Why are TE probes so long?

I'm sure somebody may correct me, but as I recall, the TE probe produces
a signal equal and opposite of the pitot (total pressure). Unless you have
separated flow, or flow inside the boundary layer, the total pressure doesn't
change anywhere around the plane. It would seem to me to be a case
where angular changes (due to deflected airflow) would be the cause
of the shift in the TE pressure. The shape and angle of the TE probe is
not as forgiving as the pitot end.



In article ,
wrote:
(Hank Nixon) wrote:

Experience has shown that TE sensors need to be at least 1/2 chord
length in front of the fin to avoid negative effects.

In addition to the disturbance from the tail surfaces, there
can be disturbances from the wing. You usually want your
probe above an imaginary line connecting the leading edge of
the horizontal stabilizer to the wing. Length (and turning
the tip up) helps get the probe into that undisturbed air.
Todd Pattist - "WH" Ventus C
(Remove DONTSPAMME from address to email reply.)

TE is opposite to Ptot, in other words TE sucks, but nowhere near equal in
magnitude.

Ian

Why is it, then, that you can calibrate a TE probe by driving down the road
with it sticking out of the window, hooked up backwards to an airspeed
indicator (static is pressure, TE is static port) and it reads the
same as a pitot probe with a normal hook up?



In article , "tango4"
wrote:
TE is opposite to Ptot, in other words TE sucks, but nowhere near equal in
magnitude.

Ian

"tango4" wrote in message ...
TE is opposite to Ptot, in other words TE sucks, but nowhere near equal in
magnitude.

Ian

Not So
Properly functioning TE probe is equal in magnitude to pitot and
opposite in sign.
In fact you could connect airspeed static to probe and airspeed
"pitot" to airfarme static and get the same reading as conventional
hookup. This is actually a fairly useful test tool.
UH

d b wrote:

I'm sure somebody may correct me, but as I recall, the TE probe produces
a signal equal and opposite of the pitot (total pressure).

Not exactly, total pressure is static + dynamic, TE is static - dynamic.

In article , Robert Ehrlich wrote:
d b wrote:

I'm sure somebody may correct me, but as I recall, the TE probe produces
a signal equal and opposite of the pitot (total pressure).

Not exactly, total pressure is static + dynamic, TE is static - dynamic.


Let's put it this way. The indicator reads the difference in pressure between
the hole called P and the hole called S. When I hook the hole called P to a
pitot tube and the hole called S to a perfect static source, I get a number,
let's say 70.

Now I hook a TE probe to the hole called S and the perfect static source to
the hole called P. The indicator again reads 70. Obviously, the TE probe is
the exact opposite of the pitot tube.

It is agreed that the pitot is the total energy static + dynamic
The obvious conclusion is that the TE probe is - (static + dynamic)

It is NOT static - dynamic, it IS -(static+dynamic)

Kinda sucks, doesn't it?

d b wrote:
In article , Robert Ehrlich
wrote:
d b wrote:

I'm sure somebody may correct me, but as I recall, the TE probe
produces a signal equal and opposite of the pitot (total
pressure).

Not exactly, total pressure is static + dynamic, TE is static -
dynamic.


Let's put it this way. The indicator reads the difference in
pressure
between
the hole called P and the hole called S. When I hook the hole called
P to a
pitot tube and the hole called S to a perfect static source, I get a
number,
let's say 70.

Now I hook a TE probe to the hole called S and the perfect static
source to
the hole called P. The indicator again reads 70. Obviously, the TE
probe is
the exact opposite of the pitot tube.

It is agreed that the pitot is the total energy static + dynamic
The obvious conclusion is that the TE probe is - (static +
dynamic)

It is NOT static - dynamic, it IS -(static+dynamic)

Kinda sucks, doesn't it?

Yes it IS (static - dynamic). Go study your fluid dynamics again
Cheers, John G.

Perhaps it isn't obvious. The airspeed indicator is a DELTA pressure guage.
It is not an absolute pressure guage. If the indicator reads the same delta
pressure, the two devices have to be producing the same delta pressure.
One sucking, the other blowing.


In article , "John Giddy"
wrote:
d b wrote:
In article , Robert Ehrlich
wrote:
d b wrote:

I'm sure somebody may correct me, but as I recall, the TE probe
produces a signal equal and opposite of the pitot (total
pressure).

Not exactly, total pressure is static + dynamic, TE is static -
dynamic.


Let's put it this way. The indicator reads the difference in
pressure
between
the hole called P and the hole called S. When I hook the hole called
P to a
pitot tube and the hole called S to a perfect static source, I get a
number,
let's say 70.

Now I hook a TE probe to the hole called S and the perfect static
source to
the hole called P. The indicator again reads 70. Obviously, the TE
probe is
the exact opposite of the pitot tube.

It is agreed that the pitot is the total energy static + dynamic
The obvious conclusion is that the TE probe is - (static +
dynamic)

It is NOT static - dynamic, it IS -(static+dynamic)

Kinda sucks, doesn't it?

Yes it IS (static - dynamic). Go study your fluid dynamics again
Cheers, John G.

d b wrote:

Perhaps it isn't obvious. The airspeed indicator is a DELTA pressure guage.
It is not an absolute pressure guage. If the indicator reads the same delta
pressure, the two devices have to be producing the same delta pressure.
One sucking, the other blowing.

Or, if you have a good static system, one that neither blows nor sucks,
but just sits there!
--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

d b wrote:

In article , Robert Ehrlich wrote:
d b wrote:

I'm sure somebody may correct me, but as I recall, the TE probe produces
a signal equal and opposite of the pitot (total pressure).

Not exactly, total pressure is static + dynamic, TE is static - dynamic.

Let's put it this way. The indicator reads the difference in pressure between
the hole called P and the hole called S. When I hook the hole called P to a
pitot tube and the hole called S to a perfect static source, I get a number,
let's say 70.

Now I hook a TE probe to the hole called S and the perfect static to
the hole called P. The indicator again reads 70. Obviously, the TE probe is
the exact opposite of the pitot tube.

It is agreed that the pitot is the total energy static + dynamic
The obvious conclusion is that the TE probe is - (static + dynamic)

It is NOT static - dynamic, it IS -(static+dynamic)

Kinda sucks, doesn't it?

No, the reading of your indicator is P - S, converted into whatever speed
unit you like. If P is connected to a pitot tube and S to a perfect static
source, P - S is total - static = (static + dynamic) - dynamic) = dynamic).
When P is connected to a perfect static source and S to a TE probe,
P -S = static - (static - dynamic) = dynamic again, so the reading are the
same.