The yaw/slip thread (merged) aka Aerodynamics 101

[Keith.Williams.]

Let's forget aerodynamics for a moment and get back to basic mechanics.

Consider a pendulum swinging from right to left and back again. At one point in time it is moving to the left. At a later point it is moving to the right. I order to change its condition from moving left to moving right it must (however briefly) have stopped moving. This requirement applies to all physical objects. It makes no difference whether the frame of reference against which we measure the motion is space or the air around us, the fundamental requirement remains the same.


Now let's consider our aircraft. We are initially flying straight and level in still air. We throttle back on the left engine and throttle up on the right. Because the thrust line has moved out onto the right wing, it generates a yawing moment to the left. So the aircraft starts to yaw to the left.

To arrest the left yaw we apply right rudder. This generates a force to the left acting at the tail of the aircraft. This has (at least) two effects. The first is a yawing moment to the right. If we have applied just enough right rudder the right yawing moment generated by the rudder will exactly balance the left yawing moment generated by our right engine, and the yawing will stop. (For purists, yes we will actually need a bit more rudder initially to actually stop the yaw). We now have a situation in which the aircraft is not yawing.

But the second effect of the rudder is a side force to the left. This will cause the aircraft to sideslip to the left.

If we now start to bank the aircraft to the right, we will tilt the lift towards the right. This initially small horizontal component of lift will cancel out part, but not all of the side force from the fin. The overall effect will be to reduce sideslip to the left. If we now gradually increase right bank in small increments the left sideslip will reduce to zero, then be replaced by right sideslip.

Now if we look at this sequence we will see that just like the pendulum, there was a point at which the aircraft was not moving to the right or left. It was not sideslipping at all. If we look a bit closer we will see that this did not happen with the wings level, but with a slight bank to the right. This is the situation in which the aircraft is banked to the right but not sideslipping.