Variometer
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The term Variometer also refers to a type of tunable electrical transformer
A variometer (also known as a rate-of-climb indicator, a vertical speed indicator (VSI), or a vertical velocity indicator (VVI)) is an instrument in an aircraft used to inform the pilot of the instantaneous rate of descent or climb. It can be calibrated in knots, feet per minute (1 ft/min 
kt) or metres per second, depending on country and type of aircraft.
In powered flight the pilot makes frequent use of the VSI to ascertain that level flight is being maintained, especially during turning manoeuvres. 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 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.
Glider pilots call the instrument a variometer (or vario for short), while power pilots tend to call it a VSI.
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[edit] Description
Variometers measure the rate of change of altitude by detecting the change in air pressure (static pressure) as altitude changes. In its simplest form, the instrument consists of an air bottle connected to the external atmosphere through a sensitive air flow meter. As the aircraft changes altitude, the atmospheric pressure outside the aircraft changes and air flows into or out of the air bottle to equalise the pressure inside the bottle and outside the aircraft. The rate and direction of flowing air is measured and displayed to the pilot. The faster the aircraft is ascending (or descending), the faster the air flows. Air flowing out of the bottle indicates that the altitude of the aircraft is increasing. Air flowing into the bottle indicates that the aircraft is descending.
Newer variometer designs directly measure the static pressure of the atmosphere using a pressure sensor and detect changes in altitude directly from the change in air pressure instead of by measuring air flow. These designs tend to be smaller as they do not need the air bottle. They are more reliable as there is no bottle to be affected by changes in temperature and less opportunity for leaks to occur in the connecting tubes.
The designs described above, which measure the rate of change of altitude by detecting the change in static pressure as the aircraft changes altitude are referred to as "uncompensated" variometers. The term "vertical speed indicator" or "VSI" is most often used for the instrument when it is installed in a powered aircraft. The term "variometer" is most often used when the instrument is installed in a glider or sailplane.
An "Inertia lead" VSI or ILVSI compensates for relative "g" forces experienced in a turn (powered aircraft) and provides appropriate mechanical compensation to remove otherwise erroneous indications of climb or descent.
[edit] Purpose
Human beings, unlike birds, are not able directly to sense climb and sink rates. Before the invention of the variometer, sailplane pilots found it very hard to soar. Although they could readily detect abrupt changes in vertical speed ("in the seat of the pants"), their senses did not allow them to distinguish lift from sink, or strong lift from weak lift. The actual climb/sink rate could not even be guessed at, unless there was some clear fixed visual reference nearby. Being near a fixed reference means being near to a hillside, or to the ground. 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. The most useful forms of lift (thermal and wave lift) are found at higher altitudes and it is very hard for a pilot to detect or exploit them without the use of a variometer. The invention of the variometer (by Max Kronfeld) moved the sport of gliding into a whole new realm.
[edit] Total energy compensation
As the sport developed, however, it was found that these simple "uncompensated" instruments had their limitations. The information that glider pilots really need to enable them to soar is not the vertical speed of the glider itself, but the vertical speed of the air through which it is flying. When the pilot chooses to dive or to pull up, as is often required for effective use of the rising air that the glider uses to stay aloft, a simple variometer will faithfully indicate a corresponding change in climb or sink rate. This means that you can only use an uncompensated variometer to detect areas of atmospheric lift or sink when in level flight. Pulling up or diving masks the true motion of the air that the glider pilot is interested in and makes the uncompensated variometer readings effectively meaningless.
The action of diving and/or pulling up a sailplane affects its speed. A sailplane pilot can exchange height for speed or speed for height. In energy terms this means exchanging kinetic energy for potential energy or vice versa. When attempting to find rising air and gain altitude, a sailplane pilot is far more interested in the total energy (potential + kinetic) of his aircraft and far less interested in the how much of that total energy is currently stored as speed or altitude.
For this reason most modern sailplanes are equipped with a type of instrument known as the total energy or compensated variometer, which adjusts its measurement of the rate of change of altitude (potential energy) by subtracting out the rate of change of speed (kinetic energy). In most sailplanes, this is achieved by connecting the variometer to the atmosphere through a "total energy probe," which is a device that produces suction that varies as the airspeed changes. Alternatively, the subtraction may be done electronically by measuring the airspeed and calculating the corresponding change of pressure resulting from the change of altitude expected due to the conversion of speed (kinetic energy) to altitude (potential energy).
The total energy probe may be shaped as a classical venturi (like two small funnels connected at their narrow ends), or simply as a slot or pair of holes on the back side of a vertical tube. The geometry of the TE probe is such that air flow generates suction (reduced pressure). If the pilot causes the sailplane to dive, the increase in air-speed increases the suction and causes a reduction in pressure at the variometer. When carefully set up, this reduction in pressure due to increased airspeed exactly cancels out the increase in the external static pressure due to decreased altitude. The net result is that there is no change in the reading on the variometer due to the change in altitude and the influence of changing aircraft velocity is eliminated.
To maximise the precision of this compensation effect, the TE probe needs to be in airflow that is as far as possible undisturbed. Hence the long cantilevered tube with a kink in the end that can be seen projecting from the leading edge of the fin on most modern sailplanes.
Very few powered aircraft have total energy variometers. The pilot of a powered aircraft is more interested in the true rate of change of altitude, as he often wants to hold a constant altitude or maintain a steady climb or descent.
A second type of compensated variometer is the Netto or airmass variometer. This instrument also accounts for the intrinsic sink rate of the glider at a given speed (based on the polar curve) such that the display will always read zero in still air. This gives a much more accurate picture of the air in which the glider is flying. However, when the glider stops to circle, the indicated rate of climb is not the actual rate of climb. The pilot must mentally subtract the sink rate of the glider (often around 1.6 kt but 2 kt is a good approximation for practical purposes).
To solve this problem, a relative Netto (rarely super Netto) variometer imposes a constant negative 1.6 kt on the reading. This allows to pilot to decide very quickly if a thermal is worth stopping for. A relative Netto effect can be simulated by marking the 1.6 kt position on the instrument panel next to the variometer and taking that value to be zero.
[edit] Electronic variometers
In modern gliders, most electronic variometers generate a sound whose pitch and rhythm depends on the instrument reading. Typically the audio tone increases in frequency as the variometer shows a higher rate of climb and decreases in frequency towards a deep groan as the variometer shows a faster rate of descent. When the variometer is showing a climb, the tone is often chopped, while during a descent the tone is not chopped and the rate of chopping may be increased as the climb rate increases. The vario is typically silent in still air or in lift which is weaker than the typical sink rate of the glider at minimum sink. This audio signal allows the pilot to concentrate on the external view instead of having to watch the instruments, thus improving safety and also giving the pilot more opportunity to search for promising looking clouds and other signs of lift. A variometer that produces this type of audible tone is known as an "audio variometer".
Advanced electronic variometers in gliders can present other information to the pilot from GPS receivers. The display can thus show the bearing, distance and height required to reach an objective. 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.
There is an increasing trend for advanced variometers in gliders to present other information such as controlled airspace, lists of turnpoints and even collision warnings. Some will also store positional data during the flight for later analysis.
