9
4.2. Zeroing of the Transducer
Remove upper halfshell after having closed the pneumatic connection tightly. The sensor board is
located next to the indicator. The sensor board has got two adjustment potentiometers, that can be
reached from the top. One of them is secured with red paint. It must not be misplaced. For adjusting
turn the other adjustment potentiometer with a small srew-driver until the zero-point is correct. Install
the halfshell back and reconnect the SC-7 to the TE-probe.
4.3. Temperature calibration
At first delivery the SC-7 is calibrated to the enclosed temperature sensor. When changing the sensor,
the instrument should be readjusted. A comparing thermometer is to be brought to the same
temperature together with the sensor. Therefore both of them should be put in a glass of water. For
adjustment the lower halfshell has to be removed. On the left side of the instrument is a switch, that is
inscribed with numbers from 0 to 9 and letters from A to F. Turn this switch until the temperature
display is correct. Now install lower halfshell back on.
5. THE SC-7 VARIOMETER IN FLIGHT
The following chapter has been written in order to help the user to draw a maximum of benefit from the
information delivered by the instrument. It is always worth while to read it, as the matter treated here is
rarely or not at all covered in the general literature on soaring.
5.1. The 1-second- and the 3-second-response
With the 1s-3s-switch the display filters can be switched from the quick 1-s-filter (a filter of second
order) to the slower 3-s-filter (a filter of first order). Figure 3 shows what happens upon flying through
an idealised thermal updraft, and the corresponding responses of the two different filter outputs, or
otherwise, the indication of the vario needle when one or the other response has been selected. For
the example a standard class glider with normal wing loading has been assumed. Airspeed be 90 km/h
(50 kts), and constant during passing the updraft. The square trace 1 shows the updraft as a function
of time: in front of the jump, there is calm air, within it, the air rises at 2 m/s (4 knots). The diameter of
the updraft be 100 m (100 yards).
Before entering, the plane sinks steadily at 0.7 m/s (1.4 kts). Upon entering, it is accelerated upwards,
the pilot will feel this clearly on the seat of his pants. The transition to the new vertical speed is fast, the
time constant being only 0,4 seconds. The acceleration at the beginning is 0,5 g, the g-meter will jump
from its steady state indication of 1 g to 1.5 g. Upon leaving the same sequence will happen, however
this time downwards. Curve 2 shows the response of the glider. Curve 3 describes the indication by the
1-second filter: after a short hesitation of about 0.2 s the needle swiftly swings upwards, after 2
seconds already 90 % of the change are reached, after 2.5s 100 % of the real climb rate of the glider,
now being 1.3m/s (2.6 kts) upwards, are attained. The indicator needle remains there to the very end
of the updraft, and returns to its original sink of 0.7 m/s (1.4 kts) as fast as it mounted upon leaving the
thermal.
For a first order filter to be as fast, its time constant would have to be 1 second. Such a filter would be
useless in normal thermal conditions as one would not be able to read it because of its permanent
random jitter induced by turbulence. Curve 4 shows the behaviour of the slow 3-s filter. This one
corresponds to the response of a good moving vane variometer: The output creeps up slowly. For it to
reach 90 % of the change in input,one would have to wait 7sec. At the end of our thermal it just arrived
at 0.8 m/s (instead of 1.3 m/s).
Which response to use to search for thermals? As the main problem here consists of discriminating
between "gusts" and useful lift, a short consideration: diameter of the general useful thermal is about
150 m. This distance at a speed of 90 km/h (50 kts) will be covered in about 6 s, at 180 km/h (100 kts)
in 3 seconds. With this in mind, one can say that it is worth while to take on a "thermal" when the climb
lasts at least 3 to 5 seconds, and when on top of that it has the strength looked for. Be it, one is
convinced to have cut the lift just on its border. If now we look at figure 3, we can set up a simple rule
for day to day use very quickly: When the 1-s-vario-display climbs to the lift waited for, then count to 3.
If the needle is still there, then take it on. This thermal is large enough to be centered in 9 out of 10
cases. If not, continue straight on.
