aft of the wing leading edge. In particular, while the flow aft of the leading edge is going substantially downstream, the flow just ahead of the leading edge is not.

There is a strong aerodynamic interaction between the forward portion of the fuselage and the wing leading edge that begins well forward of the wing itself.

A lifting wing distorts the airstream ahead of the wing as well as on and behind the wing. The wing generates upwash forward of the leading edge, and this upwash gets stronger with increasing lift coefficient. The fuselage nose forward of the wing is immersed in this upwash, which increases the local angle of attack on the nose.

Forward fuselage geometry also contributes. Noses of most single-engine aircraft are upswept. The thrust line is typically near the top of the cowling with the cooling air inlets on either side of the propeller/spinner. The bottom of the nose starts low at the firewall and sweeps upward as we move forward to the cooling inlet. This puts the nose itself at a significant positive angle of incidence relative to the chord line of the wing.

The exhaust soot streak on this Skyraider shows the upwash at the leading edge of the wing root.

All of these effects combine to generate an area of high air pressure under the nose. This high pressure naturally seeks release. At the outer edges of the nose, the air naturally wants to flow from the high-pressure zone under the nose into the lower pressure along the fuselage sides and then toward the low-pressure peak above the leading edge of the wing. This generates a strong upwash along the fuselage side as the flow approaches the wing leading edge. This upwash significantly increases the local angle of attack at the wing root above and beyond the local angle of attack further outboard.

This effect decreases relatively rapidly as we move outboard from the fuselage side. There is a significant change in the local angle of attack of the leading edge of the wing in the zone between the wing/fuselage junction and a point about one body width outboard of the fuselage side. Accordingly, the local angle of attack at the wing root is significantly higher than elsewhere on the span.

This phenomenon can have one positive effect: The high local angle of attack at the wing root leading edge will tend to cause the wing root to stall before the rest of the wing. A stall that starts inboard is preferable to one that starts further outboard. A root stall ensures stable roll damping during the incipient phases of the stall and minimizes any tendency to roll off and depart from controlled flight.

Although this premature root stall can be favorable in terms of controllability at the stall, it can cause an unacceptable increase in stall speed.

The high angle of attack near the root can also induce significant flow separation in the junction at airspeeds above the stall and cause poor junction flow and higher drag in cruise.

This flow-control vane helps turn the upwash from the nose flowing upward along the fuselage side ahead of the wing. The vane turns the local flow parallel to the wing upper surface skin. The converging channel between the vane and the wing surface accelerates the local flow, promoting better flow attachment in the wing root junction.

Fuselage Geometry

The shape of the fuselage ahead of the wing root junction has a significant effect on the local flow. As we have already seen, the natural upsweep of the nose places the nose at a higher angle of attack than the airplane as a whole. The shape of the bottom of the nose and the cross section of the fuselage just forward of the wing root also have strong effects.

If the fuselage has a relatively flat bottom, the flow is more likely to be disrupted because of two factors:

First, the flat bottom will generate higher local air pressure than a shape with a more rounded cross section.

Second, if there is a hard corner between the bottom and sides, the flow is very likely to separate because of the tightness of the corner as the air flows around the hard corner and is deflected upward by the pressure field. Even on a flat-bottomed fuselage, local rounding of the corner between the fuselage bottom and sides helps keep the flow attached.

A local wing root leading-edge fairing that softens the corner and helps direct the flow around the leading edge can also significantly improve the airflow near the leading edge of the wing root.

Wing Root Airfoil Effects

When the oncoming airstream reaches the leading edge of a wing, the job of the forward part of the airfoil is to smoothly turn the airflow parallel to the surface of the wing. When the wing is at a positive angle of attack, the flow over the leading edge turns down and back to flow smoothly over the upper surface of the wing.

The higher the angle of attack, the more the leading edge of the wing must turn the flow.

Because of the upwash coming off the nose, the local angle of attack is higher than it is further outboard on the wing.

The optimum wing shape near the root will be different from most of the rest of the span because of this higher local angle of attack.

The leading edge cuff on this Piper Pawnee is used to control the flow at the wing root. The wing has a constant chord and constant airfoil. The cuff implements a local increase in wing chord and leading edge camber. In addition to the flow turning the camber provides, the chord extensions ensures that the turbulent wake of the landing gear legs goes under the wing and does not disrupt the sensitive flow over the upper surface of the wing root junction.

