Re: New JF-17/FC-1 thread
A bit late now but some thoughts of mine I made up during that aerodynamics discussion several days ago.
Somehow I'm of the impression that aircraft inteded for high AoAs, at least in greater speed regimes, should have their intakes somewhere under the wing(root)s or the fuselage where the airflow can be forced into the induct.
This design you speak of, is actually pioneered by Jack Northrop. The idea is to use the LERX to create a boundary layer that flows into the engine intakes. The first planes to ever use this principle, was from Jack Northrop's company. Of course everyone copied it since then and the Russians have their own independent and parallel development.
Please note that the intake does not necessarily have to be under the LERX, an idea started by the YF-17 Cobra, but also on top. As a matter of fact, this is exactly the same principle on the F-5, the YF-17's direct ancestor, introduced first on the T-38 Talon trainers.
But this all moot for the JF-17, which uses a different principle in scooping air to the intakes, a process not unlike that of the Rafale and the F-35. Rather, the nose is V canted. That actually draws the air into the intakes at high AoA. The concept goes back to the original Grumman Super 7 design, which does have this V shaped walls along the side of the nose, and the concept is recentely resurrected on the JL-9.
Furthermore, the DSI intakes acts like Northrop's LERX by diverting and compressing the boundary air layer into the intake. It is also interesting to note that the edge of the intake at the bottom is forward of the edge on top, making it look like a "\", in contrast where in many planes, looking at the side of the intake, the edge looks like a "/". Clever, daring move, the idea again, harks back to the intakes Northrop designed for the Talon---moving the air upward, rather than downward.
For the F-16, the Typhoon and the J-10, it's different though, they use the underside of the nose to divert airflow into the engine.
Is it therefore correct to assume the FC-1's LERX are not primarily for vortex generation since the aircraft is not really meant to operate in high AoAs?
Since I have explained that the design is indeed suited for high AoA, then this is not true.
One should note however that the design of the LERX is closer to the F-18 Hornet than the F-16, despite the similarity of the plane's planform to the F-16. The F-16, like the MiG-29 and the Su-27, uses an inverted or concave LERX that forms the vortices close and over the fuselage. The JF-17 uses a convex shaped LERX which creates vortices in two areas, one directly at the root where it meets the intake. The intake itself is shaped like a dogtooth, meaning the outside sweeps forward and ahead of the root. That creates the vortice. Chances are the DSI intake may also be routing some of the air layer into this tooth, energizing this vortice layer further but I really cannot go further in understanding the vortice interaction between LERX and DSI.
The next vortice should appear where the LERX meets the wing, and that's fairly outward of the body. So the vortice generation is a bit more complex. But we know the benefits of creating a vortice or break outside and away from the fuselage, since that was one of the ideas behind the double delta. Also check the F-14 when the wings are swept forward. The break between glove and the swing wing happens to be away from the fuselage. Putting the vortice away from the fuselage turns out to improve maneuverbility, and this phenomenon puts the Tomcat ahead of other VG designs like the F-111 and MiG-23. It should be noted that the F-18 does the same thing too.
The curved shape of the new JF-17 LERX itself is intended to add more lift, moving the center of lift forward. Chances are the design of the aircraft now makes it more negatively unstable.
There is some reasonably complex aerodynamics involved, and shows you someone definitely doing their homework with vortice behavior.
My complaint about the JF-17 isn't located in the same area, but at the tail. Why use an F-16 like configuration where the end of the tail elevator neatly aligns with the end of the engine exhaust? Is it because it looks neat? But this is not optimal. I would have swept and extended the elevator well past the tip of the nozzle, much like the MiG-21 or the MiG-29. This tends to give better control authority on the elevator, and helps you maintain control at high AoA. The F-18, F-22 and F-35 have all gone to sweeping the elevator surface as far back as possible beyond the tip of the engine nozzle. As a matter of fact, one of the planform drawings of the JF-17, shortly revealed after the DSI version was first shown as a model, had exactly extended the elevators. I think I may find the picture somewher but I'm sure many of you would recall this drawing.
