Why are TE probes so long?

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

"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

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

Hank Nixon wrote:

Easier to learn not to bend probe than try to make TE system work with
stubby probe.

Last year, our Astir lost its TE probe due to a rough landing. We did not
find it and invented a replace from an old glider that was out of service.
We had to bend it to make it fit to the Astir. After this had worked fine
for several weeks, we found the original probe laying in the grass just
beside another landed glider, in perfect shape. So we could replace it
again! But the bended probe had done a good job.
Eggert

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.

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.

Derrick Steed wrote:
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)

Should be: "=(static pressure) - [-(static pressure) - (dynamic
pressure head)]"
= 2(static pressure) + (dynamic pressure head)

Still the wrong result. I think, when people refer to TE probes
providing minus the pitot pressure, they are talking "gauge pressure"
not "absolute pressure"
i.e. the pressure difference from static, such as is measured by most
pressure gauges. (e.g. the gauge on your service station tyre
inflation device)
Cheers, John G.

Yikes, my head hurts!

Static pressure is simply that. The static pressure of the ambient
air around you. A good aircraft static system should give you the
same value for any given ambient pressure (ie altitude) no matter how
fast your glider is going.

Dynamic pressure is the pressure due to velocity and it varies by the
square of the velocity (ie twice the velocity = four times the
pressure). The ASI measures the difference between the dynamic
(pitot) pressure and the static pressure to give velocity.

Keep in mind that the static pressure at the pitot and at the static
port are the same and cancel each other out in the equation (unless
like most of us you have a horrible static system).

The difference between the pressure at the TE probe and static
pressure should be the same as the dynamic pressure but of opposite
sign. Again static pressure at both locations is the same so it
cancels.

I have driven many miles with an automobile test rig consisting of a
pitot, a static and a TE probe connected to a pair of side by side
U-tube water manometers. The manometers share the same static and one
uses the pitot signal while the other uses the TE signal. A properly
calibrated TE probe will cause exactly the same pressure difference as
the pitot, just with the opposite sign. The mounting bracket for the
probes hung out way in front of the car to get clean air for the
testing. I must say it got quite a few stares.

In practice I usually build TE probes to overcompensate slightly to
make up for poor static systems. The probes are easy for the user to
tame down slightly and they can be tailored by the individual pilot to
match their preference and sailplane.

"Craig Funston" wrote in message
om...
Yikes, my head hurts!
snip

Dynamic pressure is the pressure due to velocity and it varies by the
square of the velocity (ie twice the velocity = four times the
pressure). The ASI measures the difference between the dynamic
(pitot) pressure and the static pressure to give velocity.


No. The ASI measures the dynamic pressure as the difference between the
total pressure and the static pressure. The pitot measures total pressure.