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4 Answers

Negative AOA when descending and upside down flight.

Asked by: 3908 views Aerodynamics, Commercial Pilot, Flight Instructor, General Aviation, Private Pilot, Student Pilot

In which situation do we say the wing is at negative AOA?

Secondly, when aircraft is abruptly brought down into descent, is the wing momentarily flying at negative AOA?

 

Steve.

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4 Answers



  1. Eric Hawicz on Apr 03, 2016

    A negative AOA would be if the relative wind is coming from *above* the chord line of your wing. Although I haven’t actually tried to calculate it, I suspect that you would need to be *very* abrupt with your pitch change to end up with a negative AOA.
    Assuming you’re in level flight, you’d need to pitch down fast enough to get the leading edge of your wing below horizontal before the downward acceleration of your plane due to loss of lift counteracts that and causes the relative wind to tend to be more and more from below.
    If you do manage it, I imagine it would be quite brief, unless you are actually upside down (or flipping tail over nose 🙂 ).

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  2. Eric Hawicz on Apr 04, 2016

    Actually, scratch the last answer, I forgot that we fly airplanes with airfoils, not flat boards for wings. Even at 0 AOA, the wing will still generate lift, so if you go “fast enough” you can fly with a negative AOA in level flight.
    Although I can’t seem to find an actual number for coefficient of lift at 0 AOA for a Cessna 172, eyeballing a value of 0.4 from the graphs at http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml, then plugging in that, 1.00649 kg/m^3 (pressure at 2000 ft), and 16.2 sq ft wing, to
    http://www.ajdesigner.com/phpwinglift/wing_lift_equation_force.php#ajscroll
    gives enough force to hold a 172’s max gross weight at around 113 knots.
    (no guarantees about the accuracy of that)

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  3. Brian on Apr 05, 2016

    As with most things related to flight dynamics it all depends on how your definition.

    One way to look at AOA is how Eric described and later adjusted in his second post realizing the effect of camber. If we base AOA specifically on it’s most accepted (basic pilot) definition than many aircraft with highly cambered wings have the potential to fly negative angles of attack in cruise flight. At the very least these aircraft can produce positive lift even at moderately negative AOA values.

    The other way AOA is often defined is relative to what is termed the zero lift angle of attack. If your angles are based on this AOA than any angle that creates positive lift would be defined as positive and any AOA that created negative lift would be defined as negative. This later case is easier for pilots as anytime g-load is posative, AOA is posative — negative then AOA is negative.

    This idea is similar to asking the question: What temperature is freezing? It depends, are we talking Kelvin, Celsius, or Fahrenheit? Temperature, just like AOA can have many scales of measure. The scale you wish to use will determine the method in which you analyse the problem.

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  4. Brian on Apr 05, 2016

    Eric I think you mean 16.2 sq m, not feet.

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