Aerodynamics of Wind Turbines: Stall and Drag Stall Now, what happens if an aircraft tilts backward in an attempt to climb higher into the sky quickly? The lift of the wing will indeed increase, as the wing is tilted backwards, but in the picture you can see that all of a sudden the air flow on the upper surface stops sticking to the surface of the wing. Instead the air whirls around in an irregular vortex (a condition which is also known as turbulence). All of a sudden the lift from the low pressure on the upper surface of the wing disappears. This phenomenon is known as stall. An aircraft wing will stall, if the shape of the wing tapers off too quickly as the air moves along its general direction of motion. (The wing itself, of course, does not change its shape, but the angle of the the wing in relation to the general direction of the airflow (also known as the angle of attack) has been increased in our picture above). Notice that the turbulence is created on the back side of the wing in relation to the air current. Stall can be provoked if the surface of the aircraft wing - or the wind turbine rotor blade - is not completely even and smooth. A dent in the wing or rotor blade, or a piece of self-adhesive tape can be enough to start the turbulence on the backside, even if the angle of attack is fairly small. Aircraft designers obviously try to avoid stall at all costs, since an aeroplane without the lift from its wings will fall like a rock. On the page on power control we shall return to the subject of how wind turbine engineers deliberately make use of the stall phenomenon when designing rotor blades. Drag Aircraft designers and rotor blade designers are not just concerned with lift and stall, however. They are also concerned with air resistance, in technical jargon of aerodynamics known as drag. Drag will normally increase if the area facing the direction of motion increases.
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