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Do you have any info on this strange wing endings used on Stemme glider?

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Andrius
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  • These wing endings were part of an experimental series aimed at reducing induced drag. These wing endings offer the performance of much longer wings. – DSarkar Jul 31 '15 at 10:22
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    @D_S If you have any kind of reference, I would upvote that answer. – DJClayworth Jul 31 '15 at 16:26
  • "Courtesy of" means you got permission to repost the picture. Did you get permission or are you just saying where you found it? – David Richerby Aug 29 '15 at 21:29

1 Answers1

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This was a test aimed at measuring the effect of feathered wingtips. The Stemme S-10 has a four-part wing, and for the experiment the outer wing panels were removed and replaced by five staggered small wings, as shown on the picture in your question. This reduces wingspan from 23 m to only 11.4 m plus the span of the wingtips, in total maybe 14 m.

A feathered wingtip shows less induced drag than a straight wing of identical span, similar to a winglet, because it can involve a bigger mass of air into the creation of lift. But it cannot perform wonders, the effect is small.

The concept behind this experiment is called "winggrid" and the brainchild of Dr. U. La Roche. See here for one of the few still active sites on the topic - the original site http://www.winggrid.ch has long been abandoned.

Sad fact: This aircraft was destroyed in a mid-air collision in 2007. At that time, it had been reverted to the original wingtips.

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Peter Kämpf
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  • So feathered wingtips were not very effective? – Andrius Aug 03 '15 at 08:12
  • @Andrius: Don't expect miracles. They work well in special circumstances, and most birds are presumably very happy with them - only seafowl and swallows have pointed tips. As soon as you need to fly through the occasional thicket of branches, they are the preferred solution. Then of course without the tip reinforcement shown on the Stemme. – Peter Kämpf Aug 03 '15 at 08:56
  • @Andrius: Now I found one site which still mentions the winggrid concept behind this experiment. I edited the answer accordingly. Don't click on the winggrid.ch link - this site is abandoned and now full of annoying ads. – Peter Kämpf Aug 29 '15 at 21:11
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    @PeterKämpf, IIRC the main reason why most birds have feathered wing tips is that they are quieter, which is why they are so prominent in owls and birds of prey. Swallows and swifts that hunt in the air and so need high manoeuvrability and sea birds that can't hope to get help from thermals, and thus need efficiency most, have pointed wingtips. – Jan Hudec Aug 31 '15 at 05:49
  • @JanHudec: Ah, good point! My understanding so far was that feathered wingtips give more area at the tips and better maneuverability around twigs and branches, and all that with limited span. Seafowl have more span and raked wingtips, because the ocean surface has much less underwood. – Peter Kämpf Aug 31 '15 at 06:32
  • @PeterKämpf, that likely also plays a role, especially for the birds that hunt in forests. It still doesn't show it would improve efficiency though. Most efficient are probably albatrosses who use dynamic soaring over sea between the boundary layer just above the waves and faster wind higher up and they have high aspect ratio wings with raked tips. – Jan Hudec Aug 31 '15 at 06:53
  • @JanHudec: Sparrows or pigeons don't hunt. Maneuverability is important for every bird on land. And spreading the feather tips vertically does help with efficiency (only a bit, but every bit counts). – Peter Kämpf Aug 31 '15 at 08:49
  • @JanHudec - I know it's an old comment but-- when a Red-tailed hawk "stoops" on prey the wings are held back and the slots on the wingtips are basically closed-- so any quieting effect is lost. Consider also the very prominent wingtip slots on Condors and Vultures-- which hunt rarely if ever. Consider also the wingtip slots seen on soaring cranes like Sandhill Cranes and Whooping Cranes, which don't hunt from the air. It's pretty clear that the slots are a good solution for birds that do a lot of thermal soaring, regardless of whether hunting or not. – quiet flyer Nov 19 '21 at 14:31
  • The slots may also help a large heavy bird accelerate into flight when taking off. The increase in "effective aspect ratio" comes into play here-- if a bird evolved to have a longer narrower wing with pointed tips and no slots (like the albatross), lifting off from the ground might in many situations be much more difficult than with the shorter, broader, lower-aspect-ratio wing with the "effective aspect ratio" boosted by slots. – quiet flyer Nov 19 '21 at 14:34
  • The slotted wings are also optimized for rapid acceleration from a standstill-- I've read a description of how the extended slotted primary feathers bend in extreme way that acts like a propulsive spring when an accipiter hawk takes off from a perch to chase prey. On the other hand examples of birds with pointed wingtips w/ minimal slotting would be Ospreys, falcons (both of which also soar in thermals, but also are optimized for windy environments, and prolonged flapping flight (crossing water bodies during migration), swifts (which literally spend all day on the wing), and seabirds – quiet flyer Nov 19 '21 at 14:34