Winglets: Why Up, not down?

[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.