Aerodynamic Flow Fences and Wing Design

Aerodynamic flow fences are thin, blade-like devices that are mounted normal to the skin of an airplane. They are usually aligned approximately parallel to the free stream.

Spanwise Flow

Ideally, the airflow over the surface of the airplane should flow smoothly from front to back. This is usually the case at low angles of attack when the flow is fully attached.

Lateral (or spanwise) flow tends to develop at higher angles of attack. Almost all separated flows start initially with the flow moving sideways before full flow reversal and separation appear. Once the flow starts to separate, spanwise flow can cause the initial flow separation to propagate away from its point of origin and trigger additional separation and turbulence.

Lateral and spanwise flow is not only a characteristic of developing flow separation. Even without causing separation, it can degrade aerodynamic performance.

Spanwise Flow on Swept Wings

As the air flows around a lifting airfoil, it first accelerates around the leading edge. This acceleration reduces the pressure of the air, and accordingly, the area of lowest pressure on a lifting airfoil is near the leading edge.

On a straight wing, this zone of lowered pressure extends spanwise, perpendicular to the oncoming airstream.

Because the pressure across any cut normal to the direction of flight is constant, there is no initial tendency for the flow to move laterally.

On a swept-back wing, this is not the case. Because of the sweep, as we move outboard on the wing, the area of low pressure near the leading edge of any station is directly outboard of an area of higher pressure on the more inboard stations. This creates a spanwise pressure gradient with lower pressure outboard and higher pressure inboard on any given cut normal to the direction of flight. This spanwise pressure gradient tends to drive the flow to turn outboard as it flows aft. As an angle of attack increases, the spanwise flow becomes more pronounced. At low angles of attack and low lift coefficients, the spanwise flow is small enough that it is not a significant issue.

Extensive spanwise flow is detrimental to the aerodynamic performance of the wing in several ways.

The first is that air flowing outboard does not generate lift like the air flowing aft because it is not being turned by the upper surface of the airfoil. This means that the rate of increase of lift with increasing angle of attack decreases as spanwise flow develops, even if the flow field with the large spanwise flow is fully attached.

The second detrimental effect is that as the air flows spanwise, it is slowed by friction with the skin and will be more prone to flow separation and stall.

Because separation will first appear at the outer end of the spanwise flow, swept-back wings will tend to stall outboard near the tip first. Tip stall on any wing will tend to cause roll instability and a tendency to roll off or spin at the stall. This is exacerbated on a swept wing because the tips are further aft than the root. As the tips stall, the aerodynamic center of the wing moves forward, and the tip stall will tend to cause the airplane to pitch up, increasing the angle of attack and worsening the stall.

The benefits of swept-back wings for high-speed flight became known immediately after WW-II. The second generation of jet fighters, like the American F-86 Sabre and the Soviet MiG-15, used swept-back wings to enable them to fly at higher Mach numbers than earlier designs. They initially exhibited instability and tip stall at higher angles of attack. This hurt their stall characteristics and degraded their maneuvering capability compared to their straight-winged predecessors.

Designers of early swept-wing airplanes came up with several aerodynamic solutions to solve the tip stall/pitch up problem caused by spanwise flow. Three types of aerodynamic devices came into use:

• The F-86 had aerodynamically deployed leading-edge slats.
• The Boeing B-47, which was the world’s first large swept-wing airplane, initially had severe aerodynamic problems due to the effects of the sweep, and it is widely reported that vortex generators were invented by a Boeing engineer to solve those problems.
• Flow fences were used on the MiG-15 and subsequent swept-wing MiG fighters, as well as many other swept-wing airplanes that followed.

On a swept wing, the fence has two effects that reduce spanwise flow. The first and most obvious is that a fence oriented fore and aft physically opposes any lateral motion of the air and forces the air flowing near the fence to move parallel to the fence.

The second effect of the fence is more subtle: Because the fence is solid, the air flowing on one side of the fence is not directly affected by the pressure of the airflow on the other side of the fence. Accordingly, the fence interrupts the spanwise pressure gradient that drives the spanwise flow in the first place.

