reduced angle of attack when flaps lowered

[subsidence]

Hello,

I don`t understand why the stalling angle of attack is reduced, when the flaps are lowered.

Can anybody help me,
thank you in advance!!

[palgia]

subsidence,

I'm not sure if ALL flaps reduce the stalling AOA. But they all reduce the stall speed (increase in Cl, allows reduction in speed to produce the same lift)
The thing to remember is that when you lower the flaps, you are changing the wing. You are therefore flying a different wing with different stalling characteristics. I do not know what are the characteristics of an airfoil that determine the stalling AOA.
One way to look at it is that by lowering the flaps, you are increasing the downwards angle towards which you are deflecting, requiring a creater turn in the airstream (this is what gives you the extra lift.) By increasing the downwards deflection, you will hit flow separation sooner. Rather simplistic explanation. Maybe it might even be wrong.
I welcome any corrections to what i wrote above.

palgia

PS. maybe i'm wrong, but i was under the impression that lowering flaps could both increase or decrease the stalling AOA. (depending on the type of flaps, and varies from wing to wing)
Please correct me if i'm wrong.

[Taylor G]

Flaps don´t REALLY decrease the max AOA - they reduce the angle the pilot can play with.
Just remeber the basic definition of an AOA; the angle between the relative wind and the chord line (chord line being a STRAIGHT line connecting the leading- and trailing edge of the wing). So basically the AOA increases immediately when you put the flaps down because a new chord line is born i.e. one from the leading edge of the wing to the trailing edge of the FLAP that now is lower than when in was buried in the wing. So you already have increased the angle "between the relative wind and the chord line" somewhat by lowering the flaps, so it follows theat you, as a pilot, are left with a smaller angle to play with (less upward pitch) before you hit the max AOA.

Taylor

[Oktas8]

Most flaps do change the maximum angle of attack actually Taylor G - either increasing or decreasing depending on the type of flap. Refer AC Kermode's Mechanics of Flight or other principles of flight textbook.

I think Palgia has it right.

O8

[the wizard of auz]

maybe I'm reading this all wrong, but I thought the stalling or critical AoA remained the same. the stalling speed decreases with the application of flap and the actual AoA increased, but it will still stall at the critical AoA.......around 16deg.

[BEagle]

The Cl versus alpha curve of an aerofoil is dependent upon many factors. One of these is camber. When any flap is extended, the camber increases; this modifies the lift curve slope; effectively moving the Cl max point 'up and to the left' on the overall Cl versus alpha graph. The aerofoil will then stall at a lower value of alpha with flap extended than when clean, but at a higher Cl max value. As Cl max is higher with flap extended, at the same lift value (L=W) at the stall, Cl 1/2 rho v**2 x S, it follows that v will be lower than for a clean aerofoil.

This should be explained to PPL students prior to Stalling 2..........

[palgia]

Beagle.

I believe I'v seen Cl alpha curves where the addition of flaps resulted in a higher stalling AOA and higher CL. In this case, the flaps "elongate" the normal curve causing the maximum point to move up and right.
Again, I might be wrong. (its been a while since I took perf at uni)

palgia

[John Farley]

palgia

You have accurately described the effect on the lift curve of leading edge devices (either flaps or slats)

BEages' description is correct re trailing edge flaps

JF

[bookworm]

Figure 9 (http://adg.stanford.edu/aa241/highlift/highliftintro.html) illustrates this quite well.

The zero of AOA is somewhat arbitrary, but convention dictates that it is the chord line of the aerofoil with flaps retracted. If you recalculated the AOA based on the chord line with the new aerofoil shape with flap extended, you'd change the zero reference.

[Genghis the Engineer]

I suspect that it may just be because AoA gauges are pretty much universally mounted on the fuselage. Selection of the flaps moves the chord line nose-down, and so a (for example) 13° AoA at the wing, will read more nose-down (lower AoA) on a fuselage mounted gauge than it did with flaps up.

