Why are TE probes so long?

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.