From this post, I've learned that the leading edge flap works by increasing the camber of the airfoil. But how exactly does it work? Since it is a flap, I can guess it works by changing the chord line, just like the trailing edge flap does. But how exactly does the chord line change? Does the leading edge move down to below the original leading edge when the leading edge flap is extended?
1$\begingroup$ Your graph shows an exceptionally poor representation of a typical LE device. This answer contains a realistic view of several different types. $\endgroup$– Peter KämpfJul 15, 2017 at 11:11
Leading edge flaps and droops do increase the camber because when they move, the leading edge actually droops, and with it the leading edge extremity of the chord line, while the mean camber line remains in place for most of the airfoil length:
The airfoil is dramatically bent down to the point the bottom side is now concave and the top side has a larger convexity.
Effect on the lift coefficient:
For the same angle of attack α, CL is decreased, but it is now possible to reach larger α and with them larger CL. It also means lower speeds are required to generate the same amount of lift.
LE flaps are usually found at the outboard portion of the wing.
More: Aircraft Design, a conceptual approach, Daniel P. Raymer (around page 278 - view it)
$\begingroup$ Ideally, the zero-lift angle of attack does not change, regardless of LE device angle. Your text and graph suggest otherwise. $\endgroup$ Jul 15, 2017 at 11:06
$\begingroup$ @PeterKämpf: So Raymer (page 278) is inaccurate? $\endgroup$– minsJul 15, 2017 at 11:13
$\begingroup$ No, but you picked the wrong graph and confused the labels. The top right picture on page 278 is relevant here. Otherways he would be in direct contradiction to his Fig. 12.4 on page 264. $\endgroup$ Jul 15, 2017 at 11:27
$\begingroup$ Ah, I see, you use the center right picture. That effect is only possible if you redefine the airfoil chord (as you do in your upper picture) but that is confusing and very unusual. You do not redefine the chord line when TE flaps are extended, so you should not for LE devices, either. After all, the wing is bolted to the fuselage the same way, regardless of LE or TE deflection. $\endgroup$ Jul 15, 2017 at 11:52
3$\begingroup$ What you show in your first picture isn't a leading-edge flap, but, rather, a leading-edge droop. $\endgroup$– VikkiMar 2, 2019 at 4:23