How to make America’s Cup catamarans ‘fly’
The impressive win of the America’s Cup by Team New Zealand is testament to the design and engineering skills in this country, hopefully ensuring a bright future for its boat building industry.
The design of the sails of these yachts is based on the same principles that govern aircraft wing design.
It may therefore be a surprise to know that the explanation given in many aeronautical engineering books of how planes fly and how yachts sail, is wrong.
In 2003, Professor Holger Babinsky of Cambridge University was determined to debunk a pervasive myth about the theory of wings.
Nervous flyers and yachting enthusiasts may find this sentiment somewhat disconcerting, however 100 years after the first flight, Babinsky corrected an important detail about how aircraft fly and yachts sail.
There is no debate in general terms about how wings enable flight.
When air encounters the front of a wing it divides into two streams. Imagine two parcels of air as they simultaneously leave the front of the wing, one above the wing and one below.
The shape of the wing causes the air flowing over the top of the wing to spread out, that is occupy a larger volume, lowering the pressure of this air.
Lowering the pressure allows the air to move much faster over the wing. On the other hand, the air flowing under the wing slams into the wing, pushes air in front of it and is squeezed.
This squashed mass of air is denser and higher pressure. The consequence of these air flows is that they produce a difference in pressure across the wing.
A physicist would couch this in the following terms - the shape of the wing causes a lower pressure above the wing and this accelerates the air.
An accelerating mass of air passing over the wing delivers a downward force on the wing which is accompanied by an equal and opposite reaction force, upwards, called the lift force.
An example of the power of pressure difference is given by a commercial jet flying at high altitude – if the door was suddenly opened, the pressure difference between the pressure inside and outside of the aircraft would cause anyone near to the door to be hurled by a huge force into the low pressure abyss outside.
So far so good, but the detailed key question is: do both air parcels that simultaneously hit the front of the wing, one above and one below the wing, take the same time to reach the back of the wing?
A widely held theory is that they do, it is called ‘‘the equal transit theory’’. It is wrong and this was the error that Professor Babinsky went to some pains to highlight.
The mistaken rationale is that because of the curvature of the front of the wing, air passing over the top of the wing has further to travel than that flowing below the wing – if their transit times are the same this means the air above the wing must move faster.
It is indeed faster but the truth is, the air flowing across the top of the wing flows much faster and is long gone by the time its sister parcel reaches the back (trailing edge) of the wing.
If you assume an equal transit time, the air speed (more accurately, velocity) you use to calculate the pressure difference would be too small and result in you calculating a smaller lift force than actually occurs. ‘
Professor Babinsky carried out a series of detailed experiments to debunk the ‘‘equal transit theory’’. In his words this theory ‘‘.. introduces misconceptions and uses a nonsensical physical argument…....’’.
The wing-shaped sails of the America’s Cup yachts are designed to encourage as large a pressure difference as possible across the sail to lift and drive the yacht forward.
The skill of the yachtsmen is to steer the yacht into winds that generate higher pressure differences across the sail, increasing the velocity of the air over the sail and maximising this force.
Thanks to outstanding aeronautical and hydrodynamic design, yachting skill, clever hydraulic and electronic innovations, plus terrific team work, Team New Zealand triumphed.