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The Bernoulli

BERNOULLI

Most physics teachers explaining fight will begin with Daniel Bernoulli, a Swiss mathematician and renowned theorist of his day (1700-1782), who pioneered work in harmonic vibration, kinetic gases, and, of course, fuid dynamics. I like to mention that he was hired by some monks to get water out of some deep wells. Understanding pressure differentials inside of pipes was critical to the task. Bernoulli's work with fuids was eventually adopted as the correct way to conceive of and predict airflow. His body of work was completed nearly 180 years before the Wright brothers flew. It took even longer for somebody to connect Bernoulli's work and flight since the discoveries were separated by time, distance, and function. Ah, science . . . My brother Jim, who just received an electrical engineering degree, was taught the following: Consider the air to be like a slab of jelly. An air craft wing will slice through the jelly, which will part as the leading edge of the wing penetrates. The jelly on top and bottom get deflected by the surface of the wing they touch.

A wing that's curved more on the top will cause the upper jelly mass to move a greater distance as it follows the curve. From Bernoulli's work, we know that a faster-moving fuid in a closed environment causes a lower pressure. So, naturally, the pressure on top of the wing is lower. What dowe call that lower pressure? Lift. The causal relationship is usually stated something like this: the faster-moving jelly (air) causes a lower pressure. If this all sounds confusing, which illustrates this concept.Then the professor whips out all the differential calculus to quantify the jelly, the wing surface, and the speed. It's all integrated into Bernoulli's equation. The professor takes a bow, and as the chalk dust settles, you believe another mystery has been solved through the wonders of math and science. Uh, not quite. Let's review. Bernoulli was working with a closed system: water in pipes. The sky doesn't feel all that closed to me. Some argue that the static pressure from the atmosphere creates a closed system. Perhaps, but that would beg the question

"What constitutes an open system?" Next, the idea implicit in the jelly metaphor is that the jelly on top of the wing gets to the trailing edge of the wing at the same time as the jelly on the bottom. It's an idea referred to as the EQUAL TRANSIT THEORY. That ain't true either. If you color the air streams differently and send bursts of colored air over and under the wing, you can see the air on top gets to the trailing edge first! It's definitely moving faster than the air on the bottom, so the equal transit notion is twaddle. Oops! Let me say, at this point, it's much easier to poke holes in theories than to stitch together a really good one.

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