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How exactly do leading edge extensions cause more drag?


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We've all been told that leading edge extensions energizes the airflow, creates vorticies, improves high AoA handling characteristics at the expense of increased drag, etc, etc.
Problem is, what exactly is causing the drag? Vorticies breaking up and hitting the airframe? Or maybe the forward motion being transferred into creating the vortex?

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I would assume your guesses are correct, but I would also add to that the larger surface area also creates more induced drag because of the extra lift provided, as well as some frictional drag from the increased surface area.

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LERX create vortices the same way that wingtips create vortices, by having higher pressure below the surface and providing an avenue for the high pressure air to leak around the tip.  The vortex caused by the LERX improves life over the main wing by disturbing the boundary layer and reducing flow separation at high AOA, much like vortex generators you might see on small planes or things like the V-22 Osprey.

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They create drag in the same way that any lifting surface creates drag at high AoA. (vortex creation is also an energy-consuming process and adds a drag factor of its own as well - "induced drag")

But "increased drag" is not entirely appropriate as you have to compare it to a wing with equivalent area, and take into account the different stall characteristics. In forward, low-alpha flight, drag would not be significantly increased.


The purpose of a LERX is to maintain lift in a regime where the entire wing would normally stall, so making comparisons of "added lift" is also problematic.

In essence, you cant just "add" a LERX to a wing, the LERX is part of the wing and the whole wing must be designed in concert with the entire aircraft (the entire body of the aircraft is a lifting surface) to attain optimal performance, so its hard to quantify.


So, at high alpha, a LERX-wing has comparable (give or take) drag over a "normal" wing of comparable area, but lift is dramatically increased (as the normal wing would stall.)



related FunFact: LERXs are an obvious example of vortex generators used to improve lift and handling characteristics, but vortex generators do not have to be very large, or part of the wing.

Take this picture of a MiG29 nose. You see those tiny fillets attached to the base of the pitot probe? Vortex generators, exactly like tiny LERXes. They improve airflow over the fuselage, and ultimately, over the LERXs themselves. Kinda like "Pre-LERXs".



And those small, fixed fins on the side of the fuselage of this Typhoon, near the cockpit - effectively tiny LERXs.




The takeaway here is that airflow all around the aircraft must be taken into account to create a desired/optimal flight characteristics and that no individual part can really be considered in isolation.


Edited by p1t1o
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Also, look at the nacelle of a typical airliner engine, and you'll find one or two VGs attached to the nacelle's upper half. They generate a vortex that largely counteracts the disruption of airflow over the wing during moderate to "high" (for an airliner) alpha flight, such as takeoff/landing, from the nacelle. They first appeared on the DC-10, IIRC. But, during cruise flight, they do essentially nothing, and contribute little to no drag.

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