On Friday, June 30, 2017 at 11:47:55 AM UTC-7, Garrett McEwen wrote:

There are so many shapes and sizes and I've always
wondered how and why that came to be and what
advantages and disadvantages they each carry.

The classic TE probe carries only one pneumatic signal from an orifice on the aft-facing surface of a cylinder. I think that the signal is essentially the inverse of pitot pressure. When plumbed to a vario instead of a static port, it cancels out climb or descent effects due to changes in the glider's inertial state caused by pitching up or down. The effect is to make the vario display the net gain or loss of total (potential plus kinetic) energy state.

More modern probes may also feature orifices positioned to measure static air pressure and even pitot pressure. Positioning these orifices on the probe in clean air out away from the glider makes it possible to get more accurate measurements of these pressures. The downside is that such probes can be difficult to retrofit to older sailplanes. They are also a bit fussy about connecting or disconnecting the probe itself during assembly and disassembly. The O-ring seals have finite lifespan, and when they start to leak the different pneumatic signals corrupt each other, often with confusing or misleading symptoms.

As for the TE probe orifice, there are a variety of claimed and illusory advantages for different shapes and patterns--holes, pairs of holes, slots and pairs of slots, ad infinitum. Oran Nicks, working with Dick Johnson, demonstrated that a single hole of diameter 1/3D works about as well as anything. When it comes down to it, all TE orifices suck.

--Bob K.