Stop Abusing Bernoulli! - How Airplanes Really Fly (Paperback)

This reader concurs with the review provided by Erik Zapletal from Maroota, Australia. It is complete, factual and extensive, and favors Newtonian based aerodynamics teaching. However, some of the other reviews are not accurate, and deserve comment. A few reviewers have not really read the book, are incapable of understanding it, or are prejudiced with vested interest. Richard Asher's review has mostly only vague generalities, with only one specific but erroneous statement-- that the author claims Bernoulli's laws are contradicted by Newton's laws. In fact, the author derives Bernoulli's equation from Newtonian principles, beginning on page 139. Asher should have found this if he really read the book. Also, beginning on page 129, the lift equation is derived logically and methodically from Newtonian principles, producing exactly the same result as classically derived from Bernoullian considerations. Contrary to Asher's claim, these derivations confirm agreement between Newtonian and Bernoullian concepts.

The Scott Johnson review, applauded by Asher, and the Jan-Orlov and Jeff Noall reviews, deny the reality of circulation, claiming it to be a fiction of classical aerodynamics for mathematical purposes. Although the classical explanation of how circulation is produced is fictional, the circulation is in fact real, as Newtonian principles and common sense demand. Reacting to force of pressure difference, air ahead of a lifting wing accelerates upward around the leading edge, away from higher pressure below and toward lower pressure above. This upward movement is the basis of stall warning systems. Above the leading edge, air accelerates rearward into reduced pressure, while below the leading edge air accelerates forward, away from increased pressure. Behind the wing, air following the surfaces is left with downward movement. Thus instantaneous air movements occurring in still air around a passing wing include upward movement ahead, downward movement behind, rearward movement above and forward movement below. Overall, this movement is of circulatory nature around the wing, and is appropriately referred to as "circulation." Pitot instrumentation evidence on page five of the book supports circulation as fact. The classical explanation, using fictional induction to explain circulation, rather than Newtonian principles of force, mass and acceleration, apparently leaves Asher and others believing circulation is not real.

Aerodynamics must be treated open-mindedly as science, not as dogma or unquestionable religion. The most popular layman-level theory of wing operation, religiously taught to pilots and public school students for about 100 years, falsely claims transit time of flows above and below a wing must be equal. This, if true, would require flow over a curved upper surface to be faster than that at a more flat, and therefore shorter, lower surface. Hence, Bernoulli's law would require pressure above to be less than that below so as to produce lift. Indeed upper surface flow is faster, but much faster than equal transit time would require or even permit. This most popular but false teaching of lift is an abuse of Bernoulli's law. That abuse is alluded to in the book title.

The middle section of the book - Chapters 5 to 8 - give a simple and accessible account of most of the design features relevant to light aircraft. Airfoil basics, wing design - taper, sweep, washout, dihedral, control surfaces, etc. - and overall configuration of the aircraft, are covered, albeit briefly. However, the gist of the book is Craig's attempt to give an, as realistic as possible, explanation, of just how wings can lift an airplane off the ground. Craig does this, relying only on Newton's Laws of Motion.

Newton's laws are some of the most fundamental cornerstones of modern technology. Bernoulli's theory is itself derived from Newton's laws. But Bernoulli's theory is based on several assumptions - the "fine print" - for instance, no energy can be put into, or taken out of, the flow, and the flow can not be rapidly curving. These assumptions yield a simple theory, but in practice, the "Bernoulli" explanation of wing-lift, is best described by the quote from Von Karmann, (page 8) - "When you are talking to technically illiterate people, you must resort to the plausible falsehood, instead of the difficult truth".

Craig accepts that the "induction theory", as taught in universities, does give the correct results. However, he is not happy with the attribution of "cause and effect" that is given in many of the standard texts - namely, that wing tip vortices are the "cause", and "induced" downwash and drag, are the "effects". Craig's view is that the moving wing "causes" the downwash, which in turn is diverted sideways, then upwards behind the wing tips, which in turn results in the wing tip vortex "effects". Conventional texts use the "vortex first" approach as an expeditious means of arriving at the right formulae for induced drag. However, as Craig states in Chapter 9 (titled "Does It Really Matter?") - "students who are paying for.. an education.. deserve more than to be taught these unsound concepts..".

