Questions, Not Answers Or The Physics of Flight

Richard Feynman has been quoted as saying something like, “I think I can safely say that nobody understands quantum mechanics.”  I’ve also heard it phrased as, “If you think you understand quantum mechanics you clearly don’t understand quantum mechanics.”

Bee Movie, another typically mediocre Dreamworks Animation product, begins with a title card reading,

According to all known laws of aviation, there is no way that a bee should be able to fly. It’s wings are too small to get its fat little body off the ground. The bee, of course, flies anyway. Because bees don’t care what humans think is impossible.

Cute, although not true.  There was a time when when science couldn’t explain the flight of a bee – in fact there was a time when science couldn’t explain anything – but thanks in part to high-speed digital photography the flight of the bee is now understood.

It took until 2006 for science to understand how bees fly.  I think there are two primary reasons for this: 1) the technology didn’t exist to properly examine the motion of the wings, and 2) it really didn’t matter.  Point #2 is the important one here.

Ira Flatow of NPR’s Science Friday gave a talk called, “Why Airplanes Fly – A Modern Myth” on an InSight Cruise.  I’m not aware of any place this is available in writing, but the talk is available in audio format at podcast.geekcruises.com.  Look for episodes 137 – 139.

Flatow explains the two primary schools of thought on how flight works (the  Bernoulli camp and the Newton camp) and presents a compelling argument against the prevailing wisdom (the  Bernoulli camp).  One of the more interesting bits to me about the talk was was his description of the origin of the Bernoulli idea in popular wisdom.  Flatow explains that it dates to the beginning of commercial aviation.   When the commercial airlines first started people weren’t so excited about jumping in planes because they weren’t understood.  To help ease people’s fears the public was taught about the Bernoulli principle.  Using Bernoulli it takes about 2 minutes to explain how planes fly, and you can do it even quicker if you have a pencil and paper.

But, there’s a problem.  It’s not that the Bernoulli principle is necessarily wrong, it just leaves a lot unexplained.  For example it doesn’t explain inverted flight, helicopter flight, or the low takeoff speeds of large aircraft.

For more on the Bernoulli vs Newton debate I recommend this great little article on the NASA site.  But my point is that the physics of flight are very, very complicated, and there is hardly anyone on the planet who understands it.  Pilots certainly don’t.  They know which levers to pull, but that doesn’t mean they understand the physics of lift.

Feynman and Einstein are both credited as saying something along the lines of, “If you can’t explain it to a six year old, you don’t really understand it.”  I’m going to combine that with Feynman’s idea on quantum mechanics in the opening of this post and say, “If you can explain the physics of flight to a six year old you don’t understand the physics of flight.”  Why?  I’ll quote from the same NASA article:

The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton’s laws of motion are statements concerning the conservation of momentum. Bernoulli’s equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems. For example, from the conservation of mass, a change in the velocity of a gas in one direction results in a change in the velocity of the gas in a direction perpendicular to the original change. This is very different from the motion of solids, on which we base most of our experiences in physics. The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations after Leonard Euler. Euler was a student of Johann Bernoulli, Daniel’s father, and for a time had worked with Daniel Bernoulli in St. Petersburg. If we include the effects of viscosity, we have the Navier-Stokes Equations which are named after two independent researchers in France and in England. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations.

It’s never been critical to anyone that we understand how the flight of bees works, so we never had to make up a story about how it works.  That’s not how science happens.  When science doesn’t have an answer it seeks one.  If doesn’t find an answer, it keeps looking.   The journey is the reward.

And one more thing Mr. Feynman said – “God was invented to explain mystery. God is always invented to explain those things that you do not understand.”

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