Raked wingtips?

[Crossunder]

What's the deal with raked wingtips?
Can anyone explain how they work?

[HotDog]

Winglets reduce wingtip vortices. High pressure on the lower surface creates a natural airflow that makes its way to the wingtip and curls upward around it. When flow around the wingtips streams out behind the airplane, a vortex is formed. These twisters represent an energy loss. Winglets produce an especially good performance boost for jets by reducing drag, and that reduction could translate into marginally higher cruise speed. But most operators take advantage of the drag reduction by throttling back to normal speed and pocketing the fuel savings.

[avioniker]

Try this link. Nice visual.

http://www.b737.org.uk/winglets.htm

[Crossunder]

Yeah, but that's winglets. I'm talking about raked wingtips, like on the B777...

[cirrus01]

777 has not got the raked type wing tips...... did you mean the 767-400 ?

Comparison.....777....http://www.airliners.net/open.file/354884/L/ (great picture by the way )

767-400....http://www.airliners.net/open.file/332347/M/

Boeings claim that the raked design has a lot of the benefits of winglets without the weight.......You make you own mind up.

:= := :=

[Crossunder]

Exactly (although the 777-300ER will have them), but no-one seems to know what all the fuzz is about?! How do these wingtips affect the vortices?

[HotDog]

It does the same thing as winglets only better, something to do with the rake incidence angle. If one gets damaged, you can still dispatch by taking both rake tips off. Not quite sure how it works exactly but will do a bit of research and let you know. HD

[jungly]

Its my understanding that the drag induced by two small vortices is less than the drag induced by a single large one.
Winglets have a few functions, sail effect, spanwise flow etc but they also create 2 smaller vortices.
I suspect that the raked design does the same thing?

The sudden increase in sweepback is sufficent to create a small vortice, leaving a smaller component of spanwise flow to effect the tip ie: a small tip vortice.

I suspect some joe-90 has calculated that the extra weight of the winglet, the drag it creates and the decrease in spanwise flow are not as significant as a lighter design with less form drag even taking into account the extra spanwise flow penalty.

Thats my 2 cents worth..... looking fwd to your findings HD

[Crossunder]

Looking fwd to hear from you HD!

Could it be that the raked tip will act like the leading edge of a delta wing and create "vortex lift"?

[LEM]

Winglets are there to counter the main vortex.
Like everything else they have some disadvantages too.
Probably what the designers try to do is balancing the counter of the main vortex to the creation of their own vortex...
If you put the winglets orizontally they will do the same job as the vertical ones, but slightly less efficiently.
But the benefit is that being orizontal they contribute to lift also, so this design seems to privilege an increase in MTOW (increased performance), loosing something in cruise.
Anyway they've reinvented the wheel, check this Dornier design which has been around for more than 20 years...
http://www.airliners.net/open.file/348779/M/ (http://www.airliners.net/open.file/348779/M/)

[HotDog]

Raked wingtips have been around for about 50 years but it seems modern wing design has only adapted this philosophy in relatively recent years.
The fore runner of modern raked wingtips was called the Hoerner wing tip, in common use on STOL wing designs.
With a conventional rounded or square wingtip, the vortex is centered around the wingtip. If the wingtip is cut at 45 degrees , with a small radius at the bottom and a relatively sharp top corner, the high pressure secondary air flow travels around the rounded bottom but can't go around the sharp corner and is thus pushed outward.
The performance of the wing depends on the distance from the right to left tip vortices (the effective wingspan) and not the actual measured geometric span. Raked wingtips provide the longest effective span for a given geometric span or a given wing weight.
The raked tip is more efficient in reducing tip vortice drag and is lighter than winglets.

I hope this explanation answers your question. Cheers, HD.

[ft]

It is my understanding that while a longer wing would give more of a benefit than winglets giving the same weight penalty, winglets are used since it is more difficult to retrofit a wing with an extended tip. You'd have to reinforce the wing to handle the added lift at the tip while a winglet mainly reduces the drag load on the wing. I have not meddled with such designs myself, but that is how I’ve had it explained to me.

