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While most aircraft have upward-pointing winglets

enter image description here
Source

there are also some UAVs (I couldn't find any manned aircraft$^1$) with downward pointing winglets.

enter image description here Source

I'm pretty sure that other considerations such as ground clearance, propeller/camera system protection and stability were the determining factors in the examples showed above.

However, according to a study (p.11) referenced in this question downward-pointing winglets are more efficient.

This also means that winglets sticking up are not as efficient as folding them down.

I couldn't find the sources mentioned in the paper.

What makes downward pointing winglets more efficient? What are the efficiency-related differences between the two directions?


$^1$: No, neither the A-10's nor similar wingtips count.

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For efficiency the orientation of winglets is not important. Best would be to stretch them out horizontally, but on a canard they do double duty as vertical tails.

Things look different when maneuvering is added: Now the downward-pointing winglet is superior especially if it carries a rudder. When roll is commanded by deflecting the ailerons, the lift change on the outer wing will carry over to the winglet. On upward-facing winglets this creates an opposite yawing moment while on downward-facing winglets this yawing moment supports the intended turning maneuver.

Also, they look cooler.

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    $\begingroup$ They look very cool indeed. I read somewhere that in aeronautics, if it looks good it usually is. $\endgroup$ – Koyovis Jul 9 '17 at 2:30
  • $\begingroup$ I'd have though that the direction doesn't matter too (vortex induced vertical speed component on the wing should be equal which implies equal induced drag). After reading the paper again I'm thinking that it probably is a comprehension problem on my side and the folding them down of the paper just means no winglet at all instead of pointing downward. The drag advantage you mention in the question I linked is then only meant with deflected ailerons? $\endgroup$ – Gypaets Jul 9 '17 at 20:43
  • $\begingroup$ @Gypaets: Yes, only when maneuvering, i.e. with ailerons or rolling motion. And they need to be behind the cg to do that. $\endgroup$ – Peter Kämpf Jul 10 '17 at 6:41
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One aspect of efficiency that downward pointing wingtips improve is increased bottom lift from increased compression of air underneath. "End caps", wing tip fuel tanks, anhedralled wings, Lippisch ears, downward folding wing tips (as seen on the XB-70), and downward pointing winglets achieve the same effect.

These be benefits must be weighed against the increased drag (with the exception of the XB-70) adding them to the design produces.

However, for the smaller drone pictured above, the downward pointing wingtips also serve a very important function best described by one of my mentors as follows: "If you were a seagull, you would not want a cross wind blowing you into a cliff". The downward pointing wingtips roll and yaw the plane into a cross wind gust, whereas the "classic" upward pointed vertical stabilizer design will tend to roll away from the gust, especially if the gust gets "under" the windward wing. For a very light model (or bird), the result is getting rolled and pushed downwind. Not a good day for that seagull.

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