User`s manual

5 GLIDE COMPUTER
The QNH is only updated if the aircraft is on the ground for more than 10 seconds, so that if
XCSoar is restarted during flight, QNH will not be adjusted. The update only occurs also if
the terrain database is valid at the current aircraft location.
• Max Temp The maximum forecast ground temperature is used by the convection forecast algo-
rithm (see 6.7) in its determination of estimated convection height and cloud base.
5.5 Speed to fly
When used in conjunction with an intelligent variometer that produces indicated airspeed measure-
ments, a speed command chevron is drawn on the variometer gauge.
XCSoar continuously calculates two types of speed to fly:
• Dolphin speed This is the instantaneous, best speed to fly in rising or descending air, adjusted
for wind if in final glide mode.
• MacCready speed This is the best speed to fly during cruise in still air, adjusted for wind if in
final glide mode.
The user can specify a maximum manoeuvring speed in the configuration settings, which limits the
speed-to-fly in MacCready calculations to realistic values.
Different pilots have personal preferences as to whether they prefer to fly in so-called ‘block Mac-
Cready’ style, in which they fly constant speed between thermals according to the MacCready
speed; or to fly in ‘dolphin’ style, in which they fly at varying speeds according to the continuously
changing Dolphin speed value.
A configuration option ‘Block speed to fly’ (see 11.5) can be used to specify whether dolphin or
block speed to fly is used. The InfoBox ‘V Opt’ shows the optimum speed according to whichever
mode is selected. When connected to the Vega intelligent variometer, the speed command sounds
are based on this optimum speed value.
49 XCSoar Manual (Altair version) • XCSoar-A-EN