Stop abusing Bernoulli when describing lift

Cecil's Columns/Staff Reports
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Many references and encyclopedias, even some published by the FAA, give incorrect explanations of lift. Cecil repeats one of the more common theories in his explanation of why golf balls have dimples. He says, “The Bernoulli principle tells us that slow moving air has higher pressure than fast moving air. The difference in pressure forces the golf ball up.” This is untrue. Lift is a force, created by turning the flow of air. If Bernoulli were the governing principle of flight, golf balls would not fly at all, Magnus notwithstanding. Golf balls, after all, are spheres. Bernoulli would predict that air pressure on the bottom of the ball would be the same as that above it.

Bernoulli does not explain why, when you blow across a piece of paper held horizontally, the paper rises whether you blow above it or below it.

Neither does Bernoulli explain why, if you place a water glass on its side in the sink and run water over just one side, the glass will roll towards the stream of water despite the fact that the water pressure on the side creating “lift” is demonstrably higher than on the dry side. (Hint: the water pressure on the dry side – going out on a limb, here – approaches “zero.”)

A better explanation of lift is found at the John Glenn NASA center web site, along with a really cool Java app that shows the effects of angle of attack, airspeed, and airfoil shape on lift.

http://www.grc.nasa.gov/WWW/k-12/airplane/right2.html

This site on subsequent pages explains exactly why Bernoulli is a poor description of the principles of lift. The Wright brothers discovered this early on. They used Bernoulli in projecting the lift created by their first airplanes and found that the Bernoulli equations understated lift by a considerable amount.

Cecil’s error is understandable; there are many pilots and flight instructors out there still who cling to Bernoulli with a religious fervor that would put Arnaud Amalric to shame.

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While I stipulate to much of what you say, you do not seem to be allowing for the fact that the golfball is already spinning, which separates this case from the standard airfoil example.

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I’m looking right now at an essay written by Einstein on the Flettner ship, which uses the Magnus effect, caused by the Bernoulli principle. I fail to see why this wouldn’t apply to a golfball.

I agree that the Bernoulli principle isn’t the operative principle for an airplane.

BTW, please post a link to the Straight Dope column you’re referring to.

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PS: golf balls don’t fly. They’re principally ballistic, but the trajectory is affected by spin.

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The Magnus effect is not a demonstration of the Bernoulli principle. Using Bernoulli to explain Magnus is the same error as using Bernoulli to explain all other forms of lift. Convenient, plausible sounding, but definitely wrong.

Yes, golf balls do generate lift. They fly. They will fly sideways if you put any sidespin on them – we call this a hook or a slice. All objects moving through the air generate lift. Golf balls are dimpled in order to maximize this lift. And they spin, which increases the amount of airflow bent downward.

Einstein was not an aeronautics expert. The Magnus effect does not demonstrate Bernoulli, either. It is a demonstration of the bending of airflows due to the viscosity of air. Heck, even Wikipedia has it right this time. Magnus effect - Wikipedia (I wondered why I was getting pig droppings on the roof.)

The Flettner ship works the same way: it is not a demonstration of the Bernoulli principle, but depends instead on the viscosity of air and the resultant bending of air flows. All sails, including Flettner sails, are airfoils. If Bernoulli were correct, fans would blow air to the side, not to their front. An airplane wing, golf ball, and all other airfoils bend air more or less perpendicular to their line of travel. The thing that fools people is smoke trails in wind tunnel tests, which appear to stream out behind the airfoil. But that is because the smoke trails are driven by a large fan, not by a wing moving through them. The fan blows the downward turning smoke trails straight again.

If you fly an airplane low over clouds, you will see that it creates a canyon in the clouds almost directly below it. Low flying aircraft also ripple water almost directly below them. The same is true of a golf ball. When you see it flying low (very low) over a water hazard you will see ripples in the water directly underneath it – generally just before you lose your ball. As a terrible golfer, I am all too familiar with this phenomenon.

Golf ball aerodynamics are more fully explained here: Golf ball - Wikipedia (Whoa! Wiki got it right again!) To quote: “Second, backspin generates lift by deforming the airflow around the ball, in a similar manner to an airplane wing. This is called the Magnus effect.”

Now then, if dimples help golf balls fly farther by reducing drag, why don’t we put dimples on airplane wings? See “vortex generators” and similar devices to find out why. Vortex generators are basically dimples for wings.

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Hmm. Evidently the Bernoulli principle isn’t the primary driver of the Magnus effect, due to viscosity. Einstein actually says

He then goes on to explain the effect the way Magnus did. My apologies.

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Two columns: airplane flight

golf ball dimples

waddlingeagle, I suggest you read his column on airplane flight. I think you will be happier. Essentially, when people say Bernoulli, they really mean “equal transit time”, i.e. the air molecules on top of the wing are somehow supposed to take the exact same time as the molecules under the wing, and since the distance is supposedly greater over the wing, this means the air has to travel faster. Cecil agrees this explanation is bogus.

Rather, the coanda effect pulls the air against the top surface of the wing, which speeds up the air faster than the air under the wing. Thus, Bernoulli is at work, but not necessary for the explanation. Cecil agrees that deflecting air is the Newtonian explanation, as long as you allow that both surfaces of the wing deflect air. Pretty much in agreement with the Goddard site you link.

Learjeff, Cecil cites The Physics of Golf Balls, which states that the spin imparted to the golf ball drags air around the golf ball, and thus imparts lift to the ball via the Magnus effect. Dimples help reduce drag by reducing turbulence behind the ball.

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Holding a water glass on its side under a stream of water is an excellent demonstration of lift using the Magnus effect. The water runs down only one side of the glass, but it does not drop straight off the side. Instead, it follows the curve of the glass all the way to the bottom before falling into the sink. This effect would be greatly magnified if the glass were dimpled like a golf ball. Set the glass on its side on a flat surface and it will roll towards the water stream. This is also a demonstration of the Magnus effect. It has nothing to do with Bernoulli.

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The Coanda effect is explained by flow turning created by air viscosity. Bernoulli is applicable only to non-compressible fluids. Air, being compressible, is subject to Bernoulli only at speeds below Mach 0.3 and even then it is a minor effect. The fact is, Bernoulli’s equations cannot predict lift without serious modification. Airplanes (and all other airfoils) do not fly because the air pressure is higher on the bottom of the wing rather than the top. They fly because they force air flows downward by dragging air off the top of the wing and sending it down off the trailing edge. Induced drag is another way of measuring lift. Airplanes would fly even if there were no air under the wing at all.

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That would be the Coanda effect, not the Magnus effect.

Correct.

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So when people say, “You are the wind beneath my wings,” what they really are saying is that you provide very little lift and a great deal of drag on my life. :smiley:

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Hmmm. Good points.

Agreed.

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Cute, but articulated wing flight is a bit different than fixed wing flight.

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Spoilsport. Yes, articulated wing flight is dependent on air under the wings for propulsion. Perhaps we should ask Cecil why birds have never evolved propellers.

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Well, I think you’re a little late in giving Cecil the message. Apparently he got it a while ago:

[QUOTE=Cecil at Why does the shower curtain blow up and in instead of down and out? - The Straight Dope]
I have, however, been discussing this question with Jearl Walker, who in addition to writing the Amateur Scientist column for Scientific American is chairman of the physics department at Cleveland State University […] he also thinks it unwise to appeal to the Bernoulli principle, which, from the standpoint of explanatory power, he regards as on a par with water sprites.
[/QUOTE]

I blame Ed for adding Bernoulli in to the golf ball column (I’m sure Cecil had Magnus alone, which is a fair description, and Ed thought it was too obscure)