![]() ![]() Soft stall criteria will limit the variation in \(C_L\) and \(C_D\) around \(\alpha_M\). If/when airfoils stall, we don’t want huge loads or even huge gradients in the loads. This is where criteria 2 and 4 come into play. On the flip side, too much stall margin is often associated with excess max \(C_L\) and extreme loads.ĭespite having a stall margin, the airfoil will likely end up in stall conditions (hopefully rarely). If \(\alpha_M\) is too close to \(\alpha_0\), then small changes in the environment can cause airfoils to stall, resulting in poor performance and undesirable loads.įor this reason, a sizable stall margin is attractive. One of the criteria above was about the stall margin, which is \(\alpha_M-\alpha_O\). The value of \(\alpha\) which will achieve \(L\) is important and denoted by \(\alpha_O\) here.Īnother important angle of attack is the one which gives the maximum \(C_L\), denoted \(\alpha_M\) here. Typically, a desired lift force \(L\) is known apriori, based on more preliminary rotor and system design.įor wind turbines, a simple BEM analysis may be done beforehand to determine \(L\). This is sometimes used as a surrogate for acoustics and/or for roughness sensitivity-a wind turbine airfoil should perform well even after its shape is contaminated by insects, dirt, erosion, etc….
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