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7 Answers

stalling speed

Asked by: 4380 views Aerodynamics

please anyone help me about following quiry which i have in my mind

with trailing edge flaps lowered 

is stalling speed of the aircraft  reduced?if yes please help me understand this aerodynamically....

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7 Answers

  1. hardergm on Apr 28, 2012

    Yes. The stalling speed is lowered with the flaps down. The reason is that the chord of the wing relative to the airflow is increased (increased angle of attack – therefore more lift)


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  2. Wes Beard on Apr 28, 2012

    This question is best answered using the lift formula.  Lift = .5 * p  * CL * V^2 * SA where p is air density, CL is coefficient of lift, V^2 is velocity squared and SA is surface area.
    If we increase a single variable in that equation, total lift will increase.  For example, if the density of the air increases (i.e. we descend to a lower altitude), the wings automatically create lift.  But if we were able to increase the coefficient of lift or surface area of the wings the result would be the same.
    The four different types of flaps: plain, split, slotted and fowler each increases the coefficient of lift and/or  surface area of the wing when deployed.
    Plain flaps change the coefficient of lift by increasing the angle of attack.  As a result, more lift is created when the flaps are deployed
    Split flaps are more designed to increase drag than to increase lift but some lift is attained through a small increase in the angle of attack. 
    Slotted Flaps are just like plain flaps except they allow high pressure air on the bottom of the wing to flow to the top of the wing.  This has the added benefit of increasing the coefficient of lift more than plain flaps.
    Fowler flaps are designed like slotted flaps except they not only allow air from the bottom of the wing to travel to the top of the wing but the flaps extend out from the wing effectively increasing the surface area of the wing.  Fowler flaps have both variables working for them and they increase lift better than the rest of the flaps. 

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  3. Brian on Apr 28, 2012

    Lift is a function of airspeed, lift coefficient, air density, and wing area. Lift coefficient is determined by the design of the wing and is a directly proportional to the wings AOA. Stall, in level flight, occurs when the maximum available lift coefficient is reached.
    Flaps do many things, but of importance for this question is their increasing the maximum available lift coefficient. If we view wing area, lift (=weight recall), and air density as constants then it becomes evident that a higher lift coefficient will allow for a lower airspeed. 

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  4. khoda bakhsh on Apr 29, 2012

    thanx all for your answers but my question is when lift is increased by using flaps how thw stalling speed is reduced? please guide me in detail…
    with regards

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  5. Brian on Apr 29, 2012

    Angle of attack is defined as the angle between the chord line and the relative wind where:
    Chord line is the line drawn from leading to trailing edge of the wing on an unflapped airfoil. and,
    Relative wind flows parallel, but opposite, the aircrafts flight path.
    Flaps increase the virtual angle of attack and, some flaps increase wing area among other things. Flaps benefits/draw backs occur only at the point of locations along the wings span where flaps are located. For instance, flaps cause a negligable effect on lifting qualities of the wing at points in front of the ailerons.
    Wes gave a complete definition list of each type, so you can reference that for the design information. I’ve only one aspect to pick on: Since a wing’s AOA is defined by the chord line of an unflapped airfoil, flaps cause the wing to stall at a lower AOA.
    That said, it is their virtual AOA increase that causes all flaps to provide at least some increase in lift coefficient. Where virtual AOA is defines the chordline as the point from the wings leading edge to the trailing edge of the flap on a flapped/slat airfoil. As Wes’ definitions note, some flaps also increase wing area among others.
    Now a question for you: Do you understand lift formula Wes’ posted? You also understand that lift, in this formula, is a constant number (=to weight) for the level flight stall calculation? Understanding this will let you realize that if either lift coefficient or wing area increase, then airspeed for stall will decrease.
    Let me know if you seek more detail or have questions. 

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  6. khoda bakhsh on Apr 30, 2012

    thanx brian for providing guidance in detail. i look forward for your cooperation in future as well.

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  7. Bob Reser on May 01, 2012

    I hope you haven’t forgotten to keep flying the airplane.  All kinds of airplanes and configurations.  Why don’t you just go up to a safe altitude with your airplane and stall the thing.  Note the power it takes to fly level with different flap configurations.  Practice a go-around or two from minimum approach indicated-airspeed simulated approach to see if you have power enough to stop descent, accelerate and clean up the flaps.
    As a Pilot, your first concern is control.  Theory is for the classroo.  Don’t worry about it.  The engineers did that.  They actually designed and built it to fly.

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