Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Wed, 28 Aug 2002 09:06:18 -0400
Message-ID: <redirect-1713325@logan.com>
From: "Fred Moreno" <fredmoreno@bigpond.com>
Subject: Induction air temperature and OAT
MIME-Version: 1.0
Content-Type: text/plain;
	charset="iso-8859-1"
Content-Transfer-Encoding: 7bit

Walter Dodson wrote in part:
"I think the ram pressure
might make an insignificant difference in the readings but don't know."

Lancair IV's fly fast enough that frictional heating effects become
significant at normal cruise speeds.  Air is heated when being
compressed at leading edges or when entering scoops or inlets.  This
heating is roughly 20 degrees F (at about 300 knots TAS) and is
unavoidable.  Elsewhere on the airframe, the air is not stopped and so
not compressed, but produces aerodynamic heating from frictional
dissipation in the boundary layer that slides along the surfaces.  The
amount of heating depends on details of the boundary layer, but a rough
rule of thumb is to assume it is about 80% of the compression heating
rise, or about 16 degrees F for typical Lancair IV high speed cruise.
The result is that no matter where you put the temperature sensor, it
will report a temperature between 16 and 20 degrees above the true
ambient.  Unless you put it some place where there is warm air exiting
the airframe.  Then the error is even greater.

It turns out that the compression heating/friction heating is the
largest contributor to error in computing true air speed under these
conditions.  If you take the indicated air speed, altitude, and the
"apparent" outside air temperature as detected by your not-so-trusty
sensor, you will compute a true air speed that is approximately 10-15
knots higher than the correct figure.  Compressibility adds another 5-7
knots of error in the pitot tube (and thus the indicated air speed
reading) but this is another topic for another day.  The net result is:

1) The actual outside air temperature is lower than you think.  The
frictional heating actually give you a little bit of icing protection if
the true OAT is a bit below freezing.  The airframe will be a bit above
freezing.  This is one reason that jets have fewer icing problems since
they will typically climb at 250-330 knots IAS.  (Having unlimited heat
when icing is severe helps a lot too, but that is a different topic.)
2) The TAS you calculate will be erroneously high, typically about 20
knots high, based on your OAT and IAS readings, and your E6B.  Same
error arises for those cute true airspeed computer rings around the
indicated air speed instrument.

The temperature error is there - learn to live with it.   Jeppesen makes
a high speed (as in jet) whiz wheel like an E6B that takes all this into
account and lets you compute TAS from the IAS and indicated OAT numbers
you get in the cockpit.  I had one given to me.  I had to spend $30 on
the instruction book before I could be certain I was using it correctly.

With regard to induction air temperature, I think it is worth monitoring
only on turbo airplanes to be sure there are no problems with the
intercoolers, blockages in the induction system, or problems with the
manifold pressure controls like mis-positioned throttle valves that are
excessively throttling the air flow.   Learn what the temperatures are
normally, and then when they change suddenly, you will know about it
before you have a detonation problem from high inlet temperatures.

Fred Moreno