Effect of External Stores

External stores have been found to have large effects on spin and recovery, especially for asymmetric loadings in which stores are located asymmetrically along the wing, resulting in a lateral displace­ment of the center of gravity of the configuration. For example, some aircraft may not spin in the direction of the "heavy” wing but will spin fast and flat into the "light” wing. In most cases, model tests in which the shapes of the external stores were replaced with equivalent weight ballast indicated that the effects of asymmetric loadings were primarily due to a mass effect, with little or no aerodynamic effect detected. However, very large stores such as fuel tanks were found, on occasion, to have unexpected effects because of aerodynamic char­acteristics of the component. During the aircraft development phase, spin characteristics of high-performance military aircraft must be assessed for all loadings proposed, including symmetric and asymmet­ric configurations. Spin tunnel tests can therefore be extensive for some aircraft, especially those with variable-sweep wing capabilities. Testing

of the General Dynamics F-111, for example, required several months of test time to determine spin and recovery characteristics for all poten­tial conditions of wing-sweep angles, center-of-gravity positions, and symmetric and asymmetric store loadings.[513]