We investigate the friction force of the oscillating liquid plug in a self-oscillating fluidic heat engine (SOFHE) in the laminar regime using numerical simulations, compared to analytical relations and experimental measurements. We study the effect of varying the length to diameter ratio (LID) and compare the results to the analytical solutions for an infinite length liquid plug, in order to clarify the impact of velocity field near the menisci (end effects). The results showed that the analytical solution for an infinite liquid plug can be used for L/D>10 with acceptable accuracy, despite the velocity not being fully developed. Modifications are required to account for end effects for L/D<; 10.