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enter image description here On a B787 vertical stabilizer shown in the image, there's this small fin at the leading edge that connects to the fuselage.

It seems too small in size to be considered a dorsal fin. Simply adding this small fin at the leading edge will only increase the sweep back, and thus reduce the vertical stabilizer effectiveness.

I am assuming that this is for improving structural strength? Or does it have other purposes?

Frank
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1 Answers1

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...thus reduces vertical stabilizer effectiveness

Any area added increases stabilizer effectiveness. The structure is similar to a strake on a wing.

The area where fins (and wings) attach to the fuselage can be a source of drag. Wind tunnel studies can show improvements by shaping these junctions in certain ways.

The greater sweep of the strake can also improve the stabilizing characteristics at higher angles of relative wind, such as in a cross wind gust, where a straight stabilizer might stall.

Robert DiGiovanni
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    It this case it's more likely the designer liked the look of a gradual transition. There is a little bit of "styling" going on. or there is a duct that needs to run up there that needs to be enclosed. – John K Apr 22 '23 at 11:43
  • @JohnK Apart from aesthetics, as someone in the comments previously mentioned, this gradual transition may also have the effect of reducing interference drag here. – Frank Apr 22 '23 at 13:33
  • If the intersection is 90 deg or more, interference drag isn't supposed to be a thing. It's why you often don't see fillets on wings that meet the fuse at a 90 deg angle, like a Corsair. – John K Apr 23 '23 at 00:37
  • @JohnK the upper part of the fuselage (when the aircraft has a positive AoA) may be a different world than a wing juction. Vertical stabilizer design is an interesting field. Many of the variable sweep shapes of 1930s vertical stabilizers (such as from the B-17) became ... the "ogive" of the Concorde. I guess "it depends" what shape is ultimately best for a given application, but it does seem to have something to do with off-center relative wind. – Robert DiGiovanni Apr 23 '23 at 08:13
  • Beyond transonic considerations, sweeping the fin moves the aerodynamic center aft without extending the fuselage. Also it looks cool. Most GA airplanes got swept fins in the 60s because of the look. Going from the straight tail to swept tail made no detectable difference whatsoever to the handling or performance of the 172. Strictly styling. Other problem is, as you increase the sweep angle, rudder displacement starts to have a pitch up effect. – John K Apr 23 '23 at 16:31
  • @JohnK yes, it does look good. I actually had some good results with square Vstabs on my FF models (so did the Vikings with their ships). I don't think it matters much with the 172, but have a look at the Fokker D7. Strakes on both the vertical and horizontal stabs and winglets at the ends of the upper wings. They knew something about high AoA flying. – Robert DiGiovanni Apr 23 '23 at 16:50
  • @RobertDiGiovanni Those are just fixed stabilizers surfaces where the triangular shape is most structurally efficient when you make it from tubing. The wing tips are just aileron aerodynamic balance horns that provide a mass balance location (if one is used) and also lighten aileron forces. – John K Apr 23 '23 at 18:32
  • @JohnK Is this gradual transition( wing root fairing) https://www.flickr.com/photos/a380spotter/5643760646 here also for styling only? Does it have aerodynamic benefits? – Frank Apr 24 '23 at 10:13
  • @Frank At very high speeds yes, it can help, as with the Citation 10 https://en.wikipedia.org/wiki/Cessna_Citation_X#/media/File:Cessna_750_Citation_X_AN0983363.jpg where they provided a really exaggerated fairling ahead the wing box going under the fuselage. But looking at the wing root on the C-10, you can see it just has a straight joint with the vertical side of the transition fairing. In the A-380 the fairing in the pic is also likely enclosing plumbing like bleed ducts, etc. When I say "styling", there is always going to be some attention to aesthetics. Designers are humans. – John K Apr 24 '23 at 11:55
  • Sort of interesting that one can't "wash out" a straight vertical stabilizer (on both sides), so the sweep may give it a less sharp stall. As for horizontal stabilizers, why not give them slats and Fowler flaps too ;-). And sweep is aesthetically appealing. – Robert DiGiovanni Apr 24 '23 at 18:55