Re: Total Air Temperature question

[Julian Scarfe]

"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

[Mike Rapoport]

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
>
>

[Julian Scarfe]

"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

[Mike Rapoport]

"Peter" > wrote in message
...
>
> "Julian Scarfe" > wrote
>
>>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?
>
> I am no nearer to the truth...
>
> The owner of the Shadin ADC doesn't know which model he has.
>
> In the KLN94B manual, page 5-39, which describes the page which
> becomes operational when an ADC is connected, it says
>
>>SAT Static air temperature (the actual temperature of the
>>surrounding air mass).
>
>>TAT Total air temperature (the air temperature including heat rise
>>due to compressibility. This is the temperature measured
>>directly by the outside air temperature (OAT) probe.
>
>>Prs Pressure altitude (to nearest 100 feet or 10 meters).
>
>>Den Density altitude (to nearest 100 feet 10 meters).
>
> which doesn't make a lot of sense because when the ADC is fitted,
> there is a 2nd OAT probe.

Either way, the Shadin should be reading the same or lower than the OEM OAT
guage. I am flying on Wednesday and if the opportunity presents itself (VFR
here in the PNW) I will try leveling off recording the spread between the
OAT guage and the temp displayed by the Shadin at two different speeds to
verify that the spread widens as speed increases.

Mike
MU-2

[Mike Rapoport]

"Peter" > wrote in message
...
>
> "Mike Rapoport" > wrote:
>
>>Either way, the Shadin should be reading the same or lower than the OEM
>>OAT
>>guage.
>
> I am sorry but I still don't get that. Total Air Temp should be SAT,
> adjusted UPwards for the airflow heating. But different parts of the
> aircraft will have different mach numbers. Which bit is TAT supposed
> to represent?

Exactly, TAT represents SAT adjusted upwards for Mach. The problem with
your senario (160ktas, 10K', ISA and Shadin reads 3C higher) ) is that the
difference in readings is the same as the *total* calculated difference
between SAT and TAT. The OAT guage would need to have a local flow of
*zero* and the Shadin would need to have a local flow equal to the
freestream Mach number. Think about it this way:

Possibility #1 Both OEM OAT and Shadin displaying TAT and Shadin reads 3C
higher (stated condition)

Since total difference between SAT and TAT is 3C (calculated by Julian
earlier in thread) and the Shadin reads 3C higher, for the OEM guage to be
reading correctly the mach number at its location would have to be 0. To be
exact, the difference in Mach number has to be exactly the same as the speed
of the airplane, pretty unlikely.

Possibility #2 The OEM OAT is displaying TAT and the Shadin is displaying
SAT.

SAT is always lower than TAT so the Shadin should show a lower temp.

> You could be right, which would presumably mean the stock OAT probe
> has a higher mach number than anything else.

Two possibilities, either the airflow over the OEM OAT probe is zero or one
(or both) temperature readings are inaccurate, a much more likely senario.

Mike
MU-2

[Mike Rapoport]

"Peter" > wrote in message
...
>
>>
> I need to wait for a reply from Shadin as to what their "True Air
> Temp" is supposed to mean.
>
Lets us know what they say.

Mike
MU-2

[Mike Rapoport]

Thanks. This is consistant with what I observe in my airplane. It is
unfortunate that they chose to name things differently from everyone else,
and particularly inappropriate to use TAT to mean SAT when there is a "TAT"
in common use.

Mike
MU-2

"Peter" > wrote in message
...
>
> "Mike Rapoport" > wrote
>
>>> I need to wait for a reply from Shadin as to what their "True Air
>>> Temp" is supposed to mean.
>>>
>>Lets us know what they say.
>
> Shadin's reply:
>
>>Shadin's OAT = = Other's TAT (Total Air Temp)
>>Shadin's TAT (True Air Temp) = = Other's SAT (Static Air Temp)
>
> They also sent me the installation manuals for models 200 and 2000.
>
>
> Peter.
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