If you watch a heavy river bird such as a swan, it will often fly just above the water, with the tips of its wings on the edge of dipping in. In doing so, it harnesses two powerful effects: (1) If its wing tips just touch the water’s surface, the leak path is sealed, the low pressure on the suction surface is not compromised and the wing hence becomes much more efficient. (2) The downwash of air behind the wing (created by circulation) reacts against the river’s surface, creating a higher pressure underneath the wing – a phenomenon known as ‘ground effect’. Turn this upside
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