Conventional aerodynamic and thermal engineering models treat energy transferand force generation as largely separate processes, assuming that thermodynamic en-ergy exchange influences mechanical forces only indirectly through pressure, viscosity,and entropy considerations. While this approach is effective in near-equilibrium con-ditions, it obscures the fundamentally causal relationship between energy transportand momentum exchange and leads to persistent confusion in systems involving strongthermal gradients, active surface heating or cooling, and microstructured interfaces.In this work, we present a unified and quantitative framework based on CausalLorentzian Theory (CLT), in which all mechanical forces arise from local, causal mo-mentum flux associated with energy transfer. Within this framework, no modificationof the Navier–Stokes equations, Maxwell’s equations, or classical conservation laws isrequired. Apparent “missing forces” are shown to arise from incomplete accounting ofthe local stress tensor under non-equilibrium boundary conditions.We develop the theory from first principles, apply it to representative aerodynamicconditions, introduce scale-dependent criteria based on dimensionless parameters, andprovide explicit numerical estimates relevant to engineering design. The results clarifywhen thermal effects can meaningfully reduce drag or produce thrust-like contributionsand when such effects are negligible. The framework offers engineers a principled toolfor evaluating unconventional drag-reduction and propulsion concepts while preservingstrict causality and conservation laws.