Stalling

Wing flying normally. Air over curved upper surface moves faster, reduces pressure, and generates lift.	Wing approaching the stall. Air above the wing starts to become unstable and generates less lift.	Wing fully stalled. Air above the wing is now turbulent - with no pressure difference there is no lift.

(Photos by Jon N. Steiger)

From my first flight onwards I occasionally encountered stalling. Half-way through my pre-solo training a couple of lessons were devoted to experiencing and recovering from various types of stall and other related matters.

Note that before any manoeuver involving loss of height it is necessary to conduct the HASSELL checks.

Straight Stall

We were flying straight and level at a cruising speed of 45 knots. The instructor asked me to stall the glider by reducing our speed by about one knot per second (the standard rate for flight testing a stall) and to explain what was happening. I slowly eased back the stick to gently raise the nose while monitoring the airspeed indicator. The nose high attitude put the glider into a gentle climb - and since there was no engine, the height gain resulted in a corresponding loss of speed.

I noticed that as we lost speed, a faster rate of backwards stick movement was required in order to maintain the same rate of nose movement and speed loss. This meant that the elevator was becoming less effective. I continued to move the stick steadily backwards. Now the wings were becoming harder to keep level - I needed much more left or right stick movement to counter any tendency of the glider to roll. This meant that the ailerons were becoming less effective. I heard the noise of the slipstream becoming fainter as our speed decreased. As we approached the expected stall speed of 37 knots, a pre-stall buffet began like a sensation of light turbulence. As I moved the stick right back towards the back-stop, the last bit of backwards movement had no effect - and then the glider stalled. The angle of attack of the wings into the oncoming airflow was greater than the critical angle, and the wings were no longer generating lift. The glider's nose suddenly dropped and we began a rapid descent. At the same time the left wing began to drop causing a roll to the left. As trained, I made a standard stall recovery by pushing the stick steadily forwards to unstall the wings. I resisted the temptation to use the ailerons to level the wings while they were still stalled - as that can make things worse (see spinning!). As soon as we were flying again, I levelled us out of the gentle dive with normal stick & rudder movement until we were once again cruising straight & level at 45 knots.

I went through this experience once more, but reacted much more quickly to the stall recovery. When we were through, I realised we had only lost about 50 feet of height.

Mushing

I was asked to start the process of stalling once more, but this time to hold-off at the pre-stall buffet and not let a full stall develop. It was hard to keep the glider balanced on the knife-edge between stalled & unstalled, and I accidentally stalled us fully so needed another attempt. When I managed to hold the correct attitude, I realised we were in a high rate of descent, contrary to the visual references in particular as the nose was above the horizon. The altimeter was steadily unwinding and the variometer was showing a fast rate of descent. We were falling like a big pancake. But because we were high and our vertical speed was constant (i.e. no acceleration force), there was no sense of falling. It was like being in an elevator moving steadily between floors.

It is possible to get into a mushing attitude by accident, and not to notice the rapid loss of height until too late. So being aware of the other stall symptoms leading up to mushing is essential: specifically a lack of elevator and aileron authority, a quieter slipstream, and the pre-stall buffeting.

Negative G

I was then talked through the difference between a stall and negative G. First of all I had to reset the accelerometer which measures the vertical G forces on the glider during flight. Then I put the glider into a shallow dive and quickly pulled up - and as we climbed I immediately pushed the nose down again. I felt a familiar sense of weightlessness or reduced G as we went 'over the top' like a roller coaster or hump-backed bridge. As we went over I was asked to demonstrate that I still had elevator authority - indeed, I could still effectively raise or lower the nose.

The instructor stressed how important it is to notice the difference. Some pilots react to reduced or negative G by believing they are stalled and try to recover with the usual stick forwards movement. Some react more instinctively (like a falling child putting out its arms) by pushing the stick forwards and kicking the rudder pedals. Either are potentially disastrous as the negative G only gets worse, encouraging the recovery attempt to continue. The glider will enter a steep dive and gain speed very quickly.

Reduced or negative G is rarely experienced in stall, or if so only briefly at the start of the descent. A stall is usually more gentle and involves fairly constant velocities, hence little change in G forces. But turbulence or sudden spots of lift and sink can create sudden reduced or negative G forces which simply need to be flown through - without reacting by trying to recover from a non-existent stall !

Turning Increases Stall Speed

When a glider is turning, all of the the lift force generated by the wings is not acting vertically to support the glider's weight. Some of the lift force is pushing the aircraft sideways into the turn, so there is less lift force actually keeping the glider airborne. To generate the 'missing lift' in order to prevent additional loss of height, the nose is normally raised slightly in a turn. The steeper the turn the more the nose is raised and the closer the wing's angle of attack comes to the point when the wing will stall. So in a turn the wing is required to generate more lift force than usual, which increases the wing loading, which increases the stall speed.

