Winglets
 

For another authoritative explanation of modern wing design(with quite a bit on winglets) try:

Wing Aerodynamics and the Science of Compromise.
Jeff Jupp
RAeS Lanchester Lecture 10 May 2001.

I have heard Jeff described as the father of the A340 wing! His lecture explains clearly what factors are involved in modern wing design. He also makes it clear that although elliptical loading is the theoretical optimum for a wing of given span it is most emphatically not the optimum for a wing of given weight.

Owain Glyndwr

"Elliptical". As in: Spitfire? Does this refer to plan, section, chord, or Station?

I'm lost.

Lyman

Elliptical, as in lift distribution. How you achieve that is up to the designer, Spitfire wing is the 'easy' way but not the only way.

Onglide is able to do the comprehensive answer, but then he dioes make his living as an aero consultant.

Lyman

None of those. Elliptical as in elliptical span loading, usually expressed as:

[local lift coefficient (at any spanwise station) times local chord]/ [wing lift coefficient times mean aerodynamic chord] plotted against spanwise station as a decimal fraction of semispan.

Aerodynamicists obtain the span loading they desire by combinations of planform (local chord) local AoA variations (wing twist) and wing camber (no lift angle)

Sorry if you are lost!

Once lost, now found. Good Scout!

The only questions remaining in my mind are -

Why do the 777-200LR & 777-300ER have raked wingtips rather than winglets?

And how does the raked wingtip provide any benefit? Is it similar to a delta in that the wing ends at a point rather being squared off?

The raked wingtip is precisely a winglet rotated coplanar to the wing.

OG: Great explanation mate.

Regarding retrofits to early designs, am I right in thinking that their wings were typically more highly outboard loaded than current practice (to minimise wave drag with their early transonic aerofoils)? So these wings have stronger tip vortices to work with and lift distributions that start closer to the non-planar ideal.


A question remains: Once you realise you have your 6% strength margin to spend on a wingtip modification, what are the considerations that would lead you to add a winglet rather than merely extending the span?

Would it have to do with newer and lighter materials available, also advanced modelling to suss risk/benefit in development instead of on wing test programs?

OG mentioned gate width and taxiway clearance re: added span?

Airport restrictions? Greater bending moments at wing root during extreme loading?

OG,
Interesting post...
I am currently working with the real-time wake turbulence measurements, so I understand the roll-up, creation, advection, and decay of the vorticies, and the effect of winglets and sharklets quite well. (crunching data on hundreds of multi-variant arrivals, incl A380)

Would be interesting to converse on these matters, especially in regards to wingtip vorticies.

It is a common misconception that the wingtips themselves create the vorticies. Winglets and sharklets do nothing to the creation of the vortex.
The vortex is a rollup combination from the bottom surface of the wing structure, (for lack of a better example, a boat plane in the water) coupled with the rollup from the top surface of the wing.
Just as a boat planing through the water, the surface area in contact, the shape of that surface, and the directional flow as a resultant of that surface, are what create the vortex.
The winglet or sharklet, tend to reduce the drag associated with the outer portion of the wing, where the camber and length are at their greatest discontinuity, winglets/sharklets reducing the turbulent airflow in the last portion of the wing, ie cavitation, thus reducing drag.
Winglets/Sharklets, in reducing the cavitation, in smoothing the airflow, will tend to optimize the particular wing for its particular vortex creation. This is not necessarily a bad thing, and each configuration must be looked at distinctly.
Just as the angle of attack differs with conditions and aircraft loading, so does the resultant vortex. (Leave out the weight, focus on the aircraft config at load)

Current designs of winglets/sharklets rely on a certain configuration...
this is similar to fixing flaps at a certain configuration... hardly optimized flight.

FlightPathOBN

How close are we to articulating Winglets, and are they indicated?

Lyman,

I would suppose that given the wingflex of the 787, that may be a resultant.

If the wing was designed for the aircraft, then the winglet would be moot, but perhaps the resultant wingspan may be too great, hence a turn-up to artificially lengthen the wing.
In reality, we are looking at a blend from Bernoulli to Newton.

FlightPathOBN: Sorry, my wording was less than rigorous. I was really alluding to the spanwise gradient of the lift distribution at the tip tending to be steeper if the wing is outboard loaded. There is therefore likely to be a bigger gain to be had from diffusing the tip vortex and/or shedding the vorticity out of plane.

The 777W has 'Raked" wingtips, basically horizontal winglets, which, because of their tapered shape, serve the same basic purpose as the traditional winglet

yes, with the wingflex on the 777, that would be the only option...

think if the 787 had vertical winglets like a 737...

http://operationsbasednavigation.com...mb-476x237.jpg

I can imagine a reason for having the winglets pointing up, (avoidance of the runway surface perhaps) but would they be more effective if they pointed down? ie containing the higher pressure.
I have a propeller on my paramotor with the 'winglets' pointing to the higher pressure side of the blade.

Downward pointing winglets would, I think, create more zero lift drag in the form of interference drag (where there are sharp angles between surfaces)...