Wind tunnel models do not have complete similarity to the full scale airplane. Part of the dissimilarity is due to the difference between the stationary model in the artificial wind stream of the tunnel and the moving airplane in still air. There are a number of other reasons for departing from exact geometrical similitude. For reasons of accuracy and economy, all minor parts of the full scale airplane, such as struts, wires, fittings, control horns and other parts whose scale corrections are large are removed from wind tunnel models.

Note presenting an investigation to determine the precision of wing sections of wood fabric construction used on a number of airplanes. All of the wing sections were found to deviate more or less from their respective prototypes. The aerodynamic effects resulting from consideration of thickness variation are then estimated from existing empirical information.