Human-powered aircraft brings to mind anything from Icarus crashing into the sea to the gloriously flimsy Gossamer Albatross skimming the waves of the English Channel - or Icarus-like costumed "birdmen" flinging themselves off Brighton Pier.
But as a 21 September conference at the Royal Aeronautical Society revealed, progress at the cutting edge of aviation today owes more than a token gesture to the engineers who realised the dream of very low-powered flight. And, while human-powered flight is arguably an esoteric pursuit, modern materials and control technologies are poised to usher in a new era of sport aviation.
Paul MacCready winning the first RAeS Kremer prize for sustained human-powered flight with Gossamer Condor in 1977 and a second Kremer prize in 1979 with Gossamer Albatross's English Channel crossing kicked off a flurry of activity in the field, culminating in 1988 when the Massachusetts Institute of Technology's Daedalus 88 project set the human-powered aircraft distance and endurance record with a 115km (71.5 mile), 3h 54min flight from Iraklion air force base on Crete to the island of Santorini.
The Daedalus team did much work at NASA's Dryden research centre, exploring lift and drag in low-speed aircraft and investigating the aero-elastic behaviour of light-weight aircraft. As NASA puts it: "The information obtained from this programme had direct applications to the later design of many high-altitude, long endurance aircraft."
MacCready's work, with support from DuPont, certainly helped open the era of advanced materials. As his pilot, cyclist Bryan Allen, recalls: "Carbon fibre was at that time a somewhat exotic material and there was very little practical knowledge of how to craft parts out of it."
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Indeed, it is no stretch to recognise the similarity between Condor, Albatross or Daedalus and the solar-powered Pathfinder and Helios projects later carried out by MacCready's company, AeroVironment - or, indeed, current programmes like the Boeing Phantom Works/Defence Advanced Research Projects Agency Solar Eagle or André Borschberg's solar-powered, manned Solar Impulse.
Technology developed for human-powered flight may also prove influential in smaller unmanned aircraft. Fred To, an architect and engineer who worked with Southampton, UK-based human-powered flight pioneer David Williams to build the Solar One solar-powered aircraft in 1978, told the RAeS conference about his subsequent development of a successful human-powered aircraft featuring an inflatable wing in 1982. Afterwards, he told Flightglobal that later work included the development, with Qinetiq, of an inflatable UAV for surveillance, that could be fired from a mortar tube. The concept did not progress to a fieldable product, but is an ingenious solution to the challenge of providing infantry soldiers with an aircraft that can be easily carried and survive a hard landing.
RAeS Human Power Group member John Edgley readily concedes that after Daedalus, interest in human-powered aircraft waned, if for no other reason than it had been proved possible and the engineers moved on to other challenges. But Edgley and his RAeS colleagues are optimistic aviation is on the cusp of a new era of human power. The group is hoping to encourage teams from as far afield as Japan, which has a particularly active human-powered flight community, to come to London in 2012 for a fringe activity to the Olympic Games.
To that end, the group is keen to flag up the fact that there is another Kremer prize yet to be claimed - £100,000 ($158,000) - for the first aircraft suitable for athletic competition in "normal reasonable weather conditions, as encountered in the UK". The aircraft will also have to operate safely at low to moderate altitudes, be suitable for small batch production or for assembly from a kit, quickly dismantled or assembled, and stowable "into a roadworthy vehicle ... not longer than 8m (26ft) internally". Intriguingly, "an onboard source of electric power is permitted for the sole purpose of control, including auto-stabilisation and propeller governing".
All of that is a long way from human-powered aircraft to date, which have been delicate, very low-altitude machines flyable only in virtually still air. The assembly, storage and transport requirements, in particular, should not be dismissed. To's inflatable project was driven in large part to get around these challenges, and he says modern fabrics open many interesting possibilities to improve on his earlier work.
This Human Powered Aircraft for Sport (HPAS) challenge also calls for lots of muscle power, to complete a course involving repeated turns, a high-power manoeuvre for an aircraft operating in ground-effect at the so-far achieved altitudes of about half a wingspan. Allen was a trained long-distance cyclist and Daedalus set its endurance record under the power of Greek Olympic cyclist Kanellos Kanellopoulos.
In the short term, a more promising way forward may be the Soaring Human Powered Aircraft (SHPA), a a pedal-assisted hang glider concept outlined to the conference by Bill Brooks, technical director at microlight builder P&M Aviation. Brooks calculates that a pilot in prone position would have to generate about 2hp (1,500W) to sustain level flight. That is impossible - Olympic-class cyclists can generate about a quarter of that in short bursts - but a typical pilot could probably sustain altitude between thermals. Brooks himself has managed to extend his glide this way in the Brighton Pier birdman gala.