  The normal-mode analysis of the Reynolds-Orr energy equation governing the
stability of viscous motion for general three-dimensional disturbances has been
revisited. The energy equation has been solved as an unconstrained minimization
problem for the Couette-Poiseuille flow. The minimum Reynolds number for every
Couette-Poiseuille velocity profile has been computed and compared with those
available in the literature. For fully three-dimensional disturbances, it is
shown that the minimum Reynolds number is in general smaller than the
corresponding two-dimensional counterpart for all the Couette-Poiseuille
profiles except plane Couette flow.