HD, I’d love a source for that for a more thorough explanation than what I can get on a web forum?

I’d assume that a raked wing tip would gain some of the benefits of a delta wing, with the vortex shedding along the entire leading edge of the raked section and providing lift on top of the airfoil.

Doublechecking my definition of a Hoerner wing tip: A relatively straight (square) wing tip with the outer edge cut off at an angle to the vertical, leaning out from the aircraft?

Cheers,
Fred

[Crossunder]

HD:

Checked! Thanks.

[HotDog]

ft
Winglets also increase the effective wingspan and reduce the wingtip vortex. They are also bolted to the wing and you can MEL them at an approx. 2.5% performance penalty, however, they are heavier than raked wing tips. You are correct about the Hoerner wingtip, the raked wingtip concept is a derivative of that design.

If you wish to delve into this deeper, try:

http://www.desktopaero.com/appliedaero/wingdesign/nonplanar.html

Cheers,HD

[used2flyboeing]

You asked for it .. sit down an let the professor pontificate ..

767-400 has raked wingtips - more importantly they have a blunt profile. The 777 doesnt have them ( I think ) because the wingspan is sooo damn long .. ( 144 feet - I think .. )

Anyway - The performance of a commercial transport airplane is typically measured in terms of mission capability and operating costs. Mission capability can be improved by reducing airplane drag during takeoff climb and cruise, and by utilizing designs that minimize structural weight. Operating costs can be reduced by reducing airplane cruise drag (hence, resulting in less fuel burn and less fuel costs) and by utilizing designs that are inexpensive to manufacture and maintain. Further, for commercial operators, higher profits can be achieved by being able to transport more customers and/or goods for a given flight. Because the additional payload increases takeoff weight, it is even more desirable to reduce takeoff drag for takeoff-climb-limited missions. - nothing new here right ?
The objectives of reducing drag, reducing weight, and reducing complexity (hence manufacturing and maintenance costs) are often in conflict. Adding a wingtip extension member can reduce the drag of a given airplane, but this will usually require increasing structural weight - IE winglets bend the hell out of wing roots because of the increased moment arm - therefore you need to beef up the wing judiciously - on ****ty wing designs - the weight increase can "washout" the efficiency increase by additing a winglet in the first place.. Sooooooo - Weight increases are due to the weight of the wingtip extension member and also due to strengthening required of the existing wing structure in order to support the increased bending moments exerted by the wingtip extension member. Additional weight penalties can also occur if the extension exacerbates flutter. - IE if you fiddle with ANY WING design after flight test - you change its natural resonance - IE it becomes a new tuning fork - heaven forbid it will like to resonate at cruise mach - IE flutter.. 747 has a speed advantage over the A340 because of flutter - the old crusty 747 hauls ass - the A340 could haul ass if it didnt like to shake itself apart at VMO.. ( my personal opinion based on 2nd hand info .. )
This conflict between the benefits of reduced drag and the disadvantages of increased weight has motivated designers to find an optimal balance between the two when designing a wingtip extension member. One such attempt is described in U.S. Pat. No. 5,039,032, incorporated herein by reference. The '032 patent describes a number of wingspan extensions termed "High Taper Wing Tip Extensions". These are also known as "raked wingtips". Raked wingtips are generally characterized by leading-edge sweep angles that are greater than the main wing sweep angles and are significantly tapered (i.e., the chord length decreases in the spanwise direction.)
Raked wingtips offer several advantages, some of which are outlined in the '032 patent. These advantages include the aerodynamic benefit of drag reduction due to increased wingspan, and a number of weight-reduction advantages (relative to simply extending the wingspan of an existing conventional main wing.) Two weight advantages are attributed to the wingtip taper. At high-load-factor structural design conditions, the smaller chords are subjected to less load and they result in less induced loading on the outboard main wing. These are both factors that reduce the bending moment that the inboard wing must support. Two more weight advantages are attributed to leading-edge sweep. The leading-edge sweep of a raked wingtip results in the center of pressure being located further aft than for a simple extension of an existing conventional main wing. At the high load-factor structural design conditions, this relative aft-movement of the center of pressure causes the sections of the main wing adjacent to the raked wingtip to be twisted more leading-edge-down, thus reducing the loading on these sections and the bending moment that the inboard wing must support. The relative aft-movement of the center of pressure also acts to attenuate flutter. The raked wingtips described in patent '032 range from moderate span extensions (e.g., 6% increase in span) to large span extensions (e.g., 12% increase in span). It is the large span extensions that offer the greatest benefits.
