28th Apr 2012, 20:53
permalink Post: 1612
Here's the cockpit temp gauge I photgraphed today:
So the TAT probe provides TAT (obviously) which effectively is skin temp (as evidenced by the TMO legend of 127C just below the TAT window?).
(TAT being static air temp plus the temp due adiabatic heating).
So the TAT probe provides TAT (obviously) which effectively is skin temp (as evidenced by the TMO legend of 127C just below the TAT window?).
(TAT being static air temp plus the temp due adiabatic heating).
Last edited by Shaggy Sheep Driver; 28th Apr 2012 at 21:14 .
3rd May 2012, 15:13
permalink Post: 1621
Quote:
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Er - no, the TAT probes measure just what they say Total Air Temperature.
They are mounted off the skin and in freestream, so they measure the same temperature as would a probe on the nose. Somewhere near the nose (not exactly on it, as the aircraft flies with a small AoA) there will be a 'stagnation' streamline where the oncoming air is brought to rest. At this point the skin temperature will be equal to the stagnation temperature (TAT). Behind that it gets more complicated! The skin temperature would depend on SAT, local Mach No, local skin friction coefficient (Mach and Re dependent, so varies with distance from nose), amount of heat radiated into space (paint colour!) and the amount of structure available to conduct heat away from the skin into the fuel (so roughly varying with thickness/chord and fuel distribution perhaps? |
The temperature shown in the top window of the flight deck gauge is TAT, with the legend 'TMO 128C' beneath it. So the aircraft was flown with reference to TAT, and provided TAT was no greater than 128C then the skin rearward of the stagnation point would be <128C?
4th May 2012, 18:15
permalink Post: 1625
Minor correction, though: TMO was 127, not 128.
4th May 2012, 18:30
permalink Post: 1626
Indeed. Typed in a hurry. Didn't even check my own photo of the temp gauge which clearly shows 'TMO 127C'.
15th May 2012, 10:03
permalink Post: 1631
The TAT thing
Quote:
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OK, so the skin temperature at the stagnation point will be equal to TAT. This can be taken as the hottest part of the aircraft (behind it, the skin temperature will be less than the TAT).
The temperature shown in the top window of the flight deck gauge is TAT, with the legend 'TMO 128C' beneath it. So the aircraft was flown with reference to TAT, and provided TAT was no greater than 128C then the skin rearward of the stagnation point would be <128C? |
MACH 0.5 . ISA -5: TAT = -50.6 \xb0 C . ISA: TAT = -45.3 \xb0 C. ISA +5 TAT = -40 \xb0 C
MACH 1.0 . ISA -5: TAT = -18.5 \xb0 C . ISA: TAT = -12.5 \xb0 C. ISA +5 TAT = -6.5 \xb0 C
MACH 1.5 . ISA -5: TAT = 34.8 \xb0 C . ISA: TAT = 42 \xb0 C. ISA +5 TAT = 49.3 \xb0 C
MACH 2.0 . ISA -5: TAT = 109.5 \xb0 C . ISA: TAT =118.6 \xb0 C. ISA +5 TAT = 127.6 \xb0 C
Hopefully it all makes a little more sense with some 'real' numbers. You can see that as Mach Number increases the gap between SAT and TAT increases hugely. The Mach 2, ISA +5 case was particularly significant for Concorde, as it breached the 127 \xb0 C/400 \xb0 K airframe temperature limit (TMO) and Mach Number would therefore be automatically reduced by the autopilot. (An overspeed warning would be generated at TMO +7 (134. \xb0 C). Fortunately sustained ISA +5 or above conditions were relatively rare over the North Atlantic but not unheard of either.
Last edited by M2dude; 16th May 2012 at 23:06 . Reason: ISA+5 Typo