C l max affected by camber of airfoil

[barrichello72]

hi guys
just a quick question that has been bugging me - possibly cos i am overthinking it!
Why is the cl max increasing as the camber of the airfoil is increased? Does it have to do with the greater kinetic energy over the top side of the airfoil as the cross section of the stream tube is reduced in virtue of the principle of continuity, energizing the laminar flow and thus hindering its sepatarion from the airfoil and the onset of the stall?

many thanks

[FCeng84]

barrichello72 - I am trying to understand the scope of your question. Which of the following are you considering:
1. For a given wing, use of leading edge devices such as slats?
2. For a given wing, use of trailing edge devices such as flaps?
3. Both (1) and (2) in combination?
4. A wholesale change to the wing cross section?
For those of us familiar with the range of characteristics for a given airplane's wing we will be much more familiar with Items 1, 2, and 3 from the list above. If your question is focused on Item 4 above the range of possibilities is much greater and thus will be harder to address.

[barrichello72]

Hi
mainly with reference to the trailing edge devices and to the kruger flaps as they are reaponsible for increasing the camber of the wing, amongst other things.
the slats in their extended position- b737ng - do increase the camber as well as they droop down, while in their fully extended position they allow high pressure air from the under side of the wing to flow over the top, energizing the laminar flow and thus decresing the stall speed - if i am correct.

many thanks

Thanks

[FCeng84]

Thanks for the clarification. I'm sure there are others among the PPRUNE community that can provide a more learned response than I, but I will offer a couple observations that may help motivate the conversation:

1. For sub-sonic wings, leading edge slat extension has very little impact on the slope of the Cl-Alpha or the lift generated at a particular AOA below that for stall. The main impact of leading edge slats is that their extension helps the flow stay attached to higher angles-of-attack. This pushes the stall AOA to higher values and similarly allows a given airplane to fly at slower speeds prior to stall.

2. Trailing edge flap extension increases effective wing camber and also causes an effective rotation of the wing in the trailing edge down / leading edge up direction. Both effects provide an increase in Cl at a given AOA. This is important when considering pitch attitude at low speed flight, particularly during approach, flare, and landing. With a clean wing (leading and trailing edge devices retracted) zero lift occurs at/near zero AOA. With trailing edge flaps extended, the zero lift point moves to a negative AOA, in some cases below -5 degrees. This allows generation of significant lift at low speed at pitch attitudes that are appropriate for approach/landing without undue risk of tail strike.

Hope this helps,

FCEng84

[barrichello72]

Hi thanks for your reply
I do understand the reason behind the increase in lift due to the usage of the TE flaps, however i am having a hard time understanding why the CL max would increase - is it because a given AOA will generate a greater lift causing the whole lift curve to shift upwards resulting in all the CL to move up and thus the CLmax as well?

thanks

[Pilot DAR]

The extension of flaps (other than plain or split flaps), and including leading edge flaps will increase both the camber of the airfoil, and the wing area. Increases in both of these will increase CL. The path which air must follow over the top of the wing is lengthened with the increased camber resulting from flap extension. If CL at the approach to stall is being considered, the increased leading edge radius resulting from Kruger flap extension will delay the separation of airflow, similarly to extended slats, or fixed slot. Delaying the separation of airflow will delay the stall (and seem to increase CL), though this characteristic is only really important approaching stall.

[dook]

Increased circulation

[washoutt]

A flap extension with zero flap angle will increase the lift L, not necessarily the lift co-efficient CL. When the extended flap are turned down with a certain angle, only then will the circulation increase, as Dook says. This will lead to a higher CL max.
I am not sure whether extension of slats have no effect on CL max, I would think that with an increase in AOA the circulation will increase at higher AOA, and therefor resulting in a higher CL max.

[dook]

Slats will increase CL max due to an increase in boundary layer energy.

[KenV]

hi guys
just a quick question that has been bugging me - possibly cos i am overthinking it!
Why is the cl max increasing as the camber of the airfoil is increased? Does it have to do with the greater kinetic energy over the top side of the airfoil as the cross section of the stream tube is reduced in virtue of the principle of continuity, energizing the laminar flow and thus hindering its sepatarion from the airfoil and the onset of the stall?

many thanks

The complete answer is complex and depends on a lot of factors.
One SOME aircraft, leading edge devices not only increase the camber of the wing, but also introduce a slot in the leading edge which energizes the air flow over the wing. However, this is not true of all aircraft. The F-16, F-18, Mig-29 and Su-27 families have leading edge devices that are continuously variable, but do not introduce a slot in the leading edge. The leading edge devices on these fighters essentially rotate on hinges along the leading edge rather than translate on tracks embedded in the leading edge. For a complete answer to your question you need to identify the specific aircraft/wing system you are referring to.