In experimental fluid mechanics, a sting is a test fixture on which models are mounted for testing, e.g. in a wind tunnel. A sting is usually a long shaft attaching to the downstream end of the model so that it does not much disturb the flow over the model. The rear end of a sting usually has a conical fairing blending into the model support structure. For minimum aerodynamic interference a sting should be as long as possible and have as small a diameter as possible, within the structural safety limits. Critical length of a sting is mostly dependent on Reynolds number. If the flow at the rear end of a model is laminar, the critical sting length can be as much as 12-15 base diameters. If the flow at model base is turbulent, critical sting length reduces to 3-5 model base diameters. Source also suggests a sting diameter of no more than about 30% of model base diameter. However, this may not be possible in wind tunnels with high dynamic pressures because large aerodynamic loads would cause unacceptably large deflections and/or stresses in the sting. Shorter stings of larger relative diameters must be used in such cases. A good rule-of-thumb is that, for acceptably low and test-conditions-independent aerodynamic interference in a high-Reynolds-number, high-dynamic-pressure wind tunnel, a sting should have a diameter "d" not larger than 30% to 50% of model base diameter "D" and should have a length "L" of at least three model base diameters, e.g. as specified for the AGARD-C calibration model), :Image:AGARD-C model on a sting fixture.png|see figure. If the test object is to be placed at high angles of attack relative to the airstream, a bent sting can be used, :Image:AGARD-C model on a bent sting.png|see figure. Bent stings usually produce higher aerodynamic interference than straight stings. If the test object has a "boattail" rear end without a well-defined base through which a sting shaft can enter the model, a so-called Z-sting can be used, having a form reminiscent of the Latin letter "Z". The part of the sting entering the model is a thin aerodynamically shaped blade so as to minimize disturbance of the flow; :Image:Wind tunnel model on a Z-sting.png|see figure. Stings often attach, at the front end, to internal wind tunnel balances to measure the forces on the model. Therefore, most stings have a central bore through which the cables from a balance or other in-model instrumentation can be conducted without exposure to the airflow. When a model is mounted on a wind tunnel balanceattached to a sting, care must be taken that no parts of the model touch the sting during a wind tunnel test; the only support of the model must be through the balance.
