Editing Variometer (section)

==Total energy compensation==
[[File:Vertical speed indicator in airplane instrument panel.png|thumb|right|The VSI in this [[Van's Aircraft RV-4]] light aircraft is within the yellow rectangle.]]

As the sport of gliding developed, however, it was found that these very simple "uncompensated" instruments had their limitations. The information that glider pilots really need to soar is the total change in energy experienced by the glider, including both altitude and speed. An uncompensated variometer will simply indicate vertical speed of the glider, giving rise to the possibility of a "[[stick thermal]]," i.e., a change in altitude caused by stick input only. If a pilot pulls back on the stick, the glider will rise, but also slow down as well. But if a glider is rising without the speed changing, this is an indication of real lift, not "stick lift."

Compensated variometers also include information about the speed of the aircraft, so the total energy ([[potential energy|potential]] and [[kinetic energy|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. But the pilot could pull back on the stick, trading the extra speed for altitude again. A compensated variometer uses both speed and altitude to indicate the change in total energy. So the pilot that pushes the stick forward, diving to gain speed, and then pulls back again to regain altitude will notice no change in total energy on a compensated variometer (neglecting energy loss due to drag).

According to [[Helmut Reichmann]], "The word 'variometer' means literally 'change meter,' and this is how it should be understood. Without further information it remains unclear what changes are being measured. The simple variometers...are rate of climb indicators.  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|angle-of-attack]] changes ([[elevator (aeronautics)|elevator movements]])...This makes it virtually impossible to extract useful information, such as - for instance - the location of [[thermal]]s.  While rate of climb indicators show altitude changes and hence changes in the potential energy of the sailplane, total-energy variometers indicate changes in the total energy of the sailplane, that is, both its potential energy (due to altitude) and its kinetic energy (due to airspeed)."<ref name="hr"/>

Most modern sailplanes are equipped with ''Total Energy compensated'' variometers.

===Total energy compensation in theory===
[[File:Variometer4.png|thumb|300px|metric variometer on a towed glider]]
The total energy of the aircraft is:

1.  <math>E_\text{tot} = E_\text{pot} + E_\text{kin}</math>

where <math>E_\text{pot}</math> is the potential energy, and <math>E_\text{kin}</math> is the kinetic energy. So the change in total energy is:

2.  <math>\Delta E_\text{tot} = \Delta E_\text{pot} + \Delta E_\text{kin}</math>

Since

3.  Potential energy is proportional to height

<math>E_\text{pot} = m g h</math>

where <math>m</math> is the glider mass and <math>g</math> the acceleration of gravity

and

4.  Kinetic energy is proportional to velocity squared,

<math>E_\text{kin} = {1 \over 2} m V^2</math>

then from 2:

5.  <math>\Delta E_\text{tot} = m g \Delta h + {1 \over 2} m {\Delta V}^2</math>

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

<math>{\Delta E_\text{tot} \over m g}= \Delta h + {{\Delta V}^2 \over 2g}</math>

===Total energy compensation in practice===
[[File:Total Energy Variometer with Braunschweig Tube.jpg|thumb|300px|Total Energy Variometer with Braunschweig Tube]]
Total-Energy Variometers use a membrane compensator, compensation by [[venturi effect|venturi]], or are electronically compensated.  The membrane compensator is an elastic membrane, which flexes according to the total pressure (pitot plus static) from airspeed. Thus, airspeed effects cancel out an increase in sink, due to acceleration, or a decrease in sink, due to deceleration.  The venturi compensator supplies a speed-dependent negative pressure, so that the pressure reduces as speed increases, compensating for the increased static pressure due to sink.  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."  Venturi compensator types include the [[Frank Irving|Irving Venturi]] (1948), the Althaus Venturi, the Hüttner Venturi, the Brunswick Tube, the Nicks Venturi, and the Double-Slotted Tube, developed by Bardowicks of [[Akaflieg]] Hannover, also known as the Braunschweig Tube.<ref name=hr/><ref name=faa/><ref>Nicks, Oran, A Simple Total Energy Sensor, NASA TM X-73928, March 1976</ref><ref>{{cite journal |last1=Brandes |first1=Tom |title=The Braunschweig Tube |journal=Soaring |date=1975 |volume=39 |issue=1 |pages=37–38 |publisher=Soaring Society of America}}</ref>

Very few powered aircraft have total energy variometers.  Pilots of powered aircraft are more interested in the true rate of change of altitude, as they often want to hold a constant altitude or maintain a steady climb or descent.