One day after about twenty flights it was getting harder to find any decent lift and so the instructor asked me to head back towards the airfield for something new - spinning. When we reached the airfield we were down to about 2,800 feet and the instructor talked me through the HASSELL checks required before aerobatics (Height, Airframe, Straps, Security, Engine - if motorglider, Location, Lookout). We did the final action (Lookout) by making two steeply banked 360 degree turns in both directions to check that there was no traffic or high ground below, close to the sawmill just south of the airfield. Then I was talked though the spin drill and the instructor gave me my first demonstration.
Then it was my turn:
The instructor raised the nose steeply, and as our speed dropped we started to stall. Then he applied left rudder which yawed the glider to the left, slowing the left wing and speeding-up the right wing while causing left roll. As the stall started, the nose dropped, then the left wing dropped and as we pitched down hard the glider began to roll ('autorotate') to the left. We were very nose-down, almost vertical, and as the spin developed the ground features began to smear into streaks. There was a real sense of falling as the ground rushed upwards. In fact the glider was simultaneously rolling, pitching and yawing as it quickly fell.
The instructor called "You have control"...
I quickly took the controls and started the recovery:
All of the above happens quickly, about 10 seconds from start to finish.
"That was really good, in fact that was excellent for a first spin recovery" called the instructor from the rear. That was nice to hear, but more than that I had really enjoyed the roller-coaster ride of these two wild spins (demo and practice). And there was thankfully still no sign of my breakfast reappearing.
A few flights later we were at it again:
We'd spent a fair bit of time thermalling and exploring around local airspace, so the instructor reckoned I should learn something new. He performed the HASSELL checks required before any significant height-losing manoeuver and then entered a fairly typical left turn. However, he raised the nose a little higher than usual and banked the glider over rather more than usual, and I heard the slipstream noise fade away as we lost speed.
Then suddenly the inner (left) wing stalled and dropped - followed a moment later by the nose - and we entered a hard left-hand spin and dropped like a glider-shaped stone. This time the the instructor quickly performed the standard spin recovery (as above) and then handed back control to me. He explained that he'd just given a standard demonstration called 'spin off a steep or thermal turn'. He said I'd have the chance to try it myself on a later flight...
Note that if a spin doesn't develop fully the glider instead may enter a spiral dive.
(Extract from leaflet "Avoiding the Stall/Spin Accident" from the European General Aviation Safety Foundation.)
A spin is caused by two primary factors that must both be present:
It is essential to realise that a glider cannot spin unless you first allow it to stall and then introduce some element of yaw (although in practice there is always some yaw - so a spin is usually inevitable once a stall is maintained).
A spin is divided into two phases - incipient and steady state. The first, the incipient phase, is that portion after stall when the aircraft commences a spin-like motion. In this phase, the aerodynamic and the inertial forces have not achieved a balance. In the second phase, the steady state or fully developed spin, the aerodynamic and the inertial forces are in balance and the attitude, angles and motions are repetitive from turn to turn.
In a spin, the view looking out of the cockpit is generally a steep, nose-down, attitude, with a yawing/rolling motion about the spin axis. With the airspeed near stall airspeed, the angle of attack indicator (if the aircraft has one) shows a fully stalled condition. The turn needle is deflected fully in the direction of the spin and the rate of descent is significant. The "G" force acting on a spinning aircraft is essentially One. The spin is a recoverable manoeuvre in aircraft approved for spinning but the recovery does require altitude. To understand a spin, you need to be familiar with the following terms:
As the angle of attack increases, the coefficient of lift also increases. When angle of attack reaches a certain point, the airflow separates from the airfoil, and lift starts to decrease. As angle of attack continues to increase, lift is still generated, but decreases even more. The stall occurs at peak.
When you are beyond stall angle of attack, if the aircraft experiences any rolling displacement, the up-going or outboard wing will experience a decrease in angle of attack. Conversely, the down-going or inboard wing has increased angle of attack.
The difference in angle of attack of the two surfaces is due to the vertical component of the relative wind in the rolling condition. The difference in angle of attack results in differences of lift and drag for the two surfaces; the up-going wing is less stalled - and the down-going wing more stalled.
This causes a rolling and turning tendency at angles of attack beyond stall. The tendency is called AUTO-ROTATION and is self-feeding. The roll at stall may be initiated by adverse yaw. Let's see what that is:
If you are near stall angle of attack and a wing drops, and you attempt to raise it by applying aileron alone, the aileron, going down, will increase the lift on the wing. But the increased lift increases the induced drag causing a yaw toward the down wing. This is adverse yaw. The down wing , with an increase in total drag, becomes more stalled. This produces even more roll, contributing to auto rotation. To prevent auto rotation, you must eliminate any slipping or turning input at the point of stall. Coordination of aileron and rudder is the key.