2005-04-21

Microlift from CX Magz

Successful cross country flying has a deceptively simple recipe: avoid landing as long as you can. But when you´ve done everything in the book, by the book, and hunger for a higher education, come fine-tune all your senses through the mystical arts of microlift flying, dynamic gust soaring and glide maximization. Adrian Thomas enters a Zen-like state.

Glide-maximization

Whatever the wing, some pilots always glide better than others, letting the glider flow through turbulence, finding better lines, gaining height and speed from air movements that pitch others back and stop them dead.
But beyond that, on another level of flying, is the fabled microlift technique; dynamic soaring that allows albatrosses to make 1000km out and return foraging trips across the Southern Atlantic. Microlift is an aspect of soaring flight that is just beginning to be explored, in an experimental way, by free fliers like Gary Osoba.
Some aspects of microlift flying require negative G loading, or extreme speeds, and are unavailable to hang-gliders or paragliders. Others aspects require the low flight speeds, low sink rates, agility and sensitivity to local air movements that our craft offer, especially given their ability to land virtually anywhere when it all goes horribly wrong.

Still air glide

The air is rarely really still, but when it is, on final glide late in the day or on a winter sled ride, some pilots always glide better than others, even on the same wing. There isn’t any energy available from air movements, so there is only one way to maximize still-air glide; by minimizing the energy lost to the air.
Almost all wings; sailplanes, rigids, hang-gliders, or paragliders, give maximum glide in steady flight at a constant speed, usually trim speed or a little higher. Any movement of the wing accelerates the air flowing round it, and that costs energy.
For a glider, the only source of energy is potential energy, extracted from gravity by losing height. So any movement of the wing loses height, therefore degrading glide. The faster the movement, the higher the acceleration, the more energy is stirred into the air and the greater the height lost.
Fluid smoothness is the secret to maximizing glide in still air. We need to minimize the movement of the wing in order to maximize its glide. The best pilots’ wings move steadily through the air, with fluid motions, and no oscillations.
Nothing kills glide more effectively than an oscillating wing, but the long lines of paragliders make them natural pendulums, and tightening the VB to maximize glide on a hang-glider reduces directional stability and can lead to pilot-induced oscillations. The main difference between good pilots and the best pilots is their ability to damp out oscillations with the least effort in the shortest time.
Fortunately this key skill is also one of the easiest to practice because oscillations are so easy to trigger on a hang-glider or paraglider. In either case the principle is the same: select a flight speed, deliberately trigger an oscillation, and find the fastest, most efficient way to damp it out.
On a paraglider simply stab on the brakes to pitch the glider back, then release them so it pitches forwards and starts oscillating. How little brake do you need to catch it at the top of the surge? When is the best time to apply brake? Is it best to use an abrupt stab of brake at the right time or a little brake applied earlier and for longer?
Any change in speed or vertical air movement causes pitch oscillations. Damping them quickly makes a big difference to performance. Oscillations in yaw or roll are just as damaging but it is just as easy to induce them deliberately to practice counteracting them even while ground handling.
Only one thing kills glide more effectively than an oscillating wing and that’s a steeply banked wing. In still air if you have to turn a wide, sweeping, gently banked turn minimizes height loss.
How you minimize losses in turns varies from wing to wing. On most wings the most efficient technique is one recommended by Bob Drury: "Leave the brakes alone, clench one buttock and wait." Eventually this gets you pointing the right way with minimum fuss and minimum height lost in the turn.

