Effect of leading-edge flap on lift curve

[issaccph]

Hi,

The graph below is captured from a aerodynamics book, and I got a question on the curve of L/E flaps.

http://i.imgur.com/I95VpDT.jpg

From what I have learnt, leading edge flaps (Droop or Kruger flaps) are operated through changing the effective curvature. When L/E flaps are extended, the leading edge is rounded therefore the airflow can be encouraged to remain attached. Additionally, the camber of the wings are changed, and thus the same value of CL can be achieved by a lower angle of attack than clean configuration, like trailing-edge flaps.

However on the graph above, the curve of L/E flaps (Also the "Slats" curve but it seems correct for slats) does share the same amount of CL with a given AoA with the "Clean" curve unless it has a higher critical angle.

In my opinion, the "L/E flaps" curve is partially correct, it is true for having a relatively higher critical angle, but the curve should be somewhere ahead the "Clean" curve, in order to match the camber characteristic I mentioned above. Am I correct?

Thanks,
Issac

[andy148]

Hi Issac,

I'm only learning so please take what i say with a pinch of salt...

Slots and slats re-energise the airflow travelling over the wing. As air travels over the wing, the air can be called laminar (not sure but i think it extends 1mm form the surface of the wing) where each air molecule follows the one in front. Until it starts to separate (separation point) from the wing and starts to becomes turbulent. As the AoA increases, the separation point starts to move forward along the wing. Eventually the laminar air flow gets smaller and the area of turbulence gets bigger. Slots and slats, help to prevent this happening by keeping the air flow, flowing over the wing and preventing it from going turbulent.
About stalling, as the AoA increases. The Centre of Pressure (CoP) moves forward towards the leading edge. The CoP is where the lift component acts, think (thrust, drag, lift,weight forces), when the CoP gets close to the leading edge the wing, the wing will be reaching it's critical AoA (usually around 16deg's for training aircraft). The CoP moves sharply rearwards and the wing is stalled, lift is no longer greater than weight, and the nose drops and you lose height.
Im not sure if your book goes in to coefficiency of lift and drag?
The formula for CL is lift=CL x ½ x air density x velocity squared x surface area of the wing. You might be getting confused with camber and surface area? As the only two things pilots can do to affect lift and drag, is too change he surface area of the wing and it's velocity.

I hope this helps in someway?

Andy

[Bluescan]

Hi Issac,
I agree with your conclusion.

The main effect of the LE flap is to increase critical AoA and CLmax by delaying flow separation. In addition, some additional lift is created at lower angles due to the increased camber.

A slat, however, only re-energise the boundary layer and therefore does not help at low AoA.

Edit:
Found this picture at theairlinepilots.com
http://www.theairlinepilots.com/forumarchive/principlesofflight/flapcurve.jpg

[Volume]

Changes in camber are much more efficient close to the trailing adge, this is why the Cl does almost not increase with the deflection of a leading edge device.
You may also understand the deflection of a 10% LE flap as an deflection of a 90% TE flap and a rotation of the airfoil nose down (lower AoA) to bring the TE back to its position, in that case the effect of the 90% flap and the change in AoA cancel each other out more or less.

Despite the slats (slotted leading edge flaps) energizing the boundary layer by the high speed airflow through the slot and thus delaying separation, also drooping down the leading edge to the stagnation point reduces the pressure peak near the LE and the pressure increase gradient on the upper surface, also delaying separation. (you pressure distribution with droop nose down is less "peaky" and more smooth).
If the stagnation point moves below the physical leading edge at high AoA the airflow around the nose moves opposite to the direction of the general airflow producing very strong pressure peaks.

[issaccph]

Thank you for all the efforts!

[Baikonour]

Whereas drooping LE slats increase camber which should increase lift, this is partially (in many cases wholly) offset by the fact that the effective angle of attack of the wing is also reduced by the drooping leading edge. Hence, the main effect of the drooping slats is to delay the stall both by by delaying the separation point through through energising the airflow over the wing as described above and by actually having reduced the effective angle of attack of the wing, combined with an increase of its cord.

Plain trailing edge flaps, on the other hand, do nothing for the stall characteristics (excepting Fowler and similar) but merely increase the CL for a given AoA.

Hence, the graph shown in your book, where LE slats extend the CL/AoA curve upwards, whereas TE flaps shift it to the left without significantly affecting the critical AoA.

B.