So we all know how planes fly, right? The top of the wing is rounded and the bottom of the wing is more straight. Air takes longer to travel over the top of the wing than the bottom, which results in more pressure on the bottom, hence the lift. Right?

As it turns out, no.

This is what I was taught, and it’s what I’ve always believed (it’s even in most lower-level text books), but it’s simply not true. This concept, called the Equal Transit Time Theory does not generate nearly enough lift to keep a plane in the air.

The main reason planes fly is far simpler: wings force air downward, which in turn force the wing (and therefore the plane) upward.

The primary actor at play here is called the Angle of Attack (AoA), and it’s easy to conceptualize. We’ve all put our flattened hand outside a car window while the car was in motion. You noticed that if you angled it straight on you could hold it steady, but if you angled the front edge upward you created massive lift that forced your arm up in the air.

The same concept works for kites, helicopter blades, sailboats, ceiling fans, and planes. In fact, winglike surfaces can generate lift almost regardless of the shape of their top and bottom surfaces. Notice that ceiling fan and helicopter blades are basically symmetrical, yet they create a downward columns of air just fine. You can actually use a barn door to generate lift…if you were so inclined.

In other words, lift simplifies nicely to Newton’s third law of equal and opposite reactions: air goes down, wing goes up.

Finally, if you’re still harboring any fond feelings for the equal transit time theory, ask yourself a simple question:

If the top vs. bottom wing shape is so important, how can planes fly upside down?

Alas, there are actually multiple ways of describing, and calculating with varying degrees of precision, the way in which lift is generated. There is the mathematical/engineering approach, and there is the physical approach, which is what I’ve described above. John D. Anderson said it best:

It is amazing that today, almost 100 years after the first flight of the Wright Flyer, groups of engineers, scientists, pilots, and others can gather together and have a spirited debate on how an airplane wing generates lift. Various explanations are put forth, and the debate centers on which explanation is the most fundamental.

The one thing the experts agree on, however, is that the way most have been taught about how planes fly (the Equal Transit Theory) is absolutely incorrect. ::

[ There are, of course, other factors at play other than angle of attack, e.g. the Coandă Effect. The point of this piece is to illustrate that the primary factor with flight is the forcing of air downward (to which Coandă contributes) rather than equal-transit. ]

References and Notes

• [ Aerodynamic Lift | Wikipedia ]

• [ See How It Flies | John S. Denker ]

• [ List of Works Citing the Equal Transit Time Theory | Wikipedia ]

• Keep in mind that the fact that Equal Transit Time theory is incorrect does not mean that the Bernoulli Principle is flawed. The Equal Transit Time Fallacy is based on a misunderstanding of the Bernoulli Principle, and does not diminish the principle itself in any way.

This is sickening. Fermilab, The United States’ main particle physics lab, is being hamstrung by congress’s latest omnibus bill. This is the same bill that approves billions for the war in Iraq but will result in hundreds at the lab being laid off permanently and countless others having to take extended time without work.

The bill reduces Fermilab’s budget by $62 million dollars in 2008, which is roughly 17% of the $372 million they expected. Oh, and that $372M, which is what their total yearly budget was *going* to be, is less than what it costs to stay in Iraq for a single day.

The cuts are going to halt research and development for the new International Linear Collider — a proposed multibillion-dollar facility that Fermilab hoped would secure the lab’s future and allow it, i.e. the United States, to compete with Europe’s CERN project.

But no. Instead we’re going to pull money from crucial scientific R&D programs, causing us to become an absolute non-force in the particle physics world. All for the war in Iraq. If this bothers you as much it does me, please make your voice heard by writing your congressional representative.

[ Write Your Representative ]

Here’s what I’m sending:

Dear $representative,

I am quite disappointed to hear that congress allowed the recent omnibus bill to pass with huge cuts to Fermilabs’ particle physics programs. The bill cuts their budget by nearly 17% and will result in the halting of our country’s premier effort to stay competitive with other countries in this important field.

What disturbs me most is the fact that the same bill authorizes billions of dollars for the war in Iraq, which in a single day pulls more money from the United States than an entire year of scientific research at Fermilab.

Please re-prioritize. I am tired of seeing the United States fall further and further behind in science — especially at the expense of a war that never should have happened.

Kind regards,

–Daniel Miessler

So we all know how planes fly, right? The top of the wing is rounded and the bottom of the wing is more straight. Air takes longer to travel over the top of the wing than the bottom, which results in more pressure on the bottom, hence the lift. Right?

As it turns out, no.

This is what I was taught, and it’s what I’ve always believed (it’s even in most lower-level text books), but it’s simply not true. This concept, called the Equal Transit Time Theory does not generate nearly enough lift to keep a plane in the air.

The main reason planes fly is far simpler: wings force air downward, which in turn force the wing (and therefore the plane) upward.

The primary actor at play here is called the Angle of Attack (AoA), and it’s easy to conceptualize. We’ve all put our flattened hand outside a car window while the car was in motion. You noticed that if you angled it straight on you could hold it steady, but if you angled the front edge upward you created massive lift that forced your arm up in the air.

The same concept works for kites, helicopter blades, sailboats, ceiling fans, and planes. In fact, winglike surfaces can generate lift almost regardless of the shape of their top and bottom surfaces. Notice that ceiling fan and helicopter blades are basically symmetrical, yet they create a downward columns of air just fine. You can actually use a barn door to generate lift, if you were so inclined.

