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Stall speed at different flap settings

Asked by: 3768 views Aerodynamics, Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Private Pilot, Student Pilot

Why does stall speed decrease with increased flap setting?

With flaps extended, it does create more lift and drag at that time but isnt't it going to reach critical angle of attack sooner than clean configuration?

 

I do not get this.

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1 Answers



  1. Brian on Apr 08, 2016

    You are absolutely right, it will stall at a lower AOA where AOA is defined as the angle between the relative wind and the chord line; where chord line is defined (as it usually is) as the line from the leading edge to trailing edge of an unflapped airfoil.

    Have a look at this graphical representation: http://www.ballyshannon.com/air/aoaliftgraph.jpg

    What is different, however, is that Clmax has increased. Stall speed is simply a derivation of the common lift formula rewritten so that velocity is the subject:

    Vstall = Sqrt (Weight / 2 * wing area * air density * Clmax)

    As such when Cl increases stall speed decreases.

    PS flaps don’t increase lift. They increase Clmax. Lift coefficient and lift are not synonymous, despite what the FAA and most laymen written pilot texts might lead you to believe; they are very different variables.

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