Why are TE probes so long?

"Mark James Boyd" wrote in message
news:4095c4f4$1@darkstar...
Why are these things so long? If they were 2" instead of
a foot long would it really matter that much? Is the airflow
really disturbed enough to make a difference?

Mark Boyd
Avenal, California, USA

Ans 1 - See Ans 3
Ans 2 - Yes
Ans 3 - Yes. It is a matter of the very small pressure change measured by
the TE versus the disturbed air.

Duane

I pulled a swivelling tailwheel off a G-103 today and was (gratefully)
reminded to not hit the long TE probe when standing up.

Why are these things so long? If they were 2" instead of
a foot long would it really matter that much? Is the airflow
really disturbed enough to make a difference?

The pitot on many power planes is maybe 2-4" long.
What's the deal here?

Has anyone tried a shorter TE probe? What's the difference
in error? Can it be compensated?
--

------------+
Mark Boyd
Avenal, California, USA

On 2 May 2004 21:05:08 -0800, (Mark James Boyd)
wrote:

I pulled a swivelling tailwheel off a G-103 today and was (gratefully)
reminded to not hit the long TE probe when standing up.

Why are these things so long? If they were 2" instead of
a foot long would it really matter that much? Is the airflow
really disturbed enough to make a difference?

The pitot on many power planes is maybe 2-4" long.
What's the deal here?

Has anyone tried a shorter TE probe? What's the difference
in error? Can it be compensated?


You want to keep the probe out of the flow field around the fin/stab.
This flow can exaggerate pitch and slip changes and the LE of the tail
surfaces have stagnation points so small changes in slip and pitch
may cause small changes in pressure over small time periods which may
in fact be large rates of change of pressure and rate of chnage of
pressure is what varios measure.

While you are at it clean the bugs off the probe before flying -
everyone cleans the wing leading edges but how many clean the probe?

Mike Borgelt

(Mark James Boyd) wrote in message news:4095c4f4$1@darkstar...
I pulled a swivelling tailwheel off a G-103 today and was (gratefully)
reminded to not hit the long TE probe when standing up.

Why are these things so long? If they were 2" instead of
a foot long would it really matter that much? Is the airflow
really disturbed enough to make a difference?

The pitot on many power planes is maybe 2-4" long.
What's the deal here?

Has anyone tried a shorter TE probe? What's the difference
in error? Can it be compensated?


Reply: There is a measurable pressure change which varies with speed
and angle of attack in front of every surface flowing through the air.
Experience has shown that TE sensors need to be at least 1/2 chord
length in front of the fin to avoid negative effects.
Easier to learn not to bend probe than try to make TE system work with
stubby probe.
Good Luck UH

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

D B wrote:
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



1. ASI has two connections: static and pitot (= total pressure = static + dynamic)

2. TE probe provides static - dynamic pressure

3. ASI measures difference between ptot (dynamic + static = total pressure) and static

4. connect TE probe to static on ASI, pitot on ASI to free air (= static pressure = ambient pressure inside your car

5. therefore ASI reads dynamic pressure (linearly proportional to airspeed below 0.3 M)

But, bear in mind that unless you hold you TE probe at least about 10 feet from you car the reading will be off quite a bit, and you should also worry about the ambient pressure inside the car. Not a good way to calibrate a TE probe. Even thought of investing in a wind tunnel?

Rgds,

Derrick.

"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

Hank Nixon wrote:
"tango4" wrote in message news:...
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

So, you are saying:

1. (TE pressure) = - (pitot pressure) yes?

But at the pitot the pressure = (static pressure) + (dynamic pressure head)

So according to your statement (TE pressure) = - [(static pressure) + (dynamic pressure head)]

OK, lets agree that the pressure sensed at the static port is (static pressure) = (ambient atmospheric pressure)

Then, connecting up an ASI in reverse:

(A) Pressure on static entry to ASI (connected to TE probe) = - [(static pressure) + (dynamic pressure head)]

(B) Pressure on pitot entry to ASI (connected to static vent) = (static pressure)

The ASI effectively subtracts the pressure on the static entry from the pressure on the pitot entry (normally this would result in it measuring the value of the dynamic pressure head which is proportional to airspeed - well, for us it is a fairly accurate measure of it)

So, we have on the ASI a reading proportional to (pressure on pitot entry) - (pressure on static entry)

Taking values from above this is
(pitot entry) - (static entry) = (B) - (A)
= (static pressure) - [(static pressure) + (dynamic pressure head)]
= - (dynamic pressure head)

This would probably damage your ASI if it was so.

Whereas, if the TE pressure = (static pressure) - (dynamic pressure head)
Then (pitot entry) - (static entry) = (static pressure) - [(static pressure) - (dynamic pressure head)]
= (dynamic pressure head)


Rgds,

Derrick.