Well that stirred up a bit of interest, which encourages me to add a bit more
of the background...there is really quite a lot of physics to be thought
about and I have done some experiments to verify some of the effects.
RonSwinden@aol.com writes:
<< Will the air column in the "pitot" line gradually "disolve" into the
petrol >>
The "dissolving" stops at saturation, so firstly- is there a difference in
behaviour between petrol free air and one saturated with vapour ? There
probably is, but as long as it is constant it would not matter. Very little
volume of liquid is required to saturate the air in the tube (22 litres from a
gram.mol if I remember rightly) so even if it came from the surface of the
fuel which first seals off the tube, the level change would not be seen.
Secondly the tank is likely to have unusable fuel in the bottom and thus the
tube will be full of saturated air even if you start from empty. But I would
need notice of how long it takes for the air in the tank to saturate the
volume in a brand new filling ! From the little time it takes to smell fuel
when you open a can, a few seconds is a guess. Certainly the "pitot" air is
never renewed until the tank is emptied again, so the connections must be
completely leak free. Yes it could be re-zeroed with a bulb to blow it back
until it bubbles into the tank, but doubt if this complication will be
necessary. I have had the system reading a constant value for several weeks
now.
Peter Lert wrote:
<>
-and Jerry had the same point.
Shouldn't do. Head is developed from mass and gravity, and the difference in
pressure between surface and bottom would be the same in a vacuum. I applied
suction to the vent/static in my test tank to verify this, but it pulled in
the sides of the plastic container I was using !. Pumping it up didn't change
it though, which comes to the same thing.
---From Jens-Achim Frei..
<<sounds great! But did you put in count that the static pressure difference
between "full" and "empty" may be compensated by the factor of ten or more
by dynamic forces on the fuel while taxiing or in flight under bumpy
conditions? Without any shock absorption you won't get a useful indication,
I guess ...>>
My experiments were only done on the bench, but I am sure you are right. But
in the same way a the sight gauge needs a restrictor to stop slopping about,
so would this.
I have looked at two geometries: Firstly a horizontal tube from the bottom of
the tank (or the drain line). I was thinking initially that the changing
occupancy of the pressured "pitot" line meant it had to be maintained
horizontal - but then realised that this is only a calibration phenomenon, not
an error. It is more convenient just to provide a double entry in a plate at
the top of the tank, with one pipe going down to the bottom.
I also calculated the error which would arise due to changing temperature of
the air in the pitot line, but surprisingly I have not able to see this by
playing a hair dryer on the tubes. In practice the fuel tank, like the cabin
,will not change much, at least while the pilot keeps himself comfortable.
Incidentally once the tubes have left the tank, they can be very small indeed,
and run through a protective tube together. I hope persuade the PFA the
system is safe by siting the instrument in the passenger headrest, behind an
armoured window if they insist.
Graham C G-EMIN
|