Q & A: The Bernoulli Theory of Flight

Q:
Did you know that the Bernoulli theory of flight is Baloney? The actual Bernoulli lift of a Cessna 152 at landing speed is about 42 pounds. The rest is Newton. The wing hump effeciently and effectively deflects air downward. Action/Reaction. Newton. Please check out the following website. http://www.aa.washington.edu/faculty/eberhardt/lift.htm
- Ron Wyett (age 52)
Baltimore, MD
A:
Ron -

The Bernoulli theory of flight is most certainly not 'Baloney.' It is, as the website that you refer to points out, a well-proven phenomenon of air dynamics that is present and should be accounted for in all flight calculations. Nor is its influence the same for all planes.

For the Cessna 152 that you refer to, its net contribution to lift is extremely small. In some older planes, the Bernoulli effect played a very significant role, however, and it is possible to design light-weight model planes that rely almost entirely upon it.

The most common description of the Bernoulli theory of flight that I have heard of indicates that the Bernoulli effect is 'part' of what allows airplanes to fly. It is also one of the elements of lift that is accessible to younger students who may not have in-depth understandings of fluid mechanics and the dynamics of flight. For this reason, it is commonly taught, and its validity should not be denied. However, it is true that there are numerous other factors that influence the total lift felt by an airplane, and the site that you referenced gives a very good description of them.

-Tamara

(published on 10/22/2007)

Follow-Up #1: Bernoulli?

Q:
You have just proved then that an aeroplane can not fly upside down. The Bernouilli effect is almost negligible in determining lift - see other websites for the complete explanation Try the link below. www.aviation-history.com/theory/index-theory.html
- Andrew (age 57)
Australia
A:
Andrew- I think you've misread Tamara's answer. Some planes, not all, rely significantly on the Bernoulli effect. Those planes cannot fly upside down. Ones that can fly upside down obviously use little or no Bernoulli effect.

BTW, even when the Bernoulli effect is important, the standard popular descriptions of how it works for planes are wrong. It is not true that the air flowing over the top part of the wing has to arrive at the rear of the wing at the same time as the air flowing over the bottom.

The URL you cite looks very good.

Mike W.

(published on 08/26/2008)

Follow-Up #2: Bernoulli pedagogy

Q:
If the Bernoulli principle is only a small part of lift and is only significant in limited situations, wouldn't it be a much better approach to instead explain lift in terms of something that is always correct? For example, Newton's principles embodied in his 3 laws are as accessible to kids (actually much more so) than Bernoulli's principle, they are easily demonstrated, and wing lift can always be explained in terms of these principles. Pressure, force, lift, angle of attack, e.g., all make mores sense in light of Newton's 3 laws and is much more teachable as well. As a teaching tool, I see little benefit in always pushing a principle that, while applicable under certain circumstances, avoids the reals reasons why most airplanes fly. All one has to do is look at the airfoil of a 747 or even the Wright Flyer and appeals to Bernoulli seem untenable.
- John Strong (age 43)
Lindenhurst, IL USA
A:
I tend to agree.

I wouldn't distinguish, however, between Newton's laws and the Bernoulli principle. The Bernoulli principle is one particular phenomenon within that framework. I think the real distinction you want to make is between the simple push provided by wind on a tilted surface and the more complicated Bernoulli effect. Both are plain Newtonian forces.

Mike W.

(published on 10/11/2009)

Follow-Up #3: Bernoulli effect and gas expansion

Q:
When a parcel of air speeds up then it stretches out so that pressure decreases in accordance with Bernoulli principle. ( http://www.boeing.com/companyoffices/aboutus/wonder_of_flight/airfoil.html ). But when air enters a narrow valley from an open field , it speeds up. So according to Bernoulli principle the pressure in narrow valley should be lower. Thus in first case spreading out of air decreases pressure while in 2nd case compressing decreases the pressure . How can both , compression and expansion , decrease the pressure ? One should yield results opposite to the other. Where is the fault in above argument? Please help.
- Saurabh (age 25)
India
A:
nice question.

I find it easiest to grasp the Bernoulli principle by thinking of the opposite direction of causation. It also helps to think initially of a nearly incompressible fluid (a liquid) rather than a gas, just to simplify things at first.

What makes something speed up? It must be acted on by an external force. What are the external forces on the flowing fluid? The most interesting ones are the pressures from the fluid behind it and in front of it, since those act where the fluid is moving and hence can do work on it. In a region where the fluid is speeding up, the pressure in front must be lower than the pressure behind. Then when it's slowing down the reverse is true. So the pressure is lowest just where the fluid is flowing fastest. The expansion or contraction of a highly compressible fluid (a gas) is a further complication, but not the core of the Bernoulli effect.

On the link you gave, the words "stretch out" are ambiguous. In your example of air flowing into a valley, someone might also say the air column stretches out, since it gets longer, although also narrower.

Mike W.

(published on 07/03/2011)

Follow-Up #4: Bernoulli and airplanes

Q:
Bernoulli's principle states that air travels faster over the longer top of an airplane wing than under it. This assumes air takes the same amount of time to travel either side of the wing. But why is this the case? Couldn't the air above the wing just take longer to pass it thus maintaining constant velocity on either side?
- Tim (age 17)
London
A:

It's not the Bernoulli principle that says that the air flows faster on the top of the curved wing. The Bernoulli principle just says that if the air flows faster on top, then the pressure on top will be lower.

You're right that the usual argument about why the air flows faster on the curved side is bogus. There's no requirement for the time of the flows on each side to be the same. The real argument is more compluicated- and I don't know it. One of the links above describes it.

Mike W.

(published on 09/11/2014)