Figure 10. Tail control effectiveness for biplane posture for tail angles of -15 (down triangle), 0 (square), and +15 (up triangle).

At low angle of attack, tail up produces a nose up moment relative to zero tail angle, while tail down produces a nose down moment relative to zero tail angle. Tail movement is effective in trimming, by moving the point where the curve crosses . The small effect on lift suggests the tail is primarily effective because of moments generated by its long length.