4. LESSON 5 - COORDINATED FLIGHT, SECONDARY CONTROL EFFECTS
- Both aileron and rudder have secondary effects
- Deal with the secondary effects by using coordinated stick and rudder
SECONDARY EFFECT 1: YAW CAUSES ROLL
When we push the left rudder pedal, the nose of the glider will yaw to the left. You will see the nose moving to the left. But what else will happen?
The outer wing will move forward. Compared to the inner wing it travels a larger distance. Therefore the outer wing moves faster and generates more lift than the inner wing. The result is that the outer wing will lift, meaning that the glider will start to roll in the direction of the yaw.
SECONDARY EFFECT 2: ROLL CAUSES ADVERSE YAW
Left stick means right aileron down to get the roll you want. Trouble is, that down aileron involves a higher angle of attack and so more drag, yawing the nose to the right. On the other side, the associated up aileron involves a smaller angle of attack and thus less drag, reinforcing the result: yaw to the right.
Move the stick to the right and the same combination of effects produces yaw to the left. So the stick not only rolls the glider (which you want) it also yaws it in a direction you do not want. We call this adverse yaw.
In this lesson, we will demonstrate adverse yaw. First we fly straight with wings level towards a specific landmark on the horizon and then we will move the stick to the left.
The glider will roll to the left and at the same time the nose will yaw to the right (1)
and after a moment slide to the left (2).
Every time we initiate a turn, we have to correct for this adverse yaw by using some simultaneous rudder in the direction of the turn when we are moving the stick.
SECONDARY EFFECT 3: BANK CAUSES YAW
Sliding creates a crossflow on the fin. This makes the glider yaw in the direction of the bank (3).
When we use the ailerons or the rudder there is always a secondary control effect, but there is also a simple solution: use the stick and pedals together. Every input of the ailerons needs a simultaneous input of the rudder and every input of the rudder needs an aileron input.
A sharp reader will already have asked: ‘Yes, but how much?’ There is a simple answer, and then a refined one. Simple: if using half left stick, use half left rudder; if full right stick, use full right rudder, and so on. This is not the final answer, but it’s a good start.
Refined: without letting your lookout suffer, the string will tell you if you are using the correct amount of rudder. In the blue diagram, the pilot needs more right rudder.
Some pilots find it helpful to imagine the string as a weather vane, pointing at the foot that needs more effort. Whatever the method, with the string in the middle the fuselage will be aligned with the airflow, the glider will fly better and you’ll stop hearing the instructor’s reminder: coordinated flight!