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Date: Tue, 03 Apr 2007 14:36:14 -0400
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From: "Paul Lipps" <elippse@sbcglobal.net>
Subject: IAS error
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As someone wrote, check for a static leak in the cockpit first. =
Sometimes you can have a leak in the case of the VS; this shows up as an =
indicated climb during level flight even though the meter was zero'ed on =
the ground. I've seen a VS with a cracked case before. Generally, if the =
error is seen only on the IAS, as opposed to a computed TAS error on =
avionics, it is usually due to a static-port placement problem. If you =
indicate too high, move the port rearward to higher pressure, or if it =
is too low, move it forward to lower pressure. Alternatively, you can =
put several layers of cellophane tape ahead of the hole to reduce the =
pressure and make it indicate higher, or behind the port to raise the =
pressure to make it indicate lower. If your computed TAS is in error on =
the high side, that will also stem from static port errors, but, in =
addition, it will come from an OAT error due to stagnation temperature =
rise. It is TAS^2 / 5521, TAS mph, or TAS^2 / 4167, TAS kt, deg.F So if =
you're clippin' along at 200kt, you can have as much as a 9.6 F error in =
OAT; at 300kt it's 21.6 F! Those amounts, if the avionics don't know =
your OAT probe rise, can give errors of 605' and 1361' dalt. You can =
check this easily enough. Once you reach a certain baro altitude, slow =
the plane down to your lowest comfortable IAS and remain there for =
whatever time it takes for your OAT to settle down to a constant level. =
Note your OAT and dalt. Now accelerate to cruise speed and again note =
the OAT and dalt. If they're the same, congratulations! But most likely =
they won't be, and that will be due to the placement of your OAT probe =
and the avionics compensation of its estimate of your temperature rise. =
You can eliminate this error by mounting your OAT sensor in the tailcone =
or ahead of the flap in back of the spar. Then you won't get this rise. =
Now, however, your avionics programmer, who's probably programmed in =
some percentage of the rise, will give you a lower TAS and dalt. Oh, =
well! Just work with them 'til they get it right!
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<DIV><FONT face=3DArial>As someone wrote, check for a static leak in the =
cockpit=20
first. Sometimes you can have a leak in the case of the VS; this shows =
up as an=20
indicated climb during level flight even though the meter was zero'ed on =
the=20
ground. I've seen a VS with a cracked case before. Generally, if the=20
error&nbsp;is seen only on the IAS, as opposed to a computed TAS error =
on=20
avionics, it is usually due to a static-port placement problem. If you =
indicate=20
too high, move the port rearward to higher pressure, or if it =
is&nbsp;too low,=20
move it forward to lower pressure. Alternatively, you can put several =
layers of=20
cellophane tape ahead of the hole to reduce the pressure and make it =
indicate=20
higher, or behind the port to raise the pressure to make it indicate =
lower. If=20
your computed TAS is in error on the high side, that will also stem from =
static=20
port errors, but, in addition, it will come from an&nbsp;OAT error due =
to=20
stagnation temperature rise. It is TAS^<FONT size=3D1>2</FONT> / 5521, =
TAS mph, or=20
TAS^<FONT size=3D1>2 </FONT>/ 4167, TAS kt, deg.F So if you're clippin' =
along at=20
200kt, you can have as much as a 9.6&nbsp;F error in OAT; at 300kt it's =
21.6 F!=20
Those amounts, if the avionics don't know your OAT probe rise, can=20
give&nbsp;errors of 605' and 1361'&nbsp;dalt. You can check this easily =
enough.=20
Once you reach a certain baro altitude, slow the plane down to your =
lowest=20
comfortable IAS and remain there for whatever time it takes for your OAT =
to=20
settle down to a constant level. Note your OAT and dalt. Now accelerate=20
to&nbsp;cruise speed and again note the OAT and dalt. If they're the =
same,=20
congratulations! But most likely they won't be, and that will be due to =
the=20
placement of your OAT probe and the avionics compensation of its =
estimate of=20
your temperature rise. You can eliminate&nbsp;this error by mounting =
your OAT=20
sensor in the tailcone or ahead of the flap in back of the spar. Then =
you won't=20
get this rise. Now, however, your avionics programmer, who's probably =
programmed=20
in some percentage of the rise, will give you a lower TAS and dalt. Oh, =
well!=20
Just work with them 'til they get it right!</FONT></DIV></BODY></HTML>

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