A conventional fighter airplane was equipped with a special device for varying the effective dihedral in flight. A survey of pilot's opinions was made to determine which values of effective dihedral were intolerable. The relation between the findings and the present Air Force-Navy stability and control specifications is discussed.

The aerodynamic characteristics of 19 isolated tail surfaces have been determined by wind-tunnel tests and tests have also been made of rectangular airfoils of various aspect ratios with and without double end plates of various shapes. Thes data from these tests have been collected and analyzed.

An investigation has been made in the Langley full-scale tunnel to determine the thickness and shape of profile of the boundary layer on the fuselage of a typical monoplane fighter airplane with a single liquid-cooled engine. The results showed that, for the range of angles of attack and fuselage stations investigated, the maximum displacement thickness was nearly 1.2 inches and was at the most rearward station (81.6 percent of the fuselage length). The displacement thickness was found to be greatly affected by the pressure gradients over the windshield-canopy combination and in the wing-fuselage juncture. An average value for the shape parameter between 1.3 and 1.4 was obtained for the turbulent boundary layer.

It may be concluded that the stalling of vertical tail surfaces is not in itself a dangerous condition. Provided sufficient directional stability exists at large angles of sideslip, the tail stall may occur with modern airplanes, as with those of the past, without the knowledge of or concern to the pilot.