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Here are two airfoils (lets focus on Re 100,000):

  • SD7043-il which is simple to build and has a glide ratio of about 60
  • e376-il a birdlike and much harder to build if using paper or similar due to the hollow inside but claims to have a glide ratio of about 105

I have never seen such birdlike airfoils on an aircraft (RC or other). Is it because of construction difficulties?

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    $\begingroup$ You might want to start by defining what you mean by "birdlike airfoils." Never heard that before. $\endgroup$
    – Juan Jimenez
    Oct 11, 2020 at 3:29
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    $\begingroup$ Can you define what you mean by "birdlike airfoils"? Neither of the two you linked to look very birdlike to me. To me, the definition of a "birdlike airfoil" is one that can radically change its shape multiple times a second, and I don't see that happening in the ones you linked to. $\endgroup$
    – Jörg W Mittag
    Oct 11, 2020 at 9:14
  • $\begingroup$ To me a birdlike airfoil is a high camber, thin hollow underside airfoil. Here is an example: semanticscholar.org/paper/… $\endgroup$
    – GRASBOCK
    Oct 11, 2020 at 9:39
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    $\begingroup$ Excuse an amateur but what is the significant difference between them? In my amateurish eyes they look very similar? $\endgroup$
    – d-b
    Oct 11, 2020 at 10:28
  • $\begingroup$ @d-b The ones I posted in the link are all birdlike airfoils (from real birds). The e376 is very thin and has high camber like the bird wings. However the SD7043 is compared to the e376 very thick and has very little camber. $\endgroup$
    – GRASBOCK
    Oct 11, 2020 at 10:34

2 Answers 2

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Early biplanes did use similar airfoils. Not as extreme as the Eppler 376, but still very thin and highly cambered.

When Otto Lilienthal started his glider experiments, he tried to copy storks. He experimented with different airfoil shapes by using exchangeable ribs on the gliders and by testing model wings on a rotation test stand (Rundlaufapparat). There he discovered that thick airfoils with a blunt nose were actually better than the thin, birdlike airfoils he had used so far. But he didn't believe his own results and continued with birdlike airfoils.

Farman-Voisin biplane

Farman-Voisin biplane, 1907 (picture source).

The same happened with all other airplane designers until 1915, when scientific work started to influence airplane designs. The highly cambered airfoil works very well in a very small range of angles of attack when the local direction of flow is parallel to the local contour of its nose. But while birds can adjust the camber and area of their wings, airplanes of those times could not. In order to combine fast flight with high lift for take-off and landing, the thick airfoil is better.

While birds are small enough to structurally get away with thin wings, the much larger man-carrying airplanes need the thicker wings to accommodate their bending loads without bracing. Scaling laws show that loads grow faster with size than the dimensions do and only thicker wings make the unbraced, cantilever designs necessary for efficient transportation possible.

Heron in flight

Heron in flight (picture source). This picture shows nicely that 90% of the area is feathers, so a thick wing becomes impossible for birds.

Birds cannot be hollow inside, except for hollow bones. So they have no choice of using thick wings - they have to work with thin wings and adjust camber and wing area to the flight conditions. Being much larger, airplanes need thick wings for aerodynamic and structural efficiency.

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    $\begingroup$ Though there have been fairly modern airplanes with very thin wings, e.g. the F-104: en.wikipedia.org/wiki/Lockheed_F-104_Starfighter#Design $\endgroup$
    – jamesqf
    Oct 11, 2020 at 2:46
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    $\begingroup$ Great point about scaling laws, I think this is something not commonly understood and it affects numerous physical phenomena which people think their intuition should explain. $\endgroup$
    – StayOnTarget
    Oct 11, 2020 at 12:34
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    $\begingroup$ @jamesqf Right, all supersonic aircraft use very thin airfoils. But those have very little camber, too. This optimizes the design for supersonic flights, something that cannot be learned from birds. $\endgroup$
    – Peter Kämpf
    Oct 11, 2020 at 16:57
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    $\begingroup$ @nick012000 They do have this - see the darker feather area of the heron wing behind the bone and muscle part at the leading edge. But it doesn't extend far, just enough to have some transition to the single-layer feather area. Nature decided it's not worth to have this double-feather area extend all the way to the back. It makes the wing heavier than necessary. $\endgroup$
    – Peter Kämpf
    Oct 12, 2020 at 4:10
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    $\begingroup$ @jamesqf F-104: German dark joke long ago. "Q How do you acquire A Starfighter? A: Buy a plot of land and wait". // I saw the only one I've seen in a park in Taichung - looking out across the Formosa strait and dreaming of days of past glory. Around here $\endgroup$
    – Russell McMahon
    Oct 12, 2020 at 12:10
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This is more an addendum than an answer, regarding "birdlike" airfoils.

Ignoring the fact that birds can modify geometry, chord and camber of their wing when required, what can at best characterize a bird's wing airfoil, in addition to camber, is maximum thickness' location, very close to the leading edge, and constant minimal thickness between roughly mid-chord and trailing edge. (Heron picture in accepted answer displays this pretty well)

Here's an other illustration, section of a bird wing at the lower arm level

enter image description here

(source)

Close match to this airfoil configuration do exist in manmade airfoils and are used in light aero models (RC or free flight) Some examples are Erich Jedelsky's and Georges Benedek's airfoils.

In picture below : E.J-75 and B-6407-E

enter image description here

(source)

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  • $\begingroup$ The key point being that RC models (especially sailplanes) intended to operate in similar flight regimes to birds do actually sometimes use such undercambered airfoils, but only when the performance improvement is hoped to be worth the greater complexity of construction. You also sometimes see a very thing balsa sheet curved in such a profile and held by a few external ribs, but that's more a construction simplicity than explicitly leveraging an undercamber. $\endgroup$
    – Chris Stratton
    Oct 11, 2020 at 16:58
  • $\begingroup$ @ChrisStratton at some point, and mostly very low Re number < 10000, airfoil has to be a curved flat sheet. Any attempt at adding thickness leads to frontal area creating huge drag. Like sticking a knife into quince jelly, vs. sticking a pencil into quince jelly $\endgroup$
    – jkztd
    Oct 11, 2020 at 18:06
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    $\begingroup$ An Re of 10000 would seem to correspond to say a 5 cm chord moving at a jogging pace of 3 m/s (10 kph)... unless I'm misusing the first calculator I found. $\endgroup$
    – Chris Stratton
    Oct 11, 2020 at 18:12
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    $\begingroup$ @ChrisStratton exactly, see F1D category ( indoor free flight rubber band planes ) Chord is a bit bigger than 5cm yet speed is far slower than 3m/s $\endgroup$
    – jkztd
    Oct 11, 2020 at 18:13
  • $\begingroup$ @ChrisStratton F1D $\endgroup$
    – jkztd
    Oct 11, 2020 at 18:25

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