Winglets: Why Up, not down?

[Draculas Teabag]

We are all aware of the reason why many jet transport aircraft e.g. B744 have winglets, but why do they point up and not down?

I am under the impression it is not due to ground clearance.

As an example, the MD11 has a large winglet that points up and a small winglet that points down, why not the other way around?


:confused:

[admiral ackbar]

Totally newbie answer, but I will give it a shot :)

From my understanding of how these things work, the vortices created from the wing tips strike these and create additional lift. I would surmise that the shape of the wing on most winglet-equipped aircraft generate larger vortices on the top side of the wing than the bottom, hence the winglets being up.

[Babi Melayu]

Because when you view from the back of an aircraft, the resultant of the flow on the upper and lower wing causes a swirling type of motion that twists inwards (clockwise for PORT wing and Anti-clockwise for STARBOARD wing) called vortices.

So I suppose by having winglets bent upwards, it assists in "smoothing" out the upward flow of the vortex as it leaves the wing tip and due to viscosity, the vortex will "stick" to the winglets. Drag is thus reduced.

Whereas having a downward bent winglet has the effect of "stopping" the continuity of the vortex flow but unfortunately results in additional drag.

Can you picture it ?

[m&v]

Food for thought-if the resulting Vortice drag is from the wingtip.In the case of upsweep the drag vector would be above the thrust line(possibly increasing lift)..downsweep would provide the converse result -increase in natural (nose down)slipstream effect from the wing(that the stabalizers job to correct)..
Havind said that, it's up to be out of the way of the ground handlers(Hitting them):rolleyes:

[jtr]

Tip strike on X wind landing and t/o may be a factor

[patience]

my two pence:

the spillage flow pattern is predominant from the high (underwing) pressure zone to the low pressure zone.. from my shadey past I know wing tip clearance is a significant factor in them not going downwards.. indeed me thinks downwards is actually a fair bit more effective than upwards.. (the high pressure can't spill laterally at all, instead of a half roll round to the upward winglet).. the C-17's (overwing) winglets are upwards presumably due to protection from military ground vehicles...

I have heard the winglet kits available for 737's (as on some BBJs) take 3-4 years to pay for (cost saving from reduced fuel consumption v investment) for the average 737 operator..

King Patience:cool:

[Keith.Williams.]

DT,

To understand this matter we need to look more closely (than we usually do) at the pressure patterns above and below wings. We often say that there is a high pressure area below the wings and a low pressure area above them. We then go on to say that the wing tip vortices are caused by high pressure air leaking from the high pressure area below the wings to the low pressure area above them. As with most aspects of the teaching of aerodynamics, this is not entirely true.


At normal angles of attack the air is accelerated both below and above the wings, so the pressure is reduced both below and above the wings. The resultant useful lift force is the upward lift produced by the upper surface minus the downward lift produced by the lower surface.

The important thing in considering tip vortice is that the pressure below the wings is reduced and the pressure above them is reduced even more. So we can say that there are three pressure areas to be considered. These are high (ambient) pressure outboard of the tips, medium pressure below the wings, and low pressure above the wings. These are of course arbitrary titles, but they reflect the general situation.

Air naturally migrates from high pressure areas to lower pressure areas and the migration rate is proportional to the pressure difference between the two areas. So at the wing tips there will be three migration effects. The greatest of these will be from the outboard high pressure area, to the low pressure area above the wings. The second will be from the high pressure outboard area to the medium pressure area below the wings. The third will be from the medium pressure area below the wings, to the low pressure area above them. The second and third will oppose each other, such that the resultant will be the difference between the two. The overall effect of all of this will be a large flow from outboard to the area above the wings and (probably) a much lower flow from below the wings to above them.

So winglets which point upwards will experience the greater part of the vortex flow and hence produce the greatest benefits. This is the main reason why most winglets point upwards.

It is only at high angles of attack that the pressure below the wings is greater than ambient. This is why stalled wings still produce some lift. So downward pointing winglets will be most useful at low airspeeds. But even in these conditions, the pressure envelope below the wings is relatively shallow, so there is little point in having long downward pointing winglets. This is why those aircraft which have both upward and downward pointing winglets, have the greater part of the winglets pointing upwards.

Although ground clearance is not the primary consideration, there clearly a greater danger of damage to downward pointing winglets. Even an impact with a service vehicle when the aircraft is parked on the ground will delay operations and cost money. This makes the economics of downward pointing winglets even less atractive.

[Capt.KAOS]

The purpose is to reduce turbulence at the tips of an airplane's wings. The air pressure on the bottom of a wing is greater than the pressure on top, so when air flowing across the two surfaces meets at the wing tip, it forms a vortex-a miniature tornado. The vortices created by a large airplane are strong enough to flip a smaller plane that is following too closely.

By breaking up vortices, winglets reduce the drag on an airplane, which translates into fuel savings. So why don't all airplanes have winglets? The airflow around winglets is complicated, so designing them is tricky. It's easier to improve an airplane's lift-to-drag ratio by simply making the wing longer, though this can lead to other problems, such as fitting into gates.

Conventional upright winglets are currently used on a number of airliners, including the Boeing 747-400 and the Airbus A330 and A340. On some of its 737 models, Boeing uses "blended" winglets, which curve up from the wing instead of sticking straight up.

Blended winglets have a counterpart in nature: Some birds curl the tips of their wings upward to reduce drag while gliding.

CaPt.KaOs

PS I believe the first winglets are used on a Learjet.
pss F1 use winglets too with a complete different purpose, but hey! I'm starting to get off topic again ;)