In other words, lift simplifies nicely to Newton’s third law of equal and opposite reactions: air goes down, wing goes up.

But even this is just one approach to explaining flight. There are actually multiple ways of describing, and calculating with varying degrees of precision, the way in which lift is generated. There is the mathematical/engineering approach, and there is the physical approach, which is what I’ve described above. John D. Anderson said it best:

It is amazing that today, almost 100 years after the first flight of the Wright Flyer, groups of engineers, scientists, pilots, and others can gather together and have a spirited debate on how an airplane wing generates lift. Various explanations are put forth, and the debate centers on which explanation is the most fundamental.

The one thing the experts agree on, however, is that the way most have been taught about how planes fly (the Equal Transit Theory) is absolutely incorrect. ::

References and Notes

• [ Aerodynamic Lift | Wikipedia ]

• [ See How It Flies | John S. Denker ]

• Angle of Attack also explains how planes can fly upside down, which is not explained at all by Equal Transit Time.

• Keep in mind that the fact that Equal Transit Time theory is incorrect does not mean that the Bernoulli Principle is flawed. The Equal Transit Time Fallacy is based on a misunderstanding of the Bernoulli Principle, and does not diminish the principle itself in any way.

[ 2009-07-27 : Sadly, the first version of this post replaced one fallacy (Equal Transit Time) with another (The Coanda Effect). The bad news is that I make mistakes. The good news is that I fix them when they're shown to me. An updated version of this post (without all the confusing comments) can be found here. ]

Post reconstructed here.

Never let school interfere with your education. — Mark Twain

[ Update: The following story is not true. I knew when I posted it that it was the kind of story that gets disproved by Snopes constantly, but I went ahead for a very simple reason: the details of the story as well as the punch-line of who the story is about doesn't really matter. It's the lesson that's important. Still, I should have checked it first and posted it in another context. ]

Some time ago I received a call from a colleague. He was about to give a student a zero for his answer to a physics question, while the student claimed a perfect score. The instructor and the student agreed to an impartial arbiter, and I was selected. I read the examination question: “SHOW HOW IT IS POSSIBLE TO DETERMINE THE HEIGHT OF A TALL BUILDING WITH THE AID OF A BAROMETER.” The student had answered, “Take the barometer to the top of the building, attach a long rope to it, lower it to the street, and then bring it up, measuring the length of the rope. The length of the rope is the height of the building.”

The student really had a strong case for full credit since he had really answered the question completely and correctly! On the other hand, if full credit were given, it could well contribute to a high grade in his physics course and to certify competence in physics, but the answer did not confirm this. I suggested that the student have another try. I gave the student six minutes to answer the question with the warning that the answer should show some knowledge of physics. At the end of five minutes, he had not written anything. I asked if he wished to give up, but he said he had many answers to this problem; he was just thinking of the best one. I excused myself for interrupting him and asked him to please go on. In the next minute, he dashed off his answer which read: “Take the barometer to the top of the building and lean over the edge of the roof. Drop the barometer, timing its fall with a stopwatch. Then, using the formula x=0.5at^^2, calculate the height of the building.” At this point, I asked my colleague if he would give up. He conceded,and gave the student almost full credit.

While leaving my colleague’s office, I recalled that the student had said that he had other answers to the problem,so I asked him what they were. “Well,” said the student, “there are many ways of getting the height of a tall building with the aid of a barometer. For example, you could take the barometer out on a sunny day and measure the height of the barometer, the length of its shadow, and the length of the shadow of the building,and by the use of simple proportion, determine the height of the building.” “Fine,” I said, “and others?” “Yes,” said the student, “there is a very basic measurement method you will like. In this method, you take the barometer and begin to walk up the stairs. As you climb the stairs, you mark off the length of the barometer along the wall. You then count the number of marks, and this will give you the height of the building in barometer units.” “A very direct method.” “Of course. If you want a more sophisticated method, you can tie the barometer to the end of a string, swing it as a pendulum, and determine the value of g at the street level and at the top of the building. From the difference between the two values of g, the height of the building, in principle, can be calculated.” “On this same tact, you could take the barometer to the top of the building, attach a long rope to it, lower it to just above the street, and then swing it as a pendulum. You could then calculate the height of the building by the period of the precession”.

“Finally,” he concluded, “there are many other ways of solving the problem. Probably the best,” he said, “is to take the barometer to the basement and knock on the superintendent’s door. When the superintendent answers, you speak to him as follows: ‘Mr. Superintendent, here is a fine barometer. If you will tell me the height of the building, I will give you this barometer.” At this point, I asked the student if he really did not know the conventional answer to this question. He admitted that he did, but said that he was fed up with high school and college instructors trying to teach him how to think. The student was Neils Bohr.

A MIRACLE material for the 21st century could protect your home against bomb blasts, mop up oil spillages and even help man to fly to Mars. Aerogel, one of the world’s lightest solids, can withstand a direct blast of 1kg of dynamite and protect against heat from a blowtorch at more than 1,300C. Scientists are working to discover new applications for the substance, ranging from the next generation of tennis rackets to super-insulated space suits for a manned mission to Mars.

[ Scientists hail ‘frozen smoke’ as material that will change world ]

e^iπ = –1

(Euler’s Equation)

I disagree, though, and find that Maxwell’s are more compelling. Less simple, sure, but more tangible and more clearly evidence of God.

Utterly cool stuff.

Anyone into physics needs to check out this explanation of the classic particle vs. wave light phenomenon:

VIDEO: Is Light a Particle or a Wave?