4. LESSON 4 - AIRSPEED, HORIZON AND TRIM
 
Key points of attention: 
  • The position of the horizon at different speeds
  • Using your ears as airspeed indicators
  • Position of the wings during straight flight 
AIRSPEED
Most gliders do not have engines. In a motorized glider, the propeller pulls the glider forward. The illustration below shows a two-seater glider with a retractable engine on top of the fuselage and a normal two-seater (bottom picture).
 
 
L = Lift
TR = Total Reaction
P = Propulsion/Engine
D = Drag
W = Weight
W1= Vector of the Weight opposite to Lift
W2= Vector of Weight in the direction of flight replaces Propulsion
 

When a motorized glider is flying straight, wings level and at a constant airspeed and altitude, Lift (L) equals Weight (W) in the opposite direction. To maintain a constant speed Propulsion (P) has to be equal to Drag (D).

The bottom picture shows a normal glider gliding downwards. You can compare a glider to a soapbox racer or a car without an engine. Both can only drive when they roll down a slope. A glider can only fly when it glides down. The steeper it goes down, the faster it gets. You will find out soon that modern gliders do not need a steep slope to fly fast and far.

In a normal glider (also called a pure glider), 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). W2 can be seen as the Propulsion of a non-motorized glider.

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, the airspeed is decreasing -> high pitch attitude.
  • When the horizon becomes high, the airspeed is increasing -> low pitch attitude.
 
 
The position of the glider's nose relative to the horizon tells you the airspeed 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, therefore the noise will go down. You will notice that it becomes silent around you. At higher airspeeds a lot of air hits the surface of the glider and you will hear a lot of noise. In other words, it is possible to use your ears as airspeed indicators, simply by listening to the changes in air volume. Changes in the volume 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 replacement. You can use the airspeed indicator to cross-check 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.

If you are sitting in the glider just before take-off, you will see the horizon as a horizontal line in the canopy, and the wings will be level. During flight, when you look at the wingtips, 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.

WHAT IS TRIMMING?
The trim lever or "trimmer" enables us to to maintain the associated airspeed at a selected pitch attitude, without us having to put in any control pressure with the control stick. After you have set a new pitch attitude and the airspeed has stabilized, you can use the trimmer and let go of the control stick. The glider will now maintain this pitch attitude. Check if the trim is set correctly and re-trim if necessary. Trimming helps you to fly more relaxed.

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 keep pulling the stick continuously 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.