SAT TAT and Free Air Temp

[eji]

Hello, I was just going through an advanced aviation book and came across these terms. Could somebody make this a little easier to understand? Also, If a jet is going maximum speed at sea level, and an identical plane is doing the same at 37000 ft. which jet is going faster. (why) Explanation is certainly appreciated. Thanx :confused:

[Flyingcircus.]

Before I start, let me say that I am a student myself, so anyone with more experience feel free to correct me on anything.

Now. SAT is Static Air Temperature. ie. the temperature that is stationary relative to the thermometer. Also known as OAT, or TOAT, ie Outside Air Temperature, or True OAT.

If the aforementioned thermometer is attached to a fast-moving object (ie a jet a/c) then the act of the probe pushing through the air creates a localised increase in air pressure. This air warms adiabatically - check a met text for adiabatic warming/cooling. Basically it is the process by which a fluid increases/decreases temperature due entirely to a change in the pressure of that fluid, without gaining/losing heat energy.

Anyway, the increased air pressure around the thermometer will cause the temperature of the air to rise somewhat, with the result that the thermometer will overread. What the instrument will read, then, will be the TAT, or Total Air Temperature. This is the SAT increased by the temperature rise described earlier.

At least that's how I understand it! :)

Now, for the second question. It would seem to me that the jet at altitude would have a higher top speed - due largely to the greatly degreased drag - and therefore would be going faster. However, I suggest you check the exact wording of the question, because it could have something to do with TAS vs IAS. What I mean is that if a jet has a Vne of (say) 430 kts IAS, and it is travelling at that max speed at sea level or at altitude, then at sea level, it's TAS will be 430 kts, while at FL370, it's TAS will be significantly greater. That could be what the question is referring to. Either way, the higher jet is going faster.

If you wanted to be really pedantic, you could argue that the higher jet is farther from the centre of the Earth, and depending on whether it was flying East or West is therefore assisted or retarded by the Earth's rotation. :rolleyes:

I hope that helps.

[eji]

hey, thanks a lot, that makes things a lot easier for me. :D :D :)

[chiglet]

Just to put the feline among the avians...
Don't forget that TAS [True air Speed], IAS [Indicated AirSpeed] and Ground Speed are ALL different :confused:
Your speed through the air is IAS. This is corrected to TAS, but if you have a head or tailwind, then your Ground Speed is [rather] different. also youe Mach number limiting speed differs both with altitude AND temperature.
EG, the Supermarine Swift record attemt on the Airspeed record was in the desert, at LOW altitude/HIGH temparature. [Less dense air] The Fairey Delta 2 was at HIGH altitude/LOW temperature over the South Coast :eek: {also less dense air]
Hope that this helps :D
we aim to please, it keeps the cleaners happy

[ 18 October 2001: Message edited by: chiglet ]

[on the glide]

and now you might got a chance to get an
interview...and pass. :)

[Kaptein Max]

When TAT is higher than SAT, it is mainly because of friction.