scope:

Description of Driving Event:

On January 15, 1991, a failure occurred in the 8' Transonic pressure tunnel during testing of a model wing with attached flap. During the test run, the flap peeled away from the wing, broke free, and proceeded down the tunnel. Extensive damage resulted.

It is hypothesized that the east wall flap fitting pin was damaged when the flap was allowed to bend during installation. These pins are stainless steel and approximately 0.38" square. They are inserted into square holes in the tunnel side plates. When the model was installed, the flap was lifted to complete the attachment, was left unsupported during the attachment process, and deflected in the center. This deflection could have caused the square flap fitting pin to become overstressed. The Metallic Materials Branch did, in fact, identify an area of tensile fracture related to a single load incident, but was unable to conclusively establish when the fracture occurred. In any case, engineering should strive to use the lowest stress concentration configuration feasible. In this instance, a round pin on the flap end fitting would have reduced the stress concentrations from the level present on the square pins and perhaps prevented test failure.