Why are TE probes so long?

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.

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.

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?

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.

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:

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.

(Craig Funston) schrieb:

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.

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?
Eggert

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.

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?
Eggert

Eggert, right. You are correct and some electronic instruments do just that
- they take the pitot and the static (instead of static and TE) and subtract
them electronically, and it's not just mechanical instruments which use TE,
the B50 has three inputs: pitot, TE, and static.

A TE probe isn't the only way of compensating for airspeed changes, ancient
glider pilots used diaphrams to achieve the same thing.

I sent some notes to Eckhard yesterday relating to how you might try and
achieve the calculation in Cumulus.

Rgds,

Derrick.