Winglets?

[XLR8]

What's the purpose of the A319/320 winglets?? They don't look like they could do enough to save fuel...are they just for good looks?

 

[OCP]

Here's too much:

Why do some aircraft have winglets?
Tip devices have become a popular technique to increase the aerodynamic performances of lifting wings, short and slender alike. The idea behind all wingtip devices is to diffuse the strong vortices released at the tip and optimise the span-wise lift distribution, while maintaining the additional moments on the wing within certain limits. Investigations and experiments, indicated that the use of vertical lifting surfaces placed at the wing tips produce a beneficial effect on both lift and drag characteristics. This is found at the cost of increased bending moment. The increase in root bending moment is found to be lower than for an equivalent tip extension. Winglet sections can be airfoils with their own design.

At the tip, due to the pressure differential between the upper and lower surfaces, there is a significant span wise component to the airflow. On the lower surface, the span wise component of flow is outwards, away from the wing root, and on the upper surface, the span wise component tends to be towards the root. Lift is defined as acting perpendicularly to the flow local of the airfoil and the surface plan form, then with a bit of cunning engineering, the lift on a vertical surface at the wing tip, in a flow with a span wise component toward the root such as occurs on the upper wing surface, could be directed "forward" - in the direction of flight, - and "inward" - toward the wing root. The forward component of lift manifests itself as a reduction in total aircraft drag. Of course, the benefit is reduced somewhat by the component of winglet drag acting aft, but nonetheless, the net result is a reduction in total aircraft drag. And as mentioned elsewhere, winglets will indeed reduce the strength of the shed vortices in the tip region, but only as a consequence of the generation of a lift force on the winglet. For a given angle of attack, installation of winglets can also increase lift, but since aircraft mass is approximately unchanged, the aircraft would have to fly at a decreased angle of attack to maintain the same lift as in the pre-winglet case - which further decreases drag.

Winglets can be used to produce extra lift, besides lower drag. The winglets must be mounted on the rear part of the wing (region of lowest pressure), to minimize interference effects. Drag reduction rates are of the order of 5 %.

Winglets are applied in the latest generation of Boeing 747, MD 11, Airbus, and most executive jets and many sailplanes. Data available for the Boeing 747-400 indicate that without winglets the aircraft. suffers about 2.5 % drag losses, which corresponds to +9.5 tons at take-off.

 

[Lear70]

What's the purpose of the A319/320 winglets?? They don't look like they could do enough to save fuel...are they just for good looks?

Main purpose on the "short bus" winglets is to stop span-wise flow and the resultant spill-off drag.

Not so much lift generating as simply proper airflow directing.

 

[Bavarian Chef]

What's the purpose of the A319/320 winglets?? They don't look like they could do enough to save fuel...are they just for good looks?

They are not winglets, they're stall fences.

 

[Fins Up]

The stall fences I have seen are usually mounted on top or bottom of the wing at various locations, not at the end. But heh, I ain't no rocket surgeon.

 

[Bavarian Chef]

I meant wingtip fence, but for some reason I recall hearing they have the same benefit as a stall fence.

I should be smarter on this stuff...

 

[SkiFishFly]

I believe the Airbus wingtip "fences" (they aren't really winglets) only influence the span-wise flow thereby reducing the wingtip vortices and producing a small decrease in drag.

 

[BornFromJets]

So does anyone know for certain why an aircraft like the 777 doesn't have them? I was asked that not too long ago, but my answers were guesses more than anything.

 

[Lear70]

More than likely because of the wingspan and the twisting motion on the wing.

That would just be an educated guess, however...

 

[Ben Johnson]

the 777 doesn't have winglets because the wing is designed more or less perfect. Metal needed to renforce the energy forces created and the weight on the wing exceeds the usefullness of having a winglet on the 777. the 747-440 winglets actually are for marketing. the extra weight to renforce the wing actually counter acts the usefullness of having them. at that time boeing was struggling to compete with airbus... a big wintip fan. although the 73 and 75 series seem to be a different case. the addition of a windlets has increased range and efficentcy such that the mod. is worth the money or atleast that is what the bean counters are telling us...