VORTILONS

  APPLICATION OF AERODYNAMIC INNOVATIONS: VORTILONS

THE VORTILON

Vortilons on an unknown aircraft

The latest innovation in the area of wing-tip stall, and control thereof, is the Vortilon. Vortilons are very simple devices that perform an important function, somewhat like vortex generators, but without the penalty of drag. They are small fences fitted on the undersurface of an aircraft’s wing, “but their main function is to generate a vortex of air over the top of the main wing only at high angles of attack. When the angle of attack on the main wing is raised, the lower surface airflow starts to move outboard at an increasing angle.” They have no loss in performance in all other phases of flight. “The vortilons stick up and move forward as the wing angle increases and they start acting as little fences to the spanwise airflow. They don't stop it, they ‘trip’ it - causing a vortex. This vortex has the effect of keeping the airflow attached to the upper surface of the wing - reducing the wing's local stall angle and increasing aileron effectiveness at low speeds/high Ø.” Vortilons also work with different parts of the wing to enhance stall behaviour.

Vortilons on the DC-9: The DC-9 wing featured vortilons on the lower wing surface that improved control at high angles of attack up to 30º.” In most attitudes, the vortilons were aft of the area where the airflow ‘stagnated,’ so they had little effect. However, when the aircraft was in a potentially dangerous, nose-up attitude, the vortilons “poked past the stagnation point and triggered vortices.”  The vortices extended over the upper wing surface and limited the span-wise flow, thereby preserving lift on the outboard wing sections, so the inner wing would stall first. In a swept wing design, this makes the nose pitch sharply down, enabling the crew to recover control quickly. The vortilons also reduced the downwash from the wing on the tail, which helped crews recover from potential deep stalls.

Vortilons on the HS 125-800: These were also relatively small. The objective was to replace wing fences used on previous models, which minimized span-wise flow and tip stall, and predominantly maintained aileron effectiveness. The standard wing fence ‘fix’ on the -800 required more vortex generators just in front of the aileron hinge line; the combination added drag. “The vortilon solution had less drag than the wing fence and required fewer vortex generators. Also, there were advantages at low speed, and possibly with high-speed cruise performance.”

Vortilons on the Lear Jet: As stated earlier, Vortilons work with different parts of the wing to enhance stall behaviour. According to a Learjet Newsletter (August 2006), the inboard pair of vortilons on the Lear Jet 35 and 35A are placed halfway between the stall strip and the stall fence. The latter forces the inboard portion of the wing to stall first, while the outboard section continues generating lift, giving the pilot better control of the ailerons for a longer period.  The inboard vortilons act as a second stall fence, creating a high-energy vortex, with concomitant benefits.  The outboard vortilon is placed directly in front of the ailerons, which we know is a desirable factor. Pilot workload during an impending stall is minimised, permitting simple recovery.

Vortilons on Boeings: As the basic Boeing 737 evolved with time, extra performance became necessary. The 737-200 NGs have three vortilons on the underside of the leading edge slats to restrict the spanwise flow of air, as shown in the figure infra. 

Three barely visible Vortilons on the Boeing 737-200NG

The Boeing 767-400ER also features three vortilons under the leading edge of the outboard slats. “Results of stall testing were not satisfactory, in that stick forces became light near the stall, and uncommanded and undesirable roll at the stall would tilt the aircraft up to a 20-degree bank. Installation of the vortilons eliminated the problem.”   

Vortilons on the Embraer 145: “The shape and position of vortilons is not yet an exact science and requires considerable flight-testing and knowledge to locate them optimally.” Continuous experimenting is required with various shapes, sizes and positions to arrive at a decision. According to Embraer, Brazil, who integrated vortilons on the ERJ145, the aircraft faced very much the same problem as the Boeing 767-400ER in terms of stall characteristics. Their test pilots found one wing dropping as α reached 20 degrees. Vortilons solved this problem.

Yellow Vortilons are clearly visible

Furthermore, the ERJ145 uses state-of-the-art lifting devices, yet it fell short of ‘the maximum lift coefficient values to meet the short take-off and landing field lengths required for regional airline operations’. Market surveys provided design margins to allow the leading edge to be modified with a fixed 'droop' and the four vortilons on the lower surface leading edge of the outboard wing panel contributed significantly towards achieving their aim. Their interaction with the wing sidewash at high angles of attack produces strong vortices that are convected to the upper surface, where they modify the pressure distribution and boundary layer development, postponing flow separation and increasing maximum lift. Their shape and position were defined using advanced 3D programming. The combined effect of the leading edge droop and vortilons allowed an improved take-off and landing performance without resorting to more complex variable geometry leading edge devices (such as slats), for a small cruise performance penalty.

Figure supra shows a close-up view of four yellow vortilons while the Figure following is a schematic representation of the ERJ145 droop and vortilon. Note that the aircraft is flying from your left to the right.