Take a window seat over the wing of a Boeing 737 Next Generation and look just outboard of the engine. A short row of small blades stands on the upper skin, each angled across the airflow like a miniature keel. Passengers photograph them, post the pictures online and ask the same question: what are those for?

They are vortex generators, and their job sounds like sabotage. Designers work hard to shape wings so that air flows over them smoothly; vortex generators deliberately stir it up. The trick lies in which air they stir, and why.

These little vanes now ride on everything from Cessna 182s to RAF Hawk trainers and Boeing airliners. The first ones were built not for an aircraft but for a wind tunnel. And some of the best-known examples were never in the original drawings: they were added after flight testing, or years into service.

The trouble with slow air

Air touching an aircraft’s skin is slowed by friction, forming a thin, sluggish layer called the boundary layer. Towards the rear of a wing the pressure rises, and that tired air has to push uphill against it. If it runs out of energy, it stops, reverses and peels away from the surface. That is flow separation.

Separation sits behind a whole family of problems. On a wing at high angle of attack, it is the stall. Over an aileron or rudder, it robs the control surface of authority. At high subsonic speeds a shock wave can trigger it, and the intermittent separation and reattachment of the flow shakes the airframe as buffet.

Samm Sheperd’s popular explainer covers the boundary layer, separation bubbles and how a small angled vane re-energises the flow.

A vortex generator attacks the problem with a wing of its own: a small, stubby blade standing upright on the skin, set at an angle to the local airflow. Barnaby Wainfan, a Technical Fellow in Northrop Grumman’s advanced design organisation, recommends 15 to 20 degrees. Like any wing it makes lift and sheds a vortex from its tip, so a row of them trails what AVweb once called “little horizontal tornadoes” along the skin.

Stirring the boundary layer

That spinning air is the whole point. Each vortex carries fast-moving air from outside the boundary layer down to the surface and lifts slow air away from it, topping up the energy the layer needs to keep flowing against rising pressure. Writing for homebuilders in KITPLANES, Wainfan put it plainly.

Placement is everything. The row must sit a short distance ahead of the point where the flow would otherwise separate; if separation starts upstream, the vanes sit in dead air and do nothing. They must also poke just above the boundary layer: Wainfan suggests a first-try height of about 1.2 times its local thickness, which is usually only 1 to 2 percent of the distance from the leading edge. Hence the small size.

They are not free. A vane in the airflow makes drag all the time, and on a clean wing that would otherwise enjoy laminar flow the penalty can be significant, though on many aircraft the healthier flow behind them offsets most or all of it. Designers also choose where the benefit lands: a full-span row lowers stall speed, while a row on the outer wing makes the tips stall last and keeps the ailerons working. Passengers notice: one r/aviation question about a Jet2 737-300 drew 75 comments.

Invented to fix a wind tunnel

The first vortex generators never flew. Harlan D. Taylor devised them in the Research Department of United Aircraft Corporation, the Connecticut group that owned Pratt & Whitney and Sikorsky. His February 1948 design memo is explicit: “The vortex generator mixing method was first conceived for the purpose of eliminating the flow separation then existing in the U.A.C. 8-ft. tunnel diffuser.” He reported that fix in June 1947; within months the vanes were also improving the tunnel’s fan and mixing fuel and air in burners.

NACA took the idea into the air. In 1950, Langley’s Lindsay Lina and Wilmer Reed fitted rows of them to the wing of an F-51D Mustang and flew it at Mach 0.71 to 0.77, where shock waves made the flow separate. Every arrangement delayed the separation, and they saw a bigger prize: “Since the problem of buffeting is recognized as being associated with intermittent separation and reattachment of the boundary layer, it seems possible that buffeting may be reduced by the use of vortex generators.”

