Many aircraft wings have their tips turned up to reduce turbulence. I have not observed the same on wind-turbine blades – do they not experience the same problem?
• The upturned tips on aircraft wings are called winglets, and are intended to reduce drag, rather than turbulence.
Wings generate lift through the difference between low and high pressure on the top and bottom surfaces, but at the wing tip, these two pressure fields meet, causing air to flow from high pressure to low. This flow generates a tip vortex, which drags airflow at the rear of the wing downwards. Lift acts perpendicular to this airflow, so if the flow is dragged downwards at the trailing edge, a component of the lift force now pulls backwards, opposing the forward motion of the wing. This is known as induced drag.
Wing shape can be designed to minimise this effect, for example, the beautiful elliptical wings of the second world war Spitfire fighter plane. Winglets, on the other hand, create a physical barrier to airflow between the upper and lower surfaces, reducing induced drag at the cost of increased friction and mass at the wing tips.
The longer and thinner a wing is, the less the bulk of the wing is affected by what's going on at the tip. Because wind turbines tend to have very long, thin blades, the structural disadvantage of adding winglet mass at the tips outweighs any reduction in induced drag.
Daniel Summerbell Cambridge, UK
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This article appeared in print under the headline “Twisted tips”