4. LESSON 23 - SLOW FLIGHT AND STALLS

Key points of attention:
  • If the stick will not raise the nose, ease it forward, regain flying speed, then return the glider to the normal attitude
  • If the stick will not lift a wing, ease it centrally forward, regain flying speed, then level the wings and return to the normal flying attitude
  • Preventing a stall can be more fun that dealing with one
  • Before deliberately stalling, be sure that pilot and glider are prepared.

During the following exercises you will experience what happens when the glider wing stalls. A pilot can stall the glider by trying to get the wing to do more than it is capable of providing. You will learn to recognize the symptoms of an approaching stall, so that you can take action to avoid stalling, and recover promptly from an inadvertent stall. 

LIFT (L) AND ANGLE OF ATTACK (α)
The wings create the lift that is necessary for a glider to fly. Let’s just briefly repeat what you’ve learned in Chapter 2. The air on top of the wings flows faster than the air below. In faster moving air, the air pressure is lower. The different airflows on top of and below the wings create a pressure difference, which in turn results in an upwards force on the wings: Lift (see illustration).

 

The easiest way to describe ‘angle of attack’ is the angle a wing makes with the oncoming airflow.

NB: THIS DIAGRAM MUST BE REVISED TO SHOW DOWNWASH BEHIND WING.

If you fly faster, the air around the wings will flow faster. The pressure difference between the top and bottom of the wing increases and therefore the lift increases. The lift also increases when the angle of attack increases. However the angle of attack cannot be increased endlessly. If we reach the limit (the “critical angle of attack”) the airflow can no longer follow the top surface of the wing and it will separate from the wing (see illustration). When this happens, the lift will decrease significantly and the drag will increase.
 

The angle of attack is α. The lift is marked with an L, drag is marked with a D, the total resultant of lift and drag is marked TR. Here, the angle of attack α exceeds the critical angle of attack; the airflow separates from the wing, lift decreases and drag increases. This is called a Stall.

Both airspeed and angle of attack influence the lift. Basically, they complement each other: if the airspeed increases, the angle of attack decreases; if the airspeed decreases, the angle of attack increases. If the critical angle of attack is exceeded, the wing stalls. If the glider stalls as a result of flying too slow, only the wings stall, not the tailplane (horizontal stabilizer). The tailplane is positioned on the fuselage at a smaller angle than the wings. Therefore the angle of attack of the tailplane is smaller and the wings always stall first.

The angle at which a wing or tailplane is mounted to the fuselage is described as the angle of incidence. The angle of incidence is the angle between the chord line  (= a straight line from leading edge to trailing edge of a wing) and the longitudinal axis of the glider. The angle of incidence is fixed and cannot be changed during flight. Gliders are produced in such a way, that the angle of incidence is smaller for the tailplane than it is for the wings.

 

The illustration shows the difference in angle of incidence between the tailplane and the wings.

If the airflow separates from the wings because the critical angle of attack is being exceeded, the airflow at the tailplane remains. As a result, you can still use your elevator to change your pitch attitude. When the glider stalls, the nose wants to drop. This is because the centre of gravity of the glider is located just in front of the wings. When the nose drops, the angle of attack decreases and the glider will accelerate again. Do you remember how we mentioned in Lesson 14 (Winch launching): “The dangers of climbing too steep at low altitudes are cable break and stall”? If you pull the stick back too quickly during a wind launch, you can exceed the critical angle of attack. The angle of attack alone is decisive for a stall. In other words, a stall can occur at every airspeed, low or high. As soon as the critical angle of attack is (nearly) reached, your glider will respond and you will have to prepare for recovery.

HIGH SPEED STALLS

Even though you won’t need to practise high speed stalls before flying your first solo flight, it is good to know how they work. High speed stalls are very popular in advanced aerobatics and you will see them in every air show. Just after stalling, the glider can reach very high roll rates, which allows the pilot to carry out all sorts of spectacular manoeuvres. The stall speed of a glider is affected by its weight and load factor, which are in turn influenced by manoeuvring and environmental conditions . A heavily loaded glider stalls at a higher airspeed than a lightly loaded glider. We can understand this by imagining how, in order to support the extra weight, the glider will have to adopt a higher angle of attack to generate more lift and fly at the same airspeed. In some gliders we can add extra weight by adding trim weights or putting water into the glider's tail. Trim weights are used to compensate for a forward shift in centre of gravity and to keep the stall characteristics more or less the same at different cockpit loads.  However, bear in mind that you should never add too many trim weights or put too much water in the tail, because your centre of gravity would shift too far backwards. This could potentially be very dangerous as it would make your glider unstable!

 
A higher risk of stall is expected in the following cases:
  • during a winch launch (caused by the higher wing load)
  • when flying steep turns (a larger bank angle causes a higher wing load) (See Lesson 24)
  • when the wings are wet, dirty or covered with ice (water or ice changes the aerodynamic properties of the wing’s profile; the critical angle of attack can be smaller than it normally is)
THREE EXERCISES – THE PREPARATION

Before you undertake exercises that involve large changes in speed and altitude, you will have to perform the pre-manoeuvre/aerobatic checklist (see Lesson 3). To save valuable flying time, you can prepare for the checklist before flight, but you will have to actually perform it just before the exercise. In addition, we advise you to trim the glider for a normal speed and to not perform exercises in the direction of the sun – the bright light would limit your vision.

 1: STALL AND RECOVERY

You will start this exercise by making “clearing turns”; a steeply-banked turn to the left then the right (or vice versa) that allows you to look all around and vertically downwards to make sure that no other aircraft are near or beneath you. After these clearing turns, your instructor will show you the symptoms of the approaching stall with the wings level, while you follow through on the controls. If the nose is raised even a little above the normal attitude (1), the glider’s airspeed reduces, and the noise of the airflow changes, becoming quieter. The effects of the ailerons may change, and the glider buffets. In spite of the instructors attempts to hold up the nose with the elevator, it will drop (2). To recover from the stall, the stick is eased to regain normal flying speed (3), then used to return the glider to the normal gliding attitude.

2: MUSHING STALL

Your instructor will have already shown you what to do if the stick will not raise the nose. This is the same if the nose has dropped or not. The second case we call a ‘mushing stall’.

Recognition is the same; recovery is the same.

3: RECOVERING FROM A WING DROP

Your instructor will now show you what to do if the ailerons do not pick up a wing (perhaps better described as not rolling the glider.

Recognition is the same; recovery is the same

We don’t like spending time telling you what not to do, but worth emphasising in this case: don’t try to use either ailerons or rudder to pick up a wing.

It is important that you are able to recognize and react to the warning signs of an impending stall, so that you will always recover quickly. Your instructor will ask you to practice stalls and recoveries very frequently, perhaps even on every flight. This is to ensure that recognition of the approaching stall becomes second nature, and you will instinctively recover quickly if this ever happens unexpectedly in your later flying career.

MUCH MORE IMPORTANT: STALL PREVENTION

Easy to say: fly the aircraft.

Easy to do for a pilot who is paying attention: maintain the speed you want, in balance.

Pilots who allow themselves to get distracted, can, particularly when stressed, unthinkingly apply the control inputs that stall the glider.

Deal with it at any stage by easing the stick forward.