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Considering a Ju-52 wing in some hypothetical swept back flying wing configuration using the same strucural design as the Ju-52.

What would be the effects of that corrugated skin, regarding spanwise flow deviation ?

enter image description here (hypothetical corrugated flying wing)

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  • $\begingroup$ the increased strength, stiffness and weight savings may take the model world by storm. Please go ahead and build it, and test it. $\endgroup$
    – Robert DiGiovanni
    Commented Oct 11, 2023 at 19:18

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The corrugation would not be much of an obstacle to crossflow, but would thicken the boundary layer, leading to low maximum lift and early flow separation.

The corrugated surface of Junkers-type airplanes increases friction drag by 20%, about the increase in surface area due to the corrugation. At minimum drag, total drag is, therefore, about 10% higher compared to the same airplane with smooth skin. However, the smooth airplane would have a higher structural mass, so its induced drag at the same speed would be higher. Corrugation is similar to a fixed landing gear or a braced wing: It is the better solution at low speed but becomes worse with higher speed and longer range.

Interestingly, the Junkers J 1 and J 2 all-metal monoplanes of 1915 rsp. 1916 had corrugated steel as a substructure under their smooth skins. The airplanes flew, but were very heavy for their size. Junkers drew two conclusions:

  1. Do away with the skin sheets, let the substructure be the skin at the same time.
  2. Switch to lighter aluminum. Steel was initially chosen because of lower cost and experience at Junkers with building gas bath ovens from thin sheets.

On the swept wing the local pressure field would still dictate the direction into which the air flows: Always to the lowest, away from high pressure. Since the corrugation looks to the air at grazing angles like a stretched sine wave, it would not stop the crossflow. It also does not work like a fence: Its sharp corners force a local separation and a vortex which will change the local pressure field and, consequently, the flow pattern over the wing. All the corrugation does is to introduce pressure ripples which will increase the thickness of the boundary layer compared to a boundary layer with a smooth pressure distribution. This thicker boundary layer will be weakened, separating at shallower pressure gradients in the pressure recovery region of the wing.

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    $\begingroup$ I can see where the boundary layer would become so thick and chaotic near the TE that the elevons may barely work. You might need a Junkers type elevon surface below the TE, like the Mitchell Wing ultralight. $\endgroup$
    – John K
    Commented Oct 11, 2023 at 4:07
  • $\begingroup$ Scaling and airspeed may have huge effects on the predicted characteristics. Interestingly, some swept wings do have fences. I would build it as a free flight model for starters. $\endgroup$
    – Robert DiGiovanni
    Commented Oct 11, 2023 at 19:14
  • $\begingroup$ @RobertDiGiovanni But fences have sharp edges which easily create vortices. Corrugation is only adding a bit undulation. $\endgroup$
    – Peter Kämpf
    Commented Oct 12, 2023 at 6:35
  • $\begingroup$ @JohnK Right, and the only consolation is that they keep working on the lower side, so their effectiveness is halved. However, sweep is much worse than corrugation, so a slightly swept corrugated wing would still be quite similar to an unswept one. $\endgroup$
    – Peter Kämpf
    Commented Oct 12, 2023 at 10:11
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    $\begingroup$ @jkztd I doubt it. Streamlines on swept wings are not straight but curved. Sure, you can tailor a corrugated surface for just one flow condition (Re and alpha), but that is rather academic. Better keep the surface smooth so local streamlines can curve as they want. $\endgroup$
    – Peter Kämpf
    Commented Oct 13, 2023 at 7:33

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