Author Topic: What Causes Lift? And the Seeming Misconception of the Bernoulli Principle  (Read 522 times)

Offline Matthew_Baker

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I consider myself a fairly intelligent person. That's why I was surprised when I read something completely contradictory to what I had been taught all my life in a Cracked Article (of all places ;))

http://www.cracked.com/article_20669_6-ridiculous-science-myths-you-learned-in-kindergarten_p2.html

Most of the article was pretty unsurprising to me except number 2. Up until this point if someone had asked me what causes a plane to lift into the air on takeoff I'd have described the Bernoulli principle; the idea that the faster air moving on top of the wing causes a low pressure system above and a high pressure system below, causing the plane to lift off the ground.

This principle is what I've been taught all my life and seemed like a very simple and reasonable explanation for why planes fly. I never questioned it until I read some of the articles that were linked.

http://www.allstar.fiu.edu/aero/airflylvl3.htm
http://www.telegraph.co.uk/science/science-news/9035708/Cambridge-scientist-debunks-flying-myth.html

These articles basically state otherwise; that the Bernoulli principle is not a good description for the cause of lift, and instead, newton's third law of motion (for every action there's a equal and opposite reaction) is more of a factor. The downwash of air (caused by the angle of attack of the wing) and the deflection of that air puts an equal and opposite upwards force on the wing, causing the plane to lift.   

Ik a lot of the people on these forums are very into science and math and a bit more intellectual than I to speak on matters of physics and engineering ;D that's why I wanted to know what you guys think.

I can't be the only one that unquestioningly subscribed to the idea that Bernoulli's principle was the soul cause of lift :P I'm wondering if it's a common misconception that's taught in other parts of the world as well. It's what me and many other people are taught at a very young age (probably because of the simplicity of the explanation) and since I learned it at a young age I never questioned it.

I could be totally ignorant :P but I was pretty surprised to think about lift in a more complex and scientific way.
 
 

Offline luftwaffe.be

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Bernoulli is not a good law to use when it comes to gasses. In fact, in engineering, doing this is a big no-no. The parameters used in Bernoulli's law can be acquired by a bunch of different equations, depending on the compounds used in the system itself.

Bernoulli's law is only valid for non compression liquids, hence you cannot use it for gasses. Instead you need to calculate stuff with the (non-)ideal gaslaw, and other laws depending on what the system wants.

Offline Zoologic

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I was also wondering, why the equation of lift force never really involve variables like pressure.

See here: http://wright.nasa.gov/airplane/lifteq.html

But the Wikipedia article here: http://en.wikipedia.org/wiki/Lift_%28force%29#cite_note-44

Explains what Cracked apparently failed to elaborate further about the "popular" myth, which is the "Popular" explanation based on equal transit-time. I extract the excerpts of the article here:

Quote
An explanation of lift frequently encountered in basic or popular sources is the equal transit-time theory. Equal transit-time states that because of the longer path of the upper surface of an airfoil, the air going over the top must go faster in order to catch up with the air flowing around the bottom, i.e. the parcels of air that are divided at the leading edge and travel above and below an airfoil must rejoin when they reach the trailing edge. Bernoulli's Principle is then cited to conclude that since the air moves faster on the top of the wing the air pressure must be lower. This pressure difference pushes the wing up.

However, the hypothesis of equal transit time has no basis in theory and is universally contradicted by experiment. Although it is true that the air moving over the top of a wing generating lift does move faster, there is no requirement for equal transit time. In fact the air moving over the top of an airfoil generating lift moves much faster than the equal transit theory would imply.

Now this is the part where Bernoulli's principle is wrongly referenced. Actually, the theory itself kinda stands. But Bernoulli has been criticized because it relied on later principle (Coandă effect) to explain his theory of the effect of speed on pressure.

It is not that:

Quote
most alarmingly of all, goddamn pilot manuals have the wrong explanation for flight

Read more: http://www.cracked.com/article_20669_6-ridiculous-science-myths-you-learned-in-kindergarten_p2.html#ixzz2jAhW0CcC

But my pilot friends, which not all of them are actually bright science students, explained it the wrong (or popular) way as Wikipedia pointed out.

Offline Tankbuster

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Bernoulli is not a good law to use when it comes to gasses. In fact, in engineering, doing this is a big no-no. The parameters used in Bernoulli's law can be acquired by a bunch of different equations, depending on the compounds used in the system itself.

Bernoulli's law is only valid for non compression liquids, hence you cannot use it for gasses. Instead you need to calculate stuff with the (non-)ideal gaslaw, and other laws depending on what the system wants.

Bernoulli's law is a small derivation of the law of conservation of energy, and if something has variable vol/mass like gas, I wonder how it is applicable without a system of tubes and pipes and all that shit to keep the mass constant.