Winglets

Winglets – marketing or necessity?

Tim Takeoff
05.07.2019
8 pictures
7 minutes

Almost everyone who is at least moderately enthusiastic about aviation or has flown frequently can name the wingtips which are bent upwards: winglets. Naturally, however, not all winglets are identical, as the invention is already much older than its use in commercial aviation. But is this merely a clever marketing strategy to address fuel economy or do winglets offer further advantages?

Of resistance and buoyancy

At first glance, every additional component on a wing adds weight and resistance. So isn’t it nonsense to build more parts onto an existing wing and, in so doing, deflect air upwards? This actually leads to even less lift! This, too, is true. But why does it make sense anyway?

The answer is, besides a slight improvement of stability and increased manoeuvrability (due to lower moment of inertia), efficiency brings advantages:

Edge vortex on the wing

When lift is generated, the wings create a negative pressure on the curved upper side and a positive pressure on the underside. The process is dynamic because pressure differences always try to balance each other out. This pressure discrepancy can only balance out at the end of the wing. This leads to the occurrence of what we call “wake vortices” behind the aircraft at high angles of attack – and also during cruising. These do not only provide a high “induced drag”, but also pose a danger to following air traffic.

A “fence” against pressure equalisation

If you now insert a kind of “fence” or limiter at the end of the wing, the effect of the pressure equalization is strongly suppressed. As you can already guess, the winglet is such a limiter.

The overpressure on the upper side of the wing already balances out at the transition from wing to winglet, while the overpressure on the underside balances out at the end of the winglet. This, so to speak, “divides” the vortex. However, the total resistance cannot be reduced. The advantage of this division, in the end, is simply that the wing profile can work much better on the exterior of the aircraft wing. A simple “fence” is already helpful, but modern winglets also have a real “profile” – like a further small wing.

One effect complements the next

The reduction of the pressure compensation results in many further advantages for the aircraft operation. Among other things, this makes the wing more efficient. It can already develop higher lift at lower speeds and low angles of attack. This in turn, reduces the induced drag and thus the size of the resulting vortices. This reduces the energy requirements of the aircraft: it needs less power and therefore less fuel to achieve the same result.

Why don’t you just bend the winglets outwards and increase the span even further?

This is one of the most interesting aspects. On the whole, it generally makes more sense to simply increase the wingspan of the aircraft. Larger span widths mean an increased aspect ratio, i.e. the ratio of span width to surface depth, colloquially also known as the “slenderness” of the surface. An increase in aspect ratio already reduces the wing’s end vortex and thus the induced drag. In the subsonic range, aspect ratio is particularly important. These advantages are significantly reduced at high speeds.

So the lower the aspect ratio, the more important winglets become. Or vice versa, the slimmer a wing is, the more winglets help to compensate for the aerodynamic disadvantages. However, many aircraft are limited in their maximum wingspan. This may be due to the strength of older models, or because the aircraft is limited by its class rating or certification. In this case, winglets make sense because they increase the aspect ratio without increasing the wingspan.

Everything just copied?

In addition, findings from bird flight research as well as numerous tests in wind tunnels make it possible to increase the effectiveness of winglets.

Retrofit solutions for existing aircraft

The problem of airport categories

A prominent example of the approval problem is the upcoming Boeing 777X. Boeing increases the efficiency of the wing by increasing its wingspan and thus its aspect ratio. The wing appears significantly “slimmer” than its predecessor and, due to the increased aspect ratio, completely dispenses with winglets.

With this increase, however, the 777X would fall into a new category for aircraft on the ground at airports. Category “E” provides for a maximum span of 65 metres – the old 777 fulfils this requirement. For the new span of 68.6 metres, however, each airport would have to meet new conditions for the 777X (such as the A380 or the 747-8). Numerous airports would therefore no longer be accessible for the 777X. Boeing avoids this problem by making the wing ends foldable on the ground. You can read more about the 777X in this WingMag article.

Aerodynamic masterpiece

The development of winglets is an absolute art by engineers. Numerous forms and stages of development testify to the meticulous research on these aerodynamic assistants. When a manufacturer announces a new winglet concept, there is usually much more behind it than merely a clever marketing trick. And let’s be honest: Winglets simply look cool! Not without reason does an old glider proverb say, “What looks good, flies well”.

by Tim Takeoff

Related Posts