# efficiency, reducing tip vortices

thought that others might be interested in this photo of a wingtip extension on Boeing’s 767-400… I outlined the shape in blue because I wasn’t sure how well it would show up in picture… the purpose of this tip extension is to reduce tip vortices and induced drag.

I think I see what they’re doing. I have a question for any aero engineering folks on board. Does the wingtip extension create it’s own tip vortex?

I don’t believe in coincedence, rather more along the lines that Ambrose works on, that is: the spirits touch those who are of a common mind in the same time continum. This may sound, far fetched, surreal, or somewhat metaphysical but that’s ok. The part of it that applies is that the last two templates I’ve drawn up have tip extentions not because of Boeing’s work but because of our sharkey friends and enemies, and not with any consideraton for vortex generation or drag reduction but just to put more power at the tip of the fin where is seems to me you get the most out of it.

Pictures are on on the way soon.

Fighting off the flu, Rich

It might… but I think there are other things going on.

here’s one possible explanation, lifted from: http://www.aerospaceweb.org/question/aerodynamics/q0148.shtml

Since the tips are so highly swept, the lift force generated by the extensions will act further back than the lift generated by the rest of the wing. This disparity creates a torsion motion causing the extensions to twist nose down. While this sort of wing “warping” is usually not desirable for structural reasons, perhaps Boeing engineers have used this torsional effect to create an aerodynamic washout.

Here’s another one:

When a wing ends abruptly such as on a non-winglet 727 or 737 or DC-9 or =

DC-10, there is a great amount of drag produced. Boeing recognized this =

designing the 767-400 “raked” wing. The wing tapers off much like a =

winglet…result…less drag.

More detailed information on “Whitcomb winglets” can be found in “Aerodynamics, Aeronautics, and Flight Mechanics” (Barnes W. McCormick, John Wiley & Sons, Pub. 1979. p215-211).

The original references are:

Whitcomb, R.T. "A Design Approach and Selected Wind-tunnel Results at High Subsonic Speeds for Wing-Tip Mounted Winglets. NASA TN D-D-8260, July, 1976.

Flechner, S.G., Jacobs, P.F., and Whitcomb, R.T… “A High SubsonicSpeed Wind-Tunnel Investigation of Winglets on a Representative Second-Generation Jet Transport Wing.” NASA TN D-8264. Nov, 1976.

Heyson, H.H., Riebe, G.D., and Fulton, C.I… “Theoretical Parametric Study of the Releative Advantages of Winglets and Wing-Tip Extensions.” NASA Tech. Paper 1020. Sept. 1977.

Progressive reduction in chord length minimizes tip vortices.

Airplane wings, such as the one shown, also have bars oriented parallel to the chord length. These block flow perpendicular to the airflow that contribute to induced drag.

Although vortices are worst at the tip, they exist throughout a planar wing. Air (or fluid) on the high pressure side moves towards the tip - air on the low pressure side moves away frrom the tip. When the air (or fluid) re-attaches behind the foil, the momentum of the two sides of fluid is different, and this also creates vortices which also creates induced drag.

Creating a progressive decrease in chord length towards the tip minimizes currents (elliptical is best), but anything that blocks flow over the foil that is perpendicular to motion helps.

I think most surfboard fins are created in a way that is hugely inefficient for induced drag - but one that serves another purpose by altering the toe-in angle at the tip relative to the base.

I knew that lots of different wingtip designs had been experimented with in aviation, but I was not aware that these were that old of a design, none of the articles that I read made that connection.

I was interested in it because it appeared to be a kelp-free variation on winglets, when applied to surfboard fins, and also because it appeared to be similar to some fins I’ve seen that were perhaps stumbled upon rather than theoretically designed (but then, who knows)>

Have you played around with fins w/“Whitcomb” tips, or does that not make sense to you for some reason?

yes, “fences” is the term I’ve heard for the “bars” on top of the wing. I have seen fins w/ similar features there, too.

I am particularly interested in reducing “induced drag”.

Anyone have any idea the fractional drag reduction planar winglets can achieve?

How about the difference in an elliptical template from a more standard dolphin pectoral fin conceptual template (assume the depths and chord lengths are adjusted to keep the drive comparable) ?

How about the reduction from non-planar winglets (a la winged keel)?

Or the reduction from curved planshapes (like the Webber CRV) ?

Or the reduction from having horizontal protrusions on the foil?

Or the reduction from using MVGs or bumpy humpback protrusions?

Which is most effective and practical?

where I live there is lots of kelp, which makes winged keels, and probably by extension (although I’ve not tried it) any kind of non-planar winglet (finlet) less practical.

What I’ve read seems to imply that non-planar winglets (the Whitcombs) are in the range of perhaps 7% more efficient (depends, though, on speed & angle of attack, more efficient at higher levels of both)…

Boeing 767 used the raked wingtip extension in lieu of the winglet, w/ claims that it was about as efficient. (However it would catch a lot less kelp if used on a fin, being simply a different tip shape)… the Boeings are made of composites but for weight savings purpose…seems like a no brainer to use that tip design if it’s even a few % more efficient, although it’s obviously not optimized for water use…

Elliptical templates - you’d think it all would have been tried by now but maybe not.

Whale bumps, theoretically very promising (highly efficient), but hard to make compared to others…

MTB probably has some good thoughts on your questions…

I’ve compared elliptical to non-elliptical (on fins with normal mean chord rake), and found quite a noticeable difference at the moments of highest induced drag (mid-turn, on a hard turn).

However, as I mentioned earlier, I think the progressive raked tips serve in a flex capacity and the benefits of this flex probably outweigh the benefits of an elliptical planform for fixed fin systems. In my system, though, it is all benefit.

I’m just wondering if adding fences or a non-planar winglet would help. I rarely surf places with kelp.