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Andy Rylance
19th Feb 2008, 10:16
Sorry guys, another misunderstanding on TAT.

TAT is defined as bringing an amount of air stationery as if it was next to the aircraft and not moving and measuring its temperature. (I think). But why is it different from outside air temperature?

You see if I put a thermometer and dangle it out of the car window it measures the temperature of the air it goes through. If I make the air go the same speed as the car, it is still the same temperature. Or if I stop the car and measure the temperature, it is still the same.

So what difference here?

Mark 1
19th Feb 2008, 10:30
If you bring moving air to rest (or accelerate stationary air to the speed of the aircraft), then you are imparting energy. That energy will appear as a temperature rise.

For a given mass of air the temperature rise will be the kinetic energy change divided by the specific heat (constant pressure).
i.e. 0.5*V^2 / Cp

Your thermometer stuck out of a car window will be stagnating a small proportion of the air flowing onto it, so will show a slightly higher temperature than if you measured it while stationary (at 70 mph it will be about 0.5 degrees rise).

TopSwiss 737
19th Feb 2008, 10:35
Hi Andy, found the following on Wikipedia (http://en.wikipedia.org/wiki/Total_air_temperature) after a quick search.

"Total air temperature is a term used generally in aviation. In other applications it is called stagnation temperature. Total air temperature is measured by a specially designed temperature probe mounted on the surface of the aircraft. The probe is designed to bring the air to rest relative to the aircraft. As the air is brought to rest kinetic energy is converted to internal energy. The air is compressed and experiences an adiabatic increase in temperature. Therefore total air temperature is higher than the static (or ambient) air temperature.

Total air temperature is an essential input to an air data computer in order to enable computation of static air temperature and hence true airspeed."

The probe they are referring to is generally known as a Rosemount probe.

Hope this helps :ok:, regards TS

Andy Rylance
19th Feb 2008, 10:49
Ok so I am I right with this:

If I happened to be in an aircraft right next to the metal on the inside of the aircraft that the outside air was rushing past, I would only ever notice the temperature of the metal going down to the TAT and not the OAT - i.e. because the air is moving so fast the temperature it will bring the metal down to would be the TAT and not the OAT, unless the air was stationery?

Chris Scott
19th Feb 2008, 11:49
Quote from Andy Rylance:
If I happened to be in an aircraft right next to the metal on the inside of the aircraft that the outside air was rushing past, I would only ever notice the temperature of the metal going down to the TAT and not the OAT - i.e. because the air is moving so fast the temperature it will bring the metal down to would be the TAT and not the OAT, unless the air was stationery? [Unquote]

I very much like your idea of putting an observer on the inside of the metal skin, but in the particular case you pose - touching the side of the fuselage - it wouldn't give you an accurate TAT.

As Mark 1 says, you've got to "stop" the oncoming ambient "moving" air completely, to convert ALL its kinetic energy into heat energy (thus causing its temperature to rise). This only happens, presumably, at forward-facing nul points, e.g., the nose, the leading edges of the wings, et cetera.

This is why it has been discussed, elsewhere on this forum, whether wing-tank fuel temperature, on a very long flight, will ultimately stabilise at the TAT or the SAT/OAT. My understanding is that, ignoring solar heating (or outgoing radiation), it will be somewhere between the two. There must be some skin friction (loss of kinetic energy into heat energy) on the side of a fuselage and the surfaces of a wing, even allowing for the boundary layer (stationary) air. But it won't be enough to raise the skin temperature to the theoretical TAT.

By the way, TAT is normally sensed nowadays by a probe (one type called a Rosemount) which effectively "catches" the air somewhere underneath the nose of the fuselage. A computer then calculates the SAT, to save the laid-back modern pilot the trouble of doing it.