6 McCready Functions
If the TASProbe is available then the NAV SD provides some data related with the McCready theory like
the Speed to Fly, or McCready setting. The Speed to Fly is the TAS value which maximizes the average
Cross Country speed considering a certain average thermal speed. This value is independent of the wind
speed, because the fastest average speed achievable through the air corresponds to the fastest achievable
average ground speed. To calculate the Speed to Fly the NAV SD takes in account the polar and the
average thermal speed. The value is shown on the SpeedToFly data eld.
Note: Traditional the average thermal speed used to calculate the Speed to Fly is manually
set by the pilot (McCready Ring). On the NAV SD this value is calculated by averaging the
last thermals climbed.
Conversely, for each TAS (Speed to Fly) value there is a average thermal speed which maximizes the
Cross Country speed. This value is also calculated by the NAV SD and shown on the McRdyNxtThrm
data eld. Additionally, the McRdyNxtThrm value can also be visualized on the Double Bar Analog Vario
(see Section
3.1.4
). On the example of the Figure
6.1
Next thermal indicator points to 3.5m/s, while
the Average Thermal shows something around 2.1m/s. Ideally, they should point the same value, so the
pilot should reduce their speed. Naturally, he could maintain speed if he expects that the next thermal is
stronger.
Figure 6.1: McCready Indicator
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Summary of Contents for Flymaster NAV SD
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