REFLECTOR:Low fuel light

[Scott Derrick]

I think  you guys are fixing something that ain't broke.  But as long as 
you have the time go for it.

Scott


John Dibble wrote:
> Yes, I agree that the  pressure will be equal, under static conditions.  
> The problem is keeping the pressure equal under dynamic conditions like 
> climb, descent, and leaking caps. If connected directly to the manifold, 
> the sump pressure will change faster than the tanks because there is 
> very little air space in the sump.
> 
> John
> 
> Chuck Jensen wrote:
> 
>>  John,At the risk of interpreting what you wrote, I believe you are 
>> concuring with my earlier post; that being, if the tanks and sump were 
>> all connected to the same vent-line (a manifold if you like), then the 
>> pressure in both tanks and sump, by definition, must be equal.   In 
>> your case, you were proposing to vent the sump to each of the tanks, 
>> though this might allow water/air traps, depending on configuration.  
>> I would be more comfortable with each being vented to a collection 
>> manifold.With a common vent manifold, it really matters little whether 
>> the atmosphere above all the tanks/sump is slightly positive or 
>> negative; the draw of the fuel pump will overwhelm any slight negative 
>> pressure that could hypothetically be present.  And, since fuel 
>> responds to gravity, I would think it would dependably flow to the 
>> sump from both tanks and the sump would always be 100% full. After 
>> all, how could a pocket of air exist in a vented sump???(that'll start 
>> another round!) There would be a vertical column of fuel in the sump 
>> vent line that rises up to the equal height of the fuel in the 
>> strakes.  This fuel column would need to let any air introduced to the 
>> sump (ie. sharp descent which uncovers the tank(s) outlet) ,pass out 
>> the vent.  Of course, if the strake outlets are uncovered and fuel is 
>> drawn by the engine from the sump, the fuel column in the vent line 
>> would empty into the sump.  When level flight resumed, the air in the 
>> sump would be pushed out the evacuated sump vent line by the ample 
>> fuel flowing from both tanks. Just to be sure, if there was any 
>> question of the ability of air to vent through a fuel filled 3/8" vent 
>> line (and Al's earlier posting indicates there probably isn't), a 1/2" 
>> vent line could be used instead to rise vertically from the sump to 
>> the common vent manifold, which is connected to the other two tank 
>> vents.And, this configuration certainly couldn't siphon from the sump 
>> tank, as experienced by SB during his test flight (when the backward 
>> facing sump vent drew a vacuum and started a siphon from the sump 
>> overboard), since siphoning depends on a hi/lo differential and there 
>> is none because all of the tanks are the same pressure.  No more sump 
>> sloshing.  No more concern about a leaky fuel tank cap.  No more 
>> unbalanced tank feeds from different pressures in the tank strakes 
>> from variation in pressurization from the vent line or leaky fuel 
>> cap......unless the plane is unbalanced in cruised, but that's a 
>> different problem and different post. Simple, no?Chuck
>