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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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