REFLECTOR: Engine failure causes

nmflyer1 at aol.com nmflyer1 at aol.com
Mon Apr 6 22:38:34 CDT 2009


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.
irst, would be as you point out to make the fuel shutoff valve a cable
perated valve.  Second, rather than pick up off the bottom of the sump,
nd loop over the gear bulkhead, I would drill a hole near the bottom of
he bulkhead and route the fuel to the pump with little height change.
hird, I would ensure the fuel return flow doesn't compromise where I
ick fuel up to the pumps.
Al Gietzen wrote:
 Prime suspect #1: Fuel starvation due to fuel shutoff valve 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 amoun
t 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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