What amount have people found is UNUSEABLE FUEL in these wings.  5
gals?  10 gals?  more (per side)?

I'll be exploring this topic soon, and would like to post the
question for others before I wait patiently for a dry tank.

Jeff,

 

Consider at least 4 conditions, three of them can be tested on the ground.

 

1. Level as in level un-accelerated cruise flight.

 

2. X degrees nose down attitude that might be used in straight line extreme descents with air brakes and perhaps gear extended.  This condition can be fixed temporarily by reducing X or switching to a fuller tank if the measurement device is accurate in such an attitude. 

 

3. X degrees nose up attitude used in straight Vx or Vy climbs.  See # 2.

 

4. Slip/skid extended flight to test the size and seal of the slosh chamber when the wing fuel is "low".  Most "ball out" flight is done at low power so the fuel draw is reduced. 

 

Items 1, 2 and 3 could be tested on the ground with the wings attached (hard) or a jig could be built to test just the wing by holding the proper dihedral and incidence.  Water and a pump could be used to find when the fluid would become un-ported.  Then it is just a matter of measuring how much fluid was present when the un-porting occurred.