AS THEY do elsewhere in North America, red-tailed hawks hunt in pairs along the edge of the Santa Monica Mountains, circling slowly in the updrafts in their search for prey. From afar, they could be mistaken for turkey vultures. Their big, broad wings, held with slight dihedral, and their short fan-like tails, are all similar. The giveaway, apart from size and colour on closer inspection, is the nature of their flight. Turkey vultures are not the most adept of creatures at riding thermals; their role in life is to glide low across the ground, using their acute sense of smell to sniff out carrion. Red-tails, by contrast, are consummate soarers, gaining height steadily as they float from one thermal to the next with rarely a flap of their wings.
At times, a pair of red-tails—the female bigger than the male—will circle your correspondent’s ridgeline home while hunting for rodents, reptiles and small domestic animals. If he stands motionless in his back yard, they will occasionally come within ten metres or so (about 30 feet), fully aware of his presence, blithely unconcerned. Once in a while, they will hang stationary on the wind, perfectly balanced as they peer down into the canyon. To share their air space is a privilege.
Once locked onto a target, red-tails attack in a long, graceful dive, feet outstretched ready to pluck their prey from the ground in one continuous, sweeping motion. This elegant manoeuvre is the complete opposite of a Peregrine falcon’s roll off the top and blistering 300 km/h (200 mph) stoop from a thousand metres of more. Apart from being the fastest creatures on the planet, falcons can sustain unprecedented forces—up to 25g—when pulling out of their near-vertical dive. To your correspondent’s mind, falcons are the fighter jets of the avian world. Red-tailed hawks are the hang-gliders.
Hang-gliding is quite the purest form of flying. Sailplanes insulate the pilot too much from the passage of air. Motorised aircraft dull the senses with noise and vibration, and isolate the pilot still further from his surroundings. Helicopters move with all the grace of a washing machine. Hot-air balloons and para-gliders are exemplary. But to lie prone in a hang-glider harness, exposed to the elements and using only body motions to control the lift and direction of flight, is at once to fulfil man’s oldest of dreams and to experience the nearest thing to bird flight.
Leonardo da Vinci dreamed such dreams 500 years ago. After studying various wing structures for his “Codex on the Flight of Birds”, he sketched numerous gliders controlled by a human being slung beneath the wings, much like a hang-glider of today. It is doubtful whether any of his craft could have flown, given the materials available at the time. Even so, Leonardo grasped the essential differences between the flexible, sail-like wings of hang-gliders and the more rigid structures that were to become the staple of sailplanes and powered aircraft.
There is some controversy over the actual date, but the first heavier-than-air manned flight is thought to have taken place in 1853, when Sir George Cayley, a British parliamentarian and engineer, flew one of his kite-like gliders in the Yorkshire dales, generously allowing a coachman (rather than himself) to man the helm. By the late 1800s, other early aviators were hurling themselves off hilltops. The most famous were Otto Lilienthal in Germany, who made several thousand downhill flights of various lengths, and Octave Chanute in the United States. So successful was Chanute’s biplane glider that it became the blueprint for much of manned flight to follow.
Before adding a motor to their Flyer, the Wright brothers perfected their technique for lateral (ie, roll) control using gliders based on Chanute’s pioneering work. The Wright Flyer succeeded where other attempts to build a practical aeroplane had failed, because it could be flown in the direction the pilot desired, instead of being left largely to the whim of the wind. In short, the Wrights traded a measure of stability for greater control—which has been the rule in the skies ever since.
In the Wright Flyer’s case, an arrangement of cables and pulleys twisted the trailing edges of the wings in opposite directions, thereby increasing the camber (and thus the lift) of one wing relative to the other. That allowed the Flyer to be banked to the left or to the right.
Though subsequently replaced by ailerons, the Wrights’ patented method of “wing-warping” was the breakthrough that got aviation off the ground. From then on, everyone forgot about hang-gliders in the race to build aeroplanes capable of flying faster, further and higher with ever greater payloads.
In a sense, wing-warping is what got modern hang-gliders back into the skies in the 1960s. All credit goes to John Dickenson, an Australian who was trying to develop a more controllable kite for hoisting water-skiers into the air from behind a motor boat. By good fortune, he came across a delta-shaped flexible wing invented in America by Francis Rogallo, and tested by NASA as a means for recovering Gemini space capsules.
