Konrad
Very Strong User
In my Samsara thread I mentioned that I really really don't like the open leading edge that we often get with carbon tubed sparred wing construction!
There are a host of reasons for this chief amongst these is that the carbon tubed sparred wing is not using the carbon very efficiently over the depth of the airfoil thickness. (Yes, I think the thinner the wing the better! Well, down to 6% of the chord, that means the spars need all the help they can get).
The other problem with open wing structures, particularly the front 1/3 of the wing is covering sag. That is because of the different pressure gradients along the span as a result of the infinite variability of the airfoil shape results in micro vortices across the wing skin. This is a major source of drag. Dr. Seilg has written about this. The addition of false ribs really doesn't help much. Yes, the shape of the airfoil in the valley with the false rib is closer to the as designed rib airfoil. So the vortices aren't as strong but there are more of them.
The deviation from the designed airfoil is to be avoided. So is the introduction of so many variations of the airfoil in the Valleys. The best way to control this is with a rigid skin that spans between the ribs. This is why it is best to choose a wing built with at least a "C” or “D" tube LE. and to avoid the carbon tubed sparred open wing. (Take a close look at the Top models Samsara and Element. Most will say the Samsara flies better than the open winged cabon tubed Element). The only benefit I can see with the carbon tubed wing is ease of assembly.
Now historically we have built "D" tubed wings with the sheeting stopping at the high point of the airfoil. From a structural perspective this is optimum when looking at structural weight. But we aren't building bridges. Dr. Drela suggest that the rigid skin be brought back to at least 40% aft. This is to get the rigid skin to saggy covering transition not to act as a trip to start the premature formation of the separation bubble. (BTW this is also why spoilers should be place well aft of the high point. Better yet do away with spoiler and use flaps and the aileron on the TE. AKA Crow!)
There are a host of reasons for this chief amongst these is that the carbon tubed sparred wing is not using the carbon very efficiently over the depth of the airfoil thickness. (Yes, I think the thinner the wing the better! Well, down to 6% of the chord, that means the spars need all the help they can get).
The other problem with open wing structures, particularly the front 1/3 of the wing is covering sag. That is because of the different pressure gradients along the span as a result of the infinite variability of the airfoil shape results in micro vortices across the wing skin. This is a major source of drag. Dr. Seilg has written about this. The addition of false ribs really doesn't help much. Yes, the shape of the airfoil in the valley with the false rib is closer to the as designed rib airfoil. So the vortices aren't as strong but there are more of them.
The deviation from the designed airfoil is to be avoided. So is the introduction of so many variations of the airfoil in the Valleys. The best way to control this is with a rigid skin that spans between the ribs. This is why it is best to choose a wing built with at least a "C” or “D" tube LE. and to avoid the carbon tubed sparred open wing. (Take a close look at the Top models Samsara and Element. Most will say the Samsara flies better than the open winged cabon tubed Element). The only benefit I can see with the carbon tubed wing is ease of assembly.
Now historically we have built "D" tubed wings with the sheeting stopping at the high point of the airfoil. From a structural perspective this is optimum when looking at structural weight. But we aren't building bridges. Dr. Drela suggest that the rigid skin be brought back to at least 40% aft. This is to get the rigid skin to saggy covering transition not to act as a trip to start the premature formation of the separation bubble. (BTW this is also why spoilers should be place well aft of the high point. Better yet do away with spoiler and use flaps and the aileron on the TE. AKA Crow!)
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