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Highly cambered airfoils like the Seligs and Epplers are hardly ever used in propeller or wing sections. Why?

Does their performance reduce at high Re? Are they only adequate at low Mach, low Re?

The question only regards the aerodynamics. Ignore fuel storage and structural elements.

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First propeller use: A highly cambered airfoil would cause high pitching moments and twist the propeller blade. Of course you can pre-twist the blade so it will assume the correct shape in the desired operating point, but a propeller needs to work over a wide range of operating points, from take-off roll to high speed flight at altitude. In off-design points (i.e., most of the time) the propeller would have poor performance when compared with one which tolerates more diverse conditions.

Note that indeed thin, highly cambered airfoils are used on compressors and turbines in jet engines. Those are more stubby and enjoy much narrower variations in flow conditions, so the highly cambered, thin airfoil is indeed the best choice here.

Use on wings: Some aircraft do indeed use highly cambered airfoils. Those with low maximum speed like human powered or electric propulsion aircraft prefer those airfoils because they create the needed lift at the lowest possible speed, so the aircraft can fly with the limited installed power. As soon as the aircraft needs to cover a wider speed range, however, a lower camber is needed to keep drag low at high speed. This is similar to the use on propellers: A wider operating range requires to move away from the narrow optimum offered by those highly cambered airfoils.

Note that aircraft with a high wing loading use extensive and extensible high lift devices which turn their wings into thin, highly cambered structures for landing. Again, as soon as a lot of lift at low speed is needed, thin, highly cambered airfoils are the best choice. Adding slots between segments will make those work even better than a solid airfoil and allow to use more camber.

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  • $\begingroup$ so in regards to propeller use, it is mostly a matter of load carrying capacity, twist and 'hot shape', correct? not because of its aerodynamics performance? Does it mean that if the stiffness of the blade was increased by geometry(increase chord) or by material (composites), then such airfoil would be ideal? $\endgroup$ – toshi ba Mar 29 at 22:56
  • $\begingroup$ @toshi ba highly cambered airfoils are "one speed wonders". Great to see them in jet turbine blades though, probably one of the reasons jets are most efficient near full power. $\endgroup$ – Robert DiGiovanni Mar 29 at 23:44
  • $\begingroup$ @RobertDiGiovanni Highly cambered airfoils behave much better in a duct where there is another airfoil surface "above" the suction surface to constrain the flow. Aside from biplanes and triplanes, that does not apply to wings, and ducted propellers are also a rarity except for special purposes. $\endgroup$ – alephzero Mar 30 at 1:59
  • $\begingroup$ @toshiba: If we hypothesise a super-stiff material and build a propeller from it, and fit every blade of it with camber flaps, yes, it could be beneficial at some operating point to activate those flaps. Most likely flap angle would then vary over prop span to improve twist for the actual advance ratio. $\endgroup$ – Peter Kämpf Mar 30 at 4:58
  • $\begingroup$ @alephzero check out lower speed applications and Reynolds numbers down to around 100,000. For many aircraft and birds, highly cambered airfoils are the best thing going for L/D ratio. $\endgroup$ – Robert DiGiovanni Mar 30 at 6:45
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You can't just cherry-pick aerodynamics and exclude everything else when it comes to aircraft design. But let's entertain flight dynamics alone for this instance, and use Selig S1210 or S1223 as examples.

Selig-S1210 (for Re 0.2e6, 0.5e6 and 1.0e6) characteristics:

S1210

First, notice that the linear range of the airfoil extends from $C_l$ 0.5 to 1.9, which means that this airfoil is only suitable for low speed flight.

Second, the maneuverability of such a wing would be questionable. For a typical aircraft, we would need the ability to do at least a 0G push-over and a 2G pull-up (for Part 23 and 25 aircraft). However, the airfoil is clearly stalled before reaching $C_l$ of 0; and if we use $C_l$ 1.0 as design cruise lift coefficient, then 2G pull-up would also be difficult.

Therefore, while such high lift airfoil may serve niche applications such as high lift competitions with model airplanes, its aerodynamic utility is limited.

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    $\begingroup$ Delightful poster! Here's a clearer version. $\endgroup$ – Camille Goudeseune Mar 29 at 17:40
  • $\begingroup$ "The linear range of the airfoil extends from Cl 0.5 to 1.9, which means that this airfoil is only suitable for low speed flight". why? $\endgroup$ – toshi ba Mar 29 at 22:52
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    $\begingroup$ @toshi Because you're forced to have a lift coefficient of 0.6 or more at cruise, which either means you'll fly slow, or you'll have a very large wing loading which is impractical for takeoff and landing. $\endgroup$ – JZYL Mar 29 at 23:09
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    $\begingroup$ @toshi No, but why would you want to fly in nonlinear range where the flow is separated, drag is high, and handling is nonlinear? $\endgroup$ – JZYL Mar 30 at 0:10
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    $\begingroup$ @toshiba - Forced by efficiency concerns $\endgroup$ – slebetman Mar 30 at 4:36

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