Altair Glide Computer and Navigation System User Manual download pdf (Page 82)

XCSoar User Manual

if the display is oriented ‘Track-Up’, then steer so the blue arrow

points directly up.

The glide computer accounts for wind drift during circling to pro-

vide an ‘optimal cruise track’ vector, which indicates the track the

glider should follow during cruise such that it will arrive at the

waypoint in minimum time. This vector is displayed on the map

as a blue arrow. When the wind is negligible, or when the com-

puter is in ﬁnal glide mode, this arrow will point along the black

line that indicates the track to the next waypoint.

The calculation and display of optimal cruise track is a unique

feature of XCSoar. Commonly, when cruising between thermals,

glide navigation systems direct the glider to steer so that the glider’s

track points directly at the target. Ideally, the glider’s track is

collinear with the line from the previous to next waypoint, such

that the cross-track error is small and hence the glider travels the

minimum distance between waypoints.

However, because the glider usually has to stop cruising in or-

der to climb in lift, whilst circling the glider drifts downwind and

therefore the cross track error can increase. After several cycles of

cruise-climb, the overall track becomes curved.

For the case where the ﬁnal waypoint is active and one is above

ﬁnal glide, circling is not necessary so this simple scheme is opti-

mal.

5.12 Auto MacCready

XCSoar can adjust the MacCready ring setting automatically to

relieve the workload on the pilot. Two methods of updating the

MacCready ring setting are available:

Final glide : During ﬁnal glide, MacCready is adjusted in order to ar-

rive at the ﬁnishing point in minimum time. For OLC sprint

tasks, the MacCready is adjusted in order to cover the great-

est distance in the remaining time and reach the ﬁnish height.

Average climb : When not in ﬁnal glide, MacCready is adjusted to the aver-

age climb rate achieved in all thermals.

Additionally, both methods may be used, so that before reaching

ﬁnal glide, the MacCready setting is adjusted to the average climb

rate, and during ﬁnal glide it adjusts the setting to give minimum

time to arrival.

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Altair Glide Computer and Navigation System User Manual Page 82