"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