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I know what vortex generators (VGs) do. My question is: How come some aircraft have VGs on both sides of the engine, whereas other aircraft have them on the inside only?

For example, in this picture of Boeing 737, a vortex generator is only on the inside of the engine:

Boeing 737

On the contrary, the Airbus A319 has them on both sides:

A319

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  • $\begingroup$ What is The this kind of VG name i mean this VG have special name . Anybody know??? $\endgroup$ – Arian Feb 19 '18 at 21:12
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The vortex generator helps only at high angle of attack by energizing the airflow between nacelle and wing. During cruise it will create additional drag without much benefit, especially if the nacelle-wing-combination has been designed well.

However, in the landing configuration it can help to lower minimum airspeed by several knots and make shorter landings possible. Now the aircraft maker has to decide what is worth more: Shorter field length, or slightly less cruise drag.

The Boeing 737 started life with a much more slender engine and had a very short landing gear when more fuel-efficient engines came to market. A lot of brain- and computing-power was needed to develop its pear-shaped engine nacelles, and the vortex generator became an essential element of it. At the inside location it helps to keep the flow path between wing, fuselage and nacelle clear of separated air, whereas on the outside a second generator would make only a minor difference.

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    $\begingroup$ So why does the Airbus and the C19 have them on both sides then? The C19 even has them on the outboard engines.. $\endgroup$ – ROIMaison Jan 14 '15 at 18:47
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    $\begingroup$ @ROIMaison: I guess you mean the C-17. I am wondering myself - this is a confession of failure on the part of the aerodynamicists. After all, both aircraft were designed from the start with their respective engines. See my comment below this answer. $\endgroup$ – Peter Kämpf Jan 14 '15 at 19:59
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    $\begingroup$ If it only adds drag during cruise would it be economical to make it retractable like other lift-enhancing devices? That is if there's room inside the nacelle for hydraulics. $\endgroup$ – TomMcW Aug 19 '15 at 17:38
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    $\begingroup$ @TomMcW: Your "if" is justified - there is little room for a retraction mechanism. This mechanism would need to be redundant to ensure proper operation in case of a single failure - after all, with asymmetric vortex generators the aircraft will behave in funny ways during landing. $\endgroup$ – Peter Kämpf Aug 19 '15 at 18:31
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To complement previous answer, vanes on nacelles create vortex on high angle-of-attack, which then goes over the wing. This effectively acts as an boundary layer fence reducing span-wise flow over the wing thus improving lift and lowering stall speed at high AoA (is. low speed). The energized flow also improves down-wash in the area of the wing where there is no slat due to engine mounting.

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