Vortex generator

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

Vortex generator
After-market Micro Dynamics vortex generators mounted on the wing of a Cessna 182K
Sketch describing how vortex generators improve flow characteristics
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 was designed with two unusual vortex generators on its wing to ensure aileron effectiveness through the stall

A vortex generator (VG) is an aerodynamic surface, consisting of a small vane 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] and for improving wind turbine performance.[3]

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

Method of operation

Vortex generators are most often used to delay flow separation. To solve this problem, they are often placed on the external surfaces of vehicles [4] and wind turbine blades. On both aircraft and wind turbine blades they are installed on the front third of a wing in order to maintain steady airflow over the control surfaces at the trailing edge.[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 obliquely so 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 surface. The boundary layer normally thickens as it moves along the 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]

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.[5] After-market suppliers claim (i) that VGs lower stall speed and reduce take-off and landing speeds, and (ii) that VGs increase the effectiveness of ailerons, elevators and rudders, thereby improving controllability and safety at low speeds.[6] For home-built and experimental kitplanes, VGs are cheap, cost-effective and can be installed quickly; but for certified aircraft installations, certification costs can be high, making the kits relatively expensive.[5][7]

Owners fit after-market VGs primarily to gain benefits at low speeds, but a downside is that such VGs may reduce cruise speed slightly. In tests performed on a Cessna 182 and a Piper PA-28-235 Cherokee, independent reviewers have documented a loss of cruise speed of 1.5 to 2.0 kn (2.8 to 3.7 km/h) . However, these losses are relatively minor, since an aircraft wing at high speed has a small angle of attack, thereby reducing VG drag to a minimum.[7][8][9]

Owners have reported that on the ground, it can be harder to clear snow and ice from wing surfaces with VGs than from a smooth wing, but VGs are not generally prone to in-flight icing as they reside within the boundary layer of airflow. VGs may also have sharp edges which can tear the fabric of airframe covers and may thus require special covers to be made.[7][8][9]

For twin-engined aircraft, manufacturers claim that VGs reduce single engine control speed (Vmca), increase zero fuel and gross weight, improve the effectiveness of ailerons and rudder, provide a smoother ride in turbulence and make the aircraft a more stable instrument platform[5]

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.[5] 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 rather 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.[7]

In the USA from 1945[10] until 1991,[11] the one-engine-inoperative climb requirement for multi-engine airplanes with a maximum takeoff weight of 6,000 lb (2,700 kg) 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[4] 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.[7]

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.[7]

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.[7]

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.[11]

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.[7]

References

1. Peppler, I.L.: From The Ground Up, page 23. Aviation Publishers Co. Limited, Ottawa Ontario, Twenty Seventh Revised Edition, 1996. ISBN 0-9690054-9-0
2. ^ a b c d Micro AeroDynamics (2003). "How Micro VGs Work". Retrieved 2008-03-15.
3. ^ Wind Turbine Vortex Generators, UpWind Solutions.
4. ^ a b Clancy, L.J. Aerodynamics, Section 5.31
5. ^ a b c d Micro AeroDynamics (2003). "Micro Vortex Generators for Single and Twin Engine Aircraft". Retrieved 2008-03-15.
6. ^ "Land Shorter! Benefits". Landshorter.com. 1970-01-01. Retrieved 2012-10-09.
7. Busch, Mike (November 1997). "Vortex Generators: Band-Aids or Magic?". Retrieved 2008-03-15.
8. ^ a b Psutka, Kevin, Micro-vortex generators, COPA Flight, August 2003
9. ^ a b Kirkby, Bob, Vortex Generators for the Cherokee 235, COPA Flight, July 2004
10. ^ USA Civil Air Regulations, Part 3, §3.85a
11. ^ a b USA Federal Aviation Regulations, Part 23, §23.67, amendment 23-42, February 4, 1991
• 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