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Aerodynamic myths

 

One truism in aviation is that the more complex a theory, the more likely pilots are to simplify it in order to understand it.

I’ve done this, and I’m sure others have too. In fact, my idea of a good theory is one that requires one word for the explanation and a colour picture next to the word! Put together 50 pages of one word-picture combinations and you’ve created the world’s best aerodynamics book! Wouldn’t that be nice?

Unfortunately, the world isn’t nice and we pilots are forced to choke down various aspects of that overly complex aerodynamic hocus pocus. In our feeble attempt to unpocus the hocus, some facts about aerodynamics have slipped through the cracks and thus myths have taken the place of otherwise legitimate theory.

Take lift for example; we (the aviation community) have been advocating the wrong ideas about lift production for better than 30 years. No one person or organization is really at fault for starting the myths I’ll talk about this month, but many of us have promulgated the myths to several generations of pilots. Luckily the misinformation deals less with how to fly a plane and more with how a plane flies!

Let me start our myth destruction by saying that I didn’t bust these myths myself, (remember - I’m the one word-picture sort) but rather NASA has finally corrected our erring ways. If you are an aerodynamic engineer then I’m probably wasting your time because you already know this stuff. However, if you are the garden variety of pilot, then you might learn something from this article.

The first myth that NASA busts is what many have called the equal transit or longer path theory. This theory promotes the novel idea that those little air molecules that come into contact with the leading edge of the wing are actually really close friends. As a result of the wings’ interference with the air molecule party going on at the leading edge, some molecules have to go over the top of the wing, while some go underneath.

Being close friends, the air molecules join up with their buddies at the trailing edge of the wing. Doing so necessarily requires the air over the top of a normal wing to go faster in order to make the meeting at the trailing edge. Following from Bernoulli’s well-known relationship between velocity and pressure, the increased speed over the top of the wing reduces the pressure and it’s this pressure differential that creates lift.

Two problems exist with this three-part theory. First, we have to have an airfoil with an upper surface that is longer than the bottom surface. However, symmetrical airfoils (same shape on top as on the bottom) generate lift just fine as it turns out; and then there is the problem of explaining sustained inverted flight. The latter would be impossible if the top surface (now facing the earth) was still producing the lift. I’ll let you ponder that one on your own.

The second part of the theory is that the air molecules seek to magically join up somehow at the trailing edge. Not only has this supposed equal transit phenomenon been debunked by wind tunnel tests but, in reality, it is not required to create the pressure differential.

The third part, about pressure differential producing lift, is really the only part of this explanation that is correct. So we see that there is a part of this theory that are correct and other parts not so correct.

How did this theory come about? Most likely the result of assumptions about what must be occurring; an assumption most likely made by people who were not in possession of all the facts or didn’t understand the aerodynamics well enough to condense it into pilotese.

Myth #2 that NASA debunks deals with the so-called Newton’s third law which most of us refer to as the action-reaction law.

Typical textbook explanations show airflow hitting the bottom of the wing and deflecting back off at a downward angle, thereby producing the action, hence upward movement of the wing being the reaction.

This is not to say that Newton was wrong however. In fact, his third law does indeed exist, but cannot be correctly used to explain lift production.

"Newton’s law of lift", also called impact lift by some books, is concerned with only the bottom of the wing. In reality air molecules hit all portions of the wing, regardless of angle of attack, and thereby impart momentum to more than just the bottom.

Airflow over the upper part of the wing has been clearly shown to create a downwash effect off of the trailing edge and Newton’s law of lift has nothing to contribute on this phenomenon.

Likewise, since this law deals only with the bottom of the wing, one would expect that two wings with the same surfaces would produce the same lift. Surprise, they don’t create the same lift when the upper surface is different.

Ok, so we have some questionable aspects of Newton’s law of lift to chew on. But wait, there’s more! Given what is known about air density, the predictions of the force that should be created by action-reaction are apparently way off base when considering actual flight data. I’ve never actually proved this, but some pretty smart people have!

This has lead some to the simple-minded conclusion that Newton’s law of lift and Bernoulli’s law both play a part in producing lift. Nice try, but that’s an attempt to preserve the good name of Newton when we really shouldn’t bother. Besides, there were no airplanes during Newton’s time, so I don’t think he’d be too upset.

Anyway, you might now being wondering how any of Newton’s laws play into the lift equation. It’s not so much that the Newtonian explanation is wrong, but that it does not contribute nearly as much for which it is given credit; at least it doesn’t contribute much in the way we’ve read about for the last 30 or 40 years.

NASA does give this theory some credit, but only when the aircraft is going very fast in very thin air. But they’re talking about hypersonic conditions, and for normal flight conditions this theory of lift breaks down under scrutiny.

The last myth deals with the venturi theory of lift production. Although Bernouli’s theory about velocity and pressure relationships within a venturi type of structure are indeed true, it is not correct to replace the bottom half of the venturi with a curved wing shape and also assume that the top half of the venturi somehow exists within an actual air mass.

For starters, the venturi theory would propose that only the top of the wing is responsible for lift. If that were so, then Newton’s lift theory can’t be right. Hmmm, kind of a circular problem here.

In any event, the bottom of the wing does produce lift, but not to the extent that the Newton theory would predict, so to not consider that portion of lift production already calls the venturi theory into question.

Another problem with venturi theory is its inability to predict lift from a flat wing shape. Air tunnel tests prove that a completely flat surface will produce lift at a positive angle of attack. However, a flat plate at a positive angle of attack will not constrict the airflow within a venturi so there is no theoretical increase in velocity as a result of a constriction and hence, no change in pressure.

The only way to constrict the imaginary venturi with a flat plate structure would be to set it at a negative angle of attack with the front edge touching the bottom of the venturi. The venturi theory fails to properly predict that the negative angle should produce the opposite of an airfoil with positive angle of attack, e.g. higher pressure within the "venturi" and thereby a lower velocity.

Using the venturi theory, a flat plate at negative angle of attack will actually produce the same result as an airfoil with upper surface curvature.

The only part of Bernouli’s venturi theory that does apply to flight is the idea of pressure differential occurring between the top and bottom of the lifting surface. The relationship of velocity and pressure within a fluid holds true and thus if one of these values is known, the other can be calculated.

With those two values in hand, forces can be calculated and lift can be calculated. However, if you produce these values in flight and calculate the lift being produced, it does not match what you’d get if the airfoil were in an actual venturi. Bottom line: air just doesn’t exhibit the rigid properties of a venturi.

So what theory is correct if these three common explanations do not entirely make the grade?

NASA points to circulation theory as a better explanation of what’s really going on around an airfoil. But please don’t ask me to explain circulation theory in any upcoming articles. It’s too new for people like me to be able to explain it with any competence.

Hopefully I didn’t shatter anyone’s world with the debunking of these three aerodynamic myths. Personally, I was happy to see that NASA finally stepped up to the plate and explained the error of our ways.

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 180 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.