Anyone reading this thread who would like a quick and easy refresher on the various types of airspeed, can brush up with the following notes, and a few comments about their impact on this thread.
Airspeed is measured by determining the difference between 2 air pressures about the aircraft. One pressure measurement is taken on the side of the aircraft, or on the side of the pitot-static tube (e.g. a small tube sticking out of the aircraft with its axis parallel to the direction of flight ), and indicates the static air pressure at your current altitude. The other pressure measurement is taken in the front, open end of the pitot tube, and indicates the total air pressure acting on your aircraft as you fly thru the air mass. The difference in these 2 measurements is indicated airspeed (IAS). Airspeed gauges are only calibrated for sea-level, standard-day atmospheric conditions. So. unless you are at sea level and the atmospheric conditions match standard day conditions, your airspeed indicator is not indicating true airspeed (TAS), ie your true speed with respect to the air mass. That's why this value is called IAS, it’s what is indicated on the airspeed gauge.
Calibrated airspeed (CAS) is IAS corrected for what is called "position error". This error is due to local airflow effects about the static measuring source. As the aircraft moves through the air, it changes the pressure field around itself. So, you don't get an accurate static pressure reading from the static source. This error is different for every aircraft type, and usually changes for each aircraft with airspeed and configuration (e.g. gear up or down) changes. Through flight test, you can determine what these position errors are, and then determine the necessary corrections to get CAS from IAS. The reason this is called CAS, is because this is what you would read if the airspeed indicator was "calibrated" perfectly, i.e. no position errors.
Equivalent airspeed (EAS) is CAS corrected for what is called "compressibility effects". As you go higher and/or faster, individual air molecules can be compressed as they come to rest inside the pitot tube. This "compressing" has the effect of causing the pressure sensor inside the pitot tube to indicate a total pressure higher than the actual value. These compressibility corrections are independent of aircraft type, and depend only on CAS and pressure altitude. They only come into play if you exceed 0.6 Mach number and/or 30,000 ft pressure altitude. The reason this is called EAS, is because this is your TAS equivalent at sea level. That means, take whatever value this is at your current altitude, Star Trek transport your aircraft to sea level, and this will be your TAS. The term is important because for a given angle of attack (AOA), an aircraft behaves the same aerodynamically (ie. it generates the same amount of lift, drag, etc.) at a given EAS regardless of altitude, discounting Mach number effects.
TAS is EAS corrected for air density at your current altitude. Air density is a function of pressure and temperature. Ground speed is TAS corrected for wind velocity.
At sea-level, standard-day atmospheric conditions (and for our purposes we can a just say sea level period, if Aces High models standard-day atmospheric conditions) all of these airspeed measurements will be equal, well except for IAS which is still dependant on those position errors, which are different for each aircraft type. An easy way to remember how their magnitudes relate to one another at higher altitudes is by using the square root symbol:
IAS and CAS are usually very close to one another. For most aircraft, usually within 10 to 20 knots or less. EAS is always less than CAS. For airspeeds of Mach 1.0 or less, the maximum difference will be 30 knots. TAS is always greater than all the other airspeeds, at altitudes above sea level.
An important note, IAS/CAS tells the pilot how the aircraft will behave regardless of what altitude he's at. The same aircraft at 200 KCAS at sea level behaves just like it does at 200 KCAS at 30,000 feet. Remember what I said about a given AOA and EAS above? IAS/CAS is very close to EAS, much closer than TAS even at moderate to low altitudes, much less high altitudes. Therefore, since the aircraft behaves the same for the same airspeed even at vastly different altitudes, this makes flying one a lot easier when referencing IAS/CAS which is why when pilots talk speed, they talk IAS/CAS and in WWII that would have just been IAS. On the other hand if you want to compare different aircraft or know which aircraft is faster you need to know their TAS and so aircraft data and performance reports are normally provided in TAS.
As a side note, because TAS is normally arrived at through corrections for position error, compressibility effects, and air density corrected to standard day atmospheric conditions, values of TAS may vary significantly from the IAS recalled by pilots flying real aircraft in non standard conditions, even when we do our own conversions. It doesn’t mean anyone is necessarily wrong, just that everyone is talking about different things.
Hope that helps…
Badboy
