The basic idea is to place large vortex generators at the beginning of the tapered upsweep of the aft fuselage in order to reduce the magnitude of flow separation and hence drag. This area of the fuselage has always struck me as one that would act like a bluff body and contribute significantly to the overall drag of the fuselage.
I'm curious as to why this idea has gained so little traction? The author claims a drag reduction of up to 1% for a 747, and 2% for a C5 Galaxy. These certainly seem to be worthwhile gains given the relative simplicity in implementing the idea in practice.
Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair.
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 1, posted (9 months 1 week 6 days 7 hours ago) and read 2239 times:
Quoting jetmech (Thread starter): The basic idea is to place large vortex generators at the beginning of the tapered upsweep of the aft fuselage in order to reduce the magnitude of flow separation and hence drag.
Modern fuselages are designed to not have flow separation back there, hence little drag impact. As a retrofit it might make sense...the 737 is notorious for flow separation at the back end.
I wonder what these design features are? The upsweep of the aft lower fuselage is akin to a yawed cylinder, which is a classic case of a bluff body with flow separation. As I understand it, the 737 mainly has VG's on the upper aft part of the tailcone.
Wortman has noted an inward lateral movement of air as being one of the characteristic features of the separated region of flow in this area, which as you can imagine, is highly three dimensional. His devices aim to produce an outward lateral flow of air via large vortices, which is in addition to the effect of endowing the boundary layer with additional momentum.
The devices, as you say, certainly make sense as a retrofit. Wortman states that the full sized items would be no more than 2 feet in span at most. For typical angles of attack between the device and the local freestream of 30 degrees, the drag force on the device is calculated as around 440 N (100 lb).
Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair.
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 3, posted (9 months 1 week 4 days 7 hours ago) and read 2009 times:
Quoting jetmech (Reply 2): I wonder what these design features are?
As far as I know, it's mostly about controlling the closure rate.
Quoting jetmech (Reply 2): The upsweep of the aft lower fuselage is akin to a yawed cylinder, which is a classic case of a bluff body with flow separation. As I understand it, the 737 mainly has VG's on the upper aft part of the tailcone.
The upper VG's on the 737 help maintain rudder/fin power. On the lower side they just let it separate. A similar problem is behind the "bicycle seat" scoop for the 737 APU inlet.
Quoting jetmech (Reply 2): Wortman has noted an inward lateral movement of air as being one of the characteristic features of the separated region of flow in this area
You have to have inward lateral movement of air...the fuselage is going away. Air must come in to fill that gap. If you *don't* have inward lateral motion then you've got a vacuum.
jetmech From Australia, joined Mar 2006, 2586 posts, RR: 53 Reply 4, posted (9 months 1 week 3 days 18 hours ago) and read 1943 times:
Quoting tdscanuck (Reply 3): You have to have inward lateral movement of air...the fuselage is going away. Air must come in to fill that gap. If you *don't* have inward lateral motion then you've got a vacuum.
No doubt. I think however, that even with the free motion of air, the static pressure in this area would be less than ambient due to the aft lower fuselage upsweep acting as a bluff body. I suspect that Wortman's idea is to promote an outward flow of air using vortex structures. This outward flow of air would lower the static pressure even further (certainly not a vacuum however), and possibly move aft the point where the adverse pressure gradient becomes severe enough to cause flow separation.
I suppose it becomes a bit of a balancing act, as lowering the static pressure behind the aft lower fuselage upsweep would in itself cause an additional net force in the drag direction. Interestingly, the 777 has a drain mast and avionics antenna in this area. I would appear to be relatively simple to move these items to the required locations and provide an external aerodynamic profile as needed. This would kill two birds with one stone.
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