In general, the greater the sweep-back angle, the more severe the spanwise flow will be. Because of this, it was not uncommon to see multiple fences on early swept-wing airplanes. On the MiG series, for example, we can see that the MiG-15 had two fences on the wing, while its successor, the MiG-17, had approximately 10° more sweep angle and needed three fences.

Another interesting early jet example of the use of a flow fence is on the North American FJ-4 Fury, which was a naval version of the F-86 Sabre. Because the FJ-4 had to operate off aircraft carriers, it needed better low-speed, high-lift characteristics than the F-86 in order to fly slowly enough on approach to be carrier suitable. While the F-86 used slats on early versions and a modified hard-leading-edge wing with a tailored shape on later models, the FJ-4 needed an additional flow fence on its wing to have acceptable slow flight characteristics for carrier operations.

Advances in knowledge of aerodynamics and better understanding of the design of swept wings has reduced use of fences compared to earlier generation airplanes with swept-back wings. Designers have learned how to tailor the shape of the wing itself to mitigate the effects of spanwise flow. Despite this, fences still appear from time to time, and they are a valid and useful tool to tailor and improve the aerodynamic characteristics of swept wings.

One example of this is the small fences that have appeared near the trailing edge of a wing of some Rutan-style canard airplanes. The Rutan canards are fast as piston-engine singles go, but not fast enough that they need wing sweep for high Mach number shock control. They have a significant amount of sweep in their main wings for balance reasons, to move the aerodynamic center of the wings aft to accommodate the rearward CG position caused by the aft-mounted engine. At angles of attack typical of approach and landing, the air begins to show a significant development of spanwise flow on the aft 20% to 30% of the wing. Small triangular fences near the trailing edge, pioneered by Klaus Savier, control this spanwise flow and increase the lift of the wing and the effectiveness of the ailerons significantly in approach conditions.

Stall Control on Straight Wings

As the flow begins to separate as a section of the wing approaches stall, the flow starts to move laterally and will eventually actually reverse direction close to the skin. As the stall develops, the area of separated flow will tend to propagate away from the point of initial stall, and this propagation will be driven in large by the spanwise or reversed flow of the initially stalled region.

Fences can be used to delay the spanwise propagation of stall, which is why they are a common feature of STOL aeromodification kits for general aviation airplanes. A well-designed straight wing stalls from the root first, and the stall then propagates outboard with increasing angle of attack. A fence placed just inboard of the inboard end of the ailerons can delay the propagation of the stall onto the outer panel of the wing and the aileron. This improves roll stability and roll damping as the stall develops and also helps keep the ailerons effective deeper into the stall.

The fences can also keep the stall from propagating laterally in response to sideslip. This helps keep the development of the stall symmetric and reduces the tendency of the airplane to drop a wing or try to spin during an incipient stall.

Controlling Localized Flow Disturbances

Sometimes a local feature of the aircraft configuration can cause separation and turbulence in the immediate area of the feature. Examples of this include protruding lumps like lights or fairings, and gaps like those found near the roots of all-moving tail surfaces.

On some configurations, this disturbance stays localized and is not a serious issue. On others, however, the flow disturbance caused by the initial imperfection in the configuration can propagate onto a wing or control surface and significantly damage the quality of the flow for a significant distance away from the triggering geometric feature. In such cases, a fence can be used to confine the separated flow to a small area and minimize its detrimental effect on the flow over the rest of the airplane.

A significant virtue of flow fences is that they are easy to retrofit to an airplane after it is built. Adding the fence does not require any other change to the airplane, and if it works, it may eliminate a need for significant reshaping of the aircraft outer mold line itself. Fences are relatively easy to add, move, and adjust in size and position to provide the flow control needed to give the airplane acceptable flying qualities. Anytime the air flowing over the surface of the skin starts moving laterally or in a direction that is harmful to the overall dynamic characteristics of the airplane, a fence can be a simple solution to improve the airflow.