The accepted explanation for Bernoulli's principle is just not true. I certainly learned it that way and have never had need to examine it.
A lot of what slides into textbooks is either wrong or at least misleading and remains long after anyone who needs to know has gone on.
Thus new knowledge is often disregarded for decades..
.
Cambridge scientist debunks flying myth
Aeroplanes can fly because their
wings cause the air pressure underneath to be greater than that above,
lifting them into the air.
But engineers have for years been frustrated by a theory which wrongly explains what causes the change in pressure to occur.
The myth is commonly found in school textbooks and aeroplane 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 bogus 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 underneath
surface, meaning it must travel faster to arrive at the other side 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 front edge at the same time.
The video demonstrates that the explanation is fundamentally flawed
because the plume above the wing reached the edge much sooner than the
plume below.
If the distance the air had to travel 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 tailing 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."
