Critical aingle of attack

kh15

New Member
Does it increase when you add flaps. Reason I saw this is because, when you increase the flaps, you change the chordline and increase the angle of attack (if level flight is maintained). Now to stall the aircraft at what is supposed to be a slower airspeed, wouldn't this be the only time the critical angle of attack changes--thus rises?? Any thoughts?
 

tgrayson

New Member
Does it increase when you add flaps. Reason I saw this is because, when you increase the flaps, you change the chordline and increase the angle of attack (if level flight is maintained). Now to stall the aircraft at what is supposed to be a slower airspeed, wouldn't this be the only time the critical angle of attack changes--thus rises?? Any thoughts?
That all depends on how you measure the AoA. As traditionally measured, adding flaps does not change the AoA, because the chordline is normally measured with respect to the orginal, "unaugmented" airfoil. The reason is that it'd be confusing to convey information regarding flap performance if you are making two changes to an airfoil...changing its AoA and amount of camber.

So if you add flaps, in order to avoid ballooning, you will have to decrease your AoA to fly at the same airspeed.

To get back to your orginal question:
Does it increase when you add flaps.
The answer is "no", it actually decreases when you measure the AoA in the traditional way.
 

Sidious

Well-Known Member
It actually will stall at a lower angle of attack. The main goal of flaps is to allow the lift coefficient to increase beyond its maximum in the unflapped condition. So since is develops a higher lift coefficient we can fly slower before it stalls
 

Maurus

The Great Gazoo
Yup everyone else pretty much covered what happens. The critical AoA does decrease when putting flaps in as the resulting increase in camber is what changes the critical AoA.

Remember that airspeed and critical angle of attack are actually two completely different entities as stall speed is always changing with weight and load factor and the critical AoA remains the same unless the wing is changed. Flaps change the wing by increasing the camber hence the change in the critical angle of attack.
 

CFmike

Well-Known Member
No the critical angle of attack should decrease with flaps.

When you add the flaps, this adds more camber to the wing and in turn you get more lift at a given angle of attack.

It won't increase the critical angle because it would require more airflow/energy to keep airflow laminar over the greater camber. This would mean having to fly a lower angle of attack.
 

Maurus

The Great Gazoo
Yup everyone else pretty much covered what happens. The critical AoA does decrease when putting flaps in as the resulting increase in camber is what changes the critical AoA.

Remember that airspeed and critical angle of attack are actually two completely different entities as stall speed is always changing with weight and load factor and the critical AoA remains the same unless the wing is changed. Flaps change the wing by increasing the camber hence the change in the critical angle of attack.
Blast! I meant to hit edit instead of quote...
 

Minuteman

“Dongola”
Re: Critical angle of attack

However! (;))

Slats and leading-edge flaps do increase the critical AOA. Usually, though, the net result is that the stall AOA (say, as measured by a fuselage-mounted vane ... tgrayson's traditional way) will decrease when combined with trailing-edge flaps.
 

Sidious

Well-Known Member
Something I was thinking about though.... What about split flaps? They seem to alter the overall shape of the airfoil the least and a quick observation shows that that the air wouldn't require any more energy on its journey over the wing. So a wing with these should stall at pretty darn near the same AOA...

I know they cause the least pitching moment because of this and they aren't nearly as efficient at producing a higher CL as, let's say fowler flaps.

I don't have any proof or sources just speculation but what do you guys think?
 

tgrayson

New Member
Something I was thinking about though.... What about split flaps?
This talk of energy is a bit of a red herring. You can't gauge the amount of energy the airflow needs to remain attached (laminar isn't the right word) by looking at the wing. What the airflow is fighting mostly is an adverse pressure gradient, which will be determined by the flow around the entire wing, i.e., the circulation. Flaps strongly affect this circulation and hence the pressure distribution around the entire wing.

Moreover, this idea of a decreased critical AoA with flaps is due to the relatively arbitrary way in AoA is measured. There is another way that's sometimes used, which replaces the chordline with the 'zero lift line', which is the direction the air would have to flow to produce zero lift. With a symmetrical airfoil, that's a conventional zero AoA, but with a cambered airfoil, it would be a negative conventional AoA. With flaps, it's much more negative conventional AoA. This other way to measure AoA is called the "absolute angle of attack". Measured in this way, the Critical AoA gets much larger with flaps deployed, so the phenomenon that needs to be explained goes away.
 
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