how does an under camber wing generate lift ?

In a nutshell:
Same way all wings do it
There is a higher pressure under it and corresponding decrease in pressue above
Thats the obvious part
The shape is really only good for low speeds. becauseIt really works efficiently at a speed where the air can follow the shape
It was the first shape used for powered flight because it was easier to make both light and strong and speeds practical were not much more than take ooff and landing speeds
Birds evolved this shape for much the samereasons

Instead of "only good for low speeds", I would say an undercambered wing is most efficient at higher lift coefficient (C_L) than a flat bottomed or symmetrical wing.

But that's kinda sorta saying the same thing in technical language .

I would say the reason it was popular for early designs was because designers were copying bird wings, and didn't know any better.

For a cantilever wing, there's nothing particularly "light or strong" about an undercambered section. My Old-Timer models with undercambered airfoils have spars that are about the same size, in about the same places, as my models with flat bottom, semi-symmetrical or full symmetrical wings.

It wasn't an issue for early designs because they were externally braced.

If I may, the early adventurers into the air did not simply copy bird wings
The structural benefit of a curved surface was ell known..for at least 1000 years
Externally rigging them was the lightest strength to weight approach
The speeds permitted the drag
Just something to mull .
I dont know of any applications of undercamber for high speed use so I tried to make my explanation somewhat in keeping with the question which was very basic

Do you seriously think an undercambered airfoil section makes a cantilever (not externally braced) wing stronger than a symmetrical one of the same maximum thickness?

Not talking flat plates here.

But Richard didn't say that undercambered, cantilever section is stronger? He compared cantilever wing with externally braced one?

I'd say that an undercambered section as used in very early airplanes - single layer of fabric stretched over very thin ribs - may offer some weight saving over two surfaces and larger ribs of cantilever section.

In terms of pure strength, cantilever wins hands down. But strength/weight ratio? Not so sure.

Originally Posted by Davidz90

But Richard didn't say that undercambered, cantilever section is stronger? He compared cantilever wing with externally braced one?

Yes, I’m trying to get him to clarify exactly what he meant.

I just compared a flat sheet to a curved one
Sorry if I lost you on that one

I really doubt that an externally braced curved sheet is any stronger than an externally braced flat plate. The bracing wires take all the loads.

Originally Posted by Coupez

I would say the reason it was popular for early designs was because designers were copying bird wings, and didn't know any better.

The idea that airfoils for airplanes should mimic bird wings pretty much died with Lilienthal. Early aircraft designers were not copying bird wings. They used thin airfoil sections because the only theoretical reference they had was what we now call thin airfoil theory and the physical data was all gathered from very low Reynolds number wind tunnel investigations. Thin airfoil theory is inviscid so doesn't give drag and wind tunnel tests at Re=200,000 or lower give very pessimistic results for thick airfoils so all the early airfoil designers were putting out airfoils that worked in the test conditions available. Cambering a very thin airfoil results in a concave lower surface, any resemblance to a bird wing is simply coincidence.

[edit] or convergent evolution if you will

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According to this reference, thin airfoil theory wasn’t fully developed until the 1920’s:

https://link.springer.com/chapter/10...81-13-1678-4_5

Though I seem to recall reading somewhere that Max Munk did some work on it during WWI while he was still in Germany.

In any case, it could not have been used by very early inventors such as the Wright brothers.

simple effctive examples of structures and bracing were old at the time the first airplanes were being tried
Airfoil development advanced as needed

Originally Posted by Coupez

According to this reference, thin airfoil theory wasn’t fully developed until the 1920’s:

https://link.springer.com/chapter/10...81-13-1678-4_5

Though I seem to recall reading somewhere that Max Munk did some work on it during WWI while he was still in Germany.

In any case, it could not have been used by very early inventors such as the Wright brothers.

Fine, fall back to Euler's flow function then or Laplace's equation. I said that it was what we now call thin airfoil theory not that it was exactly that. Most of the pieces were in place just not assembled into a coherent theory.

The earliest aeronautical experimenters (pre-1910 or so) mostly weren't trained mathematicians or engineers. They knew that the lift of a surface should vary with surface area and dynamic pressure, but I doubt many of them went further into it than that.

The Wrights built their own wind tunnel when other inventors tables of lift and drag (Lilienthal's and Chanute's) proved to be inaccurate. I know the Wrights experimented with different amounts of camber, but don't know if they played with the camber distribution (location of max thickness, etc.) The Wrights were good practical engineers but had no formal theoretical background in aerodynamics or fluid dynamics.

This history lesson goes well beyond the OP's original question - which I'll give Richard due credit for answering with his first few sentences:

Originally Posted by richard hanson

Same way all wings do it
There is a higher pressure under it and corresponding decrease in pressue above

The fact that higher pressure must relate to a lower pressure must go back athousand years anyway
I nly mentioned this because it is a fairly easy concept to grasp
Old Chinese's secret