dutch roll

I would like to add a few comments about dutch roll.>Dutch roll is a phenomena that is primarily a swept wing>problem with directional instability. >I think Dutch roll was known well before significant numbers of aircraft appeared with swept wings. The effect of swept wings can add to the problem as they have a similar effect to dihedral; they increase the roll stability but not the roll damping. I believe that the difference in velocity of the forward moving and backward moving wing is mostly a small effect especially on fast aircraft. A big difference is because a yaw causes a difference in angle of attack between left and right wings. This is how dihedral works. Playing with a roughWhether it dies away or not without control inputs depends largely on the amount of yaw damping. Hence why the synthetic damping of a yaw damper helps so much.>Dutch roll originates when a swept wing (some straight wing as>well) aircraft starts to oscilate about its vertical axis.>what happens is that the aircraft starts a slow side to side>oscilation on the vertical axis.>Someting like short little>taps on the rudder in opposing directions and given enough>time will end up in a dutch roll. What happens during this is>when the yawing action on the vertical axis starts, the wing>that is moving forward (left yaw, right wing forward) the>right wing lifts and causes a roll to the left.This is correct (except I believe the moving forward is not as significant as the difference in AoA)and it is made worse if the pilot's inputs are not such as to increase yaw damping but decrease it or even produce a driving couple (One description of pilot induced oscillations).>Due to the>lateral stability of the aircraft the wings right themselves>but in the process a right yaw is developed which causes the>left wing to lift, but a bit more than the previous diversion.The level of this yaw due to assymetric lift will vary greatly depending on the configuration. The wing with the greater AoA will likely generate more drag for example.>Left unchecked, this will lead to a Duth Roll in which the>aircraft is actually rolled inverted without input from the>pilots. This is why a Yaw Damper is installed and if it is>unserviceable, speed and altitude restrictions are imposed. I>have been in a sim ride (B727) when the instructor failed the>yaw dampers and told the crew to see if they could control it>at cruise speed. It took less than 10 oscilations before we>were inverted as the late reactions of the crew intensified>the Dutch Roll phenomena.>This can happen, but an aircraft can reach a steady state Dutch Roll. Many years ago I had an uncontrolled model aircraft with a lot of dihedral that developed a most spectacular Dutch Roll but it did not diverge beyond that point. The cure in that case was an enlarged and taller fin.All the bestElderly and out of date aerospace engineer.