Half models are used for objects with a symmetrical structure and are especially suitable for focusing on particular aspects of the object, for instance the interaction between an engine and the wing of an aircraft, or the acoustic conditions near a wing.
Half models enable a more detailed model representation than full models because larger scales can be used. Moreover, testing can be performed at higher Reynolds numbers. These conditions are favorable for wing design studies aimed at investigating Reynolds number effects or the influence of design changes, but also for studies on power plant installation effects of control surface effectiveness.
Because the models used for the tests are mounted to a wall of the test section of a tunnel, aerodynamic data is influenced by the proximity of tunnel walls and the related boundary layer effects. As a consequence, the absolute levels of the measurements performed with the half model technique are unreliable, and the tests need to be performed on a relative basis by means of comparison of the results due to model changes with a reference configuration. Despite this need for additional comparison with a reference situation, half models are frequently used because of the more detailed model representation.
Force measurements for half models are always performed using external balances.