ESDU 18006 provides a method of predicting the drag due to a leak from an external stream into a plenum located within an air vehicle. A leak into an air vehicle affects drag in two main ways. First, the momentum flux in the leak streamtube upstream of the leak orifice contributes directly to the drag. Second, the effect of the leak streamtube geometry on the external flow is expected to influence drag.

The drag due to the leak inflow is the component of force due to the flow in the leak streamtube measured in the freestream direction. Its value is dependent on the leak mass flow rate and the boundary-layer thickness in the vicinity of the leak. The drag component due to the effect of the leak streamtube geometry on the external flow is dependent on the geometry and orientation of the leak orifice as well as the leak mass flow rate.

The method requires knowledge of the boundary-layer thickness in the vicinity of the leak and the leak orifice area together with values of the maximum orifice dimensions in both the streamwise direction and perpendicular to it. Leak mass flow ratio is also required and is obtained using ESDU 18005.

The method is derived using test data for flows into a plenum from an external stream through various rectangular and circular orifice geometries in thin plates. The method is applicable to rectangular, square, elliptical and circular orifice geometries. In addition to these geometries the method should also be applicable to other similar geometries. Sketches comparing predicted data with test data show that predictions lie within ±0.1 for leak drag coefficient.