The leading edge must capture the upwash coming off the nose and smoothly turn it aft without stalling. The airfoil should have more total camber near the root, and the point of maximum camber should be well forward. A larger leading-edge radius also helps the leading edge turn the upwash flow aft.

The challenge for the wing designer is to implement the rapid spanwise variation of airfoil section over the most inboard part of the wing in a manufacturable way.

In recent times, the availability of the combination of composite materials and CAD makes implementing local tailoring of airfoils easier. The spanwise variation of airfoils can be lofted in CAD, and the more complex shape of the wing can be molded into composite wing skins.

On a traditional metal wing with sheet-metal skins, the designer is constrained to single-curvature, flat-wrapped skin shapes with straight element lines between ribs. Implementing a special airfoil near the root requires a break in the skins, dividing the wing panel into at least two flat-wrapped sections.

Some airplanes use this approach, and it is common to also use a planform break, with an increase in chord at the root. The greater root chord places more of the wing root ahead of the spar, which makes it possible to incorporate some local camber change in the leading edge near the root while maintaining a constant airfoil cross-section from the spar aft to the trailing edge.

A similar, simpler approach to get the same effect is to add a cuff with a triangular planform that adds some chord and local nose camber at the wing root.

The point of maximum camber at the wing root of this Airbus A350 is extremely far forward. One of the reasons for this is to adapt to the upwash flowing off of the underside of the forward fuselage.

Flow Energizers

Flow energizers are small blade-like strakes that extend outward from the fuselage side ahead of the wing root. The outboard edge of the strake sheds a vortex that flows over the upper surface of the wing and adds energy to the flow in the junction. One significant advantage of these devices is that they can be added to the airplane without changing other parts of the configuration and are relatively easy to modify and optimize.

Flow energizers are used on multiple airplane types, like Cirrus piston-engine airplanes and the Cirrus Vision Jet.

Flow energizers are small blade-like strakes that extend outward from the fuselage side ahead of the wing root. The outboard edge of the strake sheds a vortex that flows over the upper surface of the wing and adds energy to the flow in the junction. The proper size and shape of the energizer depends on the details of the configuration of the nose and wing root.

Landing Gear Interference

The main gear legs on a taildragger with fuselage-mounted gear typically intersect the fuselage just forward of the leading edge of the wing. This means that the intersection between the gear leg and the fuselage is immersed in the area of highest upwash on the fuselage. This upwash has two aerodynamic effects.

First, the inner portion of the gear leg will be at a high angle of attack. If the gear is a typical flat aluminum or steel spring gear, the leg will have a flat cross-section, typically with a rounded leading edge. The high local angle of attack near the intersection may cause a local stall or zone of flow separation on the gear leg.

The second effect is that the upwash coming off the nose convects the wake of the landing gear leg over the upper surface of the wing root. If that wake is separated, it can cause a premature stall of the wing root.

There are two approaches to fixing this situation:

Local Fairing of Gear Legs

One way of solving the problem of interference between the landing gear wake and the wing is to fair the root of the landing gear leg by putting a cuff around it. The cuff typically increases the leading-edge radius of the gear leg, which helps keep the airflow over the gear leg from separating.

The small triangular leading-edge extension shown here extends the local wing leading edge forward over the junction between the landing gear legs and the fuselage. This ensures that the wake of the landing gear root flows under the wing and does not disturb the flow over the upper surface of the wing root junction.

Wing Leading-Edge Extension

A local wing root extension with a triangular planform can be used to move the leading edge at the fuselage side forward of the intersection between the landing gear legs and the fuselage.

This has two favorable effects:

First, the lower surface of the wing leading-edge extension turns the upwash flow aft and shields the landing gear legs from the upwash and also from changes in angle of attack. The flow is more parallel to the landing gear leg sides and is hence less likely to separate as the air passes over the leg.

Second, moving the leading edge ahead of the intersection between the gear legs and the fuselage side ensures that the wake of the gear leg passes under the wing and does not affect the flow over the upper surface. Even if the wake of the landing gear is turbulent or separated, it will not cause premature stalling of the wing root.

The airflow near the leading edge of the wing root junction on a low-wing airplane is complex and can lead to poor junction flow and premature stall, but a combination of proper design of the wing root airfoils and other aerodynamic devices can ultimately yield smooth airflow and good aerodynamic performance.

The post Aerodynamics of the Leading Edge Junction appeared first on AvBrief.com.