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Total Air Temperature question
"Peter" wrote in message ... Hi, I fly a TB20 and a friend who has the same one with a Shadin air data computer tells me that he normally gets (say 160 ktas, 10k feet, ISA) a TAT value of about 3 degC higher than that returned by the existing OAT sensor. I looked up TAT and found info like http://mtp.jpl.nasa.gov/notes/sat/sat.html which explains TAT as being Ts (static air temp) adjusted for mach effects. This raises two questions: 1) Can one really get a 3 degC difference at such a low speed? I recall a long thread here from some years ago where it was suggested the increase is below 1 degC. Accoring to their website Shadin outputs "True Air Temperature" and on the MU-2 the Shadin temp is lower than the existing OAT guage which uses a, very accurate, flush, sensor. I haven't found a definition for "True Air Temperature" but I think that it is "Static Air Temperature". The MU-2 flight manual has a correction table to convert indicated OAT to SAT and the temperature output by the Shadin is consistant with what SAT should be. Keep in mind that your friend's OAT sensor is sees Total Air Temperature and since it doesn't know its Mach number, it can't possibly produce a corrected temperature like SAT. The 3C difference is a function of instrument error. The error is greater than 3C since the Shadin should be showing a lower temp. 2) What is the use of TAT in the context of icing? Presumably the temp rise will vary over different parts of the aircraft. If the TAT is above freezing then you won't get much ice but it can still form. Keep in mind that it is SAT that determines the temperature of the droplets. You could be flying in -10C (SAT) with a +1C (TAT) and the supercooled drops would cool the leading edges below 0C and ice would form (If the mass of the -10C dropplets was large enough to cool the surface faster than the 1C air could heat it). It would melt off pretty quickly once you were out of the cloud though since it would be exposed to +1C. I heard once that if you can go 400kts IAS then airframe icing is imposible. I forget where I saw this. Yes, the temperature rise varies with location on the airplane with leading edges and areas of increasing cross section seeing more temp rise. Mike MU-2. |
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"Peter" wrote in message
... I fly a TB20 and a friend who has the same one with a Shadin air data computer tells me that he normally gets (say 160 ktas, 10k feet, ISA) a TAT value of about 3 degC higher than that returned by the existing OAT sensor. This raises two questions: 1) Can one really get a 3 degC difference at such a low speed? I recall a long thread here from some years ago where it was suggested the increase is below 1 degC. a) 3 degC is exactly what you'd expect between SAT (the temperature of the air as measured by a thermometer at rest) and Total Air Temperature at 160 KTAS (and it's approximately quadratic in speed) but b) Mike is, of course, quite right in suggesting that the "existing OAT sensor" is not measuring the SAT! It too is affected by the compression heating of the air. 2) What is the use of TAT in the context of icing? Presumably the temp rise will vary over different parts of the aircraft. Neglecting lift for a moment, there are two extremes for the temperature of the aircraft. Where the air is brought completely to rest, the stagnation points, the temperature should be the TAT. The air itself outside the boundary layer is at the SAT. Other parts of the aircraft will have temperatures in between. If the part is a flat wall parallel to the airflow and has no heating or cooling other than from the air, it will be heated above the SAT by about 85% (the recovery factor) of the TAT-SAT. The recovery factor for a thermometer placed elsewhere will depend on its location and geometry, but generally speaking the thermometer will measure an OAT closer to the TAT than the SAT. For a conducting skin, it's a fair approximation that the skin will be at the TAT. Where the pressure is lowered, for example on the upper surface of the wing, the temperature also falls because of that. Thus it is possible for runback to freeze with the TAT above freezing. The cooling effect depends on wing loading: for a typical light aircraft the effect will not exceed 1 degC, while for big jets, you may find a part of the wing 10 degC cooler than the sensed OAT. Julian Scarfe |
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Julian, do you know the meaning of "True Air Temperature"? I don't but this
is supposedly what the Shadin ADC outputs. See: http://www.shadin.com/specs/adc_200.htm Mike MU-2 "Julian Scarfe" wrote in message ... "Peter" wrote in message ... I fly a TB20 and a friend who has the same one with a Shadin air data computer tells me that he normally gets (say 160 ktas, 10k feet, ISA) a TAT value of about 3 degC higher than that returned by the existing OAT sensor. This raises two questions: 1) Can one really get a 3 degC difference at such a low speed? I recall a long thread here from some years ago where it was suggested the increase is below 1 degC. a) 3 degC is exactly what you'd expect between SAT (the temperature of the air as measured by a thermometer at rest) and Total Air Temperature at 160 KTAS (and it's approximately quadratic in speed) but b) Mike is, of course, quite right in suggesting that the "existing OAT sensor" is not measuring the SAT! It too is affected by the compression heating of the air. 2) What is the use of TAT in the context of icing? Presumably the temp rise will vary over different parts of the aircraft. Neglecting lift for a moment, there are two extremes for the temperature of the aircraft. Where the air is brought completely to rest, the stagnation points, the temperature should be the TAT. The air itself outside the boundary layer is at the SAT. Other parts of the aircraft will have temperatures in between. If the part is a flat wall parallel to the airflow and has no heating or cooling other than from the air, it will be heated above the SAT by about 85% (the recovery factor) of the TAT-SAT. The recovery factor for a thermometer placed elsewhere will depend on its location and geometry, but generally speaking the thermometer will measure an OAT closer to the TAT than the SAT. For a conducting skin, it's a fair approximation that the skin will be at the TAT. Where the pressure is lowered, for example on the upper surface of the wing, the temperature also falls because of that. Thus it is possible for runback to freeze with the TAT above freezing. The cooling effect depends on wing loading: for a typical light aircraft the effect will not exceed 1 degC, while for big jets, you may find a part of the wing 10 degC cooler than the sensed OAT. Julian Scarfe |
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"Mike Rapoport" wrote in message
ink.net... Julian, do you know the meaning of "True Air Temperature"? I don't but this is supposedly what the Shadin ADC outputs. See: http://www.shadin.com/specs/adc_200.htm I don't, I'm afraid. Like you, I've not come across True Air Temperature before. I looked briefly at a couple of the brochures on the Shadin site after you posted, and saw mention of True Air Temperature in one, Total Air Temperature in another. Maybe Peter knows which model the ADC was? Julian |
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