Microlift lines

In goal you often hear pilots talk about good and bad lines. Convergence lines are common in the mountains, where they may be correlated with ground features and may occur in the same place day after day. These can be so reliable that locals can accurately predict their positions and may even have drawn up maps.
Over the flatlands thermic convergence lines are t also common, but their positions can’t be predicted. Lines have to be found by feel. For me this hunt for an invisible, noiseless, scentless quarry is perhaps the most exciting thing in cross-country flying. Finding good lines and following them across the terrain is an extraordinary experience that requires an almost Zen-like full attention of all the senses.
I can find no written advice on microlift technique for paragliders, but this is what works for me. When setting off on glide towards a distant target, keep the chest strap wide to maximize the sensitivity of the harness. Focus on the subtle differences in lift between the two risers. You are trying to sense the lift distribution across the wing. You may find holding the stabilizer lines or the outside C or D’s might give a bit more sensitivity.
If the lift lines are strong it may even be worth applying brake to feel what’s going on, but remember, brake kills glide. The aim is to weight-shift, not just to keep the risers level, but to turn towards the strongest lift, and away from the sink. The path can become quite erratic, with significant deviations from the track.
According to Gary Osoba, sailplanes like the Carbon Dragon and Sparrowhawk are designed for this kind of flying and are optimized to allow rapid yawing movements to stay on the line. The Bateleur Eagle, Black Buzzard and Turkey Vulture all exploit this kind of lift, and their short tails and tapered wings are well designed for the kind of rapid yawing turns that are required.
Flying along a really good line can seem like balancing on a knife-edge. It is very easy to fall off. Lines often seem to be trying to push you sideways away from them. If lines are local thermic convergence, then this really could be the case, particularly close to the top of the convection where the lines diverge at an inversion layer. The narrow band of rising air would cause the wing to bank away from the convergence line as you approach it.
Dynamic microlift: soaring gusts and turbulence
Energy can be extracted from any situation where the air velocity changes over a short distance, or even over a short time. The rotor behind a sharp edged ridge, for instance, provides a particularly dramatic change in velocity across a short distance. a situation radio-control pilots frequently use by diving through it and converting speed to height.
The key is the delta V, the change in velocity between the wind flowing over the ridge and the slower upwind flow in the rotor. The maximum velocity that can be obtained by using dynamic soaring is produced when the rate of energy gained from dynamic soaring equals the rate of energy dissipation in the manoeuvre.
Imagine you are flying through turbulent air with regular up and downdrafts. If the rate of change of velocity in the gusts is fast enough, energy can be extracted by reacting in the appropriate way to the gusts as they arrive.
The process has been called dynamic microlift soaring by Gary Osoba (see, for example, www.isd.net/sadkins/20hourworkweek.htm).
Birds flap their wings to generate thrust. On the downstroke, the angle of attack is adjusted so that the net lift acts upwards and forwards. On the upstroke the angle of attack is adjusted so that the net lift acts upwards and backwards.
The upward components add up to enough lift to balance weight. The difference between the forward force, generated on the downstroke, and the backward force on the upstroke produces enough thrust to balance drag.
The downstroke is equivalent to an upward gust. The upstroke is equivalent to a downward gust. To generate thrust through microlift technique, lift generated during the upward gust must be more than the lift generated during the downward gust. This fits well with usual ’speed to fly’ practice - slow down in the updrafts and speed up in the downdrafts, but the transitions need to be flown much more aggressively than that.
The first reaction of a paraglider to an updraft is to dramatically pitch back, generating drag and slowing the wing down. Exactly the opposite is required. To extract as much energy from an updraft as possible, accelerate the glider hard as you hit the updraft, preventing it from pitching back. Then slow down to maximize the height gain in the updraft without slowing below speed-to-fly.
At the end of the updraft, as you hit a downdraft the glider will pitch forward. As you swing through under the glider, push the speed bar hard to damp out the surge. Done correctly, this can allow you to reach full speed as quickly as possible, minimizing the time spent accelerating and as well as the total time spent in sinking air.

Microlift: the future

Microlift techniques were almost unheard of a decade ago. Now there are radio modellers who do almost no other kind of flying. There are full size sailplanes specifically designed for microlift flight, and there are scientific research papers being written on the subject.
Paragliders are almost ideally designed for microlift flight. Their low airspeed means that the delta V available from gusts is relatively high even in light winds. The agility and manoeuvrability of paragliders means they can exploit relatively small gust fields and lines of lift. The ability of paragliders to land almost anywhere means that the microlift lines above the superheated air close to the ground are accessible. There are more ways to stay in the air than just ridge-lift and thermals!

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