Mr Dickenson’s great achievement was to marry a billowing Rogallo wing to a harness and control bar that supported the pilot while allowing him to shift his weight fore and aft to affect the glider’s pitch, and from side to side to affect its roll and yaw.
When this arrangement was scaled up, so that it could be launched by running with it down a slope into a slight uphill breeze instead of being towed by a motor boat, hang-gliding took off around the world. By 1974, a standard Rogallo hang-glider could be had for as little as $400 ($2,000 in today’s money). By then there were some 40 manufacturers of hang-gliders in the United States alone.
With few safety aids, little experience and such a low entry-price, the inevitable fatalities gave hang-gliding a bad name. Today, as the sport has matured and become carefully regulated and more professional, there are essentially only two manufacturers left in America, plus a handful elsewhere. The biggest by far is Wills Wing of Orange, California. The company produces around 650 gliders a year at prices ranging from $3,800 for an entry-level Falcon 4 to over $8,500 for a competition-class T2C.
Wills Wing will celebrate its 40th anniversary next year. Having been a leading light in the business since the beginning, the company has pushed the technology further than most. Early Rogallo gliders, with their billowing sails, had a lift/drag (L/D) ratio of around four-to-one, depending on the speed. Today, even a trainer such as the Falcon 4 can have an L/D of ten-to-one, while a hang-glider designed for cross-country competitions, like the Wills Wing T2C, will have an L/D of over 15-to-one. That is less than a condor’s, but much the same as a red-tailed hawk’s.
Such improvements have come mainly from taking the billow out of the Rogallo wing, reducing its sweep, increasing its aspect ratio (span divided by width)—and, above all, learning how to control the twist in the wing. A Rogallo wing’s billowing fabric imparted too much twist—with the outer sections of the wing attacking the air at a much lower angle than the inner sections. Most wings, whether on gliders or airliners, have a little downward twist (or “washout”) built into them deliberately, so that their inner sections stall before their tips do. That helps the pilot maintain control in a stall, especially when executing a roll.
But too much twist also hobbles performance. In contrast to the loose sails of early hang-gliders, today’s craft rely on high-tech Mylar fabrics stretched over thin aluminium tubing along the leading edges and shaped aluminium ribs that give the wing its camber. The art has been in finding the right amount of twist to stop the wing tips stalling, but not enough to stunt the glider’s ability to soar and stay aloft.
Such refinements over the past four decades have come largely from trial and error—plus the cumulative insights gleaned from hanging beneath a pair of flimsy wings. Here, science has had little to offer. Flight speeds are so modest and the dimensions so relatively small that the Reynolds Number (the ratio of inertial forces to viscous forces) of the airflow over the wings tends to be too low for classical aerodynamics to be of much practical use. “We’re in a domain closer to a bumblebee than an aircraft,” jokes Mike Meier, a professional test pilot and chief financial officer of Wills Wing.
When your correspondent was a young aeronautics student, an elderly professor once told him, in all seeming seriousness, that it was impossible for a bumblebee to fly. Given its tiny wing area, its 200 beats per second could not generate anything like enough lift to get it off the ground.
Fortunately for bumblebees, modern theory says they can. Present thinking suggests that the viscosity of air, on the scale of a bumblebee, allows the insect's small wings to move a relatively large volume of it for their size. That apparently reduces the power they require to sustain flight by an order of magnitude.
Birds of prey have vastly more efficient wings. Like hang-gliders, red-tailed hawks can launch themselves off an open hillside into a 15 km/h breeze with a just a hop or two. Your correspondent is envious and would give much to be able to do the same.
Fortunately, 15km to the south of where he lives, between Los Angeles International Airport and the ocean, is the Dockweiler Beach hang-gliding park. With gentle breezes off the ocean and a broad sandy landing zone, fledgling pilots can launch themselves off an eight-metre-high dune and glide gently down towards the sea. It is a small start, but an experience never to be forgotten. From now on, the only way to go is up.
THIS post is an article from the Economist.