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new section: stall
This is the first reason why high aspect ratio (slender) wings are aerodynamically desirable.
== Boundary layer and stall ==
Glider pilots usually are introduced the phenomenon of '''stall''' within the first several flights. This section explains the fundamental physics of stall.
Stalling is a viscous phenomenon: it cannot be predicted using the inviscid flow theory. As far as the pilot is concerned, stalling matters because there is a loss of lift (which results in a high rate of descent), ineffective controls, and a possibility of spinning (which will not be discussed here).
Review how lift is created: it is necessary that the streamlines follow the shape of the aerofoil and bends accordingly. If the streamlines cease to follow the aerofoil, i.e. the flow detaches from the aerofoil, lift will be reduced very significantly. This is fundamentally what a stall is. It is observed that wings stall once a critical angle of attack is reached. To understand the physics of stall, the concept of '''boundary layers''' must first be introduced.
Consider pouring honey out of a jar: can you empty the jar completely? This is not possible because the honey sticks on the inside wall of the jar and it will not come off completely no matter how long you allow the jar to drain. This is the '''no-slip condition''' of viscous flow: honey is a viscous fluid, and whenever it contacts a wall, it will not slip on it but stick onto it.
Air is not as viscous as honey, but it still has some viscosity, which means it will stick onto the surface of a wing. Now consider the air on top of the wing: at some distance away the air is flowing at the flying speed, while on the surface it sticks, i.e. flowing at zero speed. As a result, there must be some distance where the flow speed increases gradually from zero to maximum, and in this region the flow speed is less than the outer bulk flow because of the retardation effect of the wing surface. This layer is referred to as the boundary layer.
Just like glider pilots, air can trade freely between two forms of energy, namely pressure potential and kinetic energy. By releasing an inflated balloon the pressure potential is traded for kinetic energy, thus the air accelerates and forms a jet which propels the balloon forward. When air flows around an aerofoil, these two forms of energy is constantly traded for each other: air accelerates by going from high pressure to low pressure and vice versa. The quantitative description of this trading is the Bernoulli’s Equation.
Flow in the boundary layer, however, is in a less favourable position, because its kinetic energy is constantly being robbed by the friction effect of the wall. When it moves from high pressure to low pressure, it gains some kinetic energy, but when it has to go back to the same high pressure again, it finds itself having less kinetic energy than what it would need to do so. When the air has exhausted its kinetic energy but still has a pressure mountain to climb (referred to as an '''adverse pressure gradient'''), it has no means of doing so and it will refuse and go away. This is the phenomenon of boundary layer separation.
A separation of the boundary layer means the flow will cease to follow the aerofoil. If the separated region is significant, it is characterised as a stall.
Things are, unfortunately, further complicated by the fact that, while the wing can remove energy from the boundary layer, the outer flow can help the boundary layer by “dragging it along”. The interacting factors become such a mess that a mathematical description of the precise point of separation is not yet possible. However, it is generally observed that, there are two methods to make a boundary layer separate, namely a very steep adverse pressure gradient, or a less steep adverse pressure gradient over a prolonged distance.
Here it is necessary to quote without proof that the pressure gradients over the upper surface of an aerofoil is generally proportional to the angle of attack.
A leading edge stall happens when the angle of attack is so large that the boundary layer separates straight away at the leading edge under a very steep adverse pressure gradient. This stalling behaviour is very unpleasant as very little warning is given and the loss of lift is sudden and drastic. This is usually avoided by designing a thick aerofoil where the steep adverse pressure gradient around the leading edge is smoothed out.
A trailing edge stall happens when the boundary layer separation point near the trailing edge moves forward because the adverse pressure gradient is increased due to an increased angle of attack. This type of stall is more gentle with plenty of warning signs and a gradual loss of lift. This is the stall behaviour observed on training gliders, e.g. K21s.