lehpron
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Vortex Lift And Wing Sweep Angle?

Wed Jan 19, 2005 5:35 am

I always thought it was 45-degrees swept back, but watching video of the Eurofighter demo at airshows, I'm starting to think 50-degrees.

What do you folks think, at what angle does the induced (spanwise) flow appear to spill over the leading edge?
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SlamClick
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RE: Vortex Lift And Wing Sweep Angle?

Wed Jan 19, 2005 5:42 am

Not to brag here, but my ignorance of this is postively encylopedic.

I'd guess it depends on some other factors almost as much. For example, viewed from above, the fuselage forces some air to flow out and around it. This must impart some outboard vector to the airflow. I've seen inflight video of the F-117 in the cloud tops and it was producing the shock-vapor that we associate with the upper surface of a wing, but it was in streams out above the wingtips and clearly off the body of the airplane.

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2H4
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RE: Vortex Lift And Wing Sweep Angle?

Wed Jan 19, 2005 6:02 am



By the time the airflow spills over the leading edge of the wing, I'd have to think the wing would have already exceeded the critical AOA.


2H4


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lehpron
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RE: Vortex Lift And Wing Sweep Angle?

Wed Jan 19, 2005 6:21 am

Sorry 2H4, I should have noted that I meant planform angle, not angle of attack. Wrong axis there, bud. Big grin

But I think you are correct; wait, are you? I never considered streaklined flow paths, like where fluid element was or is and when or where. Sounds like something for CFD's.
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2H4
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RE: Vortex Lift And Wing Sweep Angle?

Wed Jan 19, 2005 8:22 am

Lehpron....I think we're talking about the same thing.

On a highly-swept wing, if the spanwise flow starts to spill over the leading edge of the wing, wouldn't the airflow separation (on top of the wing) occur near the leading edge? If it did so, I don't think it would reattach, and this would leave the wing in a stalled condition.

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2H4
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SlamClick
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RE: Vortex Lift And Wing Sweep Angle?

Wed Jan 19, 2005 8:27 am

Guys I believe that you are talking about a transient phenomenon here. Sudden pitch up causes it, it goes away as soon as the plane is once again flying in the direction it is pointed.

I've seen Art Scholl and Sean Tucker and a long list of others do the same thing with a straight wing.
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FredT
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RE: Vortex Lift And Wing Sweep Angle?

Wed Jan 19, 2005 9:14 pm

2H4, with enough leading edge sweep (delta wing), the spanwise flow will spill around the leading edge of the wing, just as it does around the wing tip on other wings and in other conditions. This sets up a complex vortex flow pattern on top of the wing, complete with reattachment. This generates lift, known as vortex lift. Such wings don't really have a critical AoA at all. They don't stall, there's just a significant increase in drag.

Nope, not a transient phenomenon (dynamic stall) either.

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Fred
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SlamClick
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RE: Vortex Lift And Wing Sweep Angle?

Thu Jan 20, 2005 1:17 am

FredT I remember the F-102 and F-106 having a tremendous droop to the leading edges especially out near the tips. I suppose it is for this reason.

Clearly it must be a controllable phenomenon since they keep designing and building planes with this wing configuration.
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FredT
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RE: Vortex Lift And Wing Sweep Angle?

Thu Jan 20, 2005 1:54 am

It is controllable, understood and desired. It allows for high instantaneous lift coefficients, if you can take the drag penalty, and gives good nose pointing ability. One of the main drawbacks is a susceptibility to super stalls (or deep stalls, for those who prefer that terminology).

The most common use for LE droop is to make the stall/low speed characteristics of an aircraft more benign.

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Fred

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2H4
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RE: Vortex Lift And Wing Sweep Angle?

Thu Jan 20, 2005 8:53 am



Thanks for the info, FredT. Interesting stuff.  Smile


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B2707SST
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RE: Vortex Lift And Wing Sweep Angle?

Thu Jan 20, 2005 11:41 am

I think Lehpron is talking about vortex lift as used on Concorde and a few other delta-wing aircraft (e.g. Saab Draken). I am sure there is no "magic number" -- the creation of vortex flow would depend on airspeed, wing sweep, camber, the leading edge radius, and a lot of other factors.

IIRC, another very factor is the compound curvature that induces the spanwise flow to wrap around the leading edge and twist into a vortex over the wing. I recall reading that the S-shaped curvature on Concorde's leading edge existed to produce a smooth transition from vortex lift to attached flow.

On a highly-swept wing, if the spanwise flow starts to spill over the leading edge of the wing, wouldn't the airflow separation (on top of the wing) occur near the leading edge? If it did so, I don't think it would reattach, and this would leave the wing in a stalled condition.

Yes, the airflow does stall, insofar as it becomes turbulent and separates from the upper surface of the wing. However, the rotation of the vortex over the wing re-energizes the boundary layer and creates additional non-linear lift. The only problem is that it creates tremendous drag. The lift-drag ratio on a conventional aircraft on approach with flaps and slats extended is probably around 12. On Concorde, with no high-lift devices and an ogive-delta wing, it was about 4.



Concorde actually required more thrust at 160 knots than at 250 knots, because vortex lift generation puts you on the back side of the drag curve (slower = more thrust). On a "normal" aircraft operating on the upward-sloping portion of its drag curve, slowing down will reduce drag and the aircraft will stabilize again at the point where drag and thrust are equal.

But Concorde at low speeds was in a disequilibrium situation, in that the problem is self-reinforcing: if you slow down a little, drag rises, so you slow down a little more, drag rises a little more, and so on. If you don't watch your thrust margin, you can end up running out of power to maintain level flight. The wing never "stalls" since it's already technically stalled, but drag increases so fast that the engines can't compensate. This is what happened to AF4590 -- with two engines out and the gear locked down, Concorde needed to be going at least 220 knots to maintain altitude. They never reached this airspeed.

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lehpron
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RE: Vortex Lift And Wing Sweep Angle?

Fri Jan 21, 2005 7:21 am

>>" I am sure there is no "magic number" -- the creation of vortex flow would depend on airspeed, wing sweep, camber, the leading edge radius, and a lot of other factors"<<

I guess I mean where it becomes obvious, like the figure of assuming incompressible flow up to M0.3 simply because the air compresses to 95% at that point. Like say it is barely there by 40-degrees and obviously there by 55-degrees, I think it is full blown by then, but I also feel further increasing the sweep at that point may kill of the span wise flow. I am figuring that leading edge sweep will vary the amount of induced drag/lift.

Slightly different question: is the source of the vortex on top of a differentiate delta wing from the induced flow of the previously differentiated chord times the half-span, i.e, the pix below?



The only problem I am having with is that the vortex drag/lift might not be integration but more like sumation becuase the induced piece adds to each next wing section's pattern, because of which I have postulated the crap in the red boxes.

For those of you ahead of me, was that correct of me to asert?
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