There is a method of determining how the G-loading is affecting the stall speed. Take the current G number and calculate its square root. Multiply the Vs1 (clean stall speed at 1G) by this number and you have the new stall speed Vs when in that configuration. For example, given a Vs1 of 30 knots, if you are turning steeply at 4G (about 75 degrees of bank!) or pulling out hard from a dive, then Vs = Vs1 * root (G) = 30kt * root (4) = 30kt * 2 = 60 kt. 4G turns are rare, but as you can see from the table below, by 40 degrees of bank (easily achievable in a narrow thermal) the stall speed starts to rise significantly increased:

More Exercises

Soon after the above lesson, I was asked to take the aerotow to 3,000 feet and go through the HASSELL checks. This was a warning that we'd be doing some 'rapid loss of height' exercises! Then the instructor took me through a sequence of wild gyrations around the sky:

1. Standard stall (reducing speed at 1 knot per second - demo and practice).
2. Steep stall (pulling up into a climb and stalling - demo and practice - a good sense of increased G in the pull up and reduced G in the stall).
3. Stall speed increases in a turn (demo - in a 45 degree bank we stalled at 41 knots compared to the normal 37 knots).
4. Stall with a wing drop (demo and practice - you have to quick to get the stick forwards or you enter a spin).
5. Spin entry to left off right turn (demo only - quite an unexpected result)
6. High speed stall with increased G (demo only - you can stall at any speed if the wing exceeds the critical angle of attack into the slipstream).
7. High speed stall with yaw (demo only).

Given the severity of some of these manoeuvers (ranging from almost zero G to +3.5 G) and the repetition (demo and practice and practice again so there were more than a dozen in a row) it was a wilder ride than any roller coaster I've been on - including the legendary Kumba and Montu at Busch Gardens in Tampa (Florida). I felt rather fragile after this lot (I'd had the lunchtime cheese sandwich too recently). But this was an unusually action-packed and educational experience I'd had so far at the club. Little did I know I'd have to do it all again in one of my pre-solo check flights many weeks later.

What is a Stall ?

Unlike a car, stalling a glider has nothing to do with the engine - especially as a glider has no engine (!). To understand why a glider stalls, we first have to understand how it flies. 

A glider flies because of the shape of its wing. The common explanation is as follows: The wing deflects air above and below. The upper surface of the wing is more curved than the lower side. This forces the air above the wing to move faster than the air below the wing as the upper air has a greater distance to travel in order to meet-up with the molecules it was adjacent to before the wing got in the way. This 'spreads out' the molecules above the wing compared to those below, reducing the air pressure above the wing. The greater air pressure below the wing results in a net force which pushes the wing upwards. Or you can consider that the the lower pressure above the wing sucks the wing upwards. Same thing.

A more physically correct explanation can be found here. In particular see page 3/27, paragraph starting "This may come as a....". (Thanks to Miguel Gonzalez Prada for highlighting this web site.)

This is the reason why water droplets, insects, frost or icing on the wing will affect its efficiency - the air can't flow smoothly over the leading edge and top of the wing so the pressure difference is reduced and the wing generates less lift.

The wing stalls when there is not enough lift generated by the wing to lift the wing and the glider attached to the wing. It is caused when the relatively smooth airflow over the upper wing is disrupted and becomes much more turbulent. As you raise the nose of the glider so that it meets the oncoming air at more of an angle, the angle of attack of the wing is increased. The angle of attack is the angle between the wing's chord or centreline and the oncoming airflow. It is normally only a few degrees. The air has to travel over a larger and larger hurdle before running down the top of the wing, and the higher pressure air under the wing can more easily move around the back of the wing toward the low pressure air on top of the wing, thus weakening that low pressure area further. The air on top of the wing begins to roll and burble. Eventually, the low pressure system is completely destroyed, and there is no longer anything holding the wing up at that exaggerated angle. At this point the wing is fully stalled.

When the wing is stalled, the natural tendency of the glider (by careful design) is to drop its nose. Unless the pilot prevents this happening by holding the stick back. If a stall is not allowed to recover, the glider may enter a more serious spin. As the nose drops, the angle of attack is reduced and the speed increases. The airflow over the wing becomes smoother again, the lower pressure area under the wing re-develops, and the glider is flying once more.

It is important to note that the wing will stall when the angle of attack exceeds the critical angle. This can happen in many modes of flight: When the glider flies slower than its best glide speed, it descends faster, which increases the relative angle of the airflow over the wing. A stall is inevitable when the minimum flying speed is reached. This stall speed is around 35 knots in most gliders. Similarly, if the glider is diving and the nose is pulled up too quickly, the angle of attack into the airstream can exceed the stall angle and the wing stops working. This is a problem if the dive was caused initially by a stall or a spin - especially if you are very low. Another cause of the stall is turbulence: if the airflow gusts upwards it may exceed the critical angle of attack and the glider will suddenly sink. This is a key reason why airspeed must be increased more than usual when flying the circuit and landing in windy / turbulent weather.