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  • What are the pros and cons of a thin cambered wing?

  • Does a thin cambered wing suffer from a flow separation bubble on the lower side, at cruising speed/low AoA? enter image description here

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Very roundy leading edge, can air fill hole behind this,is flow attached at lower surface behind leading edge,during crusing flight/low AoA?

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  • $\begingroup$ I don't have a reference to link, but single surface is preferred when Reynolds number gets very low, in the 1000s or 10000s, where to the wing, the air is very viscous (to the bee, the air is like motor oil). This is why insect wings are single surface membranes, and single surface works fine for boat sails. As Rn goes up, you get to birds, where the wings are a bit thicker, and ultralights and on up until you get going fast,when you need thinner again for speed. $\endgroup$ – John K Sep 3 '20 at 15:41
  • $\begingroup$ @JohnK Birds wings belong to thick-wing category,what definition tells? $\endgroup$ – user52248 Sep 3 '20 at 18:26
  • $\begingroup$ They belong to the slightly thicker than insect wings category, but there isn't really an official definition. There is a fabulous illustrated book called The Miracle Of Flight by Stephen Dalton that takes you through the Reyonlds Number scale to describe aerodynamics and mechanics of insects, thru birds, thru airplanes. It's aimed at the layperson with beautiful photography. $\endgroup$ – John K Sep 4 '20 at 2:21
  • $\begingroup$ @JohnK Hom much The Miracle Of Flight has editions,I see from cover pictures it has more books with same title? $\endgroup$ – user52248 Sep 4 '20 at 6:53
  • $\begingroup$ The one I have is this one from the 1st printing. books.google.ca/books/about/… I guess the other cover is a newer edition. $\endgroup$ – John K Sep 4 '20 at 14:13
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As that beautiful (but rather heavy) Andean condor, Vultur gryphus, is showing, this is the go to wing for maximum lift at the slowest possible airspeed. Not quite as efficient as the high aspect albatross, but much slower. These wings are all about combining slightly less efficient bottom lift with more efficient (air bending) top lift. Perfect for catching thermals and riding them to higher altitudes.

The major con is they are so good at making lift that an increase in airspeed can create surplus lift. Lowering AOA then creates major turbulence underneath the wing, as you surmised. Now imagine bringing the leading and trailing edges up, making a thin flat wing. This is exactly what airliners do when they want to go 3x faster. Condors need not worry about this.

Thin wings also lack structural strength. This becomes a major issue with scale. Early air craft replaced these wings with solid thick wings as size and speed increased, but camber remains a valuable tool today to help slow an approaching aircraft to a safe landing speed.

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  • $\begingroup$ @CamilleGoudeseune $C_L$, $C_D$ I'm pretty sure it is supported. $\endgroup$ – AEhere supports Monica Sep 4 '20 at 2:05
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    $\begingroup$ What are "top lift" and "bottom lift"? No published wind tunnel measurements have reported these independently. No textbook mentions $C_{L_{top}}$ and $C_{L_{bottom}}$ (Thanks for the MathJax hint, $\frac{Monica}{AEhere}$). $\endgroup$ – Camille Goudeseune Sep 4 '20 at 2:20
  • $\begingroup$ What are top lift and bottom lift? Heavens, look at a sail! (on a broad reach). Then look at a turbine blade! Then try building one (by folding a sheet of balsa over some formed ribs. Let's delete the condor too, right? $\endgroup$ – Robert DiGiovanni Sep 4 '20 at 6:08
  • $\begingroup$ Lift is created by pressure differential between the top and bottom of the wing. Top lift can be disrupted, bottom lift is always there (as long as you have AOA). This is why a wing still has some lift when it "stalls" (but becomes very draggy)(ahem). $\endgroup$ – Robert DiGiovanni Sep 4 '20 at 6:17
  • $\begingroup$ @RobertDiGiovanni Now imagine bringing the leading and trailing edges up, making a thin flat wing. This is exactly what airliners do when they want to go 3x faster.I didnt understand this part with lift edges up,can you explain? $\endgroup$ – user52248 Sep 4 '20 at 8:57
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What are the pros and cons of a thin cambered wing?

