5 ms·
The mistake would be that this is what provides the lift force, as stated in the quote. If this was so, birds wouldn't need to flap, which is the actual lift g
by 21 9y ago
The mistake would be that this is what provides the lift force, as stated in the quote.
If this was so, birds wouldn't need to flap, which is the actual lift generator (pushing air down).
- mikeash 9y agoThe pressure difference pushes air down. It does provide the lift force. Birds flap to create thrust, not lift. Surely you’ve seen birds flying without flapping?
- 21 9y ago> Surely you’ve seen birds flying without flapping? Isn't that when they just "ride" the winds? The wind pushing against the back of their wings which is hold angled thus pushing it higher (like a boat sail).
- mikeash 9y agoSorry, I don’t really understand what you’re saying here. Birds and airplanes work on the same principles. Airflow over the wings produces lift. That airflow also produces drag, which must be counteracted with thrust to maintain altitude. The only difference is that birds produce thrust by flapping, and airplanes produce thrust with engines. A non-flapping bird is equivalent to an airplane with the engines shut off.
- 21 9y agoI was referring to this theory, which NASA calls wrong: https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html Most interestingly, NASA also calls wrong what you seem to defend, or maybe there are some subtleties that I'm missing: https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
- mikeash 9y agoNo, that second link describes the equal time fallacy, which I called out as wrong in my original comment.
- mannykannot 9y agoThere is no equivalent, in flying (other than balloons), of a sailboat running before the wind. A gliding bird is always facing into the relative airflow.
- mannykannot 9y agoIn general, birds do not need to flap; they can glide, and flapping is more about propulsion than lift. In the case of hummingbirds hovering or birds launching into the air from the ground, I believe it is correct to say that vortex formation is an essential part of the process. In fact, the analysis of lift generation by airfoils treats the flow as a vortex superimposed on a linear flow, so in that sense, the explanation of lift generation is both inseparable from and dependent on vortex formation, though I am not sure the article's author had this in mind.
- vlehto 9y ago"the explanation of lift generation is both inseparable from and dependent on vortex formation" Are you sure about this? There are of course high lift devices and wingtip vortices are unavoidable. But I'm pretty sure you can (at least theoretically) generate lift with an airfoil in a wind tunnel while having completely laminar flow.
- mannykannot 9y agoI am referring to the Kutta–Joukowski theorem [1]. I don't pretend to understand it in detail, but here it says "the fluid flow in the presence of the airfoil can be considered to be the superposition of a translational flow and a rotating flow." Your reference to a wind tunnel might have something to do with extending an airfoil from one side to the other of a wind tunnel, thereby approximating a foil of infinite span, but I don't think it is relevant to free flight. The intensity of wingtip vortices can be reduced by making the span longer, all else being equal, but then the volume of air involved in the circulation increases. [1] https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem (I have seen another version that I think offers a more intuitive, though less rigorous, presentation, and if I can find it I will add a link.)
- icegreentea2 9y agoThe rotating flow / vortex in that sentence is probably referring to the 'circulation' of the air flow, which has some mapping to the lay idea of vorticity, but isn't fully it. It's basically a measurement of deflection through a closed boundary.