G

[Cuban_8]

As a former Aerodynamicist, I think it is important to re-iterate the points that Taylor G and Bookworm have made.

The Cl/AoA debate is an issue of definition. By convention in aerodynamics, the chord line of an aerofoil is defined for it's clean configuration with nothing "hanging out". As we begin to deploy trailing edge high lift devices, we are indeed modifying the characteristics of the aerofoil - the most significant change being the introduction of a greater camber. Commensurate with this, the aerofoil's true chord is also modified - but by convention, the reference chord remains that for the clean aerofoil. Thus, on a characteristic Cl/AoA plot, it appears that the aerofoil stalls at a lower AoA, but with a greater Clmax. In reality, the AoA at which the aerofoil stalls is usually greater than for a clean aerofoil. A secondary effect of this is very beneficial to use as pilots - the pitch angle at which we a achieve a given AoA is reduced.

The same principle is true for Fowler slotted flaps and similar devices. The effect of such devices on a Cl/AoA plot often look too good to be true compared to other trailing edge devices, in that they offer a relatively large increase in Clmax. However, you have to consider that Fowler slotted flaps are also significantly increasing the aerofoil wetted area, but this fact is not reflected in the calculations - the aerofoil reference area remains that for the clean aerofoil. The increased area is hence absorbed into the increased Clmax the devices offer, slightly misleading!

Of course, day to day, the theory offered by BEagle is all we really need to know. However, when your really trying to understand what is going on, such simplifications can be misleading.

Rgds

Cuban_8

[BEagle]

I admit that my dCl/d alpha explanation referred to deflexion of simple TE flaps as I did not want to overcomplicate the answer.

The effect of LE devices and other lift augmentation devices is a vastly more complex subject.

It was a KISS answer to a KISS question! I fail to see how it can be misleading.

[Cuban_8]

BEagle - In hindsight, I realise my response may have appeared slightly derogatory, which was not my intention at all. I merely meant to suggest that the theory you volunteered, when proposed to a ppl student, would not allow you to come to the conclusion I was suggesting. Not that it really matters. I would volunteer your exact same explanation every time. Apologies ;)

Interestingly, LE devices are indeed more complex, but also have a far more profound effect upon the lifting characteristics of aerofoils. It is quite revealing to look at the procedures one has to follow subsequent to Flap/Slat jams on large jets. In my experience (perhaps you could chip in with your wealth of knowledge BEagle), large jets can generally land at slower speeds when flapless than they can slatless - slats significantly increase the AoA an aerofoil can achieve before stalling. Incidentally, when landing with flap jams, the pitch angle on approach tends to be very high leading to a probably, if not definite, tailscrape!

Rgds,

Cuban_8

[404 Titan]

Cuban_8
In my experience, large jets can generally land at slower speeds when flapless than they can slatless - slats significantly increase the AoA an aerofoil can achieve before stalling.
Not always so. On the A330 Vapp with Slats 0 & Flaps Full = Vref + 25. With Slats Full & Flaps 0 = Vref + 30.

[Sqwak7700]

TE flaps increase the camber of the wing which increases the CL MAX, but the greater curvature means the airflow separates at a lower AoA because it simply cannot make the sharper turn (as BEagle pointed out, the CL curve moves up and slightly to the left). This is true for TE devices that only increase camber (Spilt and Plain Flaps). More efficient flaps that also increase boundary layer energy (Slotted and Fowler Flaps) raise the CL curve even further and help to maintain the airflow to the wing at even higher CLs. Fowlers being the most efficient flap types also increase wing area.

The effects of LE devices depends on the type of device. A device that increases boundary layer energy (Slots and Slats) extends the CL curve, allowing a greater CL Max and an increased critical AoA. If the LE device increases camber only (Kreuger Flaps), then it does not extend the CL curve, but does move it up and to the left (having the same effect that TE flaps have). Slats increase the BLE and also the camber therefore extending the CL curve and raising it as well.

Next time I pull out my aerodynamics book, remind me to wear a dust mask :O