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Summary: The effects of negative dihedral on lateral stability and control characteristics at high lift coefficients have been determined by flight tests of a model in the Langley free-flight tunnel. The geometric dihedral angle of the model wing was varied from 0° to -20° and the vertical-tail area, from 0 to 35 percent of the wing area. The model was flown with various combinations of dihedral angle and vertical-tail area at lift coefficients of 1.0, 1.4, and 1.8. As the effective dihedral was decreased from 0° to -15°, the model became increasingly difficult to fly. With an effective dihedral of -15° the flying characteristics were considered to be dangerous because, when there was only a slight lag in the application of corrective control following a disturbance, the unstable moments resulting from spiral instability became sufficiently large to overpower the moments of the controls so that return to straight flight was impossible. Inasmuch as full-scale airplanes because of their greater size will diverge at a slower rate than free-flight models, the amount of negative effective dihedral that would constitute a dangerous condition is expected to be greater for full-scale airplanes.
The effects of large variations of lateral area on the lateral stability and control characteristics of a free-flying model when ailerons are used as the principal control have been determined by flight tests in the Langley free-flight tunnel. The effects of the lateral-force parameter (rate of change of lateral-force coefficient with angle of sideslip) were investigated for a wide range of values of the directional-stability parameter (rate of change of yawing-moment coefficient with angle of sideslip) and the rotary-damping-in-yaw parameter (rate of change of yawing-moment coefficient with yawing angular velocity).