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.

"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, 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

"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

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

"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

"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

"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

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

"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