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698jet 01-30-2011 10:26 AM

AOA question
 
would a wing with flaps extended stall at a lower or a higher angle of attack than the same wing with flaps retracted. and if both wings were at the same angle of attack which would produce more lift. take a crack at this question I like to see if I'm getting the same answer as I think my son and his friend have differnt answer then me so I said lets see what other people say.

FlyerJosh 01-30-2011 10:33 AM

Based on what I've seen in the past, IIRC, the same wing will stall at a lower AOA. I'll have to find a reference later.

mmaviator 01-30-2011 11:08 AM

Falcon 7X Fly-By-Wire Control System

there is talk about AOA with clean and dirty configuration with just a small hint of stalling in there.

"The Falcon 7X’s low-speed envelope protection eased down the nose at 14-deg. angle of attack, or about 1.5 deg. less than maximum lift coefficient in the clean configuration. The system allows up to a 17-deg. angle of attack with slats and flaps extended, but this too is less than the stalling angle of attack."

golick 01-31-2011 05:38 AM

stalling angle of attack with flaps is at a higher coefficient of lift but at a lower angle of attack than clean. Refer to figure 5 in High Lift Systems: Introduction because the increase in the coefficient with flaps comes from an increase in camber which moves the whole curve to the left (so that the zero-lift angle of attack is more negative).

snippercr 01-31-2011 06:07 AM

Alright, let me take a crack at this one. Remember, Angle of Attack is the angle between the relative wind and the chord line of a wing (thinking light, GA aircraft wings here). Chord line is defined as leading edge to trailing edge. When we lower flaps, we have changed our trailing edge position and increased the angle of attack. Therefore we are generating more lift due to a higher angle of attack.

I would say that the wing will stall at the same angle of attack, although our relative pitch attitude will be lower than that of a normal stall.

As a really bad example: Let's say our pitch attitude and chord line were the same. So if we pitched up 10 degrees, so would our chord line and our angle of attack would be also 10 degrees. Let's also say that we find our aircraft stalls at 18 degrees. Let's also say that lowering our flaps increases our chord line by 5 degrees. So at level flight (0 pitch up), our angle of attack is now 5 degrees. Given the wing previously stalled at 18 degrees pitch up, I would say we should stall at 13 degrees pitch up (with flaps, the wing is still at 18 degrees). Thus, with flaps, we stall at a lower pitch attitude but the same angle of attack.

This assumes no power, constant weights, slotted or plain flaps (IE no change in surface area).

So that's my take on it. Could easily be way off the money.

AKASHA 01-31-2011 06:42 AM

Well I'm certainly no expert on aerodynamics... I'll take a stab too...

Quote:

Originally Posted by 698jet (Post 938803)
would a wing with flaps extended stall at a lower or a higher angle of attack than the same wing with flaps retracted.

It has been my understanding that a wing will stall when it exceeds the critical angle of attack and that the critical angle of attack is constant.

Quote:

Originally Posted by 698jet (Post 938803)
and if both wings were at the same angle of attack which would produce more lift.

The wings are at the same angle of attack thus producing the same lift, however the plane with flaps extended is producing the lift at a lower airspeed.

Cubdriver 01-31-2011 07:40 AM

Quote:

Originally Posted by 698jet (Post 938803)
Would a wing with flaps extended stall at a lower or a higher angle of attack than the same wing with flaps retracted ?...

The lift versus AoA curve goes up and to the left. So, stalling AoA goes down by a few degrees with flaps. The additional lift we get is far stronger than a lowered critical AoA, which is why we use flaps.

Denker on flaps

Someone said AoA tends to remain constant when flaps are deployed: not correct. If this were true airplanes would be crashing practically every time someone deployed the flaps at too slow a speed. Many small airplanes employ the last ten to thirty degrees of flaps on short final. Typical trainers balloon a little from the extra lift when the flaps go out, then they quickly go nose down without any help from the pilot to reach a new equilibrium between lift and weight. Obviously, the weight of the airplane stays the same. The wing wants to go to a lower AoA which also helps to maintain a safe stall margin. The airplane also tends to trace a lower glide path because of extra drag which helps in short field performance and obstacle clearance.

Quote:

...and if both wings were at the same angle of attack which would produce more lift ?...
Again, the curve goes left and up. You decide- draw it on paper if it helps.

AKASHA 01-31-2011 07:52 AM

Quote:

Originally Posted by Cubdriver (Post 939291)
Someone said AoA tends to remain constant when flaps are deployed.

No, they didn't.

Cubdriver 01-31-2011 08:07 AM

SnipperCR:

...Angle of Attack is the angle between the relative wind and the chord line of a wing... Chord line is defined as leading edge to trailing edge. When we lower flaps, we have changed our trailing edge position and increase the angle of attack. Therefore we are generating more lift due to a higher angle of attack...

This statement is ok as far as it goes, but it would have you thinking a wing keeps the same or even higher AoA after the flaps go out which is not the case. The airplane immediately noses over in an attempt to maintain equilibrium between weight and lift. Also it is not the higher AoA that creates the extra lift, it is the greater camber of the flapped wing that creates the extra lift. AoA actually goes down after a fresh equilibrium is established.

Fly Boy Knight 01-31-2011 10:32 AM

According to Aerodynamics for Naval Aviators,

Chapt. 1 - Basic Aerodynamics - High Lift Devices

"One factor common to the maximum lift condition is the angle of attack and pressure distribution. The maximum lift coefficient of a particular wing configuration is obtained at one angle of attack and one pressure distribution. This fact is sufficient justification for the use of angle of attack indicators and stall warning devices which sense pressure distribution on the wing."

In addition to the above passage, there are two figures in this section that show the CL max of a wing clean and the CL max of a wing with flaps extended being attained at a slightly different angle of attack. The numbers in their example graph and figure are...

Clean: CLmax = 1.5 AOA for CLmax = 20 degrees
Flaps: CLmax = 2.0 AOA for CLmax = 18.5 degrees

With these examples in mind, although it is not specifically addressed, I believe that in addition to lowering stall speed and increasing CL for a given AOA, lowering the flaps will also slightly reduce the critical angle of attack of an airfoil.

In the text above, the author refers to an airfoil "in a particular configuration" stalling at the same AOA and P distribution. The author does not however, specifically address the effect of changing said configuration on CRITICAL AOA.

With all of this in mind, this little reduction in critical AOA is NOT the primary reason why an airplane stalls at a much lower PITCH atttitude with flaps extended versus retracted. The effect of Lift and drag that flaps induce is the primary reason for that.


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