Thanks to those who took the trouble to reply.
The correct diagram is attached.As was pointed out you don't want the TE line connected to the static. There will be a flow through as TE is below static pressure by the same amount that pitot is above static pressure. In this case all the instruments connected to TE and static see some pressure between TE and static. i.e lower than the static pressure but higher than the correct TE pressure. This results in the altimeter over reading, the ASI over reading and hopelessly under compensated variometers.
Paul Mander pointed out the problem with switching between static and TE source for the varios, particularly the mechanical type (Winter, PZl, Sage) which will see very high rates of climb and descent which can be damaging. Most electronic varios won't care and our B300 up series don't as even the pointer mechanism won't be damaged and the sensors can be used as poor microphones. Roger Druce came up with a better scheme and that is to switch the TE between the raw source and a highly damped one (inset on attachment). This looks to be a much better idea. I must say that I never had a problem with the Ventus TOP or Nimbus 3DM where I just ran the usual TE connection. If the vario moved during climb I just checked that the string was still in the middle.
Note that when you connect your ASI to pitot and static these sources must be the ones specified in the flight and maintenance manuals as the limiting speeds are in INDICATED airspeed i.e. what you see on the ASI. Some gliders can have errors of several knots in these sources. Schempp gliders in particular have airspeed systems that under read at low speeds and over read at high speeds making you think the glider circles nice and slow and really goes well at high speed. You do however have to use these for the ASI. Paul Bikle (former boss of NASA Dryden Flight Research Center and glider world altitude record holder for many years)used to reckon leaving the altimeter in a glider on cockpit static was OK. You could use the flight manual static or a better idea is to use a Prandtl probe (combined Pitot and static probe) in the leading edge of the fin and use that static source. Many tailboom statics will fill with water during water dump so I'd avoid these unless they are shown not to fill with water by testing. Prandtl probes or triple probes (combined pitot, static and TE) are commonly fitted nowadays. If you don't use all the connections do remember to plug the unused lines from the fitting otherwise you will find you are running on cockpit static. For advanced varios and flight computers, using the Prandtl probe is by far the best idea. The errors will be less than 1%. This will give you correct netto and speed to fly and correct True Air Speed so you get accurate wind information.
This will even be required on our upcoming TE/netto system that is insensitive to horizontal gusts. Other sensors are involved too but a good TAS measurement is essential. We've flight tested the sensors and they work. Provisonal name is DYNAMIS The lack of noise in this system means a faster response is possible. All current TE vario systems need to be slowed to time constants of around 2.5 to 3 seconds to produce comfortable and useable results. I think we can use 1 second and what you see is nothing but the airmass vertical motion not a combination of airmass vertical motion and horizontal gusts. It doesn't care about bugs on the wings, if the brakes are open or the engine is running (provided the pitot/static isn't in the air stream from the prop or jet). Even works in turns. This should help to easily pick the best air between thermals and which "thermals" are real thermals and not the horizontal gusts that cause us to think there is a thermal with present TE systems.
MikeBorgelt Instruments - design & manufacture of quality soaring instrumentation since 1978
www.borgeltinstruments.com tel: 07 4635 5784 overseas: int+61-7-4635 5784 mob: 042835 5784 : int+61-42835 5784 P O Box 4607, Toowoomba East, QLD 4350, Australia
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