4. LESSON 22 – SLIPPING AND SKIDDING

Key points of attention:
  • A slipping turn: the heading is opposite to the bank angle, too little rudder input for the current bank angle. Hardly any rate of turn
  • A skidding turn: the aircraft is turning too fast for the bank angle and yaws into the turn -> rate of turn too big for the bank angle
COORDINATED VERSUS UNCOORDINATED TURNS
A coordinated turn uses the right amount of rudder deflection for the required bank angle. The nose of the glider follows the circle you are flying. The yaw string is pointing straight towards you. The fuselage stays more or less exactly straight into the airflow (see illustration below).

 

SKIDDING
Skidding in a turn is a situation in which you use too much rudder for the amount of bank you have. The rate of turn increases too fast while the bank angle remains the same. As a result the glider will yaw to the inside of the turn. You can also see that this is happening by watching the yaw string: it will point to the inside of the turn. The inner wing has a reduced speed compared to the outer wing, and the airflow on the inner wing also gets disturbed by the glider’s nose and fuselage. The inner wing is “in the shadow” of the fuselage. If you are flying too slow or close to your critical angle of attack (the angle where the airflow no longer can follow the wing, see Lesson 23), skidding may result in the air separating from this wing. A spin can be the result of skidding turns (see Lesson 25).

SLIPPING
Skidding occurs when you use too much rudder. Slipping on the other hand is the result of not applying sufficient rudder in the direction of the turn for the required bank angle. While slipping, one wing will be lower than the other. The glider’s nose now yaws in the opposite direction of the turn (remember, the secondary control effect of roll is yaw). The incoming airflow now blows against the inside of the fuselage and the yaw string will point in the opposite direction of the turn. The risk of an unexpected spin is lower when you are slipping, because the (slower) inside wing is not shielded by the fuselage of the glider. Slipping does cause additional drag. To restore a coordinated flight when slipping, you need to apply rudder in the opposite direction of the yaw string.

SIDE-SLIPPING
The airbrakes are very effective for increasing the rate of descend during an approach. But if you need to lose altitude quickly you can also combine the airbrakes with a side-slip. When a side-slip is performed intentionally and correctly, there is no turning motion because the nose is now pointing opposite to the bank angle. Nowadays, a “side-slip approach” is an advanced gliding technique. In the early days of aviation, gliders were not equipped with proper airbrakes and side-slipping was the only way to lose height on final approach.

Forward slip: the heading is opposite to the bank angle, while you maintain the original track. The use of the airbrakes increases the rate of descend even more. 

To enter a side-slip:
  1. Pick a landmark on the horizon: long and straight (rail-)roads or runways are perfect.
  2. Use the ailerons to roll the glider in the direction of the wind, simultaneously apply some opposite rudder to maintain the glider’s longitudinal axis slightly offset of the desired ground track.
  3. Use the elevator to control the pitch attitude: be aware of the fact that the airspeed indicator is not reliable when side-slipping because the airflow is now offset to the pressure tubes. Indications may even drop to zero!
To recover from a side-slip:
  1. Simultaneously release the rudder pressure and level the wings.
  2. Adjust the pitch attitude to a normal pitch.