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Quick facts

Lift force

Wind turbine blades follow the same physical fact of the aircraft wings, this is the lift force generated by the difference in pressure between both sides of the wing/blade.

The curvature of a wing is such that the air speed over the upper side is higher than the air speed over the lower side. This creates a pressure difference that 'pushes' the structure upwards holding the aircraft in the air.


If we rotate the wing to make it vertical, the lift force creates a rotating movement (torque) making the wind turbine rotor spin.


If the angle of attack is low, passing air flow over the blade is attached to the blade's surface. Given the shape of the blade, air speed on the upper side is higher than in the lower one (pressure is lower in the upper side), creating an ascending lift force (rotational in the case of a turbine).

Esquema aerodinámico

When the angle of attack increases (when wind speed rises), air flow on the upper side of the blade begins to separate from the surface creating a turbulence that inverts the flow direction in that part of the blade.

Esquema Stall

When this happens, air speed through the upper side of the blade drops suddenly causing the pressure to increase and minimizing the pressure difference that causes the lift. In this case, the rotational force falls and the turbine rotor slows down.

Omphlus lepturoides

The results of the case study carried out in Magallon 26 wind farm in Zaragoza, Spain, identified a beetle as the main contributor to the accumulation of insect debris on the blades.

Omophlus lepturoides

This flying beetle is Omophlus lepturoides, a species that feeds from the shrub roots and lives underground until the eclosion.

From this moment the beetle lives for a period of one week, feels attracted by bright colors (white) and flies on low wind conditions ascending up to a few dozens of meters.

As the image shows, it has two orange thick elitra (wing covers). When this beetle collides with the rotating blades it produces severe rugosity on the blade. The constant accumulation of rest of these insects on the leading edge of the blades contributes severely to the double stall effect.

Stall · Double Stall · Causes · Flying insects · Economic implications


Stall is referred to as the loss of lift force of a blade due to changes in its aerodynamic behavior. This is the same effect that suffers an aircraft when airspeed is too low, the wing lift force decreases and the aircraft stalls.

In the case of Pitch controlled wind turbines, stall is induced by the rotation of the blade over its axis. In Stall controlled turbines, the design of the blade produces a progressive stall effect over a certain wind speed maintaining the torque on the rotor as the airspeed increases. In this scenario, the design of the blade plays a critical role on the power output and stability of the turbine.

The power curve is the representation of the power output of a given turbine at any wind speed and it is unique to that turbine.

Grafico stall

If no aerodynamic loss (stall) were present, the power output (rotor speed) would increase with no limit, causing mechanical and structural failure on the turbine.

The key to maximize performance is to design a turbine with a power curve as steepest as possible on the ascending part of the plot (more power output at low wind speeds) and as flat as possible past the maximum power output.

Double Stall

This is a typical phenomenon of stall controlled turbines although it also occurs on pitch controlled turbines. Double stall is referred to as a second and unpredicted aerodynamic loss in the blade, not related to the blade design and that produces a decrease in the energy that is extracted from the wind by the rotor.

The following power curve shows a typical 1 MW turbine affected by double stall (blue) in relation with the manufacturer's power curve (red):

Grafico double stall

The shaded area shows the power loss as the consequence of this phenomenon that, depending on the wind speed, it can add up to 50% of the production. This effect is quite obvious at high wind speeds (large mismatch between theoretical and real power curves), however, at low wind speeds the plot is displaced to the right causing a production loss on the most frequent range of wind speeds. [Top]


There are several factors that may cause double stall. These can be a malfunction in the yaw system, a faulty blade design or the accumulation of insects, ice, oil, dust or salt on the leading edge of the blade. In any case, the result is the modification of the aerodynamic profile of the blade producing a poorer transformation of the energy of the wind into torque.

Since the publication of Corten's PhD thesis (2001) about the implications of the accumulation of insects in the leading edge of the blades, this hypothesis has gained strength and other prestigious journals like Science and Nature have published articles on the matter. [Top]

Flying insects

The following image shows flying beetles in the surroundings of a tower of Magallon 26 wind farm in May 2005:

Insectos voladores

When these 'clouds' of beetles reach the rotor swept area the blades impact on them causing the blades look like the pictures below:

Insectos1 Insectos2 Insectos3

The increase in rugosity penalize severely the aerodynamic efficiency of the blade. [Top]

Economic implications

The effect of insect debris accumulation in the blades has been under study since 2004 in Magallon 26 wind farm (10.8 MW) in Zaragoza, Spain. The owner of the wind farm developed in 2005 the Blade Cleaning system and was installed in every turbine.

It has been proved that the implications of insect caused double stall are substantial. Taking the data from this real case, the calculations for a typical 1 MW wind farm result as follows:

Nominal power: 1,000 kW
Annual equivalent hours: 2,500 h
Theoretical production: 2,500,000 kWh
Price: 0.08 €/kWh
Predicted income: 200,000 €/year
Double Stall  
Average loss on power curve: 10 %
Production loss: 250,000 kWh
Income loss: 20,000 €/year per installed MW

Therefore, assuming a 10% annual average power loss on the power curve, production can fall up to a quarter of a GWh per year (equivalent to the annual consumption of 71 households) and a revenue loss of 20,000 € per year.

The implications of the double stall phenomenon not only affect the expected revenue of the wind farm but also the National Grid system as the predictions of power output are less accurate. Besides, the increasing mismatch between expected and actual production may convey sanctions from the grid operator.

Facts and comparisons

There are a few considerations to be made: