Decreasing lift force with increasing angle of attack

    • Akismet
      I'm looking at a plot between the lift coefficient and the angle of attach (AoA) and I see that the lift coefficient increases with AoA. I understand that this happens because the net upward force is higher at higher AoAs. However, as we increase the AoA further, the wing experiences a sharp decreases in the lift force. Why does this happen? Any help here is appreciated!
    • prajput
      Ansys Employee

      Hello @bsista

      What you are seeing is typically called wing stall in aerodynamics. Stall is a condition of lift drop caused by flow separation at a wing. As the AoA increases, the flow starts separating from the top side of the wing. The flow initially separates near the trailing edge, then the separation front moves upstream as the angle of attack increases. Past the critical angle of attack, which is what you are seeing in the plot where the sharp decrease begins, the pressure distribution on the top surface of the wing starts increasing due to the large flow separation. The lift generated decreases meaning that the wing has started stalling. During stall, a small increase of the angle of attack produces a significant drop of lift generated by the wing.

      I hope this answers your question.

      Thank you.

      P.S. : Check out this free course by Ansys that talks about Lift in detail!

    • Ashish Khemka
      Ansys Employee

      Hello @bsista

      Just to add to @prajput 's answer:

      In flight control, there are two types of stall - tip and deep stall.

      Tip Stall: Wing tip stall occurs when the tip of the wing starts stalling before the rest of the wing. This is typically associate with tapered and swept wings. Tip stall for swept wings produces a pressure imbalance over the wing that leads to a pitch up moment that makes more difficult to recover from the stall.

      Wing tip stall can be generated also during rolling maneuvers, where the relative wind due to rolling increases the relative angle of attack of one of the wing tips leading it to stall. Once the tip stalls the pressure imbalance over the wing causes the aircraft to have quicker rolling motion. Wing tip stall can affect the aircraft maneuverability if the stalled region extends to the ailerons.

      Deep Stall: The deep stall is the most dangerous condition. The deep stall is a flight condition typically associated with T-tail aircraft. For certain angles of attack the wing and the tail are aligned in such way that the turbulent separated flow generated by the stalling wing covers the tail of the aircraft. This makes the elevators ineffective so that the airplane cannot recover from the stall.

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