Every body immersed in a flow is subject (besides hydrostatic lift) to a flow-induced force, which depends mainly on the velocity of flow, the size of the body and the shape of the body. The shape of the body is represented by the dimensionless number, the drag coefficient c_w.

The goal of scientific study and practical application (e.g. in vehicle construction) is to design the perfect body shape in order to keep drag low. The drag coefficient for arbitrarily shaped bodies can only be determined reliably by experimentation.

The HM 225.04 experimental unit – used in the aerodynamics trainer HM 225 – allows drag to be measured in various models so as to determine the respective drag coefficients. In the measuring section, a model (plate, cylinder and aerofoil model) is used as a drag body. The forces occurring in the air flow are measured with a beam scale with movable weight. When conducting the experiment with a cylinder, a Pitot tube can be used to record a pressure distribution of the surrounding flow.

Also, the drag can be measured indirectly via the pulse rate. The Pitot tube, movable obliquely to the direction of flow, allows pressures to be recorded so as to determine the velocity profile downstream of the cylinder and thus to gauge the so-called wake depression.

The experimental unit is attached to the HM 225 trainer, simply and precisely with quick release fasteners.