Skin friction challenge demands active drag control

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On all three key fronts in aviation's battle against fuel burn - aircraft weight, engine efficiency and drag - there are promising applications of nanomaterials. However, as a recent London conference on uses of these billionth-metre-scale particles was told, while nano-enabled materials are the focus of much practical research into making structural materials lighter and more durable - and even better-performing in the intense heat of a jet engine - when it comes to drag reduction it is proving remarkably difficult to turn theoretical benefits into practical advances.

Several speakers at the 24 April HiPerNano 2012 event at the Institution of Engineering and Technology pointed to encouraging indications from trials being run by EasyJet with a clear nano coating that makes an aircraft's skin much smoother. The nano trick is to fill in the microscopic pores in an ordinary paint surface, with the result that much less dirt sticks to the surface and what does stick is more easily washed off.

But while a cleaner skin should slip through the air relatively easily, any improvement represents only a fraction of skin drag and further improvement is proving daunting. As Dr David Birch, of the University of Surrey, explains, a "magic" paint that is easy to apply and cuts drag is, naturally, what everybody wants - but the underlying physics of skin drag are much more complicated and defy any straightforward attack.

The aerodynamicist notes that work on surface texture modifications such as machined grooves, or riblets, as small as 10 micrometres goes back to the 1980s and "sharkskin" and other textured paints are available today. However, he says that while these techniques can improve drag they only work in a narrow band of operating conditions, and can even add drag in other conditions.

Other passive, add-on devices such as winglets, fins and surface bumps are also effective only in certain flight modes. And, like surface textures, they may cost fuel burn in other modes.

Moreover, passive devices are inherently limited in what they can achieve; they can't move and are heavy. Some unnamed winglet designs, Birch notes, are even designed more for aesthetics than for function.

There is clearly no magic answer - but another assessment of the drag problem does suggest a promising way forward. Working from rough approximations - Birch has yet to find an aircraft maker prepared to divulge more detailed numbers, if it even knew them - about half the energy in an aircraft's fuel is lost to the chemistry of combustion, to internal resistance in the engines and to running auxiliary systems. Another quarter or so is absorbed by air pressure and shockwaves. Half of what is left is burnt up to overcome skin friction, or so-called boundary layer turbulence.

This skin friction, says Birch, is thus an "attractive target" for so-called active flow control techniques, which is to put energy into the airflow in the hope of achieving a net benefit in reduced drag.

Possible techniques range from "twitching" an aircraft's existing control surfaces to covering the skin with fluttering surfaces less than a millimetre across or installing a carpet of microjets of air or plasma.

None of these ideas are practical with current technology. Birch is impressed by the results that can be had by twitching control surfaces, but while this does reduce turbulence it also wears out the actuators.

To visualise the boundary layer turbulence problem, imagine microscopic "worms" of air turbulence appearing and disappearing unpredictably anywhere on the skin. These can actually be imaged, but not modelled, so to significantly reduce turbulence each one would need to be sensed and eliminated in a fraction of a second by a localised active flow control intervention, rather like a hugely complicated version of the whack-a-gopher arcade game.

Birch says laboratory work on small surfaces suggests this approach could cut skin friction drag by 30% - a degree of improvement that, if applied to all the commercial aircraft operating in the UK, would cut fuel burn by some 3 million tonnes a year, or about 5% of total UK petroleum consumption.

Sadly, the gap between small-scale laboratory work and practice looks daunting. Some recent work with electro-active polymers - materials that expand when electrically charged - suggests the technique would work, but the devices are not scalable. In practice, the sensors are too costly and too fragile - especially given the 10kHz or so flutter rate needed - as are micro air jets or plasma discharge points, which are huge power consumers.

Moreover, the number of sensors and intervention points is massive. Birch concedes that all the computer power in Europe might not be enough to monitor and control an active flow system on one airliner, which would need a control algorithm with about 1012 degrees of freedom and more power than the engines could generate simply to cool the computers.

However, abandoning the concept may be premature. Birch points to recent work at Imperial College in London, where a 4kV system survived for several minutes consistently with the 30% drag reduction target.

In the near-term, some value might be had by turning this detailed understanding of aerodynamics to the design of other types of active system that could be turned on only for take-off and landing. The objective would not be to reduce drag - perversely, drag would increase - but to reduce the wake vortices that pose such a hazard near airports, particularly behind large aircraft.

Birch likes these concepts, as they should pose no big certification issues. If such a system failed, the aerodynamics would merely revert to normal.