Todd Pattist wrote:

Eggert Ehmke wrote:


I understand we need the TE probe to generate the difference
static-dynamic because classical instruments are pure mechanical and
need the TE information to compensate the vario. But given electronic
devices/computers, do we really need the TE probe at all? The
information is given, when static and dynamic pressure are known. The
rest can be calculated. Wrong?


Not "wrong" but incomplete. The static pressure (from the
static sensor) and the total pressure (static plus dynamic
from the pitot tube) are large numbers. You would need to
calculate the dynamic pressure which is a small number by
subtracting the two large numbers to be able to calculate
the TE pressure that you want. Since the large numbers
come from two different sensors (static and pitot) they have
different errors, and the difference shows those errors. It
is simpler/cheaper and you get better data by using a single
TE sensor at a single point where the errors tend to cancel
out.

Or you could use the semiconductor equivalent of the airspeed gauge
(delta pressure sensor), connect it to the pitot and static, and measure
the dynamic pressure directly. This avoids the need to subtract two
large numbers. Even if the pressure is measured accurately, the quality
of the reading depends on the quality of the pitot/static system. This
quality is generally easier to ensure on a TE probe than a sailplane.

Nonetheless, using a Cambridge 302 on my ASH 26 E with a probe and with
electronic TE, I couldn't tell the difference. I stayed with the
electronic TE because it is much less affected while the engine is
running, since the pitot and static are not in the prop wash.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA