From Anderson, for inviscid flows,

Full Potential Equation

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\left [ \left ( \frac{\partial \phi}{\partial x} \right ) ^2 - a^2 \right] \frac{\partial ^2 \phi}{\partial x^2} + \left [ \left ( \frac{\partial \phi}{\partial y} \right ) ^2 - a^2 \right] \frac{\partial ^2 \phi}{\partial y^2} + \left [ \left ( \frac{\partial \phi}{\partial z} \right ) ^2 - a^2 \right] \frac{\partial ^2 \phi}{\partial z^2} + 2\frac{\partial \phi}{\partial x}\frac{\partial \phi}{\partial y} \frac{\partial ^2 \phi}{\partial x \partial y} + 2\frac{\partial \phi}{\partial x}\frac{\partial \phi}{\partial z} \frac{\partial ^2 \phi}{\partial x \partial z} + 2\frac{\partial \phi}{\partial y}\frac{\partial \phi}{\partial z} \frac{\partial ^2 \phi}{\partial y \partial z}=0