The Bernoulli hyothesis sounded like BS to me when I learned it in my teens. Sure, the faster moving air reduces pressure to cause lift; you can prove that by blowing air through a straw held over a piece of paper. The question I had was why the air is moving over the top of a wing faster just because it has a longer path – were the molecules that got split having conversation and the one on top wanted to continue it?
While I appreciate the accolades, I really don’t have my finger on the pulse of this discussion. Heck, I barely have a pulse.
No. Air isn’t solid and just because the wing is pushing down on the air doesn’t mean that the column of fluid under the projected area of the wing will squish you.
For a very simple layman’s explanation here is NASA’s take. https://www.grc.nasa.gov/WWW/k-12/Summer_Training/Elementary97/planearticle.html
I always took this to be a manifestation of the continuity principle. See here for more detail. If the molecules that take the longer path didn’t catch up to those that took the shorter path, you’d end up with a mass imbalance.
Yes that is a simplistic description by someone at NASA who creates web pages.
Hmmmm…. so air isn’t solid and pushing it down supports a multi-ton machine but to do that the machine would have to fall in order to push down a compressible fluid far enough to raise it’s pressure enough to support it’s weight and do it without the compressed air squirting out on all sides of the column.
As the wing moves through the air it pushes down on it. The plane doesn’t have to fall at all. If it’s flying level it isn’t falling. In level flight the lift is in balance with the weight and the lift comes from a net force on the wing that is equal to and opposite the force on the fluid surrounding the plane. The physics is undeniable. For there to be an upward force on the wing there has to be a downward force on the air supporting the wing. There has to be an equilibrium if the acceleration of the wing is 0.
Now people say what about the lower pressure on top of the wing? Great. How does that pressure on the wing get lower to begin with? If the plane is sitting on the ground there is no pressure difference on the wing. Curved wing? Not necessary. You can fly with non curved wing. What matters more is angle and pushing the air in front and under the wing which compresses it and moving away from the air on top and behind the wing which causes a pressure drop. It takes time for air to flow into the space that the wing is moving through. Mass is conserved and if there is compression in front of the wing that means somewhere, and that somewhere is behind and on top of the wing, air has to expand. Just think of a plane that’s flying with a flat angled wing instead of being distracted by curved airfoil mumbo jumbo.
Make a paper airplane with angled wings. Throw it level to the ground and watch it climb. Where’s it getting the lift?
Now what the air does after being pushed forward and down is where things get messy.
DesertDog, what you learned is not in fact the Bernoulli hypothesis. I’m sure that the people who taught you thought that it was the Bernoulli hypothesis, but they were wrong. The Bernoulli effect is just Newton’s laws, and like any correct way of phrasing the physics behind airplane lift, involves significant downdraft from the wings.
There is no supporting column of air.
Instead, there are wingtip vortices, created continuously as the high pressure under each wing rolls around the wingtip and into the low pressure found on top of the wing. This effect is apparent (from about 0:16 to 0:40) in this video.
Though they won’t damage your car, such vortices can be felt on the ground, and can persist for a while. Small aircraft must use caution when taking off or landing shortly after a heavy plane. Here’s a useful video (with useless music) that concerns an accident caused by this issue.
Pretty sad when even the wrong answer is given a wrong explanation. I’m beginning to think planes fly on pure faith.
I’ve been tempted, while reading this thread, to write a lengthy essay (even longer than this post).
No need for that. Everything I might have written is there on-line. DesertDog’s link to that Wikipedia page should be required reading – the whole page, not just that section of the page. Not to mention the NASA link in aerodave’s article and the related link that octopus gave a few posts above.
That Wiki page further includes links (among others) to the page on Lift (force) and also to Venturi effect. These are also required reading.
The Wiki page on Lift includes a lengthy portion on “simplified” explanations in plain English for us common schlubs, followed by some more technical stuff. The common “Newtonian” and “Bernoulli” explanations are described, and also criticized as being, not wrong, but incomplete. The silly “equal transit time” theory is debunked, including a cute animated graphic with moving colored dots. (I think the NASA page also includes this somewhere.)
I note that the Venturi effect has gone, heretofore, unmentioned in this thread, and in many on-line discussions of lift. I consider this to be evidence of not knowing what one is talking about. This is a necessary effect to understand toward understand the “Bernoulli” effect. This, along with compression of “stream tubes” above the wing, and not “equal transit time”, seeks to explain why airflow above the airfoil is faster than airflow below, and nothing about it implies equal transit time.
The Venturi effect states that an incompressible fluid flowing through a tube with a constriction, will flow faster through the constricted region. Air flowing over an airfoil differs in two blatantly obvious ways: Air is extremely compressible, and the “tube” through which air flows over the wing is constricted only below (by the upper surface of the wing) and seemingly not above. It’s like the lower half of a constricted pipe with the upper half cut away. The Venturi effect seems to apply, at least somewhat, because the compressible air complies at least a little bit with the Venturi hypothesis, and the upper layers of air above the wing have an internal pressure that partially constrains the flow of air over the wing a bit like in an enclosed pipe. Nothing in this explanation needs to imply any equal transit time theory. The above-cited Wiki links discuss these points.
Thirty posts, and not one mention of a treadmill?
Fine. Carry on then.
I’m sure airplanes carry treadmills all the time. No problem.