4. LESSON 24 - STEEP TURNS

Key point of attention:
  • During a turn the load factor increases, therefore the stall speed increases

MORE BANK REQUIRES A HIGHER AIRSPEED
A glider’s wing stalls more easily during a turn than during straight flight. This is because the direction of the lift changes when we turn.
In straight flight, you can visualize the lift as a force lifting the glider’s wings up vertically. During a turn, the lift is still perpendicular to the wings, but because of the bank angle, it is no longer vertical. We can divide the lift into a horizontal and vertical component; L1 is the vertical component which compensates for the weight of the glider, and L2 is the horizontal component that compensates for the centrifugal force of the turn. The steeper the turn, the bigger L2 becomes. This also means that the stall speed increases.

The load factor expresses the load to which the structure of the glider is being subjected: the gravitational force equivalent, more commonly known as g-force. This g-force also increases during turns. In a turn with a 60° bank angle the load factor is 2G. In other words; the amount of g-force is now twice as big as the weight of the glider, and the amount of lift compensating for this will also be twice as big. Like on a roller coaster, you will have a sensation of being pushed in your seat.

Normal lift distribution on a glider.

Most gliders are very strong. They can withstand positive load factors of over 5 times the weight of the glider.

Lift distribution on a glider with the airbrakes extended. 

When we open the airbrakes, the lift is no longer evenly distributed along the wings. Around the airbrakes, "load pivot points" arise. The maximum positive load factor is now 3.5 times the weight of the glider. Of course, the main purpose of airbrakes is to enable us to land safely. By causing additional drag, we can descend more steeply and shorten the gliders’ glide path. But airbrakes are not very effective in reducing the airspeed in a steep dive. You will learn more about this in Lesson 25. For now it’s important to remember that the best way of decreasing speed during a steep dive, is to pull the stick towards you. Avoid using the airbrakes.

During straight flight, 100% of the lift can be used to compensate for the weight of the glider. In a turn with a 45° bank angle, only 70% of the lift is available to compensate for the weight, whereas the load factor has increased. This means that, during turns, we need to increase the airspeed to get additional lift. The lift increases with the square of the speed. So when you double your airspeed, you generate four times the lift. When the bank angle increases, the stall speed increases too.

Bank angle        Increase in stall         Load factor
0 ° + 0%  +1
30 ° + 7% +1,15
45 ° + 20%  +1,41
60 ° + 41% + 2

 

During turns with bank angles up to 30° you will only have to fly slightly faster. But during steeper turns, you will need to increase the airspeed significantly from the moment you start turning.