After-market Micro Dynamics vortex generators mounted on the wing of a Cessna 182K

1967 Model Cessna 182K in flight showing after-market vortex generators on the wing leading edge

TA-4SU Super Skyhawk showing the row of vortex generators on the drooped leading edge slats.

The Symphony SA-160 has two unique vortex generators on its wing to ensure aileron effectiveness through the stall

A vortex generator is an aerodynamic surface, consisting of a small vane or bump that creates a vortex.^[1][2] Vortex generators can be found on many devices, but the term is most often used in aircraft design.^[1]

[edit] Method of operation

Vortex generators are likely to be found on the external surfaces of vehicles where flow separation is a potential problem because vortex generators delay flow separation. ^[3] On aircraft they are installed on the front third of a wing in order to maintain steady airflow over the control surfaces at the rear of the wing.^[2] They are typically rectangular or triangular, about 80% as tall as the boundary layer, and run in spanwise lines near the thickest part of the wing.^[1] They can be seen on the wings and vertical tails of many airliners. Vortex generators are positioned in such a way that they have an angle of attack with respect to the local airflow.^[1]

A vortex generator creates a tip vortex which draws energetic, rapidly-moving air from outside the slow-moving boundary layer into contact with the aircraft skin. The boundary layer normally thickens as it moves along the aircraft surface, reducing the effectiveness of trailing-edge control surfaces; vortex generators can be used to remedy this problem, among others, by re-energizing the boundary layer.^[1][2]

Vortex generators delay flow separation and aerodynamic stalling; they improve the effectiveness of control surfaces^[2] (e.g. Embraer 170 and Symphony SA-160); and, for swept-wing transonic designs, they alleviate potential shock-stall problems (e.g. Harrier, Blackburn Buccaneer, Gloster Javelin).

[edit] After-market installation

Many aircraft carry vane vortex generators from time of manufacture, but there are also after-market suppliers who sell VG kits to improve the STOL performance of some light aircraft.^[4]

[edit] Increase in Maximum Takeoff Weight

Many of the vortex generator kits available for light twin-engine airplanes bring with them the added benefit of an increase in maximum takeoff weight.^[4] This might seem paradoxical because installation of vortex generators does not increase the strength of the wing.

The maximum takeoff weight of a twin-engine airplane is determined by structural requirements and single-engine climb performance requirements (which are lower for a lower stall speed). For many light twin-engine airplanes the single-engine climb performance requirements determine a lower maximum weight than the structural requirements. Consequently, anything that can be done to improve the single-engine-inoperative climb performance will bring about an increase in maximum takeoff weight.^[5]

In the USA from 1945 ^[6] until 1991 ^[7] the one-engine-inoperative climb requirement for multi-engine airplanes with a maximum takeoff weight of 6000 lb or less was as follows:

“All multiengine airplanes having a stalling speed V_s0 greater than 70 miles per hour shall have a steady rate of climb of at least 0.02(V_s0)² in feet per minute at an altitude of 5,000 feet with the critical engine inoperative and the remaining engines operating at not more than maximum continuous power, the inoperative propeller in the minimum drag position, landing gear retracted, wing flaps in the most favorable position …”

where V_s0 is the stalling speed in the landing configuration in miles per hour.

Installation of vortex generators can usually bring about a slight reduction in stalling speed of an airplane ^[3] and therefore reduce the required one-engine-inoperative climb performance. The reduced requirement for climb performance allows an increase in maximum takeoff weight, at least up to the maximum weight allowed by structural requirements.^[5]

An increase in maximum weight allowed by structural requirements can usually be achieved by specifying a maximum zero fuel weight or, if a maximum zero fuel weight is already specified as one of the airplane’s limitations, by specifying a new higher maximum zero fuel weight.^[5]

For these reasons, vortex generator kits for many light twin-engine airplanes are accompanied by a reduction in maximum zero fuel weight and an increase in maximum takeoff weight.^[5]

The one-engine-inoperative rate-of-climb requirement does not apply to single-engine airplanes, so gains in the maximum takeoff weight (based on stall speed or structural considerations) are less significant compared to those for 1945 - 1991 twins.

After 1991 the airworthiness certification requirements in the USA specify the one-engine-inoperative climb requirement as a gradient independent of stalling speed, so there is less opportunity for vortex generators to increase the maximum takeoff weight of multi-engine airplanes whose certification basis is FAR 23 at amendment 23-42 or later.^[7]

[edit] Maximum Landing Weight

Because most light twin engined aircraft landing weights are determined by structural considerations and not stall speed, most VG kits only increase the take-off weight available and not the landing weight. In these cases increasing the landing weight requires either structural modifications or else re-testing the aircraft to demonstrate that the certification requirements are still met at the higher landing weight.^[5]

[edit] References

• Kermode, A.C. (1972), Mechanics of Flight, Chapter 11, page 350 - 8th edition, Pitman Publishing, London ISBN 0 273 31623 0
• Clancy, L.J. (1975), Aerodynamics, Pitman Publishing, London ISBN 0 273 01120 0

[edit] See also

Wikimedia Commons has media related to: Vortex generators

• Turbulator
• Boundary layer suction