What is the name of the instrument in a glider that tells the pilot if he is ascending or descending. Any glider pilots on the list with the answer?

From : Don Stackhouse

It's called a variometer. It's a very sensitive rate of climb indicator, with a very short time lag in its response. The original ones used a pair of tapered tubes with cork pellets in them, like a pair of Dwyer wind meters, plumbed in series to a metal container about the size of a thermos bottle. If the glider was descending, the rising air pressure outside would cause air to leak into the bottle, lifting the pellet in one of the tubes. If the sailplane was climbing, air would leak out of the bottle, lifting the other pellet.

The electric ones typically used a bottle as well, but with a pair of thermistors in the tube leading from it. One thermistor or the other would be cooled by air flowing in or out of the bottle through the tube, causing a change in their relative resistance that could be sensed and displayed on the instrument in the cockpit.

One of the problems with variometers is "stick thermals". If the pilot pulls back on the stick, the plane will convert some of its speed into altitude and climb briefly. An unwary pilot with less than smooth flying in steep turns could misread this as a change in the intensity of the thermal they're trying to core, and come to a wrong conclusion regarding the location of the true center of the thermal. To help with this, the total energy system was invented. Instead of sensing just the altitude, it used a special pitot probe that would also consider changes in airspeed. If the glider began to climb but registered an exactly corresponding decrease in airspeed, the system would know that there was no net change in the total kinetic (speed) + potential (altitude) energy of the aircraft and would therefore read zero. These systems were somewhat tricky to calibrate precisely, but once properly set up they could be very useful.

Oddly enough, although these newer, fancier systems could generate a lot of very useful information and could measure the climb and descent rates far more accurately than the old style glass tube and pellet mechanical systems, the old mechanical systems often had less time lag in their response than the fancy electronic ones. Time lag in the variometer response makes it more difficult to find the core of a thermal during a thermal turn, and it also makes it more difficult to spot lift in the first place. For these reasons, it's not uncommon to find a sailplane with all the latest electronic gadgetry on the panel that also still has an old pellet and tube vario installed right in the middle.

Don Stackhouse
DJ Aerotech