As Craig explains, the "vortex first" approach may be the result of aerodynamic "induction" theory borrowing heavily from electro-magnetic (EM) theory. Most EM textbooks claim that it is the electric current travelling through the wire, that induces ("causes") the EM flux around the wire. However, some "heretical" electromagneticists prefer to think of the EM flux travelling outwards, through space, from the battery (the "cause"), and along the way, dragging the electric current along the surface of the wire (the "effect"). The latter view is not often taught, possibly because the average student can more easily relate to the finite, almost tangible, "spark" in the wire, as being the "causual" agent, while the fluxes in the infinite, evanescent, EM field, are only passing "effects".

Some of the negative reviews of this book may be the result of Craig's terse style of writing. Terse is good, when presenting simple facts. But with the difficult task that Craig has set himself, a more patient, explicit, and long winded style, might have worked better. Craig usefully could have spent a whole chapter, very carefully worded, describing the development of the flow field around a 2-D, flat-plate airfoil, as it is accelerated from rest, up to some finite, steady speed. Another whole chapter could describe the flow field development around a 3-D wing. To best convey the information, these chapters would have to use carefully drawn diagrams, or better yet, photographs of real flows.

Unfortunately, it is with some of his diagrams that Craig makes the same serious errors, as do almost all other books on aerodynamics. Figure 1.8 shows the airflow approaching the airfoil horizontally, and then departing at a downwards angle. The main problem here, is that the departing streamlines are significantly lower than the oncoming streamlines. The "non-viscous" flow of Figure 3.7, shows this horizontal displacement even more explicitly. Even with the idealized "non-lifting" flow of Fig 3.7, such a displacement of the streamlines would imply infinite lift! With 3-D flow there is a temporary downflow of the trailing streamlines, but these soon curl sideways, and then upwards, to temporarily move above their upstream positions, and so on.

The point is, the power of the misinformation in these diagrams, is so great, that many very clever students, will swear that an airfoil intercepts horizontal streamlines, and then sends them off, on a forever descending path. The fact that Craig so often stresses the "upwash" that occurs in front of the wing, in the text, makes one wonder why he didn't emphasize the upwash, in the drawings.

Craig also questions whether viscosity is a necessary precondition for airfoil lift. The standard theory, based on the "Kutta condition", is that viscous forces resist the airflow whipping around the sharp trailing edge (TE) of the airfoil (as shown in Fig 3.7). This viscous drag is supposed to cause the trailing stagnation-streamline to move down to the TE, which results in the simultaneous generation of the starting and bound vortices, and the development of lift. Craig leaves the question open, but this writer can see no reason why inertial forces, acting on the rapidly accelerated fluid mass near the TE, could not generate the starting and bound vortices, and thus lead to lift.

Overall, this book is a good read for both students, and teachers, of aerodynamics.

However the author makes a big mistake by mixing an theoretical, mathematical calculation model and the 3D airflow around a finite wing. Real air does not circulate like the author shows on page 129. Its impossible for the real airflow to pass TE that way.

The Circulation on page 111 Figure 8.7 Biplane Circulation Interference, would make the big frontfan on Boeing 777 GE90 engine work very bad in a real world airflow.

I have always been taught, that the mathematical applied Circulation around a wing in a "perfect, hypotetic, non existing fluid" only was superpositioned to adjust the physical unreal Potential calculated flow field showed on page 43, Figure 3.7 A Concept of Non-viscous Flow Passing an Airfoil.

From 1754-1904 The Potential calculated flowfield could not be corrected to look like real fluid flow Field until a mathematical superpositioned Circulation flowfield was invented And applied.

The mistake the author also makes is to say the all wings are infinite and all airflow thereby will be 2-dimensional and upwash will be equal to the downwash.

The Circulation in real air is spanwise, not flowwise like the author writes.

Where I live I can see and hear every morning how Bae146 flies over at 800 feet. After some 50 seconds "the wake turbulence"= (wingtipvortices and the downwash from the wing together) comes down to the ground, hitting my hat. But there is no upwash lifting my hat before the aircraft arrives.

The mixing of Calculation Theory Models and real air behavior is very common in many books, especially in the US.

I hope that the author in the next edition of the book, changes his Lift explanation to a physical more real one, explaining how "pressuregradients" are created by centrifugal forces and how these pressuregradients "sucks" in air from higher pressure regions.

A wing is just an airflow deflector. A Sail makes the same for a boat. By disturbing the free airstream (changing it's direction) pressure gradients appear.

The Circulation Lift explanation here is some sort of "perpetum mobile"