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IAS, GS, And TAS Relation

Sat Jun 16, 2007 8:26 am

Hello, all.

I've been playing x-plane a lot, and I have an odd question.

Right now, for example, I am flying LGA-SXM. My Indicated Air Speed is 297 knots. There is a 21 knot wind at 170, and yet my ground speed is 461 knots. My TAS, which I don't know what it stands for, is 482 knots.

How are GS and TAS related to IAS and how are they calculated? I always thought GS was the sum (or, in this case, with a head-wind, the difference) of IAS and wind speed, but apparently that's wrong.

On a related note, if my IAS is 297 at 33000 feet, why is my Mach number .82, where as at sea level at the same IAS, the Mach number would be much less? I understand it has to do with air pressure and temperature and the like, but I'm confused as to WHY all that makes a difference.


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RE: IAS, GS, And TAS Relation

Sat Jun 16, 2007 9:38 am

True airspeed (TAS) is your actual speed relative to the air.

Ground speed is TAS plus tailwind, so in your example you seem to be flying 170 directly into that 21 knot wind (or that 21 knots is the headwind component, not the total wind).

The problem is, it isn't that easy to actually calculate the TAS by measuring atmospheric qualities on the airplane, so they display Indicated Airspeed (IAS), which is a relationship between the total pressure and static pressure. You can read about the relationship between different types of airspeeds in introductory aeronautics books by people like Anderson or Shevell, or even Wikipedia (although I haven't read that article so have no idea how accurate it is, but assume really gross errors would have been corrected by now), or if you prefer a lot of equations without much explanation, maybe this site will be interesting for you.

Quoting ANITIX87 (Thread starter):
On a related note, if my IAS is 297 at 33000 feet, why is my Mach number .82, where as at sea level at the same IAS, the Mach number would be much less?

Two reasons:

1. At sea level for an IAS of 297 you would have a much lower TAS.
2. At sea level the speed of sound is much higher, so even at the same TAS you would have a lower Mach number -- the speed of sound is proportional to the square root of temperature, and temperature drops linearly from sea level to around 36000 feet.
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