Others followed. NACA kept a YF-84A at Edwards for vortex generator research from late 1949 to 1954. According to AVweb, Boeing fitted them to the upper wing of the original 707 because, at speed, separated flow left its inboard high-speed ailerons in badly disturbed air. The 727 carries a row on its fin ahead of the rudder hinge, and tails as new as the 787’s still wear them.

The fix that turns up in flight test

Why do so many vortex generators look like afterthoughts? Partly because the boundary layer is hard to predict, and so is the cure. Small wind-tunnel models do not always tell the full-size truth, which is why NACA took its 1950 experiment into the air: the earlier tunnel work had been done at a low Reynolds number.

Der B-47 Stratojet is the textbook case. Wind-tunnel tests warned that Boeing’s six-jet bomber would pitch nose-up at maximum speed as the outboard wing stalled; flight tests confirmed it, and small vanes were added to stop the separation. Almost four decades later, a NASA-funded study at the University of Washington still described the state of the art bluntly.

Test teams find the trouble the low-tech way. They fix tufts to the skin and film them in flight; on NASA’s F-18 High Alpha Research Vehicle, tufts sorted the flow over the vertical tails into attached, unsteady and separated regions. Once the bad patch is found, vortex generators are the natural cure.

They are light, cheap and simple to fit. Kit makers bond them on with adhesive, while bigger strakes are bolted or riveted, and a row can be moved, resized or removed before the next flight. Wainfan’s advice to homebuilders is the same as any test team’s: making them work takes experimentation with size, number and spacing.

The 737’s bounce and other afterthoughts

The Boeing 737 carries a classic in-service fix. In November 1968, vortex generators became an optional retrofit to cure a “vertical bounce” in cruise, caused by airflow separating from the fuselage beneath the horizontal stabilizer. One generator under the stabilizer itself saw higher loads than expected and was removed in 1971, replaced by four on the upper aft body, and the full set went into production that year.

The 737 Classic first left the factory without the aft-body set, until the upper generators returned after line number 2277, from May 1992, to calm elevator and tab vibration. Only the 737 MAX, with a re-contoured tail cone, finally designed them out.

Modifications can create fresh separation, too. In February 2003, certification of blended winglets for the 737-300 was held up after low-speed tests in Arizona found handling deficiencies near V2 at high weights, caused by flow separation where wing met winglet. The fixes under study included outboard vortex generators. Winglets, which tame the big wingtip vortex rather than create small ones, are a story of their own.

Vortilons and nacelle strakes

Vortex generators have bigger cousins. After the prototype BAC One-Eleven was lost in a deep stall during testing in October 1963, killing all seven crew, Douglas wanted its rear-engined, T-tailed DC-9 to pitch firmly nose-down just past the stall. Part of the answer was the vortilon, a flat plate under the leading edge that grew out of a cut-back engine pylon (the name means vortex-generating pylon). Almost idle in cruise, it sheds a strong vortex over the wing near the stall; MD-80s, Boeing 717s and Embraer ERJ 145s use them too.

Nacelle strakes, also called chines, do a similar job for underwing engines. At high angles of attack, a 1983 Boeing patent explains, flow separates from the engine inlet’s upper lip and its turbulent wake spreads over the inboard wing; a fin on the nacelle sheds a vortex that stops it spreading. “The reduced separation results in increased lift at a given angle of attack and a delayed stall.” The 737 Next Generation has one per nacelle, the A320 family two, and humid air can make the vortex visible.

Fences, fighters and trainers

Fast jets use the same trick. Britain’s Gloster Javelin gained vortex generators on its wings with the FAW.4, Singapore’s A-4SU Super Skyhawks carried a row on their drooped leading-edge slats, and the MiG-23MLD, an upgrade aimed at the Flogger’s high angle-of-attack handling, added them to its pitot boom, with saw-toothed wing roots acting as vortex generators too. The RAF Hawk T1 pictured at the top of this page carries two small delta-shaped ones on its upper wing.

