I would like to open up for discussion another issue relating to the fuel
vents. I believe that it will be better to vent the sight gauge to the top of
the tank rather than to an external vent.
My reasoning is this:
The height of the liquid in the sight glass is dependent on the pressure at
the base, and the pressure at the top of the column of fuel. It will only
show the correct height if the pressure at the top of the fuel in the sight
gauge is the same as the pressure above the fuel in the tank (oh, and of
course if the plane is level..) If the top of the sight glass is vented to
the airflow, the pressure at the top of the column will not be the same as
the top of the tank when either the speed of the aircraft changes, or the
external pressure changes due to rapid climb or descent. This is due to the
large volume of air above a partly full tank taking some time equalise with a
different pressure at the vent 'pitot' head.
I intend to vent to top of the sight gauge to a connector on the polythene
part of the filler tube, which is always going to be at the same pressure as
the top of the tank.
Am I right in this thinking ? I also think that the only sensible place for
the sight tube is as close to the tank as possible - level with the back of
the seat. Not so easy to see but far less susceptible to errors due to pitch
changes.
Thanks also for all the comments about seat foam; very useful.
Roddy Kesterton #220
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