Bicester Aviation Services
FAQs on Variometry and Instrumentation Systems
1. What is the difference between 'Relative Netto' and 'Super Netto'?
The short answer is nothing; Relative Netto and Super Netto are alternative words for the same thing. However let us refresh our memories regarding the definitions of Total Energy, Netto, Relative Netto and Super Netto.
Total Energy compensation, sometimes referred to as TEK, is simply the actual sink or climb rate of the glider as measured by a basic variometer, with no alteration being shown if the glider changes speed, other than the change in polar sink With an uncompensated vario, if a glider does a 'zoomie' in still air from 100 kts to 45 kts, the vario will show a large rate of climb as the glider's kinetic energy is exchanged for potential (or height) energy. Total energy compensation simply removes this rate of climb caused by the energy exchange, and vice versa. To translate this to practical figures, let's assume that at 100 kts the polar sink of the glider is 4 kts and at 45 kts it is 1 kt. Then at 100 kts an accurately compensated vario would show 4 kts sink and as the speed is reduced to 45 kts during the zoomie, the vario reading should slowly show reducing sink until it reads 1 kt sink at 45 kts.
Netto is the vario reading shown when a pneumatic or electronic gizmo removes the glider's polar sink from the TEK vario reading. In the example above, a netto vario would show zero at 100 kts and as the speed was reduced to 45 kts, remain at zero. In other words it is telling us what the air mass is doing, in this case it is still air. For this reason, a netto vario is sometimes referred to as an 'airmass vario', which is much more accurate name for what it shows. If we now take the same glider at the same speeds but place it in a constant air mass 5 kt thermal, then at 100 kts it will show a 5 kt climb. However our TEK vario will only be showing a 1 kt climb (5 kts air mass thermal minus 4 kts polar sink). At 45 kts the netto vario will still be showing a 5 kt climb, but our TEK vario will now be reading 4 kts climb (5 kts air mass minus 1 kt polar sink). It is important that you understand this so far, because if you are not clear on these definitions, then the next one will totally confuse you!
A Super Netto or Relative Netto vario will show the actual rate of climb that you would achieve if you reduced your speed to thermalling speed. Mathmatically it is the netto vario reading less the polar sink rate at the best thermalling speed.
In summary, lets consider the same glider as discussed above flying through a true, air mass 5 kt thermal at 100 kts and at 45 kts. At 100 kts, the total energy vario would show 1 kt climb, the netto vario would show 5 kts climb, and the relative vario would show 4 kts climb. At 45 kts, the total energy vario would show 4 kts climb, the netto vario 5 kts climb and the relative vario 4 kts climb.
2. -----I have been encountering Total Energy problems on both the L-Nav vario and the mechanical Winter vario. They manifest themselves in the form of a very jumpy and erratic needle movement -----
By design, a perfect TE tube produces a pressure that is less than ambient static pressure by the same amount that the pitot tube produces a pitot pressure above ambient. It can best be considered to be a "negative" pitot pressure, and if an ASI is connected in reverse between static and TE, it should read the same as a conventional ASI. Unfortunately, the TE sensor is subject to errors that do not plague a pitot tube to the same extent. The main problems are its sensitivity to yaw and horizontal gusts. Furthermore, as it is usually mounted on the top of the fin to be in an undisturbed streamline airflow, the length of tubing back to the cockpit area is sensitive to any restrictions in its length.
All these problems cause the TE pressure, by the time it reaches the instruments in the cockpit area, to have errors. One is that the turbulence and yaw sensed by the TE tube causes the vario reading to be very 'jumpy'. This is normally more of a problem with an electric vario as it potentially has a faster time constant than a mechanical vario. There are two solutions; one is to put a restrictor in the TE line at the rear of the vario, or install a gust filter in the TE line. A gust filter is simply a small capacity of around 250 to 500cc that increases the amount of air in the TE line and thus smoothes out the gusts. A gust filter is the preferred solution as a restrictor also alters the 'time constant' of the system which has the effect that the change of TE pressure becomes out of phase with the pitching movement of the glider. Having said that, Schumann varios usually incorporate a restrictor in the TE line to reduce random pointer excursion.
3. I have a 'leak' between the pitot and static connections of my LNAV. Is this normal?
Yes, it is. The LNAV senses airspeed by measuring the rate of airflow through a calibrated capillary connected between the pitot and static connections within the instrument.
4. My LNAV is giving strange and random readings. This can be caused by corruption of the RAM over time. The solution is to refresh the RAM as follows:
a. Note the polar and any custom settings, as refreshing the RAM will delete the current settings and overwrite the polar with a default ASW20 polar.
b. Power up the LNAV while pressing the "GO" and "LEFT" buttons.
c. The screen will show "Configure". Go one screen left to "Calibrate" and left again to "Defaults"
d. Press "GO". The RAM will be refreshed and all custom settings, including the polar, will be set to factory defaults
|Contact: Dickie Feakes at 01869 245948 or 07710
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