Many modern fighter planes use a structure called
a "strake" (lately it's common to call it a LEX) to generate large
systems at high AoA (actually in this context "high AoA" is anything
than 7 or 8 degrees). These strakes are basically small deltas
sharp LEs attached to the LE of the main wing at the root. The
forms on the strake separates from its leading edge at the intersection
with the main wing and continues down stream where it produces high
and low pressure on the upper surface of the main wing. This
the AoA range of the wing thereby allowing a small wing that's
at high speed to also give reasonably good handling at low speed.
LE Leading edge
PF Potential Flow
PV Primary Vortex
SV Secondary Vortex
TE Trailing Edge
RP Reattachment Point
SF Surface Flow (boundary layer)
|These next two drawings are a bit harder to find, especially
books. They also show the starboard wing.
Vortex "A" in the drawing of a double delta bellow is the normal leading edge vortex. Vortex "B" has the same direction of rotation as A and therefor when they come into contact they wrap around each other. A tries to follow a line approximately parallel to the forward LE so when B wraps around A it is dragged farther inboard than it would go by itself, thus a larger area of the wing is affected by this type of vortex system than would have been affected by a single vortex. Since the CLmax of a wing with vorticity can be expected to be 80% higher than with potential flow alone naturally the designer wants the vortices to cover as much area as possible. In order to get this spiraling vortex pair the angle "K" must be greater that 10 degrees and the minimum sweep back of the aft portion of the wing should be more than 50 degrees.
The drawing below depicts an unstable shear layer (as opposed to a stable shear layer, which is what the other drawings show). It occurs at higher AoA than is needed to produce the type of LE vortices shown in the previous illustrations and with larger sweep angles.
|If you have any comments about this page pleas let
me know .
For a more detailed discussion of the flow about a delta see "Aerodynamics of the Delta Wing".
Also see this historically interesting page with an article about the AVRO 707A
Picture credit: Office National d'Etudes et Recherches Aeronautiques