Older swept-wing fighters often used a blunter tool. The MiG-17 carried six wing fences, flat plates that physically block air from sliding spanwise towards the tips. A vortex generator mixes rather than blocks, and fences have a price: Douglas’s vortilon patent notes that leading-edge fences “seriously decrease maximum lift coefficients and increase aerodynamic drag at cruising speeds.”

Bolt-on STOL for light aircraft

Light aircraft came late to the party. Micro AeroDynamics of Anacortes, Washington, has made vortex generators since 1989; founder Charles White told AVweb he expected to sell only to twin owners until Alaskan short-field competitors asked for kits for singles. Its Cessna 182 kit bonds 80 vanes to the wings, 40 to the fin and 40 under the tailplane, costs about $1,600, and is claimed to cut stall speed by 8 percent.

Independent reviewers broadly agree. AVweb’s Rick Durden puts realistic stall-speed reductions at four to 10 knots depending on type, plus better low-speed aileron authority, and noted that every aircraft in videos of Alaskan short-field contests wears them. The cost is a little cruise speed: Durden suggests planning on one to three knots, and testers reported 1.5 to 2 knots on a Cessna 182 and a Piper Cherokee 235.

Twins gain most. A row ahead of the rudder hinge lets the rudder deflect further before its flow separates, which can lower the engine-out minimum control speed; Durden measured a Piper Aztec’s Vmc falling from 68 to 61 knots. And because many twins’ single-engine climb requirements are tied to stall speed, a lower stall speed can raise the permitted take-off weight: AVweb cites a Cessna 340A kit that added 300 pounds.

Micro AeroDynamics president Anni Brogan tells Aero-News Network how VG kits work, how they are certified and tested, and why VGs on a twin’s rudder can lower Vmc.

Not only about lift

Some vortex generators are there for your ears. The 737 carries ten tiny ones just ahead of the windshield, which the Boeing 737 Technical Site credits with cutting cockpit noise by 3 dB. Since 2014 Lufthansa has fitted small tabs ahead of the fuel-tank pressure vents under its A320s’ wings, where passing air produced a whistle like someone blowing across the mouth of a bottle.

So the next time a row of little blades catches your eye from a window seat, read it as the record of a negotiation. Somewhere on that surface the air wanted to leave, and someone, in a wind tunnel, a test flight or years into airline service, talked it into staying.

Sources: United Aircraft Corporation Research Department, H. D. Taylor, Report M-15038-1, 16 February 1948 (via NASA NTRS); NACA Research Memorandum L50J02, Lina and Reed, 30 November 1950; NASA image E-960, 1953; NASA image ED07-0027-01, 26 February 2007; NASA CR-182511, Breidenthal and Russell, 1988; NASA TM-101734, Fisher, Del Frate and Zuniga, 1991; KITPLANES, Barnaby Wainfan, 27 July 2012; AVweb, 9 March 2014; Boeing patent US 4,540,143, filed 4 August 1983, granted 10 September 1985; McDonnell Douglas patent US 3,370,810, 27 February 1968; The Boeing 737 Technical Site (b737.org.uk), fuselage and winglets pages, accessed October 2026; Aerospaceweb.org, Nacelle Vortex Generator; Micro AeroDynamics website and Cessna 182 catalog sheet, accessed October 2026; Aero-News Network, 28 March 2014; GreenAir Online, 5 August 2015; Simple Flying; Bureau of Aircraft Accidents Archives (G-ASHG, 22 October 1963); rbogash.com (Boeing B-47); Thunder & Lightnings (Gloster Javelin); IPMS/USA Reviews, 9 October 2012; Wikipedia (Vortex generator, Vortilon, Strake, Wing fence, Boeing B-47 Stratojet, Boeing 737 MAX, McDonnell Douglas DC-9, BAC One-Eleven, Gloster Javelin, Mikoyan-Gurevich MiG-23, Mikoyan-Gurevich MiG-17, ST Aerospace A-4SU Super Skyhawk, United Aircraft), accessed October 2026