Friday, February 10, 2012

Do we need to know why airplanes fly? What does this have to do with life?

Do we know why airplanes fly?
Why do airplanes fly?  Or rather, why do airplanes fly if, as it turns out, we didn't understand how they do?

To summarize some elementary physics, the four forces of fluid dynamics, drag, thrust, lift and weight, are what allow airplanes to fly.  These forces only exist in a fluid (liquid or gas).  Thrust pulls or pushes the plane forward, while at the same time, drag inhibits it.  Thrust must be greater than drag for a plane to move forward. 

Weight and lift are also opposing forces; lift must be greater than the weight of the object.  And, the object must be moving for lift to come into force.  The classic explanation for flight is that a moving object, the wing of the plane, splits the airflow in two -- both up and over the wing, and down under it.  Because the wing is curved on top, the air that goes over the wing travels faster than the air moving underneath.  As airspeed increases, its pressure falls, which means that the air above the wing exerts less pressure than the air under the wing moving more slowly, and this pushes the wing and thus the airplane up, against the opposing force of gravity -- lift.

The relevant wing shape is called an airfoil. It's shown in the above figure we grabbed from the web.  Everyone has known this since Bernoulli proposed this principle in 1738.  However, this explanation has frustrated people who know better for a long time.  The "principle of equal transit times," that is, that the air above the wing must 'catch up' with the air beneath, is just plain wrong.  As it happens, there's no reason that the air above and below must converge.  In fact, the air above, indeed moving faster than the air below, reaches the end of the wing much ahead of the latter, as this video from Holger Babinsky's lab at Cambridge University shows.

Quoted in a story at, Babinsky said, "What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes – lower pressure on the upper surface and higher pressure on the lower surface. This is why a flat surface like a sail is able to cause lift – here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it’s the curvature that creates lift, not the distance.”

Is this nitpicking?  Does it matter that we haven't really understood this?  Planes do fly, no matter whether we've understood in detail why.  And, after all, no matter how much faster the air above the wing moves, it's still faster than the air underneath, and the end result -- lower air pressure above -- is the same.  The important thing is that planes do fly.

When Ken was, long ago, a graduate student in meteorology (training to be an Air Force weather officer), it was taught that the theory was as above but that many aspects of actual fluid flow theory were inadequate for the actual complexities of airfoils.  Things such as local eddies or turbulent areas that broke up the smooth airflow could reduce lift or even be dangerous to staying aloft under some circumstances.  So engineers -- like even the Wright brothers -- used models in wind tunnels, then tests on actual prototype aircraft to tinker with the design til it worked.  Regardless of the niceties of the theory, with lots of experimentation -- engineering -- predictable function was arrived at. This is the basis of many applied sciences.  In the case of airfoils, Babinsky is hoping that a more precise understanding of lift can lead to the design of more efficient, faster planes, relying at least somewhat less on tinkering.

Why are we talking about this on MT?
But, how does this relate to MT and what we usually write about here?  We write about cause and effect all the time, in evolution and genetics and the practicalities of observational science in particular, and how cause is so often not understood, or misunderstood, or simplified.  But, sometimes it doesn't matter -- how aspirin works wasn't understood for decades after it came on the market, and how the most effective antimalarial in history, artemisinin, works is still not understood, as discussed here, and as indicated in this figure from a paper entitled, "The Molecular Mechanism of Action of Artemisinin--The Debate Continues".  

From O'Neill, Barton and Ward, 2010

When empirical demonstration of efficacy comes before theoretical understanding, application can happen without fundamental principles.  It's when application depends on principles that things can get difficult.  Should sugar be regulated?  Should we eat eggs?  Can we predict disease from our genotypes?

You can tinker with airfoils as much as you want.  Once you get a design, you can test it as often as you want.  For all practical purposes once you work it out experimentally, every wing with the same shape works the same.  That's why you can relax and order a nice drink on your flight, without looking out fearfully at the wing to see if the one supposedly holding up your plane actually works.

But that is definitely not the way life works!  All individuals today are different, and it is difficult in most cases to isolate the same genetic variant in many study subjects and assume the rest of the individuals' causal environments are the same.  Life is inherently about difference: evolution works strictly through variation, and without evolution neither we nor airfoils would be here.  That's why the same conditions don't apply in the evolutionary past any more than they do in the present.  That's why, while evolution tinkers with what it has available at any given time, it's not an engineer working towards some preconceived goal.  And it's also why it is so hard to predict traits from genomes, and why GWAS doesn't account for most variation in traits.

We cannot even take the engineering who-cares-about-theory approach when studying life, except in some relatively clear cases of, say disease, or traits like eye color and so on.  Even they turn out to be not so simple (even Mendel's peas, from which he and we developed our basic ideas about genetic inheritance, weren't totally simple).  Compared to airfoils, there is no 'theory' of a person or of evolution, beyond some rather broad generalities.  So the lesson from airfoil discoveries is.....that they are not very relevant to what so many hope to discover about life.


James Goetz said...

Was that Newtonian gravity or Einsteinian gravity? : -)

John R. Vokey said...

Yes, and how can airplanes fly upside-down? ;-)

Anne Buchanan said...

Those are trick planes, aren't they?