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Pressure, Atmosphere and Instrumentation

2,740 bytes added, 21:44, 18 October 2020
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The International Standard Atmosphere: fix typo
In the simplified model, it is assumed that the density of air is a constant. The Equation of State clearly says otherwise: density depends on pressure and pressure depends on density. Here we run into a problem and the problem can no longer be solved by simple algebra: the powerful mathematical tool of calculus must be used.
Even this is under the assumption that the temperature is a constant. The additional complexity is that the temperature in the atmosphere varies greatly, and you can feel this quite easily by climbing onto a hill and note the temperature drop (just make sure you use a thermometer instead of feeling, to isolate the effect of windchill). At low altitudesbefore the cloud base is reached, as a rule of thumb, the temperature will reduce by 0.6°C 3°C for every 100 meters1000 feet' raise s rise of altitude. This is known as the dry adiabatic lapse rate.
The temperature of the air greatly depends on the heat transfer between the ground and the air: it is the ground that absorbs the radiation from the sun and heats up, the air is transparent so the absorptivity is quite low in the visible spectrum. Generally, the higher the altitude, the less heat the air will get from the ground, and, as a result, the air will become cooler. This applies until the tropopause is reached, beyond which the temperature ceases to decrease and, in fact, starts to increase again at higher altitudes. Gliders almost never reach the tropopause, so we can ignore this complexity.
=== The International Standard Atmosphere ===
The '''International Standard Atmosphere (ISA)''' is a '''model''' for the atmosphere widely used in aviation. It is established based on extensive observations. It is not meant to be exact as the atmospheric conditions can vary actively (especially at low altitudes, knows known as weather). Also, the temperature in the atmosphere at low levels is subject to seasonality as we well understand. Despite these factors, the ISA is a good model and perhaps the most acceptable one to be used if some aviation equipment is to be designed.
The modelling approach of the ISA is to divide the atmosphere into several layers, within each layer the static temperature is assumed to vary linearly. If \(T\) is known, it is then possible to use the theories as described before to solve for the density and the static pressure simultaneously.
=== Altimeter ===
==== Function ====An altimeter is an instrument that displays the vertical distance between the aeroplane and a reference datum, which is defined by the sub-scale setting on the altimeter. An altimeter is a compensated instrument, which means the density variation in the atmosphere is automatically corrected for. An altimeter, therefore, gives reliable readings at all altitudes with the exception of readings very close to zero, in which regime the absolute errors become significant. An altimeter needs to be read in the same way as a clock: ask an instructor to demonstrate this if you are not fluent at this. Altimeters come with 100ft and 1000ft hands, and most of them found in gliders also have 10000ft hands.
==== Setting ====
 
A datum level must be defined for the altimeter, and it is important to remember at all times that the altimeter reading is relative to the datum '''you''' the pilot have chosen. Depending on the situation in flight, the datum used may or may not be helpful.
 
Three settings are relevant to general aviation, they are QFE, QNH, and STD respectively.
 
QFE means the datum level is at airfield elevation. In this setting, the altimeter reads zero on the ground at the airfield where it flies from. In local circuit training and local soaring, the QFE is normally used. However, when the terrain surrounding the airfield is not level, the QFE setting does not guarantee a correct indication of the height above the ground. QFE is also useless if a circuit and land is planned at locations other than the home airfield, which is the reason you must be comfortable to circuit and land without reference to the altimeter. In addition, if a very long soaring flight is being made, the QFE levels at the take off time and the landing time can be different.
 
QNH means the datum level is at the mean sea level. This is a useful setting for cross country flights, because all the altitude values given on a chart are above mean sea level (with some exceptions such as obstacles above the ground, in which case a height above the ground is also quoted). It is possible to circuit, approach and land with QNH, which power pilots do (their standard instrumental terminal procedures involve checkpoints with altitudes prescribed in QNH), but the usefulness is limited in gliding as all operations are supposed to be under visual flight rules, and visual approaches rely little on knowing the absolute height above the ground.
 
STD is the setting where the sub-scale is adjusted to 1013hpa or 29.92in Hg. This setting shows flight levels. For example, FL55 is 5500ft when the altimeter is at the STD setting. This is useful if navigating at great altitudes or if attempting to avoid airspace.
=== Vertical speed indicator (power) ===
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