Costs for design changes in late stages of a project are very high and can significantly delay the project of a new aircraft. The applicability of sub-scale flight testing for initial assessment of flight dynamics of new aircraft projects, before a full-scale prototype is available in early stages of development is assessed in this work. Sub-scale flight test models shall be similar to the full-scale design in mass properties and maneuverability aspects, the effects of Mach, Reynolds and Froude number similarities and discrepancies need to be understood in order to determine the correct method for sub-scaling the aircraft being designed. A numerical assessment of sub-scale similitude requirements and effects on flight dynamics is performed for a proposed fighter aircraft for high and low Mach number flight phases. The better suitability of Froude number scaling in place of Mach number scaling for sub-scale flight dynamics similarity is verified, best similarity is found for lower Mach number flight phases in which the predominant aerodynamic aspects are potential, such as takeoff and landing, which allows the use of sub-scale flight testing for initial assessment of these critical phases.