Winglets..are they usefull?
 

What i can recall from my limited knowledge of aerodynamics, wiglets help reduce wing tip vortices and thereby cruise drag. How come Boeing went for winglets on the 747 and BBJs, but not the 777?

It could be because winglets have disadvantages such as weight, cost, their own drag and increasing wing bending loads. Whether there will be a net benefit by employing them will presumably vary between types as well as the type of sortie they will be used on.

The Hatfield Aerodynamics team (home of the Airbus A300 wing and BAe146) always stood by their statement that “a good wing design doesn’t need winglets”; although some of them subsequently had to accept ‘end plates’.

I am of the view that, for small aircraft, there is an element of marketing hype involved.

For big birds, the sail gives an effective increase in span (at some expense in fatigue etc and probably worth around 70 or so percent sail length in terms of span) but has the big advantage of not compromising a lot of very expensive airport gate infrastructure ...

I do recall flying SFO-LHR on a BA 744 that was minus its port winglet. Caused me to rub my eyes and walk to the other side -- sure enough, winglet all present and correct. I asked one of the CC and she said, oh no, you must be wrong -- then she looked herself . . .
Wonder how it affected the trim?

For a B747-400 the winglets become cost effective on flights of five hours or more. On shorter flights the cost of carrying the extra weight does not compensate for the fuel efficiency. I believe that JAL operate some 744's without winglets, but with modified wingtips, on their domestic routes.
The 744's with winglets are certified to fly with one winglet missing, but not both missing. Boeing could not justify the certification flying that would have been required for both to be removed, as they felt that it would be unlikely for both winglets to be damaged. The aerodynamics of an aircraft with the winglet removed is different to an aircraft with the modified wingtip.

Airbus products can also fly with only 1 winglet out of interest.

In terms of the 777, whilst they don't have winglets, the larger, later versions have raked wingtips (as does the 767-400), which have a similar effect to the winglet, with less weight, however they increase the span which can be a problem on some jets (such as the 737 which uses a smaller gate to the widebodies).

I believe the A340 was originally designed without the winglets, though when the IAE superfan was cancelled they had to add the winglets to compensate for the performance.

Any aircraft designed to a span limitation can have its max L/D increased by adding winglets; if done intelligently the L/D increase more than pays for the additional wing weight due to the higher bending moments. This was true for the A380, which was designed to an 80m span limit when the optimum span for the aircraft was somewhere around 84m.

Even on an aircraft which is not span limited, winglets can be used with minor wing weight penalty by soaking up some of the 'fat' in the wing strength which is evident once the ultimate static load tests are done; any strength margin over the certification requriements can be cashed in as an aero benefit via winglets. On the A340-300 the winglets were added mainly to improve the low speed performance, but they also cut cruise drag by about 3-4%. Aviation Partners claim around 7% for the BBJ winglets, but they are proportionately twice the height of the A340 winglets.

Some winglet effects are not immediately obvious. One project I worked on gained significant wing limit load relief due to the wing sweep and fact that the aft-positioned winglet pulled the aero centre aftwards at the wingtip which caused the load to shift further inboard at manoeuvre limit due to aeroelastic wing twist. The weight saving more that balanced the lower aero benefit of this smaller winglet. All jolly interesting.

On Glide

A 777 had decades more technology in its modern wing that the 747. They are quite different wings aerodynamically.

Look at the new Boeing 747-8 and 787 wing design. No giant winglets, but their design is much more efficient. More of a slender raked design at the tips.

http://www.boeing.com/commercial/747family/index.html

Never seen them on angry palm trees - wonder if it's been tried?

Some issues with winglets outlined here..

http://airtransportbiz.free.fr/Techn...hewinglet.html

It seems they don't increase the effective span very much.

Winglets were until recently common on glider designs. The conventional wisdom is that they improve the low speed handling and climbing characteristics, but that the penalty is reduced L/D at higher speeds.

Some recent designs have cranked-up wingtips with complex leading edge changes in the outer third of the the wing. These may or may not have winglets.

Examples at:

http://www.alexander-schleicher.de/englisch/e_main.htm
http://www.dg-flugzeugbau.de/technis...en-1000-e.html
http://www.schempp-hirth.com/en/flug...s_4/index.html

Didn't the record breaking Westland Lynx have winglets of sorts?

Out of interest, what sort of weight and price would a 744 or 737 wingtip be?

A retrofitted winglet on a 737-800 built without privision weighs 235kg for the pair, but only 170kg if provisioned or installed during production. Price varies hugely with contract.

We tried winglets on the Columbia 400 and found that they improved directional stability during stalls / stall characteristics, but spin recovery was worse so we dropped them.

I did read somewhere that on the 737, the winglets only start making a benefit on the longer routes (3-4hrs+), as below that, the weight was more of a penalty. Which is interesting with Ryanair running it on all their 30min Ireland-UK hops.

Can someone please tell me the technical reasoning behinf the 777 not having winglets. They fly a long way and I want to know why thaey are not used and if anyone thinks that winglets will be fitted to existing 777 in the future, like the A320 which is being tested with winglets. Are the gates/vortex generators really that effective on the 777 stopping the span wise flow.:confused:

Winglets are only really benneficial on long haul flights. Winglets themselves add extra weight and drag to the aircraft which have their obvious penalties, and on short flights they don't earn their keep! But when used on a long flight, then efficiency bennefits outweigh the drawbacks.
All wings have spanwise flow. The idea of winglets is to remove the drag inducing vorticies away from the main wing reducing the downwash effects. You'll note on the ER 777's and on the 767-400ER, a "Raked Wingtip" has been used instead which basically results in the same thing (reducing wingtip vorticies). A raked wingtip means that the wing tip is highly swept.
The 777 wing is quite different to your standard airliner wing. It has a "super critical" wing design and is many years ahead in its efficiency.
So RWT's for the 777, winglets are out! :ok:

767-400ER with raked wing tips (RWT's)
http://www.boeing.com/commercial/767...es/400_8_n.jpg
777 with RWT's
http://www.boeing.com/commercial/777...63337-08_n.jpg

Raked wing tips: I would say the 777 is years behind on that. We had them on gliders more than a decade ago. Just look at a Discus.