PPRuNe Forums - View Single Post - So many multiple correct (or none correct) answers
Old 17th Mar 2011, 07:52
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IO540
 
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there are two explanations for lift, one uses Bernoulli and pressure distribution to justify the lift produced, the other shows that an airfoil deflects air downwards and the reaction to that downwards force is what we perceive as lift. Both explanations have their plus and minus points.
Both are equally true. They have to be.

Bernoulli derives from Newtonian mechanics. The reason Bernoulli is used in this case is because it simplifies the calcs for fluids.

For a wing to generate lift, it must redirect the airflow in a downwards direction. This cannot be done without a pressure difference being created between the top and bottom, and that cannot be done without the airflow on the top being faster than the airflow on the bottom. It's all the same thing.

The problem with those questions is that the terminology is ambiguous and that makes them meaningless. I recall one question from the PPL which was something like which of the two types of drag (induced and parasitic) increases/decreases with speed. The obvious answer is they both increase. The absolute values of both must increase. But that is the wrong answer...

The IR exam material is mostly bull, put in for a laugh. Like the questions on Glonass. But one could go on and on. If one wanted questions that test knowledge without reliance on having read a specific prescribed book (to get the terms right) one would remove about 1/3 right away. If one tested knowledge relevant to aviation one would remove about 90%.
If you could somehow deflect the air downwards without getting any difference in pressure then you would have a damn near perfect aerofoil.
That is about as physically possible as a single magnetic pole
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