Winglet's

The vortices are a bi-product of generating lift. Angle of attack is the angle of the chord line of the wing with respect to the air flow it moves through. To get lift, the wing must be at a positive angle to the wind flow but this also increases drag via air hitting the underside of the and being deflected downward.

To obtain lift at a stable altitude a parcel of air who's kinetic energy constantly cancels out the force of gravity pulling the jet down, must be deflected downwards behind the wings to counter-balance the weight of the aircraft.

Trim modulates AOA to manage lift, via changing the amount of deflection of air under the wings.

Takeoff and stall are both defined by either enough deflection or not enough deflection, of a cushion of air which the jet rides over (or tries to and fails in the case of a stall). The downward deflection air movement is called a 'down-wash' and it's what triggers the 'wake-turbulence'.

The downwash forms a long narrow ramp of sinking air behind the jet. The greater the angle of attack and speed the greater the downwash energy, and resulting twin-opposed vortices rotational energy. The descending ramp of air is why the vortices sink to a lower level behind the jet.

See the first image on this page:

A helicopter makes it simple to visualize. In the case of a helicopter the downward movement of the air creates its lift, which is easy to observe. The rotor is a wing and the cyclic control alters the wing's angle-of-attack with respect to the airstream. The more angle of attack the pilot asks for, the more engine power is needed to prevent a stall, and thus the more lift and downwash is generated. So downwash strength is always directly related to lift needed to counteract the aircraft weight to remain buoyant and hover. A toroidal (donut-shape) parcel of air comes in from the top of the rotors and exits the bottom of the rotors. The surrounding air at the tips is curling over into the low-pressure zone that's created by the blade's AOA constantly deflecting air downward under and behind the rotor-wing.

So air at the tip curls in to replace air deflected downwards and dust during a landing reveals the donut of curling over air. In creates an intense 'tornado' rotation just outside the rotor blade's outer tips. The strength and size of the air's rotation is proportional to engine power and AOA required to counter-balance aircraft weight and lift desired. So helicopters are doing exactly the same thing a jet does to produce deflected downwash and a tip vortex as a bi-product of generating its lift.

So how does it create the wing tip vortex curl for a fixed wing aircraft?

You and a friend get a heavy 'plank' of plate-steel, say 2 ft wide by 10 ft long, and you hold it horizontal at the surface of a swimming pool, then you allow just one end of it to sink down and observe what the water does. It curls in from both sides as it goes down. If bubbles were present you would see the water curl into the void left from the water being pushed downwards. We have a descending ramp of contra rotating water vortices forming above the descending plate's upper surface.

So wing-tip vortices are a dynamic reaction of the air either side of the descending ramp, curling in to fill the low pressure created by the deflection downward behind the wing.

The wing tip is simply where the curl over is the most intense and sharply defined. It is the 'edge' of the descending ramp. The air being a fluid has inertia, so once set in rotational motion it does not want to stop rotating. So it keeps rotating at a rate defined by the deflection energy imparted to the ramp of air, and its rate of descent.

For as long as it descends more air must curl in, immediately either side, and this maintains the twin contra-rotating tornado of wake turbulence.

A380 demonstration of tip curl-over into intense twin visible vortices:



With no wingtip extension vortex deflection device present the air curls over onto the outside rear wing skin before the wing has passed by this initial curling air. This impact on the wing skin increases drag because the curling air imparts kinetic energy to the wing skin as it crashes down on it. This does not actually add weight to the aircraft, but it has the same aerodynamic outcome as adding more weight to the aircraft. The curling air is pushing downward, and the small downwards energy is transferred to the air frame.

How do you counteract apparent increased weight?

If you didn't counteract it you would sink. So you increase angle of attack slightly which slightly increases the ramp air energy you deflect downward to make the extra lift needed to maintain a stable altitude.

So you have thus slightly increased drag and increased the required fuel burn and reduced the range.

A wing tip device is designed to eliminate the air crashing on to the top of the wing via causing the curl to occur later and fall behind the rear edge of the wings and their ailerons. So you reduce the drag that way and use less AOA and throttle to displace less air downward, and thus you get more fuel to burn, and a lower burn rate.

Longer range at most speeds is what results, and a lower AOA means higher speeds with less drag occurring. Some wing tips also add to span slightly, which also requires less AOA than without.


Don't ask why the new 737 scimitar tips are shaped as they are though, I still haven't fully figured it out. Boeing says the new tips are significantly more efficient than earlier designs and I suspect the lower scimitar tip is meant to reduce the curl drag and buffet on the upper tip's surface.

I suspect you're referring to vortex generators which create small eddies of rough air in the boundary layer of the wing skin at low approach speeds, to lower the stall speed, via keeping the air attached to the wing skin for longer as speed deceases toward stall speed. These consequently allow shorter takeoff and landings and improve low-speed handling and are present on many aircraft as low speed performance upgrade kits.

Vortex Generators

Wing Fences

Regarding the 737 scimitar wing-tips I found a very informative site while looking around which explains it's intent (in detail) as a wing span extension which alters the lift vector distribution across the wing to more effectively reduce AOA induced drag penalties en-route. Several other cumulative performance advantages are discussed as well.

Scimitar Tips