# Forget bernoulli’s theorem - Wolfgang Langewiesche's Stick and Rudder

So says Wolfgang Langewiesche:

FORGET BERNOULLI’S THOEREM

The angle at which the wing meets the air; what does it mean?
To understand this, you have to go back to a simple idea of how a
wing really manages to fly. how lift is developed. When you studied
theory of flight in ground school, you were probably taught a good
deal of fancy stuff concerning an airplane’s wing and just how it
creates lift. As a practical pilot you may forget much of it.

All the fancy physics of Bernoulli’s Theorem, all
the highbrow math of the circulation theory, all the diagrams show¬
ing the airflow on a wing—all that is only an elaboration and more
detailed description of just how Newion’s law fulfills itself—for in¬
stance, the rather interesting but (for the pilot) really quite useless
observation that the wing does most of its downwashing work by
suction, with its top surface. Trying to understand the piloting of air¬
planes by concentrating on Bernoulli and Prandtl is like trying to
catch on to tennis by studying just exactly how the rubber molecules
behave in a tennis ball when the ball hits the court and just exactly
how the catgut behaves in the racket when the ball strikes: instead
of simply observing that it bounces!

This post is what reminded me of this book.

Question is how much, or what kind of theory is needed to master tasks which are not intuitive? Or contrary to common sense?

What’s needed is not theory per se but a useful mental model.

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Kennebec Captain,

I normally just lurk here at GCaptain, because I am only a part-time amateur mariner that can learn a bunch by listening in.

But I am both a commercial pilot and aeronautical engineer. I have always been both fascinated and appalled by how some pilots describe the theory of lift and other aerodynamics. Now I get to listen to mariners discuss the same things !

Langewiesche’s book is a perfect example of boiling down the theory into the needed information, but some people just have to hear a “back story” to retain or believe the information that they really need. ( Which in this case is the effect of angle of attack, and nothing else ! )

As a practicing aeronautical engineer, we have a saying:
“all models are wrong, some are useful”

So as you said, people who are working on a ship need mental models that allow them to correctly evaluate and act in real situations.

The naval architects need the fancy math, not the operational people.

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I agree, it’s often the case that digging in deeper is useful. It does seem like Langewiesche is yelling in that section.

That’s the other interesting part of Earl’s post. He mentions Alan Turing’s paper which is outside of Turning’s area of expertise. As it happens I ran into something about that just the other day.

1. Alan Turing contributed to virtually every field, but in some sense he did only one thing. Von Neumann did more than one thing, did he do two? He too contributed to virtually every field.

Point being sometimes digging into an area that is well known can help in areas outside our immediate area of expertise.

I always stressed that phrase “it is to be hoped” in Turing’s statement when lecturing students. It showed his depth of understanding of the modeling process: the choice of features in any model is purely arbitrary, and only time will tell if you got it right. In the case of Turing Patterns, it looks like he did.

Similar to what is known as the “Golden Guess” in cryptanalysis.

Cheers,

Earl

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The physics of aerodynamic flight versus controlled angle of attack are really two different things, but the bigger point is completely sound: a good stick and rudder guy can say, “Bernoulli who?” and not be ironic. As a matter of fact, I’ll bet I’ve known some ultralight pilots that knew very little of the physics involved but were very intuitive stick and rudder guys.

Earl also posted this - the term “connections” is apt

Has Bernoulli, and the “Kutta-Joukowski lift theorem,” but also this bit:

On December 17, 1903, the Wright brothers
achieved the first successful powered flight.
They did not use the lift theorem, but took an
empirical hands-on approach. Their airfoil
designs were guided by data they collected from
their wind tunnel. In 1909 Wilbur Wright
wrote, “…I think it will save me much time if I
follow my usual plan and let the truth make
itself apparent in actual practice.”

Stick and Rudder is without a doubt one of the seminal works of early aviation. One of my favorites and one that I shared with my flight students years ago.

I like how he called the elevator “flippers”. Also how he said rudder pedals were useless and that planes probably wouldn’t have them in another 10 years. No one’s perfect, I guess.

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In my teens I learned to sail a small boat, long before I learned the theory, dynamics and applications of fluid flow in both USN Nuclear Power School, and in my later mechanical engineering curriculum.

So I understood exactly what the telltales 8-12 inches back from the luff of the sail were telling me about my foil shape and angle of attack, and how to correct/optimize it, when I had no idea about Reynolds Number.

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Aircoupe got rid of the rudder pedals by linking the rudder to the ailerons but it didn’t last.

Coupled rudder and ailerons are quite common on light aircraft. Mooney, Bonanzas, Cirrus, Citation 510 … probably a few more that I don’t know of but the 'Coupe is the only one I know that dispensed with the rudder pedals altogether. I had one for a couple of years, it was a hoot to fly, you could turn left by sticking your arm out the open canopy.

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This topic came up elsewhere for me about a week or so ago and it basically came down to, ‘we don’t know exactly why flight works but we can correctly calculate what will successfully fly and that’s what matters.’

Indeed. The map is not the territory, and different maps are good for different things. Physics isn’t the Truth, and neither is an engineer’s model, and neither are our various mental models that allow us to get predictable results from our daily actions. They are all models which are useful for different applications.

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Was excited by Wolfgang’s book when I was getting every rating possible in the mid 70’s. Was counseled by the Chief Instructor to NOT breathe a word of that book, that man nor none of his explanations to the FAA examiner when taking my multi-engine flight test oral. Why? The feds HATED that book and the author for some reason.

I think they considered his explanations too folksy. It was an unspoken rule to not share the book with students.

So we know how solid sails work, like aircraft wings
How do you explain sailboat sails with only one side of the foil profile?

Maybe hang glider style but they are only falling out of the sky?

It’s a thin section cambered airfoil.

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yes very thin…1mm?

It’s like the celestial sphere. It’s an abstract concept that only makes learning celestial more complicated than it needs to be but almost every book discusses it.

Yup, it is a very effective profile at low Re, hence its widespread use by birds and slow flight RC designers.

Speaking of mysterious profiles, I’m very intrigued by the kfm series, mostly because of the dissent over why it works and why it doesn’t scale.

However, we’re getting into the exact opposite of the topic, so maybe a new thread is in order.

The same way. Solid foils are not necessarily ideal. They need a way to be strong enough to withstand the forces. Soft foils are supported differently for sails or kites etc with ropes or spars that are not (necessarily) part of the foil, although they can be.

The wind flows along both sides of the foil, preferably unbroken laminar flow. Because the foil is angled to the wind, the flows over the different sides are at different speeds in order to rejoin after exiting the foil. The speed differential produces a pressure difference with the low pressure to leeward at right angles to each part of the foil. This can be resolved into two forces at right angles to each other. The larger sideways force is countered by the boat’s speed and shape and a fin of some sort eg centreboard or fixed keel. The other smaller force drives the boat ahead.

It’s essentially the same principle. There are an infinitely variable options for foils, spars and rigs. Some add thickness to the foil, especially the leading edge eg by a semi-rotating mast or foam between two layers of sail cloth but these add difficult complications to the mast’s rigging and aren’t common except in high performance dinghies and catamarans.