Glider Aerobatics

"There can be little doubt that aerobatic training deepens our understanding of the flight characteristics of a glider and develops our ability to explore its capabilities." --Peter Mallinson & Mike Woollard, The Handbook of Glider Aerobatics
Normal aerobatic box

A brief introduction to aerobatics

Aerobatics have long been regarded as one of the greatest tests of a pilot's skill. While the figures themselves are nothing more than combinations of lines, curves and rolls, performing them with grace and precision demands a complete mastery of one's aircraft. It is therefore unsurpising that aerobatics serve as an important component of advanced flight training for both fullscale and R/C pilots. Precision aerobatics are normally performed within an aerobatic box, an imaginary equilateral cube in the sky. The aerobatic box is aligned such that the majority of maneuvers, also known as figures, are flown up and down wind along the primary axis. This axis is usually parallel to the runway, the judges (if an aerobatic contest), and in R/C flying, the flightline where the pilots stand. The goal is to perform all figures within the constraints of the aerobatic box while maintaining a safe minimum altitude (or floor) above ground level. Competition aerobatics involves flying a pre-determined set of figures known as a sequence or programme. Each figure has a K factor, or difficulty coefficient, assigned to it. The sequence itself also has a K factor representing the total difficulty of all figures it contains. As the pilot flies a sequence, each figure is judged for quality and given a mark from 0-10 (10 being best). This mark is multiplied by the figure's K factor to determine the score for that figure. The scores for each figure in the sequence are then summed to determine the pilot's final score. Normally, a number of sequences are flown as part of an aerobatics competition, including Known (provided to the pilot prior to the contest), Unknown (given to the pilot shortly before the sequence is to be flown the day of the contest), and Free (designed by the pilot him or her self). The winner of the aerobatics contest is the one who accrues the highest total score for all sequences flown. Regardless of whether or not a pilot is interested in aerobatics for the sake of competition, the notion of difficulty factor as well as the structure provided by the aerobatics sequence provide useful tools to guide the development of one's flying skill. After all, it's one thing to perform a flawless loop, and another thing altogether to perform a flawless loop as one element within a sequence of flawless figures!

Slope and flatland glider aerobatics

Without a motor to pull one's airplane around the sky, glider aerobatics present a unique challenge all their own. While vertical performance is obviously limited, the lack of torque effect enables gliders to enjoy a true neutrality in control response not available to their powered counterparts. In flatland glider aerobatics, the glider is normally taken aloft either by means of a winch or aerotow. R/C gliders can also be highstarted, or use a motor with folding prop to climb to altitude. Once aloft, the glider enters the aerobatics box and performs an aerobatic sequence as it descends, usually completing the sequence at relatively low altitude. A landing promptly follows. Slope aerobatics are in some ways a middle ground between powered aerobatics and flat field glider aerobatics. With the constant lift provided by the wind coursing up a ridge, slope gliders can fly for virtually unlimited amounts of time--far longer than both their powered and flat field counterparts--and can maintain a relatively constant altitude. At the same time, while vertical performance can be improved thanks to slope lift, there is still nowhere near the limitless upline potential of powered aircraft. The most notable difference, however, is that slope flying is not performed primarily parallel to the wind and runway as with flatland glider and powered flying, but rather crosswind as pilots trace back and forth across the face of the hill. At sites with sufficiently strong lift, gliders are also able to fly a good distance upwind out in front of the slope. This change in orientation with respect to the wind between flatland and slope flying presents an issue when one attempts to translate flatland aerobatics sequences to the slope context. First of all, if the sequences are to be flown as depicted on the sequence card with respect to wind direction, then they will have to be flown towards and away from the slope rather than back and forth across it. This would result in the majority of the figures being flown perpendicular, rather than parallel, to the flightline--not an ideal circumstance for pilots, judges or other onlookers wishing to view the symmetry of the figures being flown. In addition, at sites with less than stellar lift, this would require the gliders be flown well outside the best part of the lift band, potentially making the sequence impossible to complete. Slope aerobatic box To address these issues, it is possible to disregard the wind direction represented on the sequence card and instead fly the figures crosswind. This effectively rotates the aerobatic box 90° relative to the prevailing wind direction, making the main axis of flight parallel to the flightline, keeping the gliders in the strongest part of the liftband and retaining the orientation of the flat field sequences vis-a-vis the pilots and judges. While this rotation resolves the issue of visibility, it radically changes the nature of the flight task being presented to the pilot. Instead of having to focus mainly on corrections for up and downwind drift, the pilot must now account for the effects of a crosswind -- a much more challenging task in many regards, especially when one considers the strong winds and turbulence inherent to slope soaring. The effect of the crosswind component varies from slope to slope and day to day. At extremely steep slopes with powerful vertical lift, it is possible that the effects of crosswind component will be mitigated somewhat due to the significant upward deflection of the slope lift. But the opposite holds true for milder slopes, where the crosswind component will be a constant--and challenging--influence. In addition, the wind will frequently come in at less than a perfect right angle to the slope, resulting in an extremely challenging mix of head, tail and cross wind components as the glider traverses the face of the slope.

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