Where did the false "equal transit-time" explanation of lift originate from?

Post by FabHK » Wed, Apr 23, 2025, 9:36 AM UTC

jstanley wrote: post #29

smallbugfound wrote: post #2

No idea why you would provide the equal transit time theorem to students, it makes such a low amount of sense that you're inevitably going to get your students extremely confused if they are paying any attention at all. "Why does the air have to transit in the same time period?" "But _why_ is the air moving over the top faster? Weren't you going to tell me how a wing works?" Etc etc etc It is the worst kind of lie-to…

> Why does the air have to transit in the same time period? Because otherwise it would leave holes in it where one side moves too fast before joining back up. It makes more sense if you imagine air to be incompressable.

         ____
    ____/ __ \____
 -> _____/  \______
    _______________
 -> _______________

Post by FabHK » Wed, Apr 23, 2025, 9:42 AM UTC

bandrami wrote: post #48

FabHK wrote: post #30

Earlier quoted context omitted.

The very short answer is that the wing pushes the air down, thus (by Newton's 3rd law) the air pushes the wing up.

That's incomplete though; an asymmetric airfoil with 0° angle of attack does generate lift. The problem is the "equal times" -- the times are not "equal", but the air passing over the curved side does go faster.

Post by km6GEwiQqsyEKQH » Wed, Apr 23, 2025, 10:00 AM UTC

tlb wrote: post #43

nandomrumber wrote: post #28

Earlier quoted context omitted.

Problem is electricity is kinda three different things, as far as models go. 1) DC; 2) low frequency AC; 3) Radio frequency circuits With DC, the water in pipes analogy is close enough to be useful. With low frequency AC, like mains electricity at around household voltages, there are additional considerations to make the water in pipes analogy to be useless enough we generally require people to be licensed to work wi…

The pipes analogy gives people a wildly unrealistic idea of the speed of electron movement in a wire. They only move a few mm/s at max current in copper wire. Electrons take minutes to go down the power cable from your PSU to your GPU. At GHz frequencies, they move less than an atom diameter.

Post by bandrami » Wed, Apr 23, 2025, 11:37 AM UTC

FabHK wrote: post #54

bandrami wrote: post #48

Earlier quoted context omitted.

That's incomplete though; an asymmetric airfoil with 0° angle of attack does generate lift. The problem is the "equal times" -- the times are not "equal", but the air passing over the curved side does go faster.

Sure. By deflecting the air down. > Cambered airfoils generate lift at zero angle of attack. When the chord line is horizontal, the trailing edge has a downward direction and since the air follows the trailing edge it is deflected downward.

Post by dr_dshiv » Wed, Apr 23, 2025, 12:43 PM UTC

Kim_Bruning wrote: post #53

blueflow wrote: post #44

Earlier quoted context omitted.

"/usr stands for UNIX System Resources" And less technical: "The rainbow has 7 colors" "Sun is the day, moon is the night" So many things, i wish i had a name for these kind of traditional beliefs.

https://en.wikipedia.org/wiki/Lie-to-children short version: https://en.wikipedia.org/w/index.php?title=Lie-to-children&o...