4. LESSON 5 - COORDINATED FLIGHT, SECONDARY CONTROL EFFECTS

Key points of attention:
  • Moving the rudder causes the glider to rotate around its vertical axis (yaw). The side effect of yawing is rolling.
  • Moving the stick to the left or the right will make the glider rotate around its longitudinal axis (roll). The side effect of rolling is yawing in the opposite direction (adverse yaw).
  • When the glider has a bank angle (when the wings are not level) it slides in the direction of the lowest wing and will start to yaw into the turn.
 

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 turn. This is a beneficial side effect: when you step on the rudder pedal, you want to make a turn and it helps you to roll in the same direction. 

SECONDARY CONTROL EFFECT 2: ROLL CAUSES ADVERSE YAW

Moving the stick to the left (moving the right aileron down) creates more lift, and therefore more drag on the right wing. At the same time, the left aileron is up, so there is less lift and drag on the left wing. As a result, the glider rolls to the left and (because of the extra drag on the right wing) the nose yaws to the top right around the vertical axis. This negative effect is called adverse yaw.

 

In this lesson, we will demonstrate adverse yaw. First we will 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 ............

..... and after a moment slide to the left. 

Every time we initiate a turn, we correct for adverse yaw by using some simultaneous rudder in the direction of the turn when we are moving the stick.

 

SECONDARY CONTROL EFFECT 3: BANK CAUSES YAW
  1. If the wings are level, the glider flies straight (1).
  2. If one wing is lower, the glider will slide to the side of the lowest wing (2).
  3. Sliding creates a crossflow on the fin. This makes the glider yaw to the left or right in the direction of the turn (3).
COORDINATED FLIGHT
When we use the rudder pedals or the ailerons there is always a secondary control effect. To counter these side effects, we always use the rudder pedals and the stick at the same time. Every input of the rudder needs a simultaneous input of the ailerons and every input of the ailerons needs a rudder input. Both inputs need to be done in the same direction and in the right proportion to each other. If both control inputs are balanced, the fuselage stays straight into the airflow and the yaw string (the piece of string taped to the glider’s canopy ) will point straight at you. We call this balanced state a “coordinated flight”.