Customer Reviews

Stop Abusing Bernoulli! - How Airplanes Really Fly

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Stop Abusing Bernoulli! properly presents aerodynamics of airfoils, wings and airplanes in terms of Newtonian physics. In 1972, NASA scientist Norman K. Smith wrote in The Physics Teacher: "For explaining dynamic lift, the result of an encounter between a fluid and a lifting device, Newton's laws must be used. Consolidation of all dynamic forces produced in a fluid-- propulsion, lift, control, etc.-- under Newton's third law is not only correct physics but also makes the whole business far easier to teach and to learn." More recently, in a paper presented to the 2001 meeting of the American Institute of Aeronautics and Astronautics, Dr. Jaako Hoffren, of the Helsinki University of Technology, also proposed teaching more directly in Newtonian terms.

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.

In "Stop Abusing Bernoulli!.." Gale Craig gives a good explanation of the mechanics of flight and a thought provoking account of how aerodynamics is taught.

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 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.

This book describes and corrects many misunderstandings that can be found in most aerodynamic books for pilots and schools. The book shows a lot of examples where "The Bernoulli effect?" is said the create forces on wings, rotating balls/cylinders, papersheets. The "longer airflow path over the curved wing theory" and "halfventuri theory" is shown here how false the are. That's very good. These facts must be repeated over and over again to get results.

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"

The concepts taught in this book violate known laws of fluid dynamics which have been experimentally validated. There is no problem with describing the generation of lift using Newtonian momentum principles in a global sense, but this does not imply that classical circulation theory is invalid! This book is written with a great deal of misunderstanding of fluid dynamics. Fluids cannot be analyzed as a discrete number of particles which act independently of each other. Due to the continuum nature of fluids, air flow around a wing cannot flow in a "regenerative" pattern as proposed in this book. This can be seen experimentally to be impossible by analyzing the velocity field around a wing using a flow visualization technique.

I am a aerodynamisist working at Pratt & Whitney (maker of jet engines), and have a graduate degree in mechanical engineering, specializing in fluid mechanics. I design airfoils for the turbo-machinery in the jet engines we design.

As an aerodynamisist, I can attest that if the author's (Mr. Craig) speculative ideas were indeed correct, then jet engines simply would not work! We have a wealth of experimental data which demonstrate that the author's explanations of how airfoils work are simply not correct.

I read Mr. Craig's book out of a sense of curiosity. I was very disappointed to find how completely bogus the book is. I noticed several mathematical flaws in the author's derivations, but in going to write this review I noticed another reviewer (Mr. Scott Johnson) has already addressed the same topic (Please refer to Mr. Johnson's excellent review. You must select to read all the reviews to see Mr. Johnson's review since it was written a couple of years ago).

I will not repeat the same material as Mr. Johnson wrote. Instead I would like to bring out an additional point. Only those who lack an understanding of fluid mechanics would seek to argue whether the principles of Newton or Bernoulli accurately account for how wings generate lift. Bernoulli's laws do not contradict Newton's laws and visa versa. Any results derived from Bernoulli's laws will satisfy all of Newton's laws of motion. Newton's laws of motion are very general laws applicable to an incredible variety of situations. It is this generality that makes Newton's laws so powerful. Bernoulli's laws are much less general (they apply only to fluids), but they give additional insights, which Newton's laws don't provide, about how fluids behave. Bernoulli's principles give us more information about how fluids obey Newton's laws of motion. It is thus absurd to suggest that a explanation of wing theory using Bernoulli's laws contradicts a Newtonian explanation as the author of this book claims.

Unless you are looking for a reading in science-fiction, I do not recommend this book.

I read this book after a long, but independent, study of the aerodynamic literature. Trained in Physics at the University of Californa at Santa Cruz, after becoming a hang glider pilot, I wanted to seriously understand the principles of flight. I enjoyed reading this book a great deal more than many of the other texts I had read previously, but is that really saying much?
The argument continues about whether Bernoulli or Newton should be the basis of an "explanation" of how wings work and why flight is possible. For me, after long thought, the principle that a wing (whatever this is) dislaces air downward is the basis for all further refinements about how this is possible.
This book is not perfect and leaves many questions unanswered as pointed out in other reviews, but it is a perfect counterpoint to the main aerodynamic textbooks and really points out where they fail - their lack of "explanatory power." They substitute mathematical (simplified at that, and not necessarily relevant) notation when they should be trying to apply reason. They fail to answer simple questions, and you begin to wonder if the authors really know what they are talking about.
I notice many of the reviewers here have a similar problem trying to explain what they are talking about.

The flaws in this book are so numerous I shall not attempt to list them all. Instead, I will focus on just one: the author's derivation of the lift equation. This derivation is full of ad hoc assumptions that are completely without justification. First, the flow pattern proposed is impossible as stated by Mr. Newborg and Mr. Noall. The author contradicts himself in his rebuttal of Mr. Newborg because his mathematical derivation does assume a non-physical flow field. The author completely misunderstands what is meant by "circulation" in classical aerodynamic theory. Circulation is defined as the line integral around any closed path in a vector field. It is a purely mathematical definition and DOES NOT imply that the air circulates around the wing! The author has mistakenly interpreted the mathematical circulation as a physical circulation. The airflow as Mr. Noall points out does not rotate around the wing in the streamwise direction. For this reason alone, the author's derivation is invalid. Real wings do generate circulation (as defined!) in both the streamwise and spanwise directions; a point mistaken by Mr. Newborg. The author arbitrarily assigns the center of physical circulation (which doesn't exist anyway) to the cord mid point with no justification whatsoever for doing so. Also the derivation falsely defines the angle of attack in terms of the trailing edge instead of the cord line, as is the case in the equation the author is trying to mimic. The most blatant flaw in this derivation is that if the author evaluated the integrals properly (that is from 0 to 360 degrees instead of 0 to 180) in order to get the net effect of his "circulating air mass" on the wing, his integrals would cancel giving a net lift of zero! Instead the author only integrates through half of the domain, and then talks himself into doubling the answer in order to get the correct equation. By his faulty angle of attack assumption, he guarantees that the sin(theta) term will appear in the end result because his integrals are in terms of beta, and not theta. There is not room to discuss more of the falsehoods presented in this book, but I find it interesting to note that the author had to publish his own book.

The title is great, but the book disapoints. It appears to be self-published, not that that is necessarily bad, but Craig could use an editor. Craig's explanations of lift are badly written, confusing, and disorganized. There are better explanations of lift in terms of Newton's laws than what is provided in this book on a variety of websites. Save your money and instead google for "wing," "circulation," "lift", "newton" and other similar terms. Or check out NASA's explaination of lift on their website. I plan to get rid of my copy of "Stop Abusing" at the first opportunity.

A book that finally explains what really makes airplanes fly. Using Newtonian Mechanics - which is how everything else in dynamics is explained - Mr. Craig simply and logically teaches aerodynamics. A must buy!!

This book describes and corrects many misunderstandings that can be found in most aerodynamic books for pilots and schools. The book shows a lot of examples where "The Bernoulli effect?" is said the create forces on wings, rotating balls/cylinders, papersheets. The "longer airflow path over the curved wing theory" and "halfventuri theory" is shown here how false the are. That's very good. These facts must be repeated over and over again to get results.

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.

If I make a quickroll and fly inverted, what happens to the direction of the Circulation? 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.