Does flap extension increase or decrease the stall AOA

[JammedStab]

According to Fred George....

"At low altitude, a high-performance business jet wing may not stall until local AoA reaches 16 deg. to 18 deg., or more, depending upon high lift configuration. When extended, trailing edge flaps increase lift per degree of angle of attack, but stall actually occurs at a lower maximum AoA because of airflow separation. Leading edge slats, in contrast, enable the wing to achieve a high stalling AoA because they delay airflow separation."

Understanding The Angle-Of-Attack Indicator | Business Aviation content from Aviation Week (http://aviationweek.com/business-aviation/understanding-angle-attack-indicator)

Interesting. And Fred is smart.

[Mad (Flt) Scientist]

The part I would take exception to is more "When extended, trailing edge flaps increase lift per degree of angle of attack" than the following part you italicized. For a purely hinged flap, i wouldn't expect a significant change in CL-alpha. (For a translating-type flap, the increased effective wing area would have such an effect, that may be the context).

I believe it's true in general that flap deflection does tend to reduce the stall AOA, as the more heavily loaded wing tends to trip whatever the stall mechanism is, earlier

[JammedStab]

Thanks, Do you work for Bombardier?
The article also has this interesting info...Any comments?

"Aerodynamically, the minimum drag point occurs at the highest lift-to-drag ratio. Before the advent of modern super-critical airfoils, jet aircraft typically would cruise at or below the critical Mach number, the indicated speed at which local flow over some part of the aircraft, usually the upper surfaces of the wings, reached the speed of sound.

With subsonic local flow over the wings, flying at a constant, optimum angle of attack would yield the best lift-to-drag performance throughout a wide range of cruising altitudes. If, for instance, a first-generation Falcon Jet or Hawker were flown at an optimum angle of attack, it would be possible to eke out more miles per pound of jet fuel than if it were flown at a constant indicated Mach number. (See sidebar.)

In contrast, virtually all current-generation civil jets have supercritical airfoils or semi-supercritical airfoils. This means that the airfoil is designed to operate efficiently with local airflow greater than Mach 1 over a large portion of the wing chord. The wing actually produces more overall lift and lower drag than it would with subsonic flow.

Supercritical airfoils still are most efficient at the best lift-to-drag ratio, as are subsonic airfoils. But the optimum cruise speed is a function of both Mach number and AoA. Flying the aircraft at the optimum Mach number for fuel efficiency thus causes AoA to decrease as aircraft weight decreases. Flying at a constant angle of attack would cost range. Maximum range performance also depends on the thrust output and specific fuel consumption of the engines.

Boeing engineers point out that adjusting cruise speed for winds aloft is more critical to maximum specific range than flying constant AoA. Pilots are advised to fly faster into a headwind and slower with a tailwind to squeeze out optimum range, as shown on the specific range predictions programmed into a full-function FMS."