Beside analytical approaches, physical modelling represents probably the oldest design tool in hydraulic engineering. For free surface flows, the similitude according to a Froude similarity allows for a correct representation of the dominant forces, namely gravity and inertia. In parallel, fluid constants such as the surface tension and the viscosity might be incorrectly reproduced, affecting the air entrainment and transport capacity of a high-speed model flow. Consequently, small physical models operated under the Froude similitude systematically underestimate the air entrainment rate and air-water interfacial properties. To limit this deficit, minimal values of the Reynolds or Weber number have to be respected. The paper summarises the physical background of these limitations and their combination in terms of the Morton number. Based upon a literature review, these limits are listed and discussed, resulting in a series of more conservative recommendations in terms of air concentration scaling.