Impact of flaps on angle of attack

[RobertE]

Actually, I,m not sure if I should express this as AOA or as pitch angle. something tells me AOA is expressed relative to the chord of the wing and the chord changes with flap deployment. if that's true then I really mean pitch.

if clean and in level flight I stall at, say, a 15 degree pitch angle, with flaps deployed would the stall come at a greater or lesser angle? and, if so, why? I'm having trouble imagining why it should differ if stalls are produced by airflow completely separating from the upper surface. After all, the flaps come into play quite late in the game, past the point at which you've still got laminar flow no matter what the flap position is.

Thanks.

[FAST FLIGHT OPTIONS LLC]

If you remember back to your PVT pilot written test you may recall that an aircraft's wing always stalls at the same critical angle of attack

[RobertE]

That's, sort of, the question. If AOA is measured by angle relative to the chord and if the chord changes with flaps, does that mean the pitch angle that will produce a stall in level flight is reduced when flaps are deployed? That seems odd but, as you can see, I'm a bit confused on the whole topic.

[FAST FLIGHT OPTIONS LLC]

Flaps change the camber of the wing which will increase lift (and drag) thus lowering the speed at which the plane will stall vice the angle

[jetdriven]

Wouldn't flaps increase AOA when lowered? The trailing edge of the wing moves with the flap, and is lower when they are down.  You can also fly a 40 degree flap 172 at full flaps level flight with -5 degrees pitch.

[Dave Marten]

Here is a little refresh on Flap effects wrt AoA and lift.

 

http://www.nordian.net/pdf/easa_principles_of_flight_demo.pdf

 

"Flaps produce the required increase in lift at a lower geometrical angle of attack of the main aerofoil than with a clean wing.
However, the effective chord line and the effective camber changes with flaps selection. Flap deflection results in increased lift and drag at a given angle of attack and increases the maximum CL. The flaps of the trailing edge increases CL max and produce the same amount of lift at a lower geometrical angle of attack than with a clean wing. The lower stall AoA is here measured to the chord line of the main aerofoil. The stall AoA measured from a chord line from the leading edge of the main aerofoil to the trailing edge of the flaps can be higher."

 

BL: if you are out practicing power off stalls in your favorite GA airplane (level flight) you'll notice that your pitch angle (in level flight equal to your geometric AoA) at stall is greater in the no-flap config than with flaps extended. You can also see this effect in that a fully flared off no-flap landing will result in a higher pitch angle at touchdown.

 

Another fun characteristic of flap extension that we Mooney pilots live with is the nose-down pitching moment!

[xftrplt]

We should be careful not to conflate pitch and AOA, as they are often unrelated.

Flaps increase the Cl (Coefficient of Lift) for a given AOA.

A given airfoil may stall at a slightly lower AOA with flaps (but not slats) extended. But is producing more lift at that AOA than when clean.

RE, you need to get a good text and study. Even online there is plenty of information.

Edit: DM answered the question very nicely, while I was pecking away.

[RobertE]

Thank you, Dave Marten. That article you attached answered the question perfectly.