Colgan 3407 NTSB Animation

[Lilah]

Can't believe no one corrected your post till now. Please, go read an aerodynamics textbook before commenting on anything technical.

Retracting flaps REDUCES the angle of attack, and extending flaps INCREASES AoA. The Angle of Attack (AoA) is the angle between the relative wind and the chord line of the wing. The chord line is the line between the leading edge and trailing edge. The trailing edge changes when the flaps are extended or retracted. When flaps are extended, the trailing edge lowers relative to the existing chord line, the chord line steepens relative to the relative wind, and the AoA increases.

Do you want to get technical? I don't think it's as simple as you make it out to be. Every time you change configuration, you also get a whole new airfoil, complete with a new critcal AOA. The camber is effectively increased with flap deployment in most cases.

I suspect lowering the flaps at a given weight, in level flight, at a given speed will increase your AOA, but also given you a greater margin to your critical AOA. This obviously comes at a price, greater drag, especially with final flaps in most planes.

Consider a regional aircraft in it's landing configuration, that gets too slow. If you retract the flaps, it WILL lower the AOA, but you probably won't have enough lift from the resulting configuration to maintain level flight. Is reducing the AOA by retracting flaps always the best course of action?

Fly Safe,

Lilah


Edit: Crossky, I don't really think you OR embpic1 were advocating retracting the flaps as a first step of stall recovery, I just think you were a little harsh on his comment. To me, he clearly implied that retracting the flaps would bring you closer (or exceed) your critical AOA if you were trying to maintain level flight.

 

[Crossky]

Do you want to get technical? I don't think it's as simple as you make it out to be. Every time you change configuration, you also get a whole new airfoil, complete with a new critcal AOA. The camber is effectively increased with flap deployment in most cases.

I suspect lowering the flaps at a given weight, in level flight, at a given speed will increase your AOA, but also given you a greater margin to your critical AOA. This obviously comes at a price, greater drag, especially with final flaps in most planes.

Consider a regional aircraft in it's landing configuration, that gets too slow. If you retract the flaps, it WILL lower the AOA, but you probably won't have enough lift from the resulting configuration to maintain level flight. Is reducing the AOA by retracting flaps always the best course of action?

Fly Safe,

Lilah


Edit: Crossky, I don't really think you OR embpic1 were advocating retracting the flaps as a first step of stall recovery, I just think you were a little harsh on his comment. To me, he clearly implied that retracting the flaps would bring you closer (or exceed) your critical AOA if you were trying to maintain level flight.

It might or might not, that depends on the airspeed and load factor/G, I suppose. If a wing is already below the stall speed for a lower flap setting than what the flaps are at, that's a nail waiting to be driven in the coffin. And I think that's what happened here. However, with the same relative wind and body angle, the AoA always decreases with a flap retraction.

I don't want to get too technical, I'm just for pure understanding and application of aerodynamics, that's all.

In this accident, it's clear that the shaker went off artificially, because of the automatic 20 kt increase in Vref speeds when wing anti-icing is on.

Sure, the critical AoA changes with a flap position change, but I don't know how much or which way. My point was the AoA immdiately increases with flap extension and immediately decreases with flap retraction. In response to a flap change, a pitch and power change will have to occur to change the AoA back toward where it was before.

The shaker activated at about 130 kts, the A/C pitched up, slowed much further, banked steeply, and flaps were retracted. All these things made the situation worse and brought the plane much closer to a wing stall. The worst thing about retracting flaps at too low a speed is that stall speed increases as flap extention decreases, of course.

I think we're speaking the same language, just vastly different dialects. Sorry if I was harsh to embpic1.