Variometer

[1] It can be calibrated in metres per second, feet per minute (1 ft/min = 0.00508 m/s) or knots (1 kn ≈ 0.514 m/s), depending on country and type of aircraft.

In gliding, the instrument is used almost continuously during normal flight, often with an audible output, to inform the pilot of rising or sinking air.

The electronic type with audio needs a power source to be operative during the flight.

The instrument is of little interest during launching and landing, with the exception of aerotow, where the pilot will usually want to avoid releasing in sink.

However, as early as 1901, Wilbur Wright wrote about thermals, "when gliding operators have attained greater skill, they can, with comparative safety, maintain themselves in the air for hours at a time in this way, and thus by constant practice so increase their knowledge and skill that they can rise into the higher air and search out the currents which enable the soaring birds to transport themselves to any desired point, by first rising in a circle, and then sailing off at a descending angle.

"[2][3] According to Paul MacCready, "A variometer is essentially a pressure altimeter with a leak which tends to make it read the altitude of a moment earlier.

Electric variometers use thermistors sensitive to airflow, or circuit boards consisting of variable resistors connected to the membrane of a tiny vacuum cavity.

[5][6][7][8] A simple variometer can be constructed by adding a large reservoir (a vacuum flask) to augment the storage capacity of a common aircraft rate-of-climb instrument.

They are more reliable as there is no bottle to be affected by changes in temperature and fewer chances for leaks to occur in the connecting tubes.

The term "vertical speed indicator" or "VSI" is most often used for the instrument when it is installed in a powered aircraft.

An "Inertial-lead" or "Instantaneous" VSI (IVSI) uses accelerometers to provide a quicker response to changes in vertical speed.

[9] Human beings, unlike birds and other flying animals, are not able directly to sense climb and sink rates.

The actual climb/sink rate could not even be guessed at, unless there was some clear fixed visual reference nearby.

Except when hill-soaring (exploiting the lift close to the up-wind side of a hill), these are generally very unprofitable positions for glider pilots to be in.

After the variometer was invented in 1929 by Alexander Lippisch and Robert Kronfeld,[10] the sport of gliding moved into a new realm.

Compensated variometers also include information about the speed of the aircraft, so the total energy (potential and kinetic) is used, not just the change in altitude.

For example, if a pilot pushes forward on the stick, speeding up as the plane dives, an uncompensated variometer only indicates that altitude is being lost.

Since the actual sailplane climb and sink displayed on these instruments depends not only on airmass movement and sailplane performance, but also in large part on angle-of-attack changes (elevator movements)...This makes it virtually impossible to extract useful information, such as - for instance - the location of thermals.

Typically, this is converted to an effective altitude change by dividing by the acceleration of gravity, and the mass of the aircraft, so:

According to Helmut Reichmann, "...the least sensitive venturi mounting point would appear to be on the upper quarter of the vertical fin, some 60 cm (2 feet) forward of the leading edge."

In 1954, Paul MacCready wrote about a sinking speed correction for a total energy venturi.

MacCready stated, "In still air...a glider has a different sinking speed at each airspeed...it would be nicer if the variometer automatically added the sink rate, and thus showed the vertical air motion instead of the vertical glider motion.

Probably the nicest is to utilize the total energy venturi and the dynamic pressure from the pitot tube.

"[5] Tom Brandes states, "Netto is simply the German way of saying 'net,' and a Netto Variometer System (or polar compensator) is simply one that tells you the net vertical air movement with the sailplane movement or sink taken out of the usual variometer reading.

When thermalling, the sensor will detect acceleration (gravity plus centrifugal) above 1 g and tell the relative netto variometer to stop subtracting the sailplane's wing load-adjusted polar sink rate for the duration.

"[4] In modern gliders, most electronic variometers generate a sound whose pitch and rhythm depends on the instrument reading.

In cruise mode (used in straight flight), the vario can also give an audible indication of the correct speed to fly depending on whether the air is rising or sinking.

The pilot merely has to input the estimated MacCready setting, which is the expected rate of climb in the next acceptable thermal.

Each variometer system consists of a radio transmitter in the glider, and a receiver on the ground for use by the pilot.

Other forms of telemetry may also be provided by the system, displaying parameters such as airspeed and battery voltage.