The Variometer
I was furtunate enough to be invited to take a ride in glider. One instrument that caught my attention in the Grob G103 was the variometer. It looks like a VSI however, the vertical rate is not measured in ft/min but in knots. I was told that the variometer measures true lift.
None of my old ATPL notes or any other book I found even mentions this instrument. Since it is obviously very much gliding related I wonder if there are (ex) glider pilots on pprune that wouldn't mind sharing the principle of this instrument.