Saturday, 16 March 2013

Bernouille turned upside down

A year or two ago I saw a film on YouTube which appeared to debunk  the Bernouille Principle as a way of explaining the lift of a wing. Student pilots are always taught that an aeroplane flies because of the shape of a wing, which because of its shape experiences negative pressure over its top surface so that it is effectively sucked upwards (or forced upwards by the greater pressure beneath the wing). So this film said, how does that explain the fact that an aeroplane can fly equally well upside down?

I was fascinated and came away excited and confused. The film hadn't persuaded me that Bernouille was wrong, but on the other hand it obviously couldn't be the whole story.

I am reading a book called Stick and Rudder, which has apparently been continuously in print for the last 60 years and is very well regarded. The author, Wolfgang Langewiesche, says it is all due to the angle of attack, and that while Bernouille is no doubt true, it obscures the whole business: 

   "Trying to understand the piloting of airplanes by concentrating on Bernouille 
and Prantl is like trying to catch on to tennis by studying just exactly 
how the rubber molecules behave in a tennis ball"

I have only just started the book, but essentially he says that all flight is about the angle of attack of an inclined plane (surface). So that is why my hand, which is not aerofoil sectioned, rises when I stick it out the car window, or why an aerobatic aeroplane with almost no hump in its wing flies equally well upside down!

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