Why do planes have dutch roll while in flight?

Question

As far as I know, dutch roll is when one side of the wing gets more lift, therefore the other side gets drag, falls "behind" etc., and then its going to the other side....

BUT: Why does a dutch roll really exist while in flight? If the plane flies in still air, there shouldn't be that problem, right?

Answer 1

It's an effect of wing sweep.  It couples (connects) yaw (nose side to side) with roll.

What happens is that on a swept wing airplane, when there is a yaw disturbance- let's say nose to the left- the other wing moves forward.  Effectively, the (right wing in this case) gets more span as well as speed, and the other (left in this case) moves back (less speed and less span).  Thus the right wing lifts and the left drops.  This goes back and forth and (in the early days) could get quite violent.  

It's fixed by automatic yaw dampers on the rudder that prevent the yaw oscillation from building up.

Answer 2

Any design can develop dutch roll whether the wing is swept or unswept.

With a little asymmetry due to a gust or aileron deflection the wing with more lift also has more drag (lift always induces drag).  So the upgoing wing causes the airplane to yaw in the opposite direction (right wing up as if to turn left; yaw to the right).  

Some airplanes have more built in stability than others so the effect is less pronounced in some airplanes.  Typical solutions are to increase stability by adding area with a ventral fin or a larger vertical stabilizer leading edge, or to add a yaw damper to make up for the stability that the design naturally lacks.

Answer 3

Consider an aeroplane in straight and level flight. The pilot (or a gust of wind) induces a yawing motion. If the aircraft yaws nose right for example - whilst the aircraft is yawing, the port wing is moving quicker relative to the oncoming air than the starboard wing. Therefore there is more lift being generated on the port wing than starboard - hence a secondary rolling motion is also induced, with the port wing coming up.

As the aircraft begins to roll the extra lift on the port wing also creates extra drag - which acts to try and cancel the yawing motion. Now the starborad wing is moving quicker relative to oncoming air, and therefore extra lift is generated - and now the aircraft will roll with the right wing coming up. The resulting extra drag acts to cancel this motion and then we are back to where we started with the aircraft yawing nose right.

This cycle of yawing and rolling continues until damped out by natural directional stability or corrected through contol input (yaw dampers). With directionally unstable aircraft, if uncorrected via pilot / control system input - the yawing and rolling displacement will increase over time. In some cases, this may occur very rapidly!

In reality, "still air" is very rarely found!!