REFLECTOR: fuel flow sensor

Scott Derrick scott at tnstaafl.net
Tue Apr 7 07:12:39 CDT 2009


The fuel flow sensor needs to be between the fuel controller spider. 
Thats the normal place to put it as all certified planes return fuel to
a sump tank.

Scott

nmflyer1 at aol.com wrote:
> Brian,
>
> My exit line from the sump goes thru my flowmeter, then through a Low
> hole in the forward gear bulkhead. Then through the filters to the
> pumps that sit on top of my Lower NACA scoop.
>
> When I was pondering a return line to the sump, I thought about
> running it through a down=tube in the corner of the sump, away from
> teh pickup line. Seemed to make sense to me, until Airflow told me not
> to bother as I would not have a problem with their design (previous
> post to Al).
>
>
>
>
> -----Original Message-----
> From: Brian Michalk <michalk at awpi.com>
> To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
> Sent: Mon, 6 Apr 2009 8:43 pm
> Subject: Re: REFLECTOR: Engine failure causes
>
> Al,
>
> Good comments.  For prime #1, I envision three fixes.
> First, would be as you point out to make the fuel shutoff valve a cable
> operated valve.  Second, rather than pick up off the bottom of the sump,
> and loop over the gear bulkhead, I would drill a hole near the bottom of
> the bulkhead and route the fuel to the pump with little height change.
> Third, I would ensure the fuel return flow doesn't compromise where I
> pick fuel up to the pumps.
>
> Al Gietzen wrote:
> > Prime suspect #1: Fuel starvation due to fuel shutoff v
> alve arrangement.
> >  I still need to inspect the hose for evidence of kinking.  The fuel
> > return hose from the pressure regulator did not have fuel in it during
> > post crash inspection.  My theory is that my aggressive climb at 90 kts
> > resulted in the shutoff valve being higher than the level of fuel in the
> > strakes.  This could be a combination problem coupled to other theories
> > I have.  A vapor bubble might have gotten to the fuel pump, and caused
> > it to lose prime, and then cavitate when I lowered the nose.
> > 
> > Brian:  I don’t like the idea of having that line come forward to the
> > valve, and back. Although normally not likely that the level of the
> > valve getting slightly above the level in the strake would result vapor
> > bubble.  But how big is the line? Pressure drop in that line and valve
> > could result in vapor lock when the pumps are drawing high flow rates.
> > In addition, fuel returning from the engine is warm, and warming the
> > fuel in the sump. Now you have warm fuel being pulled against a flow
> > restriction.  Not good. You want minimal restriction  between sump and
> > pump  You might consider putting an emergency shutoff back at the sump
> > with a pull cable on a 90 degree ball valve.  Works for me.
> > 
> > Theory #2: Foaming fuel in sump tank
> >  It might be possible, but unlikely that the sump tank became full of
> > foaming
>  gasoline.
> > 
> > I think this very unlikely; that’s a lot of foam.  Yes the flow is high,
> > but so is engine usage. I have FI pressure regulator bypass also
> > returning to the sump.  There was never enough foam to make the low fuel
> > float switch activate.  The fuel flowing in from the strake will force
> > the foam out the vent.  I did find evidence that some small amount foam
> > was going out the vent line, so I installed a small separator ‘tank’
> > replacing the vent manifold – problem solved.
> > 
> > Unlikely theory #3: Debris in the tanks clogging filters
> > I pulled the filters and inspected several weeks before flight.  They
> > were clean, and this is the first time since building the plane and
> > doing all taxi testing and ground runs that I had inspected the
> > filters.  The tanks were found to be very clean after the building process.
> > 
> > Of course you’ll check the filters.
> > 
> > All the best,
> > 
> > Al
> > 
> >  
> > 
> > 
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