Scientists have debunked one of the most commonly held myths in science – how an aerofoil produces lift
Aeroplanes fly because the air pressure underneath their wings is greater than that above, producing lift. But engineers have for years been frustrated by a theory which wrongly explains what causes that change in pressure to occur, According to a report by David Millward and Nick Collins. The myth is repeated in school textbooks and flight manuals, and is so widely believed that even Einstein was rumoured to subscribe to it.
Now a Cambridge scientist has become so fed up with the false explanation that he has created a minute-long video to lay it to rest once and for all. The video, published on YouTube by Prof Holger Babinsky of the university’s engineering department, seeks to explain in simple terms why the myth goes against the laws of physics. According to conventional wisdom the pressure change happens because the air on the curved upper surface of the wing has further to travel than that below the flat, or mildly curved under surface, meaning it must travel faster to arrive at the trailing edge of the wing at the same time. In fact the real explanation is nothing to do with the distance the air has to travel.
The curvature of the wing causes the change in air pressure because it pulls some of the air upwards, which reduces pressure, and forces the rest beneath it, creating higher pressure. A law known as the Bernoulli equation means that when pressure is lower, air moves faster – so the air stream above the wing does move more quickly than the one below, but this is not what causes the difference in pressure.
Prof Babinsky proved his theory by filming smoke passing across a wing. If traditional wisdom had been correct the smoke above and below the wing should have reached the trailing edge at the same time.
The video demonstrates that the explanation is fundamentally flawed because the plume above the wing reached the trailing edge much sooner than the plume below. If it was simply the distance the air had to travel that was causing the pressure to change, then a boat’s sail – where the air travels the same distance on the inside and outside of the curve – would not work, Prof Babinsky said.
He added: “I don’t know when the explanation first surfaced but it’s been around for decades. You find it taught in textbooks, explained on television and even described in aircraft manuals for pilots. There is no law in physics which states when streams of particles start at the leading edge of the wing they should reach the trailing edge at the same time. I’ve even heard a story that Einstein drew a design for an aircraft wing with a long, squiggly line on top of an aerofoil to make the distance for the air to travel greater, but this would not work.”