Aircraft Speeds
One of the most important factors during navigation is speed. Without it the aircraft would not get anywhere. Aircraft speed is measured in relation to the airmass it flies in, airspeed. The movement over ground is influenced by the wind and called groundspeed. Even airspeed is not what it looks, as aircraft performance is related to changes in the atmosphere which all have their influence.
Airspeed is measured by the pitot static system, but as air density varies with temperature and pressure, and aircraft performance is related to standard atmosphere (ISA) we need to convert indicated airspeed to true airspeed (ISA speed) so that the pilot can use this speed for performance calculations and navigation.
Indicated Air Speed, IAS
Aircraft can only fly by virtue of their speed through the air and the flow of air molecules over their wings (Bernoulli's principle and Newton's laws). This speed must be indicated to the pilot and is measured by a pitot tube in the undisturbed airstream. Pressure in the pitot tube is a combination of air speed (dynamic pressure 1/2 ρ V2) and altitude (static pressure) and it is part of the pitot static system
Pitot static system, click to read more.
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The airspeed indicator can be seen as a pressure chamber divided in two sections by a diaphragm connected to the pointer mechanism. Each of the sections has a connection, one to the pitot tube and the other to the static port. And as the speed of the aircraft increases the pitot pressure (dynamic + static) increases and countered by the static pressure on the other side of the diaphragm. So the result is pitot pressure - static pressure which gives us dynamic pressure and this is indicated airspeed which is shown on the instrument to the left. |
Indicated Airspeed, IAS
When the aircraft moves through an airmass, dynamic air pressure is build up in the pitot tube and the diaphragm expands moving a pointer on the instrument. Airspeed is indicated in knots (1 nautical mile per hour) but mph and kmph can be found too. Some have dual scales, for example mph and knots. The scale usually has colored bands indicating flap speeds (white arc), normal operating (green arc) and caution speeds (yellow arc) and a radial red line indicating never exceed speed (VNE).
Calibrated Airspeed, CAS
Indicated airspeed corrected for position error is called calibrated airspeed, CAS. These errors are caused by the actual installation and position of the pitot tube on the aircraft and result in the incorrect sensing of the dynamic and static pressure. This error is usually small, noticable with high angles of attack (during level stall excercises for example). The same occurs when the airflow is disturbed by turbulence, as this is shortlived it is off no factor.
Equivalent Airspeed, EAS
Equivalent Airspeed Correction chart
Air is assumed to be incompressable, up to about 200 KIAS and in the lower atmosphere (below 5000ft). Above that the altimeter overreads (air is starting to compress) by 1/2 knot. At 10000 ft the error starts at 160 KIAS with a 1/2 knot overread. Airspeed corrected for compressibility errors is called equivalent airspeed, EAS. For most low level experimental pilots this error is of no concern.
True Airspeed, TAS
Airspeed indicators are calibrated under ISA conditions, only if these conditions exist in the atmosphere the instrument will indicate true airspeed. Some airspeed indicators have an outside air temperature (OAT) set knob. With this subscale the pilot sets OAT against pressure altitude (for that set the altimeter to 1013 hPa or 29.92 "hg temporarily) so that the airspeed indicator reads indicated airspeed (IAS) and below that true airspeed (TAS) in a movable subscale. These are somewhat more expensive but so handy. The correction for temperature and pressure is called density error.
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The formula to the left is explained as follows: TAS = IAS √ (ρ0 / ρ), where ρ0 = 1,225 kg/m3 and ρ is the actual air density. |
IAS → CAS → EAS → TAS
Thus IAS corrected for instrument and position errors is CAS, corrected for compressability errors is EAS, corrected for density errors is TAS.
IAS is important because that is what the pilot is working with and during manoeuvering the aerodynamic forces are the result of this indicated airspeed. TAS is important because it gives an idea of the actual speed of the aircraft in the airmass, which is what we need to compute groundspeed.
Groundspeed, GS
For navigation, i.e. movement over the ground, we need the speed of the aircraft over the ground. As TAS is the airspeed corrected for all mechanical, position, compressability and density errors this is the speed to use when calculating groundspeed. When there is no wind, TAS is GS. If there is any wind we need to use the circular side of the E6-B or any other navigational computer to calculate the groundspeed.
Finally, both groundspeed and distance relates into time so that the fuel needed for the trip can be calculated. It is therefore important that the pilot has a good understanding of these concepts.
