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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. |
#2
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"Peter" wrote in message ... "Mike Rapoport" wrote The 3C difference is a function of instrument error. The error is greater than 3C since the Shadin should be showing a lower temp. OK, that demolishes my argument... I will suggest he checks his OAT gauge. I check mine, the same type, regularly against a PT100 meter which is certified to 0.1C and it is always within the OAT display resolution (1C). However are you quite sure that TAT should be lower than SAT? An OAT gauge which is that far out would be pretty obvious on the ground e.g. when getting the ATIS. Not exactly...IF "TAT" is Total Air Temperature then it will be higher than SAT. But ( a couple buts actually!) 1)The OEM OAT guage reads TAT not SAT since it is not corrected for Mach effects. Thus, there is no way that the Shadin should read higher than the OEM guage if both are accurate. If the Shadin is displaying Total Air Temperature, then it should read the same. If the Shadin is displaying Static Air Temperature (which they might be calling "True Air Temperature) then the Shadin should read lower. Either way the Shadin should not read higher if both are accurate 2)If you to to Shadin's website, they state that the ADC puts out "True Air Temperature" and I think that they mean acutal air temperature or SAT. My own observation is that, on the MU-2, the OAT guage reads a higher temperature than the ADC displays and the difference between them matches the temperature correction table in the flight manual pretty well http://www.shadin.com/specs/adc_200.htm Confusing this issue further, Garmin says the TAT means "Total Air Temperature and, on the 530 the temp is displayed as XXTAT. However, Shadin doesn't claim that the ADC even outputs Total Air Temperature. I actually asked Shadin this question this morining but, stupidly, I forgot to attach my email address! The way to test all of this is to see how the temperature spread varies between the OEM OAT guage and the Shadin ADC at different airspeeds since TAT will go up and SAT will not. Keep in mind that it is SAT that determines the temperature of the droplets. I can see this but I would think it is the temperature of the airframe that determines whether the droplets will freeze when they will hit it. Yes, but the temperature of the airframe will be different depending of whether it is flying through +1C air or +1C air loaded with -10C droplets. As the number and size of the droplets increases the leading edges will get closer to -10C. This is mostly theoretical for low speed aircraft but is relevent for jets which have large spreads between TAT and SAT. Mike MU-2 |
#3
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I have the Insight TAS-1000 in my airplane.
I've noticed that there's about a 5 degree C rise in air temp when flying at the top of the green (165kts or so as memory serves) One can count on getting ice when the TAT is exactly zero. I've often wondered why the variation... the OAT thermometer measures static, I think. The reason the jets have little trouble with ice is that they go so fast that TAT remains above zero until the OAT is so low that ice is not a problem. Bill Hale |
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"Peter" wrote in message ... "Mike Rapoport" wrote Yes, but the temperature of the airframe will be different depending of whether it is flying through +1C air or +1C air loaded with -10C droplets. How is that possible? Surely if the SAT is +1C then everything hanging in the air will also be +1C. Sorry, I wasn't clear. I meant that in a case where TAT was 1C you could still get icing because SAT could be low enough (well below 0C) that the water droplets are cold enough to cool the airframe below 0C and still be supercooled themselves. I looked at the Shadin website but there is no useful information, and they have removed the detailed manual. Did you find where it lists the information that the ADC 200 outputs? Total Air Temperature is not one of them. Mike MU-2 |
#5
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"Bill" wrote in message oups.com... I have the Insight TAS-1000 in my airplane. I've noticed that there's about a 5 degree C rise in air temp when flying at the top of the green (165kts or so as memory serves) One can count on getting ice when the TAT is exactly zero. I've often wondered why the variation... the OAT thermometer measures static, I think. No, the OAT thermometer measures TAT. Mike MU-2 The reason the jets have little trouble with ice is that they go so fast that TAT remains above zero until the OAT is so low that ice is not a problem. Bill Hale |
#6
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On Sat, 11 Dec 2004 01:43:33 GMT, "Mike Rapoport"
wrote: "Peter" wrote in message .. . "Mike Rapoport" wrote Yes, but the temperature of the airframe will be different depending of whether it is flying through +1C air or +1C air loaded with -10C droplets. How is that possible? Surely if the SAT is +1C then everything hanging in the air will also be +1C. Sorry, I wasn't clear. I meant that in a case where TAT was 1C you could still get icing because SAT could be low enough (well below 0C) that the water droplets are cold enough to cool the airframe below 0C and still be supercooled themselves. I looked at the Shadin website but there is no useful information, and they have removed the detailed manual. Did you find where it lists the information that the ADC 200 outputs? Total Air Temperature is not one of them. Mike MU-2 Mike.. ALL of your temp gauges must be better than ours (OMC O.A.T) In any of the 15 MU-2s we have,, Ice forming on the taxi light is indication of below freezing (or approaching it). No wonder they want ice protection prior to reaching anything close to 0 *C.. Chuck |
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