HOW IT WORKS

An Aircraft Aerofoil

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We have all seen an aeroplane wing and we know how it works. One of the distinctive features of the aerofoil is the smooth and aerodynamic leading edge.

 

Now imagine the shape of a traditional sailing yacht mast laying along the leading edge... That is what a modern yacht's mainsail looks like!

More disturbance in airflow around the mast leads to higher drag, degrading sail performance significantly. 

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What makes a Perfect Sail?

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Why are sail forces important?

Only a small proportion of the aerodynamic force is converted into forward thrust. Upwind Drag increases the side forces and reduces the Thrust.

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The double element rigid wing sails being used in the America’s Cup have

very good aerodynamic characteristics. 

But they suffer a few practical

problems

• Difficult to install/stow

• Fragile and complex

What is Required

Good aerodynamic characteristics

– High Lift/Low Drag

– Large angle of attack range

– Must work on Port and Starboard tacks

• Robustness

Ease of handling/stowage

Low weight

Low cost (relatively)

Must be able to support other sails

– headsails and spinnakers

Must be able to be reefed to reduce area

Must be able to be stowed easily and on

board

Aerodynamics

There have been numerous studies into the development of aircraft wings. However, these findings are not directly transferable to marine applications.

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Key points for Sail Aerodynamics

• Laminar flow is unlikely to exist

– If it does it will be difficult to maintain

• Wing twist is very important due to wind shear

• High lift is important

– Asymmetrical wing sections

– Relatively high camber

• Ability to change section shape is important

– Wide range of operating conditions

• Ability to reduce area (reef) is important

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3 FLOW CONDITIONS

LAMINAR

– Molecules slipping over one another

– Surface molecules stopped/outer

layer moving at the velocity of the stream

– Low friction/low energy

– Easily disrupted

TURBULENT

– Layers of molecules mixing

– Thicker layer than laminar

– Very small eddies

– Higher friction

– Thickness increases towards trailing edge

SEPARATED

– Large eddies

– Highly disruptive to flow stream

– High energy loss/drag

Laminar to Turbulent Boundary

Layer Transition

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In perfect conditions we would predict the laminar to turbulent transition to occur at as far back as 50% of the chord (0 deg AoA). At 8 deg that drop to 5-6%. With bugs and dirt on the wing we would predict the 0 deg AoA transition to occur at 4.5% and at 0.5% for 8 deg AoA.

Laminar flow on a yacht sail is a myth!

• The above was just an example of a minor disturbance (in

this case surface roughness)

• Things that will destroy laminar flow:

– Large eddies from surface waves

– Pitching and rolling of the boat

– Small irregularities in the mast/sail such as mast tracks, stitching, creases and wrinkles

– Halyards and ropes in front of the sail

– Other sails

– Other boats

• Designers should not aim to get laminar flow at the expense of more practical requirements

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If you are going to trip – trip early

Studies have shown that forcing the early transition from laminar to turbulent can be beneficial

– Can reduce overall drag (by reducing energy in the turbulent boundary layer)

– Can make the aerofoil more robust to changes in angle of attack (the higher energy in the boundary layer at the leading edge can delay leading edge separation)

– Can make the aerofoil more robust to irregularities

(for the same reason as above)

The AWS semi rigid wing (SRW) concept

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• A counter rotating mast

• Rotation pushes leeward battens into compression and pulls windward battens into

tension

• Outhaul (tension along the boom) controls additional camber

• Other design features control span wise section shape

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