REFLECTOR: Re: Reflector Digest, Vol 9, Issue 68-Fuel Feed-blinking low fuel warning

[Jim Sower]

<... a simple flow switch in each line from the mains would give you 
vital information about which way the fuel is going ...>
And it could cause enough resistance to flow (remember, we're talking 
head pressures of a few inches of /water/ and fractions of that 
generating assymmetric flow.  Your flow switch could cause (or certainly 
aggravate) the very problem you're using it to identify. 

Do the math - 5 in head pressure of gas = .164 psi; 7 in = .23 psi.  
That gives us a 12 gal differential in fuel level generates 0.066 psi 
head pressure differential to level them out.

That ain't much to work with and it don't take much to disturb it 
further or STOP it ... Jim S.

Carruth, Joel L wrote:

> Jeepers!  I've been listening to the problems with unporting, greasing 
> the o-rings, false sump indications over 10K, and "no way" of knowing 
> what situation your fuel is in, and frankly it leaves me a little 
> worried about my future fuel system!!!  On one hand, I tend to agree 
> with Scott Baker and don't change the design, but on the other, you 
> all have convinced me that there is room for some tweaking!  I just 
> don't believe in guessing about my fuel condition.
>
>  
>
> In particular, I plan to do away with the float switch and install a 
> fuel-level sensor in the sump. Secondly, a simple flow switch in each 
> line from the mains would give you vital information about which way 
> the fuel is going! This wouldn't be an expensive flow meter, but a 
> simple flow switch with indicator. I think Omega sells these for 
> around $130 each. Two switches, plus the fuel-level sensor would take 
> the guess work out of fuel management and allow the pilot to make 
> corrections as needed... before the fan goes quiet!!  
>
>  
>
> '96 SEFG
>
>  
>
> */Joel Carruth/*
>
> Lockheed Martin
>
> JSF - Pilot System Software
>
> W: (817) 763-4337
>
> Fax: (817) 777-8378
>
> joel.l.carruth at lmco.com
>
>  
>
> ------------------------------------------------------------------------
>
> *From:* reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] 
> *On Behalf Of *Jim Sower
> *Sent:* Thursday, February 17, 2005 9:02 PM
> *To:* Velocity Aircraft Owners and Builders list
> *Subject:* Re: REFLECTOR: Re: Reflector Digest, Vol 9, Issue 68-Fuel 
> Feed-blinking low fuel warning
>
>  
>
> <... The only way I could see that happening is if the flow from the 
> mains is somehow restricted due to inadequate venting, and that the 
> fuel pump is pulling fuel out the sump tank and the sump vent line is 
> letting in air into the sump lowering the fuel level. ...>
>
>
>       *Bingo!!*
>
> And once your sump fuel level falls low enough to light the low fuel 
> light, you never know how much usable fuel you have until that light 
> goes out.  The mains might be providing 95% of demand in which you'll 
> make it home with a gallon or two in the sump.  Then again, they might 
> be only providing 50% of demand in which case you flame out in under 
> an hour.  If the strakes are only providing 25% of demand, you've got 
> about half an hour of fuel. 
> And you have NO WAY of knowing which situation you're in until either 
> you get where you're going and LOOK at the sump, or the fan goes quiet.
>
> Go ahead and ask me how I know this ... Jim S.
>
>
> Chuck Harbert wrote:
>
> To John Dibble:
>
> This is very strange that it only occurs intermittingly at high 
> altitudes, unless it is just a intermittent bad electrical connection 
> or switch. Do you have the same type of low fuel switch that I have ( 
> mine is green and floats in the sump tank)? I installed mine on the 
> pilot side of the sump near the top. I didn't quite understand your 
> description and thot maybe it was a different type switch. Are you 
> sure the warning light itself isn't just loose in it's socket?
>
> I'm trying to understand what changes at higher altitude that would 
> affect the low fuel switch. The only thing I could think of was that 
> atmospheric pressure drops, so the sump tank could possibly expand a 
> little I guess. Do you have a sump tank vent line and the main tank 
> vent lines per plans?
>
> I'm assuming that you have pretty much ruled out the possibility that 
> the sump tank is partially emptying and that the low fuel switch is 
> working properly? The only way I could see that happenning is if the 
> flow from the mains is somehow restricted due to inadequate venting, 
> and that the fuel pump is pulling fuel out the sump tank and the sump 
> vent line is letting in air into the sump lowering the fuel level.
>
> Chuck H
>
>
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