Gives actual rate of descent or ascent of the glider
poss delay) in relation to the ground. i.e. Derived
simply from rate of atmospheric pressure change. Mechanically this is by measuring
the rate of flow of air from the static source into or out of a capacity (flask).
This is really the same as a VSI in a power aircraft but glider instruments
are designed to be more sensitive, I believe.
This is really the same as a VSI in a power aircraft but glider instruments are designed to be more sensitive, I believe.
Total Energy Compensated Vario:- instead of the static source this uses the ‘suck’ of a backwards facing hole (Brunswick Tube/Scleicher multiprobe etc) to compensate for ‘stick lift’. At any constant speed the 'suck' remains constant and a climb will still be registered by the outflow of air into the lower atmospheric pressure. But if you pull up you will slow down and the 'suck' will be reduced. This will increase the net pressure in the system and will balance the reduction in pressure due to the climb, so the reading remains zero (if everything works properly!). i.e. if airspeed instantaneously changes there is no change in indicated lift/sink. (Can be done electronically in which case there is no need for complicated plumbing!)
Netto Variometer:- (Airmass Vario) In addition to TE compensation, a Netto Vario removes the sink rate of the glider itself in still air (Mechanical: using a calibrated leak from the pitot into the capacity/flask or Electronic: from stored polar) – so indicating the lift or sink of the actual airmass. This is useful while cruising so that you can see the strength of a thermal you fly through. E.g. in a 4 knot thermal a glider with a sink rate of 2 kts at the relevant speed will show 2 kts on the vario but 4kts on a Netto variometer.
‘Super Netto’ or ‘Relative’:- This indicates what the climb rate of the glider would be if you slow to thermalling speed. i.e. fly at min sink (electronic only, I think).
Speed to Fly (Flight Director/Speed Command):-
For a given McCready setting, this indicates the best inter-thermal speed to fly for fastest x/c time and is indicated by the McCready ring on mechanical varios or the push/pull ‘Flight Director’ on electronic varios.*
Mechanical this switches between Netto (‘cruise’) and Basic+TE (‘climb’) by providing an on/off tap for the calibrated leak: netto allows use of the McCready ring to directly indicate speed to fly for fastest x/c time. (If in climb mode a series of successive approximations is required.)
Electronic is more complex because there are usually many pilot configurable options. In LX terminology SC= ‘Speed Command’ and with this mode selected speed to fly for fastest x/c time is indicated for the given McCready setting. However the vario might be configured to display more or less anything when SC is selected. Maybe the most logical thing to display in this mode is relative/super netto (?).
*McCready Theory:- The ‘speed to fly’ is the inter-thermal speed needed to achieve the best cross-country speed. The latter corresponds to the fastest time between your current height and the same height in next thermal. (You can spend more time climbing high and spend less time in the glide by flying fast (but losing more height) or spend less time climbing and more in the glide by flying slower (and losing less height). Which will achieve the best result or rather what is the optimum choice of speed/climb?
By the mathematics of McCready theory this depends on the strength of the NEXT thermal – which of course you don’t know. So you guess it by using the best average you have found for the day recently. This is the McCready setting to input to your vario which then continually indicates the best speed to fly. The theory is perfection but in practice most pilots find this too optimistic and they set half the indicated figure (i.e. half the best recent average). See Bronze Slides – Polar Curves for full mathematics.
McCready Ring:- somewhat out of date now that we have
electronic varios but still seen on a few mechanical varios. It consists
of a bezel around the vario circumference which is
graduated in knots (
Final Glide:- Once you have set off on your final glide McCready is of no significance. You need the correct speed to just get you home (at your safety height as appropriate). The question is – how high should you climb before starting the final glide? This will depend on your McCready setting. If the thermal does not go high enough you can reduce your MC setting until it does!