3
$\begingroup$

Vortex lift is a kind of aerodynamic lift force due to vortices formed along the leading edge of highly-swept (usually 60 degrees of swept angle or higher) wings at high angles of attack. The "vortex sheet" is high in rotation speed and thus low in pressure, which provides the pressure difference, resulting in lift. *in subsonic speeds enter image description here "Vortex sheets" causing vortex lift

Normally an unswept wing (without wingtip devices)would have wingtips that can easily form vortices, called wingtip vortices. This kind of vortex is associated with higher induced drag, and is often avoided by adding wingtip devices like winglets. However, is it possible that wingtip vortex can also produce vortex lift? If so, why try so hard eliminating it?

Improve this question
$\endgroup$
2
  • $\begingroup$ I am a student pilot and just curious about different means of lift production. In addition to my question description, unswept wings produce lift with laminar, attached flow, at higher angles of attack, the vortices would cause stall. While vortex formed around highly-swept wings would help with lift generation at certain situations. I haven't looked into this a lot, so I would really appreciate a precise, simple answer. $\endgroup$
    – Frank
    Commented Dec 5, 2022 at 14:14
  • 1
    $\begingroup$ Just a couple of clarifications: "vortex sheet" is the vorticity sheet released behind the trailing edge of a wing. Wingtip devices do not avoid induced drag nor the formation of tip vorteces but have more or less the same effect of extending a bit the wingspan. $\endgroup$
    – sophit
    Commented Dec 5, 2022 at 15:22

4 Answers 4

Reset to default
4
$\begingroup$

Vortex at straight wing has very small contact area with wing and also at wrong position.

Vortex has low pressure, you can see blue color on upper surface where vortex touch surface. This part of wing has backward orientation, so this produce more drag than lift. Pressure leak(light blue/green color) at wing tip area reduced lift more then this little increase of lift due to vortex. End plate/winglet reduce this pressure leak.

enter image description here

Improve this answer
$\endgroup$
2
3
$\begingroup$

The picture you've chosen of a water tunnel test at Onera nicely shows how the vortices generated at the leading edge of the delta wing wash over its upper surface and coalesce to form two big vortices, one on each side of the wing. Those vortices help the airflow in remaining attached to the wing so that lift can be generated till very high AoA, as it can be seen for example in the following plot taken from this NASA report:

lift coefficient of a delta wing

The 62° delta wing tested at subsonic speeds generates lift till some 30° while a "standard" rectangular or swept wing would stall at maybe half of that angle.

is it possible that wingtip vortex can also produce vortex lift?

The same vortex phenomenon happens also on a "standard" wing but! their coalescence takes place well behind the trailing edge, where the wing is already finished and the effect of generating lift till very high AoA has therefore no time (or space) to develop.

Anyway this is not the only important characteristic of those vortices: in general, when a vortex rolls up along a direction parallel with the chord, it generates also a force aligned with it i.e. drag by definition. This drag is termed induced drag. The vortices seen on the delta wing make no exception: if on one hand they help the airflow in remaining attached, on the other hand they increase drag as well and quite a lot.

So it becomes now quite interesting to compare the polar of that delta wing with the polar of a standard swept wing (the plot for the latter is taken from this second NASA report):

Polar of a delta wing polar of a swept wing

Let's compare for example the points at $C_l=0.8$. For the delta wing it corresponds to a $C_d$ of 0.2 while for the swept wing it corresponds to a $C_d$ of some 0.07: that means that at the same lift coefficient a delta wing generates some three times more drag then a swept wing! That's why the Concorde had to takeoff with the afterburners on and that's

why try so hard eliminating it.

Improve this answer
$\endgroup$
4
  • $\begingroup$ Let's ... um ... make a winglet out of the second ... and ... have it on the end of a straight wing! $\endgroup$
    – Robert DiGiovanni
    Commented Dec 6, 2022 at 10:36
  • $\begingroup$ @RobertDiGiovanni: is it a question? I don't understand it, can you please reformulate it? $\endgroup$
    – sophit
    Commented Dec 6, 2022 at 11:04
  • $\begingroup$ It's an unswept wing question. But your data confirms why the Vulcans needed an armada of tankers to cross the Atlantic. $\endgroup$
    – Robert DiGiovanni
    Commented Dec 6, 2022 at 11:33
  • $\begingroup$ Ah now I understand it 🙂 I would say that the polar for a rectangular wing is basically the same as the swept one, maybe the stall region would be a bit delayed. $\endgroup$
    – sophit
    Commented Dec 6, 2022 at 12:10
1
$\begingroup$

The vorticity behind a wing is only a consequence of what happened before above and below the wing, but cannot alter the wing's lift or drag in any way.

I remember a breathless report of an ultralight flying wing called Kasperwing which could fly extremely slowly thanks to a stationary vortex which formed on the upper side of the wing at high angle of attack.

Wikipedia calls it a bit more soberly a

fully controlled, completely stalled parachutal descent mode

which was made possible by the airfoil shape. Note the "descent" part: This did not make straight or even climbing flight possible but needed to be bought with a high rate of potential energy loss.

Other than that, no, vortex lift is the prerequisite of highly swept leading edges.

Improve this answer
$\endgroup$
6
  • $\begingroup$ If wingtip vortices don't alter the wing's lift or drag in any way, then what is the purpose of installing wingtip devices like raked wingtip, to eliminate wingtip vortices, as an unmodified wingtip should produce more lift than a specially-shaped raked wingtip? $\endgroup$
    – Frank
    Commented Dec 6, 2022 at 10:51
  • $\begingroup$ @Frank Marketing. Mostly. They do help by spreading the vortex sheet slightly and lowering local vorticity, but most of the vorticity remains. None of them even comes close to the claimed "elimination" of wingtip vortices. Note that this claim is not made by engineers, but by marketing. That should tell you something. $\endgroup$
    – Peter Kämpf
    Commented Dec 6, 2022 at 17:46
  • $\begingroup$ Not sure if wingtip devices are all marketing? Wingtip vortices don't cause induced drag, that's correct, I totally agree with you on that, but those vortices do affect lift distribution and drag negatively, in my understanding. Check out this question also: aviation.stackexchange.com/questions/96338/… $\endgroup$
    – Frank
    Commented Dec 8, 2022 at 14:44
  • $\begingroup$ @Frank My answer was referring to all those wingtip devices pointing up or down. A raked wingtip is actually quite clever, see aviation.stackexchange.com/questions/19073/…. But I do not consider a raked winglet a wingtip device. It is a characteristic of a wing planform. $\endgroup$
    – Peter Kämpf
    Commented Dec 8, 2022 at 15:49
  • $\begingroup$ Just to clear any misunderstandings. So winglets and other wingtip devices DO reduce drag and other negative effects provoked by wingtip vortices? $\endgroup$
    – Frank
    Commented Dec 9, 2022 at 8:23
0
$\begingroup$

Yes, but only at higher angles of attack, which generally have lower overall airfoil lift to drag efficiencies.

Straight wings use Hoerner wing tips to push the vortex away from the wing tip. This allows the entire span of the wing to be productive in lift and away from turbulence. Ground effect also creates a similar result by strengthening the outflow from higher pressure under the wing.

However, soaring birds which, by virtue of the updraft contributing to their higher relative wind Angle of Attack, take full advantage of wing tip vorticies not only to create lift, but also thrust$^1$.

But, as airspeed and the Reynolds number increases, better lift to drag ratios can be had with higher aspect straight wings at lower angles of attack, where wingtip vorticies are less prominent.

$^1$ in much the same manner as sailboats create forward motion into the wind

Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .