© Flytec USA, 2003
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predetermined threshold. The difference between stall alarm threshold, and the speed for minimum sink can be very
small. Several pilots have noticed that while circling up in weak thermals at minimum sink speed, the stall alarm
sometimes sounds. For this reason there is an altitude limit the pilot can adjust in Menu>Basic Settings>Stallspeed,
above which the stall alarm will be disabled. The landing areas should then, of course, be below this limit.
E2 Netto vario
In contrast to the normal vario, which displays the vertical speed of the glider, a netto vario displays the vertical speed
of the rising or sinking of the surrounding air mass. The prerequisite for this is a correctly entered polar curve for the
glider and, of course, an air speed sensor. Let’s assume that a pilot is flying at 31 mph (50 km/h) through the air.
The 5030 determines from the glider’s polar curve that at 50 km/h there would be a sink rate of 220 fpm. Let’s say
the normal vario shows a sink rate of 100 fpm. Consequently, the surrounding air must be rising at about 120 fpm. If,
in our example, the normal vario indicates a sink rate of 400 fpm, then the corresponding air would have to be sinking
at about 180 fpm. With the correct polar curve and vertically stable air, the netto vario should show a value of zero at
all speeds. Therefore we are in the position to check our polar curve when we are absolutely certain that the air
mass is not rising or falling. If the netto vario in this example in the upper speed range indicates air that is
continuously rising by 60-100 fpm, then we know that our wing is better than the stored polar curve, and sinks
approximately 80 fpm less than the polar curve states. This can be corrected in Menu>Basic Settings>Polardata
Another example from common practice: A pilot crosses a valley at a fast glide speed. Suddenly he/she notices a
sharp decrease in sink rate, and turns instinctively. As it turns out this was a mistake because what was thought to be
a thermal turns out to be nothing more than zero sink. A quick glance at the netto vario would have kept the pilot
from wasting time and altitude. The netto vario display only makes sense during descending glides. While climbing
in a thermal, it is better to activate the averaging vario (called the integrated vario on the 5030 menu), which indicates
the average climb over a chosen time period. It is also possible to set the digital vario to serve as an averager during
climbing, and as an air mass (Netto) vario during glide. These settings can be accomplished in Menu>Basic
SettingsVariomode.
E3 Airspeed - True (TAS) and Indicated (lAS)
In general aviation, it is customary to measure the airspeed with the help of a pitot tube as a dynamic pressure speed
(IAS), and also to display it as such. The advantage of this method is that at any altitude level the maximum
allowable speed or the stall is displayed at the same position on the instrument’s scale. This is also the case for the
speed of the best glide, which will have a fixed position on the speed scale. However, the disadvantage of this
system is that the indicated speed is correct
only
at a certain altitude (usually at sea level). The glider will fly faster
the higher one flies due to the fact that the atmosphere gets thinner with altitude. At approximately 21,000 ft (6,500
m) the air weighs only half of that at sea level, and the air speed will therefore increase 1.41 times that (the square
root of 2).
To calculate wind speed, arrival altitudes, or arrival times true air speed is needed. The vane wheel sensor (propeller
type) shows the true air speed (TAS) because it runs virtually without friction. Thanks to modern processor
technology, the 5030 can calculate both true and indicated airspeed, regardless of which air speed input device
(propeller or pitot tube) is used. The pilot can set the type of speed he/she would like to see in the display without
effecting the airspeed related calculations. Therefore, if the pilot elects to display IAS, there should be no surprise
that at high altitudes with no wind, the difference between groundspeed and airspeed will correctly show zero, even
though the groundspeed provided by the GPS will be much higher than the indicated airspeed.
E4 Polar Curve and Best Glide Speed
The polar performance curve of a glider shows the relationship between air speed and the associated sink rate. The
highest point on the polar curve is the minimum sink rate. If one plots a line starting at zero sink rate, tangent to the
polar curve, it touches the polar curve at the best glide speed. If one divides this speed by the associated sink rate,
the result would yield the best glide ratio
The 5030 creates a table from the polar data entered by the pilot in Menu>Basic Settings>Polardata,
and places it in
memory. At every flight speed, the unit knows the glider’s associated sink rate. If the current sink rate measured by
the vario is more than then value found in this table, the glider is in sinking air. If, for example, using the polar curves
shown below, the vario displays a sink rate of 100 fpm (0.5 m/s) when flying at 22 mph (35 km/h), the pilot must be in
