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6 Answers

Why doesn’t the AOA affect the stall speed?

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Aerodynamics

Why doesn't the AOA affect the stall speed? 

With the higher AOA, the wing can geneate more lift if the airspeed is the same. That means when the AOA is higher, the wing need lower airspeed to generate the same lift as the AOA is low. Conversely, we need more airspeed to gain the same lift if the AOA is lower. The stall ocour when the lift is smaller than the gravity. When the AOA is higher, I think we need lower airspped to generate the lift which is eaqual the gravity. The stall speed should be lower. But why doesn't the AOA affect the stall speed?

6 Answers



  1. aaron on Mar 01, 2013

    I think you misunderstand the concept of a stall. a stall is NOT dependant on airspeed. A STALL WILL ONLY OCCUR WHEN THE CRITICAL AOA IS EXCEEDED. in a small aircraft (Cessnas, etc.) this angle is about 18 degrees, but varies based on certain things relating to the structural design of the airfoils and the plane itself. the most accurate way to judge a stall would be to have an AOA indicator installed in your aircraft. since these are crazy expensive instruments to install (most larger jets and turbo props have them). since we cannot afford to have such instruments in our light aircraft, we calculate the stall speed based again on airfoil characteristics and something called a lift curve, which plots something cashed the coefficient of lift as a function of AoA. this will tell you exactly what AoA the wings will stall at. here is a good picture of what one of those curves would look like, with the critical angle of attack labeled. http://www.aviation-history.com/theory/lift_files/fig9.jpg

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  2. John D. Collins on Mar 02, 2013

    Aaron,

    The FAA has a stated policy that an AOA system can be installed in a certified aircraft as a minor modification and encourages their installation. I have one installed in my Bonanza and love it. They can be purchased for less than $1000, which is cheap in terms of typical airplane costs. See http://www.alphasystemsaoa.com/index.html and http://www.advanced-flight-systems.com/Products/AOA/aoa.html

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  3. Ben on Mar 02, 2013

    Speed has nothing to do with whether a plane stalls or not. Exceeding AOA will stall a plane. I tested this when doing both power off and power on stalls. There are guidelines based on your plane’s POH for Vs0 and Vs1 that need to be considered as well.

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  4. Namugoni on Mar 11, 2013

    Ben.
    thank you for your answer.
    i just wondered what you said.

    Vso and Vs1 are things that we just make it consider?
    below Vso and Vs1 are no specific speed going to stall?

    what we really need to consider is originally AOA. am i right?

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  5. Mark Kolber on Mar 11, 2013

    INamugoni, I’m not sure I’m understanding your question, so I hope this helps without knowing if it answers what you are asking.

    >>below Vso and Vs1 are no specific speed going to stall?<<

    I'm not sure what airplane you are talking about and the information on stall speeds can vary.

    Looking, for example, at the most recent version of the stall speed chart for the Cessna 172, it gives power-off stall speeds at maximum gross weight for various flap settings and bank angles, as well as whether the CG is forward or rearward.

    Although the chart doesn't discuss AoA, each of those configurations represents a different AoA. An airplane maintaining altitude with flaps up will have a higher AoA at a 60° bank than at a 0° bank and so will stall sooner (at a higher airspeed). That's reflected in the chart.

    The actual stalling speed may be lower than indicated under certain circumstances. For example, in level flight a lower than the max gross weight will result a lower than published stall speed. That's because less weight = less load = lower AoA = lower airspeed before the critical AoA is reached in that configuration.

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  6. Ale on Jun 04, 2013

    Why doesn’t the AOA affect the stall speed?

    You said “With the higher AOA, the wing can generate more lift if the airspeed is the same”. This is true. Likewise, we can say with higher airspeed, the wing can generate more lift if the AoA is the same. So, any increase in AoA and or IAS will causes the lift to increase (i.e aircraft will climb if other factors remain constant).

    You said “when the AOA is higher, the wing need lower airspeed to generate the same lift as the AOA is low.” That is also true and it is just a rephrasing to what you said above.

    You also rephrase the above again and said “we need more airspeed to gain the same lift if the AOA is lower.” I agree too.

    The summary to what you said is that the lift is affected directly by both the AoA and or the airspeed (e.g. if we increases AoA, lift will increase and if we increases IAS, lift will increase. If either AoA or IAS decreased, we need to increase the other one to compensate for the lost of lift due to lose of one factor).

    You said “the stall occurs when the lift is smaller than the gravity”. This is not true. When the left is “smaller” than gravity, aircraft descends NOT stall. The stall is defined is a SUDDEN reduction in lift when the AoA exceeds its critical angle of attack not just because lift < gravity.

    increasing AoA is good for lift until it exceeds a certain value (typically 15 deg between the cord line and the relative wind, NOT pitch attitude) then instead of increasing lift, it suddenly destroys it.

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