Understanding airspeed measures
From wiki.flightgear.org
The speed of an aircraft is the amount of distance the aircraft travels in relation to a reference point per unit time. As with any velocity measurement, the questions to ask in order to understand any given velocity measure are: What is the reference point? and How is the measurement done? There are several different speed measures used in aviation:
- ground speed (GS)
- true airspeed (TAS)
- equivalent airspeed (EAS)
- calibrated airspeed (CAS)
- indicated airspeed (IAS)
In modern aviation, speed is usually measured in units of Knots (kt). However, in older planes, notably German WW II fighter planes, airspeed gauges are in kilometers per hour (km/h), which is still used in present-day European glider planes. The conversion factor is 1.852, i.e. you can roughly divide a reading in km/h by two in order to get the value in kt. If the speed is measured in kt, sometimes a 'K' is put before the acronym, so KEAS stands for 'equivalent airspeed measured in kt'.
Finally, for supersonic planes, the Mach number is often used to describe aircraft velocity.
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Ground Speed
Ground speed (GS) is the velocity with which the aircraft moves relative to a fixed point on the ground. One needs to know ground speed in order to see how long a flight from A to B actually takes. Nowadays GS can be directly measured using a GPS system, and some aircraft equipped with such a system have a groundspeed gauge. Without a GPS, GS has to be calculated from airspeed and the local wind pattern or estimated by measuring the time between passing two points on the ground with a known distance, but in Flightgear you can always cheat and get it from the property browser under velocities/groundspeed-kt.
It is perhaps also worth noting that GS measures only the horizontal component of the aircraft velocity - in a steep dive, the aircraft can move very fast, but because the motion is chiefly vertical, the groundspeed can be very small at the same time.
True Airspeed
True airspeed is the velocity with which the aircraft moves relative to the surrounding air. The difference between TAS and GS is that the air itself may move with respect to the ground (that's wind), and dependent on course relative to the wind direction a discrepancy between TAS and GS is induced. TAS can't really be measured directly but needs to be calculated.
Knowing TAS during flight is surprisingly useless - for navigation, ground speed is needed, and aerodynamic limits do not depend on TAS but rather IAS. The chief value of TAS is as a measure of aircraft performance and in pre-flight planning before the wind effect is taken into account.
Indicated Airspeed
Indicated airspeed is (usually) the number displayed on the airspeed gauge. Airspeed is usually measured by means of a pitot tube at the front of the aircraft which is exposed to the airstream and hence registers not only the ambient air pressure but also the dynamical ram pressure (created by the plane motion ramming air into the tube). This dynamic pressure component is a measure for the aircraft velocity, and airspeed gauges can be thought of as dynamic pressure gauges with funny labels in terms of velocities.
It becomes apparent that IAS is in fact not TAS once one takes into account that the ram pressure is not only a function of velocity relative to the air but also a function of the air density, i.e. it changes with altitude (or more precisely, with density altitude). The same IAS reading therefore may correspond to vastly different TAS when the plane starts climbing to high altitude. At sea level, a KIAS of 400 kt roughly corresponds to 400 kt TAS, at 80.000 ft (the cruising altitude of the SR-71), the same reading may indicate a TAS in excess of 1600 kt (is can be very difficult to reconcile an airspeed of 400 kt with a reading that one is flying in excess of Mach 3 when one doesn't know what the airspeed gauge shows).
In spite of this dependence on density altitude, IAS is a very useful quantity in flight. Many aerodynamical properties, for example drag, the stress on the airframe, stall speed or the forces on control surfaces depend on the dynamic pressure generated by the airstream, not on the actual aircraft velocity. Thus, the actual stall speed of an aircraft at sea level is very different from the stall speed at 30.000 ft - but they correspond to the same IAS reading.
Calibrated Airspeed
Calibrated airspeed corresponds to IAS corrected for the measurement error. For various reasons, airplanes do not carry 'perfect' sensors as they would be used in a scientific experiment, so usually there is some discrepancy between the actual reading of the gauge and the reading a perfect instrument would show. CAS takes into account this correction.
Equivalent Airspeed
Equivalent airspeed takes into account yet another correction, this time having to do with air properties rather than sensor errors. At high altitude, the compressibility of air changes, so even CAS becomes more and more unreliable. For the SR-71 Blackbird ceiling of 85.000 ft, that actually is an effect and the plane is flown based on a KEAS velocity measure. For more conventional airplanes, EAS is not much used. Thus, EAS is what a perfect dynamic pressure sensor would show when properly calibrated for the air compressibility at the current altitude.
Mach number
The Mach number is the ratio of an aircraft's TAS over the local speed of sound. A Mach number below 1 means that the plane moves subsonically, a Mach number above 1 indicates supersonic flight. The Mach number is interesting because a number of phenomena take place just around Mach 1, for example a sudden increase in drag induced by shockwave generation. However, since the speed of sound changes with the compressibility (and hence temperature) of air, the Mach number is dependent on altitude (as the air temperature drops at higher altitudes). This implies that Mach 2 at sea level corresponds to a faster TAS than Mach 2 at 30.000 ft. The precise relations between TAS, Mach number and altitude are rather complicated formulae and depend in essence on the local weather pattern determining the pressure and temperature gradients in the atmosphere.
