I do agree with Martin. I use my PDA (Palm M515 with soaringpilot sw.)
as a short time averager (1-1.5 sec delay) also and it has GPS input
only.
I don't think anyone really follows speed to fly values so accurately
that it would worth to build such a complex system (feed PDA with TE).
At least for me it's enough to to know if I should slow down to 120km/h
ot push over 160km/h in a stdCirrus
Regards,

/Janos



Martin Gregorie wrote:

You may be over-complicating things. The following assumes you prefer
cheap toys to expensive ones.

A simple GPS + PDA system can handle all your navigation and glide
calculations independently of the rest of your instruments. Granted,
the GPS altitude is less accurate than position, but probably still
good enough for FG calculations given all the other uncertainties and
happenstances during the final glide.

All you really need on top of that is a simple audio vario such as a
B.40, Tasman, or even an ancient Cambridge Mk 4. These are all
stand-alone instruments, so no additional interconnection is needed.

For STF, it again needs TE from a vario or flight computer,
but I suppose it could fake it as above.

If you have room for a second vario, fit a mechanical unit with a
Macready ring.

A club mate uses exactly this type of rig for club class competition
in his ASW-19.