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From: "Ed Anderson" <eanderson@carolina.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-541230@logan.com>
Subject: Re: [FlyRotary] Bubbles in fuel line from pump
Date: Sun, 21 Nov 2004 22:57:57 -0500
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Hi Finn,

    Just a shot in the dark, but what kind of fuel flow transducer is in
your line?  Could your transducer blades be causing cavitation in the fuel
line??

Ed
----- Original Message -----
From: "Finn Lassen" <finn.lassen@verizon.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Sunday, November 21, 2004 9:39 PM
Subject: [FlyRotary] Bubbles in fuel line from pump


> I'm near wit's end trying to find out why I get bubbles in the line from
> my facet pump to my carburators.
>
> To recap my setup:
> Fuselage fuel tank -> fuel selector valve -> fluel flow sensor -> G3
> automotive paper filter -> to drain point -> T-split  to each fuel pump.
> Separate lines from each fuel pump to carbs. Three carbs. The two outer
> carbs (sharing one pump) each feed a secondary port. The center carb,
> with its own pump, feeds the two primary ports. One throttle to the
> outer carbs, one throttle to the center carb. When have climbed above
> pattern altitude I turn off the pump to the center carb, ready to reduce
> center carb throttle when all fuel has been burned out of the bowl, and
> use center carb throttle to lean mixture. Output from pumps near
> fuselage bottom slopes up to carbs which are appox level with center of
> fuselage tank.
>
> Bottom of fuselage tank about 16 inches above the facet fuel pumps.
> When removing the quick-drain valve, the system gravity feeds about
> 17-19 GPH through the selector valve, flow sensor and filter.
>
> Various observations:
> Idle fuel pressure on ground about 3.8 - 4 psi.
> Max power fuel flow near sea level at full power only about 13.5 GPH. 10
> GPH at altitude.
> Fuel pressure to outer carbs is roughly inversely proportional with
> observed amount of gasses (air?) in the line from the pump to the outer
> carbs.
> Engine wants to quit when pressure drops to about .1 or .2 psi. (Only
> happened one time after a continous full power climb to 10,000' to get
> over clouds. Pressure dropped steadily to that point.)
> Removed flow sensor and filter and amount of bubbles appeared to be
> significantly less.
> Put sensor back in, and bubbles reappeared, inconsistently.
> Have mostly observed fuel pressure drop at high power climb attitude.
> Seems to get worse at higher altitudes.
> Have checked tank vent (used to be shared between the three tanks, but
> as part of debugging now goes only to fuselage tank). Vent is pointed
> forward, mounted below fuselage, center, just forward of main wing spar.
> On the last test flight today, I continued to see bubbles, even after
> reducing power for several minutes and levelling off. Were at 7,500'.
> Pressure about 1 psi. After a dive to get down below Class B 6,000'
> floor, pressure came back up to above 3 psi.
> A few minutes after landing I noticed that the fuel line to outer carbs
> appeared empty. Turning on the pump for a few seconds filled it up.
> After turning off the pump I noticed that the line emptied out. How can
> this be? By gravity feed alone, the fuel level should have been a couple
> of feet up that line. I did not delect any gas leak.
> Maybe 10 minutes later, the line stayed full after turning on and off
> the pump (or was now full - not sure).
>
> This brings me to questions about vapor lock. I actually don't know what
> that is. Carb bowls are vented into airbox. So how could the fuel in the
> bowls be under pressure, maybe boiling, and push gasses back down the
> fuel line to the pump? Unlikely? I do have a SS heat shield between
> carbs and exhaust headers. Does its ability to reflect heat lessen if it
> is no longer shiny?
>
> I guess the next step is to replace all rubber lines going to the pumps
> with transparent ones so I can see what's going on the the lines to the
> pumps.
>
> But other ideas are certainly welcome. Up till the last observation,
> I've been trying to locate air leaks in lines to pumps and was ready to
> replace the brand new facet pump with an old one, in case it somehow is
> sucking air.
> Now I'm mystified.
>
> Finn (please no jokes about wit).
>
>
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