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beinghuman
1st Nov 2011, 06:12
Hello Guys,

One question. I've read and heard everywhere that an aerofoil stalls at a constant angle, irrespective of speed. I came across a statement saying '' As the flap is lowered, Cl max increases but the stalling angle is reduced ''

How and why does the stalling angle change in this scenario ?

rogerg
1st Nov 2011, 06:51
As the flap is lowered the airfoil changes effective shape, usualy having a greater chord and thickness ratio, and stall characteristics.

bfisk
1st Nov 2011, 07:05
A very important appendix to your statement, which is often overlooked in initial training, is that a wing will stall at a constant angle for a given configuration.

Extending the trailing edge flaps normally increases the CL for a given AoA but reduces the critical AoA. CLmax increases. Ie, you are shifting the CL/AoA curve up and to the left.

Extending leading edge flaps or slats normally shifts the graph up and to the right - ie little or no change in CL for a given AoA, but critical angle of attack is increased with a corresponding increase in CLmax.

henra
1st Nov 2011, 09:48
Hello Guys,

One question. I've read and heard everywhere that an aerofoil stalls at a constant angle, irrespective of speed. I came across a statement saying '' As the flap is lowered, Cl max increases but the stalling angle is reduced ''

How and why does the stalling angle change in this scenario ?

From a pure aerodynaimcal point of view the stalling angle of the effective new chord does not decrease. Rather the contrary (due to more camber).
However in an aircraft the AoA is measured with regard to the main plane and not with regard to the new effective chord (which is altered due to the rear part of the chord being moved downward). Aerodynamic AoA 0° with flaps down would provide negative AoA readout on the instruments.

The effective increase in max AoA of the more cambered profile is less than the effective tilt of the chord on the airframe when deploying the flaps, thus resulting in a reduced max AoA wrd to the airframe.

Microburst2002
1st Nov 2011, 17:01
a clean wing and the same wing with flaps extended are different wings.

In order to compare them you have to choose a common reference. With respect to that reference, stall angle is decreased.

The truth is that in the airplane you will stall at a lower pitch with flaps down.

FCeng84
1st Nov 2011, 17:21
Stall occurs as a result of separation of the airflow over the upper surface of the wing. On most large commercial transport aircraft, extending of flaps involves not only lowering the wing's trailing edge, but also opening slots to allow airflow through the wing. This in combination with extended and/or gapped leading edges works to maintain attached airflow over the upper surface of the wing to a much higher AOA than with a clean wing configuration. A typical commercial transport wing may stall at less than 10 degrees AOA in the Flaps Up configuration while stall with Landing Flaps can be well beyond 20 degrees.

jhurditch
1st Nov 2011, 22:05
beinghuman,

hopefully this clears some things up...

You are correct in saying that the stalling angle is decreased by lowering flaps and cl max is increased aswell by the lowering of flaps. The fact that flaps being lowered decreases the stalling angle however seems contradictory.

A highly cambered wing will stall at a higher stalling angle than an aerofoil with little camber. The reason for this however is due to the smooth and gradual change of shape of a cambered wing, something like on a STOL aircraft.

When lowering flaps however, whilst the camber is increased, the change of shape occurs abruptly at the rear of the wing due to the flap slab. That is to say, the mean camber line is increased only at the rear of the wing.

The change of pressure gradient with flap extended is abrupt rather than smooth and laminar for a clean wing. For this reason, flow seperation occurs at a lower angle of attack than with a clean wing.

The camber has increased, but it has increased in an inefficient way, at a cost to the stalling angle when flaps are extended.

Hope this clears things up.

Microburst2002
2nd Nov 2011, 06:36
the classical graph of the CL curve versus AoA of the clean wing and the flapped wing would come in handy.

can someone show it here?

henra
2nd Nov 2011, 19:58
When lowering flaps however, whilst the camber is increased, the change of shape occurs abruptly at the rear of the wing due to the flap slab. That is to say, the mean camber line is increased only at the rear of the wing.



The change of pressure gradient with flap extended is abrupt rather than smooth and laminar for a clean wing. For this reason, flow seperation occurs at a lower angle of attack than with a clean wing.


If you cosider the AoA as Angle between the air stream and the line from leading to trailing edge (i.e. the TE of the flap) that is not exactly true.
It is true if you consider the Angle between the air stream and the main plane of the wing (i.e. the wing without the flap).

Btw. the slotting rather postpones flow separation.

Let's have an example in order to make it clearer.
Let's assume the flap is 20% of the chord length.
If you now deflect the flaps 20° down this tilts the angle of the overall chord (including flap) by 4°, i.e. if the pitch of the aircraft remains constant, the AoA is increased by 4°.
Or inversely in order to keep aerodynaimc AoA constant you would have to pitch 4° Nose down. Your AoA instrument would then indicate an AoA decreased by 4° although aerodynamically in reality it is still the same as initially.

The more cambered profile (including the flap) might have an AoAmax increased by 1,5°. This would mean that the AoA displayed at the stall would decrease by 2,5° as the probe keeps the aircraft as reference plane and not the (changing) chord of the wing.

Edit:
As you normally refer the AoA to the main plane (it would be absolutely impractical to do otherwise and for no benefit) the wing will stall earlier as the air will be accelerated stronger as without flaps deflected thereby increasing the lower pressure on the upper side, making it more susceptible for separation.