Regardless of these benefits, there are challenges in implementing raked wingtips on some aircraft. For example, on aircraft designed to operate at high subsonic Mach numbers (i.e., at or greater than about 0.70) there is a tendency for the boundary layer on the upper surface of each raked wingtip to separate under high-lift conditions (such as during takeoff climb or landing). This boundary-layer separation has the potential to increase drag and to generate premature buffet. The primary motivation for adding a wingspan extension is to increase the lift-to-drag ratio (primarily by decreasing drag), both during cruise and takeoff climb. If there is a significant drag increase due to large-scale boundary-layer separation under takeoff climb conditions, part or all of the takeoff-climb improvement is lost. When the raked wingtip boundary layer separates, there is also a possibility of unsteady aerodynamic forces strong enough to vibrate the airplane structure and to be perceived by the airplane pilot as buffet indicating the onset of aerodynamic wing stall. If this form of buffet occurs prematurely (that is, within what would normally be the operating envelope), stall speed must be declared at a speed significantly higher than the aerodynamic wing stall, thus degrading airplane performance.
The '032 patent acknowledges the tendency of the boundary layers on raked wingtips to separate under high-lift conditions. In the '032 patent, raked wingtips are categorized into two groups, one group with leading-edge sweep angles between 40 and 50 degrees and another with leading-edge sweep angles between 50 and 60 degrees. For the first group, the '032 patent indicates that some form of a mechanical leading-edge high-lift device (such as a slat) is required in order to avoid premature low-speed buffet. The addition of a mechanical leading-edge high-lift device avoids premature boundary-layer separation, alleviating the buffet problem, but it adds profile drag, weight, complexity, and cost. Under some circumstances, these disadvantages may outweigh the benefits of the raked wingtip. For the second group, the '032 patent indicates that the wingtip leading-edge sweep is great enough to trigger the formation of a stable leading-edge vortex, and that therefore premature buffet will not occur and no high-lift mechanisms are required.
The inventors herein have discovered that under some circumstances, leading-edge sweep angles of 50 to 60 degrees may not be adequate to ensure the formation of a stable leading-edge vortex when conventional transonic airfoils are used for the raked wingtip geometry. As used herein, "transonic airfoils" are those designed to operate at high subsonic freestream Mach numbers, with significant regions of locally supersonic flow. Additionally, even if the presence of a stable leading-edge vortex prevents premature buffet, such a vortex may result in higher drag than if the majority of the raked wingtip boundary layer could be kept attached over the range of typical operating conditions. Further, the technical viability of any raked wingtip would be improved greatly if there was no requirement for a leading-edge high-lift mechanism.
Thus, the evolution of the improved raked wingtip, particularly for use with aircraft that operate at high subsonic Mach numbers. The ideal raked wingtip would provide the aerodynamic benefits of an increase in wing span, while avoiding premature boundary-layer separation under high-lift conditions. Further, the optimal arrangement would not add significantly to wing weight or wing complexity. Both the raked wingtip & the blunt raked wingtip are Boeing inventions. Airbus A330/A340 uses the 747-400 winglet. The A320 family of airplanes uses the AIRBUS ( opps BAE - UK developed ) delta winglet that has an opposite plan-form camber on the upper & lower half of the winglet - to gracefully control the direction of the vortices comming off the top & bottom of the wings to kinda mesh them together non distructively - a clever design - by a company with a fine British aircraft heritage .. god save the queen .. All that being said - winglets on business jets regardless of eficiency increases are for sex- appeal ..

Enjoy & cheers