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I was recently reading this very helpful string What is the physical meaning of circulation found in Kutta condition? and it explained very nicely potential flow around an airfoil. My question then is this: what happens to circulation around a cambered airfoil at a small, negative angle of attack which is producing lift, but is also producing upwash. How do we reconcile Kutta-Жуковский theorem with this observation?

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  • $\begingroup$ For lift to exist, a net downwash has to be present too, because action and reaction are opposite and equal... $\endgroup$
    – xxavier
    Jul 24 at 12:26
  • $\begingroup$ Well okay sure. But Lift is equal to ρvΓl where gamma is circulation, rho is density, l is length (of chord) and v is of course velocity. Impulse accounts for only half this and is caused by the turning of the flow (which is caused by adding the circulation into a streamline mathematical model). So yes. But also. As per A. Bowers et al (in ‘On Wings of the Minimum induced drag: Spanload implications for aircraft and birds’) such airfoils do exist in nature and do produce lift if very little. $\endgroup$
    – AeroDude12
    Jul 24 at 20:59
  • $\begingroup$ You can't have lift without downwash. It's a physical impossibility... $\endgroup$
    – xxavier
    Jul 25 at 8:54
  • $\begingroup$ There is always upwash and downwash. The only impact of a negative attack angle is that the circulation will be opposite and the lift will go downwards instead of upwards. $\endgroup$ Jul 25 at 13:06
  • $\begingroup$ If circulation is opposite it means г is negative and lift is not produced. Lift IS produced no matter what is said by xxavier. I agree with them we have no idea how but I am certain that Al Bowers is correct and further experiments by amateurs confirm his experimental, and analytical evidence. There has to be positive circulation or a new nuance to the theorem of lift. $\endgroup$
    – AeroDude12
    Jul 26 at 9:46

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