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Thursday, 27 Apr 2017

Lift theory was ‘fake news’, Cambridge scientist reveals

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."

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28 Apr 17
John Lorains I never understood why 2 molecules of air starting at the leading edge one going above the aero foil the other below had to arrive at the trailing edge at the same time. Equally why does the upper surface curve pulling the air upwards reduce pressure? JL
28 Apr 17
Paul Hurst The molecules from the top of the wing push molecules in contact with the wing out of the way. Result low pressure. The air molecules from the top of the wing arrive at the trailing edge before those from below. Result is a downwash of airflow. LIFT.
28 Apr 17
Peter Day Just like Evolution, which had no scientific basis.
28 Apr 17
Brian Mellor It is the Coanda effect. The airflow above the wing is deflected downwards, so "lift" is the reaction. Newton's 3rd law. And the two particles do NOT meet at the trailing edge.
28 Apr 17
barry light Propellers are a rotating wing, the push from them is downwards. Try walk behind a running engine and note the narrow angle of the "breeze".
30 Apr 17
Jane Giffould When learning to fly I heard that the explanation given was incorrect but that we had to learn it for the exam!
1 May 17
Laura Raison Prof Babinsky's video features a symmetrical aerofoil as used on a tail fin and not on a wing, and therefore is unhelpful in explaining lift.
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