We investigate the existence of extremal solutions for a class of fractional differential equations in the area of fluid dynamics. By establishing a new comparison theorem and applying the classical monotone iterative approach, we establish sufficient conditions to ensure the existence of the extremal solutions and construct twin convergent monotone explicit iterative schemes. Generalized nonlinear nonlocal Bagley-Torvik equation and generalized Basset equation with nonlinear source functions are some special cases of our discussed problem.