Hypersonic From United Kingdom, joined Oct 2005, 149 posts, RR: 0 Posted (4 years 11 months 4 weeks 1 day ago) and read 1850 times:
Hi All,
Seems that accumulative fuel savings are being achieved by adding firstly winglets onto airliners both big & small.
More recently we're seeing either blended winglets or Raked tips.
However, the tail-plane, esp on larger aircraft like the 777, MD-11 etc, DO produce draggy vortices to a lesser degree. Therefore raking them with a positive twist, thus creating a slight forward thrust component, similar to the rakes on the 777-300 would surely help.
See this link in large view for an example of the tailplane creating draggy vortices like the wings.
ZuluAviator994 From Australia, joined Mar 2008, 510 posts, RR: 0 Reply 1, posted (4 years 11 months 4 weeks 20 hours ago) and read 1775 times:
Hm...i suppose, but I believe the extra weight would probably offset the benefits...
And as you see on the picture, the vortices aren't that great on the tailplane, and like I just said, I think more fuel would be burned than saved
Heck, let's let the guys in the biz answer...
Rgrds
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2H4 From United States of America, joined Oct 2004, 8950 posts, RR: 62 Reply 2, posted (4 years 11 months 4 weeks 20 hours ago) and read 1773 times:
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Quoting Hypersonic (Thread starter): However, the tail-plane, esp on larger aircraft like the 777, MD-11 etc, DO produce draggy vortices to a lesser degree.
They do, but not so much in cruise flight.
The aircraft in the photo you reference is in landing configuration with flaps down. The flaps create a pitching moment that must be counteracted by the horizontal stabilizer. We can see vorticies in that photo because the horizontal stabilizer is creating lift of its own, compensating for the pitching moment created by the flaps.
In cruise flight, the flaps are retracted and the horizontal stabilizer creates little to no lift of its own. Thus, little to no induced drag is created, and wingtip devices would offer little to no benefit.
BoeingOnFinal From Norway, joined Apr 2006, 476 posts, RR: 0 Reply 3, posted (4 years 11 months 4 weeks 18 hours ago) and read 1741 times:
Quoting 2H4 (Reply 2): The aircraft in the photo you reference is in landing configuration with flaps down. The flaps create a pitching moment that must be counteracted by the horizontal stabilizer. We can see vorticies in that photo because the horizontal stabilizer is creating lift of its own, compensating for the pitching moment created by the flaps.
Many airliners with autoland capability (if not all?) trim nose up on final approach (don't remember the height, modes etc, probably differ from aircraft and manufacturer), which must be counteracted with elevator down. I believe this is to speed up the transition for a go-around in case of windshear or other situations where climb pitch is crucial. Type Rated pilots might have some more insight on this subject though.
But as 2H4 states, with a variable incidence tailplane the lift created by the horisontal stabiliser is not much in cruise. As far as my knowledge goes, the raked wingtips are there to reduce induced drag (a product of lift), and would not help on form drag nor friction drag. An aircraft that can vary it's C of G by moving fuel, has almost zero tail load. An airliner with the right C of G position could also encounter a situation with minimal tail loading. Thus, not much help in trying to reduce induced drag, when there is minimal lift, which means minimal induced drag to begin with.
Rwessel From United States of America, joined Jan 2007, 1989 posts, RR: 2 Reply 4, posted (4 years 11 months 4 weeks 13 hours ago) and read 1717 times:
Also, there's really nothing you can accomplish with winglets or raked wingtips that a better (usually longer) wing can't do better. Since there's rarely any length limitation on the horizontal stabilizer, if there are issues with tip-vortex losses in cruise (and as others have pointed out, there usually isn't), you can just increase the aspect ratio.
Mir From United States of America, joined Jan 2004, 19700 posts, RR: 56 Reply 5, posted (4 years 11 months 4 weeks 11 hours ago) and read 1717 times:
Quoting BoeingOnFinal (Reply 3): Many airliners with autoland capability (if not all?) trim nose up on final approach (don't remember the height, modes etc, probably differ from aircraft and manufacturer), which must be counteracted with elevator down. I believe this is to speed up the transition for a go-around in case of windshear or other situations where climb pitch is crucial.
It's more because that's the angle of attack it takes to fly at the approach speed. If you fly slower, you have to pitch up more to maintain the same amount of lift.
-Mir
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BoeingOnFinal From Norway, joined Apr 2006, 476 posts, RR: 0 Reply 6, posted (4 years 11 months 4 weeks 6 hours ago) and read 1595 times:
Quoting Mir (Reply 5): It's more because that's the angle of attack it takes to fly at the approach speed. If you fly slower, you have to pitch up more to maintain the same amount of lift.
That is true, for a given AoA, weight and C of G the tailplane loading would be the same regardless of trim. Good observation
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