Why hasn't a wing been designed to stop wing tip vortices effectively?
To quote John S. Denker in See How It Flies:
It is a common misconception that the wingtip vortices are somehow associated with unnecessary spanwise flow (sometimes called “lateral” flow), and that they can be eliminated using fences, winglets, et cetera. The reality is that the vortices are completely necessary; you cannot produce lift without producing vortices.
Vorticity is a fundamental component of lift generation. Wingtip vortices can only be stopped by not generating lift.
Here are some attempts:
These do not actually eliminate trailing vortices completely. You do not need to have a wing tip to get them because they form all along the trailing edge of a wing. When there is positive pressure on the bottom and reduced pressure on the top of a lift-generating surface, then you get a span-wise component of flow. On the top, the air flows inward (a little) and on the bottom it flows outward (a little) and as those flows leave the trailing edge, the twist up to form axial trailing vortices and they still cause induced drag.
Unless you have an infinitely long wing span, it is impossible to eliminate trailing vortices completely. They are a consequence of the lift varying along the spanwise direction.
Ultimately, it’s the total drag that you care about. The induced drag is actually quite low at cruising conditions, so if you start adding lots of surfaces to reduce the strength of the trailing vortices, you will end up adding more skin friction drag and throw out the baby with the bathwater.
The following image is from a Caltech PhD thesis investigating the trailing vortex system behind a circular ring wing.
http://thesis.library.caltech.ed...
The dashed circle represent half of the wing. The paths with arrows represent the projection of the velocity vectors, which are mostly pointing out of the page towards the viewer. You can see that there are still trailing vortices despite having no wingtips.
Tim, and the man he quoted is right; wing vortices are a side effect of the of the act of creating aerodynamic lift.
Can they be completely eliminated? Perhaps one day. Right now the best we can do is reduce it's effects.
To better understand the phenomenon, let's see how and why a wing vortex is created shall we?
This here is the profile of a simple wing design. The top part is curved as you can see. This curvature causes the air moving through it to:
- Deflect downwards, creating an opposing reaction pushing the wing upwards
- Compress a lot more compared to the air travelling below. As a result of being forced to occupy a smaller volume, the air on top starts to go faster to maintain the flow rate due to conservation of mass and if you remember your physics classes (or your Bernoulli’s principles), the faster a fluid moves the less pressure it has.
But remember the bottom of the wing is not curved and so the air doesn't move as fast as the top one. Which means the bottom part of the wing has more pressure being exerted on it than the top one. The bottom of the wing is pushed up in effect. This difference in pressure and the downward deflection of flow is exactly what is called “lift”.
Now imagine a wing in your head. The air flowing through the bottom is pushing up over the entire bottom surface, but that surface ends at some point. This point is the tip. At the tip, this air that's pushing up basically spills over. Spilling over the bottom surface and pushes over into the top part, because it has less pressure. The air tries to fill the vacuum.
This spilling over is what's called a vortex. It swirls around the tip of the wing and tries to equal the pressure at the top and the bottom. This creates drag and causes the surface at the edge of the wing to stop creating lift. You effectively end up having a shorter wing than you have with extra drag to boot.
Now that a look at this:
See the wing tips? Do they look familiar?
Do they look something like this?
If they do (and they should) it's no coincidence. The people who came up with the winglet design were inspired by predatory birds’ wings.
Oh by the way; that upward pointing protrusions at the the tip of each wing is what's called a “blended-winglet”.
Airbus mostly uses something like this:
What a winglet does is make it harder for the air at the bottom to spill over and move it further away from the wing surface. It's basically like a wall, the air can't just walk over to the top of the wing, it has to climb over it if it wants to mingle with the low pressure air so badly. Which means the parts of the wing that couldn't generate lift before, now can.
The end result is that you effectively increased the length of your wing AND created less drag.
So you have more lift and less drag. Best of both worlds. You could of course simply lengthen your wing to increase the amount of lift, but this way you actually induce more drag, eating away at the benefit of a long wing.
There are different winglet designs out there and they all do the same thing.
So back to the question; “Why hasn't a wing been designed to stop wing tip vortices effectively?”
They have.
It's called a winglet.
Edit: At Alain Faddegon’s urging I've revised my explanation of the process of creating lift to a correcter version. This is more elegant I think. Though still not as comprehensive as a full-on mathematical explanation including angles of attack and whatnot, but I wanted to keep it as simple as possible to understand and visualise.