Pros:

  • Low drag at high lift.
  • Very little displacement effect, so the pressure rise for a given lift coefficient is lower than for a thicker airfoil. The main benefit is a higher maximum lift before the airfoil stalls.

Cons:

Does a thin cambered wing suffer from a flow separation bubble on the lower side, at cruising speed/low AoA?

Yes, absolutely. Flow separates directly below the leading edge as it cannot follow the strong local curvature and reattaches downstream when the curvature is lower. This causes pressure drag which reduces airfoil efficiency at higher flight speed.

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Thin cambered wings were the norm in the early days of powered flight, right up to around 1920. Speeds were around 100 mph, the camber spread further back than your illustration shows, and wings were double-skinned but still thin by most standards. They had to be braced to keep them both light and stiff, but at these low speeds it was worth it.

Between 1907 and 1913 JW Dunne developed a conical leading-edge profile with much sharper camber increasing progressively outboard. Its purpose was to confer excellent stability and handling qualities to a swept wing, and it entirely succeeded. His biplanes had high wing area with low wing loading despite a heavy undercarriage and were consequently slow. His monoplanes were remarkably snappy, reaching upwards of 60 mph at a time when 45 mph was the norm.

After WWII NACA developed a similar conical leading-edge droop to try and resolve the low-speed handling problems with a high-speed thin delta wing. It worked well, and the drag penalties in supersonic flight were acceptable. The Convair F-102, F-106 and B-58 all featured it.

So a little extra camber to the leading edge of a thin wing can do you a power of good at low speeds. Just, don't overdo it.

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  • $\begingroup$ Conical camber uses more camber towards the wingtips which is opposite to what Dunne's wing did. And conical camber helps to reduce supersonic drag. At low speed flow is separated anyway on delta wings, so camber doesn't matter much. $\endgroup$ – Peter Kämpf Dec 26 '20 at 5:36
  • $\begingroup$ @Peter Kampf. Dunne's biplanes had conical leading-edge camber, though his monoplanes had a different conical development. Check out his 1909 UK patent. Also his 1913 lecture to the RAeS: steelpillow.com/dunne/aero/TheTheoryOfTheDunneAeroplane.pdf For the original motivation for conical camber on supersonic delta wings, see for example NACA's Riebe and Moseley ntrs.nasa.gov/api/citations/19930089161/downloads/… Best to stay evidence-based over such niceties. $\endgroup$ – Guy Inchbald Dec 26 '20 at 8:37
  • $\begingroup$ You don't seem to know what conical camber is. Maybe this or this will be instructive. $\endgroup$ – Peter Kämpf Dec 26 '20 at 9:07
  • $\begingroup$ @PeterKämpf Thank you for the links, they reinforce what I said. But your implication that Riebe and Moseley didn't know what they were talking about is absurd. Here too is Dunne's patent I referred to, do check out Fig.8, which shows what he used for his biplanes: steelpillow.com/dunne/aero/GB190908118A.pdf As you can see, it is the same conical camber that was rediscovered by NACA and applied to the Convair deltas. Enough links, enough discussion, others may now decide for themselves. $\endgroup$ – Guy Inchbald Dec 26 '20 at 11:36
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One main drawback not cited in the previous answer is that they don't offer any storage space. Might be good for some gliders, or early planes that where slow and didn't care about range, but for an airliner you still have to put the fuel somewhere.

Furthermore the sharp leading edge restrics the range of AOA that the wing can fly without stalling, thus reducing the range of speeds the aircraft can operate. You can either fly and land slow with a highly cambered wind or fligh fast but land fast also (good exemple are old jet fighter like the f-104)

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