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On 2/8/2018 2:50 PM, Finn Lassen finn.lassen@verizon.net wrote:
Working on wing tanks for my RV-4.
Pickup lines are spec'd at -6 (3/8") in Van's plans.
Looking at a Walbro pump I replaced in my (Rusty's) RV-3B the inlet appear to be -6 but outlet is -4 (1/4") (fitting thread size -- so may actually be less).
Rusty carried -6 all the way to fuel injection rail and back to fuel regulator, but -4 back to tank from fuel regulator.
Also appears he used -4 from left wing to right wing fuel transfer. I'm using a 40105 Facet pump as transfer pump which appears to have 1/8"-27 INT fittings. It does seem to pump less than 30 GPH, but is pumping into same line as pressure regulator return line to tank. In the RV-4 I intend to have separate fittings in the right tank for fuel return and for transfer from left tank.
So the question is: is there any valid reason to have more than -4 lines after transfer pump and after high pressure pumps and filters going to the engine (and back). Higher power draw by pumps because of pumping into -4 lines instead of -6?
Obviously it'd be easier, less weight and less $ to use - 4 lines.
Finn
P.S. I think Tracy is running from the left tank, but I think advantage of running from the right tank is keep lines on the "cool" side of the engine. Probably minor.
Hi Finn,
Here are a few data points from 'the other world'. One of the most reputable Lycoming engine rebuilders says that he uses -4 (1/4") lines *after* the engine driven fuel pump on their fuel injected engines (~25-30 PSI), to minimize the volume of fuel that's near the hot engine. These are 'returnless' injection systems; the fuel deadends at the injectors. Now, everything up to the mechanical pump is -6, to minimize pressure drop (and risk of vapor formation) with the pump 'sucking' on the line.
IIRC, the plumbing around the engine on rotary cars is what amounts to 5/16" lines; perhaps someone else can verify.
My original plumbing on the -7 (not yet flying) had -6 up to the firewall, and -4 to the engine. I followed Tracy's lead on the -8 by putting the mechanical regulator right after the Walbro pumps, inside the fuselage, with a -6 bypass line returning to the main tank. The pressure reference to the regulator still gets plumbed to the intake manifold, of course. Logic for the -6 return line is that the Walbro pumps 4 to 5 times the quantity of fuel used by the engine, so I didn't want back pressure in the return line to affect pressure delivered to the engine. I ended up T-ing the transfer line from the other tank(s) into the regulator bypass line, so I'm now really glad I used -6 for the bypass/return. Logic for keeping the regulator aft of the firewall is that pumped, unused fuel never sees the heat of the engine. If you do the math, the FWF fuel is always at ~40 psi, so it'll be virtually impossible for it to flash into vapor before it exit an injector. On the other hand, with the regulator on the engine's fuel rail, the regulator is acting like the expansion valve in an air conditioner circuit, guaranteeing that the returning fuel will collect engine heat.
I did change the FWF line from -4 to -6, but only because I decided to put the final filter (same filter Ed Anderson & others are using) ahead of the firewall, and the only A/N adapters for it are -6.
I picked the right tank as the 'main' because the -7 has side-by-side seating, and I figured that when I'm solo or when Tupper's flying right seat, moving fuel to the right tank will balance a bit better. After running the lines, I almost wish I'd made the left tank the main, because plumbing would have been a bit simpler with everything on the left side of the plane.
The Walbro pumps that RWS were selling had 10 mm threaded fittings on both ends. You can probably get A/N adapters in sizes other than -6, but I definitely wouldn't go smaller than -6 on the source side, to avoid 'vapor lock' on the input side of the pump. As you probably know, smaller line does offer more resistance to flow, and any bend in the line can add the equivalent of several feet of line (in flow resistance) to the length.
I'd be surprised if there's enough cost difference between -4 & -6 line & fittings to make a perceptible difference in the finished price of the plane. And you could probably get it all back by buying the fittings on ebay instead of the usual suspects. Not sure about buying aluminum line itself from unknown sources, but the fittings I've bought seem to be just fine; I wouldn't be surprised if they come out of the same factory as the high-dollar stuff with USA branding on it.
As I said above, If I were going small, I'd go small on the delivery side and stay big on the return and supply sides.
FWIW,
Charlie
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