- Be sure not to neglect your lookout during this lesson
- Three ways to determine your speed
- Setting the trim
AIRSPEED
This picture shows a glider in normal straight flight, but with the angle exaggerated for clarity. You can compare a glider to a soapbox racer or a car without an engine: each will only drive when rolling down a slope. A glider can only fly when it glides down through the air. The steeper it goes down, the faster it gets. You will soon discover that modern gliders do not need a steep slope to fly fast and far.
- L = Lift
- TR = Total Reaction
- D = Drag
- W = Weight
- W1= Vector of the Weight opposite to Lift
- W2= Vector of Weight in the direction of flight
The Total Reaction (TR) of Lift and Drag is equal and opposite to the Weight (W) of the glider. Weight (W) can be split into two forces: the force W1 that is equal and opposite to Lift (L) and W2, a force that is opposite and equal to Drag (D).
HORIZON
You can find out what your airspeed is by looking at the airspeed indicator on your instrument panel. But it is also possible to estimate the airspeed roughly by looking at the position of the horizon from the canopy. In this lesson, you will learn how the airspeed changes as the position of the horizon shifts. It is important that you learn how to do this. For safety reasons, glider pilots spend most of their time in the air looking outside. They only check their instruments briefly from time to time. There are three ways to estimate the airspeed.
Method 1: The best way to estimate your current airspeed is the position of the horizon as seen from the canopy. When the position of the horizon shifts, your airspeed changes. The distance between the horizon and your glider’s nose is called the pitch attitude.
When the horizon is low (nose is high), the airspeed is decreasing -> high pitch attitude.
When the horizon becomes high (nose is low), the airspeed is increasing -> low pitch attitude.
The position of the glider’s nose relative to the horizon tells you the air- speed of the glider.
If you want to maintain a certain airspeed you can simply set the associated pitch attitude. It may take a while for the airspeed to increase or decrease to the desired airspeed value but with every pitch attitude below the horizon, there is an associated airspeed value. When the nose is pointing above the horizon (high pitch attitude), the airspeed will keep decreasing.
Method 2: When the airspeed decreases (when the glider slows down), less air hits the glider, making less noise. At higher airspeeds a lot of air hits the surface of the glider; it gets noisier. In other words, it is possible to use your ears as airspeed indicators, simply by listening to the changes in the sound of the airflow. Changes in the sound of the surrounding air indicate airspeed changes.
Method 3: The airspeed indicator does a very precise job, but you don’t really need it. The pitch attitude and your ears make a good substitute. You can use the airspeed indicator to crosscheck the airspeed every now and then. But don’t stare at it; doing so would prevent you from looking out for traffic and would therefore be unsafe. If you want to adjust your speed, just adjust your pitch attitude.
During flight, when you look at the wing tips, you will notice that they are both positioned a little bit above the horizon. This is because the wings are flexible so they bend upwards. This makes it hard to judge whether you are flying level by looking sideways. In fact, it is easier to detect a small bank angle (angle between the wings and the horizon) when you look straight over the nose of your glider. In the beginning, you might find it difficult to keep the wings level. This will become easier when you become more experienced. Try to adopt the same seating position before every flight so the relative position of the horizon remains the same.
TRIMMING
In the stability and controls section (Lesson 4.2) we explained that the glider is stable; left to itself it will fly at one particular airspeed. Use the trimmer to set this speed. First, select the pitch attitude for the speed that you want. Now wait for the airspeed to settle; if it isn’t quite right, adjust the attitude using the stick, and again wait for the airspeed to settle. Once stabilised at your chosen speed, adjust the trim lever to remove forward or aft stick forces. Flying in trim reduces your workload, leaving more time for lookout and other important tasks.
HOW DOES THE TRIMMER WORK?
There are two types of glider trimmers. On some gliders you use the trim lever to adjust a small trim surface on the elevator. By adjusting the trim position, a vertical force is exerted upwards or downwards on the elevator. This is called an aerodynamic trimmer. In other gliders we use a ‘spring trimmer’. The trim lever is then connected to a spring that exerts a force on the stick. When you adjust the trim, the spring force changes.
CENTRE OF GRAVITY
When you are sitting in the front seat of the glider, your weight can make a big difference. A heavier pilot has to pull the stick continually to keep the nose of the glider close to the horizon. Having to pull the stick constantly will tire you out quickly. The trim makes it a lot easier to maintain a certain pitch attitude and airspeed. Using the trim also saves you a lot of mental energy that you can now use for other tasks.
