Q5 Dutch Roll.
Lateral stability = Roll due to sideslip
Directional stability = Yaw due to sideslip.
As an aircraft rolls or yaws sideslip is inevitable, part of the control mechanism. The sideslip, however causes, has an impact on both axes.
So, DR is an oscillation in both yaw and roll, driven by sideslip. If it is perceived as "snaking" (the nose weaving from side to side) then it is mostly in yaw, and thus directional stability is greater than lateral stability and if it's a problem, the solution is probably a yaw damper.
If DR is seen as wing-rocking, then it is mostly in yaw, and thus lateral stability is greater than directional stability and the fix is a roll damper.
More normally lat-dir is fairly well balanced and DR is seen as a figure of 8 along the nose, or oval at the wingtip (or when test flying, the chinagraph cross you put on the side window). The ratio of height:width of the oval seen out the side is approximately the ratio of lateral to directional stability.
G
[ 13 July 2001: Message edited by: Genghis the Engineer ]