Mounting the Model
Given a part or complete airframe with the desired outer mold surface, the first problem faced in tunnel testing is mounting the model in the tunnel. Most tunnels use three-point mounting systems: a pair of main mounts that are side by side facing the airflow, and a smaller movable mount on the tunnel centerline either in front of or behind the main mounts, see again Figure 11.1. The movable mount can be used to change the AoA during testing. Often, the whole mounting system can be rotated about a vertical axis as well. Unless one is using dedicated tunnel test parts, some suitably strong locating points must be identified on the model for attachment. For whole-aircraft tests, it is normal to use the undercarriage mounting for this purpose, either through the wheel axle holes or where the undercarriage legs meet the main airframe. If the aircraft includes a catapult launch bar, this can also be used as a strong point for mounting. For tests on wings alone we use dedicated mounting rigs, which also incorporate a flat plate to provide the inboard boundary condition for the flow. We avoid using the tunnel wall for this because the tunnel boundary layer will be very different to that induced by the fuselage. The size of the boundary plate should be several times the root chord of the airfoil and also mounted away from the tunnel wall. In this case, we mount the wings vertically in the tunnel and change the AoA by rotating the mounting, see Figure 16.1.
Having attached the model to the tunnel mounts, great care must be taken to establish an appropriate datum AoA. For whole airframes, we tend to use the main fuselage as the datum since in the cruise condition this is ideally horizontal to minimize drag; for wings we adjust the datum so that the chord line used to define the root section is in line with the tunnel flow direction. Clearly, if tunnel results are to be compared to calculations, it is important that common AoAs are used.
Figure 16.1 Decode-1 and channel wings on wind tunnel mounting rig. Note the circular boundary plate that stands in for the absent fuselage.