What is the thickest airfoil ever used in an aeroplane wing for lift ?

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Sraight'nlevel

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I am curious...I recall B-29 had 28% thick airfoil in the root of the wing.

Could 50% thick be possible...would it bring anything...like weight savings ?

I am referring to the good old "tear drop" shape...which is supposed to be the slickest ever.
 

wsimpso1

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Four digit catalogs go to 24%, and laminar go to 24%. Thicker is certainly possible using the family definitions NACA came up with. Making your own thickness distributions should get interesting. Generally, the thicker the wing the lower the max cl and higher is cruise cd, but this effect is not huge - 24xx series went from about cl=1.60 and cd=0.006 at 12% going to cl=1.3 and cd=0.0075 at 24%.

Other Pros and Cons for this sort of foil are:
  • A deeper wing can save weight in spar caps;
  • Shear webs and wing ribs may gain some weight - they may go no lower than min gage and may require stiffeners to preclude buckling/wrinkling/crippling;
  • More volume becomes available within the wings for stuff like fuel, internally mounted engines, cockpit and passengers, etc.
Billski
 

Sraight'nlevel

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How about the spanwise flow...if you have straight wing with 50% thick root and then getting like 12% at the tip of the wing ?

Is that efficient at let's say aspect ratio of 20 ?
 

wsimpso1

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How about the spanwise flow...if you have straight wing with 50% thick root and then getting like 12% at the tip of the wing ?

Is that efficient at let's say aspect ratio of 20 ?
We are always begging for stall problems with thickness taper. With more nominal levels of taper, acceptable stall behaviour is available by incorporating aerodynamic washout, but that much taper would take an impressive amount of aero washout.
 

addaon

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I'd still be wary of going much over 24% thickness -- stall behavior will be different than typical airfoils (constant slight separation near the TE, etc), and the thickness taper issues have already been mentioned. if the goal is a thicker section towards the root, it's very common to do a more modest thickness (percentage) taper, or none at all, and instead increase the chord dramatically inboard of the innermost (or second) rib:

1668710965300.png

This gives you the local thickness to handle increased loads in this region without the weirdness of super-thick airfoils. If you're for example doing a 15% - 18% thickness taper, a 1.5x chord increase at the root gives you the same thickness there that a 15% - 27% taper would, with much less strangeness.
 

addaon

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As a side note, every reference I can find for the B-29 root section has it at 22%, not 28%. 22% is effectively "let's use the thickest tested sections" from NACA work; 28% would be novel design.
 

Sraight'nlevel

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I see...my memory failed on that then.

Could it have been Consolidated B-24 Liberator ?
 

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addaon

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If you're really interested in super-thick sections, the literature to look at would be wind turbines, which go up to 40%+ at the root due to super-high structural loads. Those tend to be at low Re (because the thick sections are limited to the root; thickness taper is highly non-linear), so hard to generalize too far; but there's a whole lot of relatively bizarre mitigations to allow using such a thick section with minimal compromise; e.g. they're often sliced off with a TE thickness of > 10%, which is a massive bluff body, but still less drag than trying and failing to maintain attachment across such a huge adverse pressure gradient.

1668712017630.png
 

Sraight'nlevel

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If you're really interested in super-thick sections, the literature to look at would be wind turbines, which go up to 40%+ at the root due to super-high structural loads. Those tend to be at low Re (because the thick sections are limited to the root; thickness taper is highly non-linear), so hard to generalize too far; but there's a whole lot of relatively bizarre mitigations to allow using such a thick section with minimal compromise; e.g. they're often sliced off with a TE thickness of > 10%, which is a massive bluff body, but still less drag than trying and failing to maintain attachment across such a huge adverse pressure gradient.

View attachment 131835
Yes those are really thick !
 

Sraight'nlevel

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The recommended thickness of the "optimal" wheel pants by Hoerner is 27% thick...but it is not a wing.
 

addaon

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Correct, the aspect ratio of 3 - 4 being optimal for fairings assumes (a) that you're optimizing for lowest drag for a given thickness, not for something else (volume, weight, lift) and (b) that there is zero angle of attack / lift generation, and therefore minimal (for a given shape) adverse pressure gradient trying to cause separation. Wings are a very different beast, and analyses consistently come up with a thickness percentage between 12% and 18% being optimal if lift is being generated and optimization is done for weight rather than area or volume.
 
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