• can sails be considered as wings?

Not everyone agrees - for example, Frank Bethwaite (1996) suggests: "because sails look like wings, it used to be believed that sails and wings worked in the same way. They don't." His main concern is that sails operate at much smaller airspeeds than aircraft wings.

However, the parallels are closer when we consider the unsteady airflow occurring across the wings of birds and bats during flapping flight.

Bethwaite, F. (1996) High performance sailing. London: Adlard Coles Nautical (p 188)

• the effects of boat movements on the sail

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Most of these movements will produce appreciable changes in the flow of air across the sail or sails attached to the boat, causing rapid accelerations and decelerations in airflow and corresponding changes in the lift and drag being produced by the sail. Thus sails often experience unsteady airflow conditions. The more stable the shape of the sail, as for example in the case of rigid wing sails, the more pronounced will be these surges and lulls in power production by the sail. The wings of animals that use flapping-flight also experience - and make use of - unsteady airflow. We can learn from their solutions to this interesting area of aerodynamics.

• wings and sails have a lot in common: what we learn from one type of foil can be useful in understanding the others
• in animals, structures concerned with flight make considerable use of resilience and elasticity to improve efficiency of energy use
• the wings of insects (especially the larger ones) and the sails we make have a lot in common: the aerodynamics of both are influenced by passive adaptations of the structural framework to external energy sources
• the wings of bats and birds have a wider range of effective geometry due to active (muscle-powered) adaptations within the wings - this approach is to some extent implemented in aircraft with mechanisms such as ailerons, elevators, rudders, flaps, spoilers, and variable sweep. There is now research into actively morphing wings for full-sized aircraft (see for example: Scientific American, November 2003)
• there is a parallel between the stability-providing geometry of the hang-glider and the geometry of some birds' wings: both can be envisaged as conforming to conical surfaces
• this awareness of conical geometry can be helpful in sail design
• the more adaptive foils can be, whilst remaining under control, the greater the potential range of activity and efficiency they have.