Calibrating the Test
Before any readings can be used from tunnel tests, it is vital that a careful and accurate calibration process is carried out. This needs to cover the following:
- • The balances or force cells that record lift, drag, and moment (normally the tunnel support team will know these constants or they will be applied automatically and readings be supplied in known units);
- • The relationship between tunnel mount vertical motion and changes in AoA (which will need measuring with an accurate inclinometer and will vary from model to model);
- • The barometric conditions in the tunnel (which are highly temperature- and weather- dependent. Most tunnels have high-quality instruments in place to record these data. It is also not uncommon for these readings to change during a single session of testing, so such readings should be recorded throughout the test program);
- • The wind speed in the tunnel (usually a Pitot-static tube is mounted on the center line of the tunnel working section ahead of the test area);
- • The forces induced on the tunnel mountings in the absence of the test specimen (under all likely operating speeds, a series of runs should be made before mounting the test specimen, but including as much of the mounting system as possible);
- • The gravitational forces acting on the mounts due to the presence of the test specimen with the wind tunnel stationary (at all likely orientations, a series of readings should be taken before turning on the tunnel fans).
Given suitable calibration, it is then possible to interpret the data being generated when the tunnel is in operation. Note, however, that the raw data from the tunnel sensors must be converted to lift and drag coefficients by suitable manipulation, which is usually best accomplished via an appropriately set up spreadsheet. It is always wise to keep all the raw data captured in case subsequent reanalysis is required. We also always have a series of estimated values at hand when capturing data so that any anomalous values coming from the tunnel instruments are immediately apparent. It is very frustrating if some error has been made in calibration or setup and this is not revealed until after the test runs have been completed and the model removed from the tunnel.