Hi Charlie;
You are correct. I just return fuel through a very small heat exchanger to the inlet side of the FI pump. The Facet pumps also pump against this pressure with no problem, but I usually just turn them on during TO or landing. The fuel easily draws through them when off. I used to have a header tank much like Ed as it seemed a simple solution to my multiple tank fuel management issues, however it was also larger to provide a usable reserve. Didn't work out so well as it was prone to vapour lock. Unfortunately when I changed the system, I changed several things at once, so I can't say that removing the tank was the lone cure to my issues. As I also have a turbo which is in close proximity to the fuel rail, which even though it has the stock heatsheild still emits a tremendous amount of radiant heat which is easily absorbed by the aluminium fuel rail. Dave Leonard provided me with his old steel fuel rails which I used to make my new single fuel rail. The theory is that steel has far lower lower thermal conductivity than aluminum and since it is made of thinner material, IIRC, the weight was very close. Not satisfied with that I also made a small simple heat exchanger for the return fuel, then I installed an aftermarket fuel regulator located downstream of the heat exchanger on the cool side of the firewall, with the low pressure outlet returning into the feed line from the Facet pumps to the FI pumps. This replaced the stock fuel regulator that was located at the end of the fuel rail. My theory here is that by keeping the fuel at a higher pressure on the return from the fuel rail until after it has gone through the heat exchanger, it is much less likely to flash into vapour. As the fuel is regulated to a lower pressure, any possible expansion due to being close to vapour point does result in cooling of the fuel (think refrigerant) but is by then in a cool location so will be unable to absorb heat due to a greater delta T. Did that make a difference? I dunno? Not sure which one did, but it works well now, and I've been satisfied with it.
With that said, however. Since I do have it at home in a large heated shop, I have been tempted to consider changes, mostly because I'll admit it is still unconventional and I want to make certain that I have the safest, most reliable system possible, the next time I haul it out to the airport because these days I just can't see myself finding time to do anything other than flying it once I haul it out there. And I plan to be flying it with my son as my co-pilot. Kinda changes things a little.
Attached is a drawing of the current layout of the system. You may have already seen it in the archives.
Todd
C-FSTB
-----Original Message-----
From: Charlie England <ceengland@bellsouth.net>
Reply-To: Rotary motors in aircraft <flyrotary@lancaironline.net>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: high/low pressure pumps question
Date: Fri, 04 Jun 2010 12:24:09 -0500
Hi Steve,
Good info; I appreciate it. If it will at least transfer faster than the
fuel is burned, that might satisfy me. IIRC, the Facet pumps are
designed for continuous duty, so that really shouldn't be too big a
problem (there are H-series Lycs flying on RV's with just 2 Facets; no
mechanical pump).
I've considered using a modified selector from a dual tank truck, but
I've got 2 aux tanks in addition to the stock pair. My present plan is
to use Van's 3-input selector to feed the transfer pump, & leave the
main to feed the fuel pumps.
I've also thought about T'ing the transfer line into the main tank's
supply line, but that's bad news when the Facet sucks the selected aux
tank dry.
I believe that Todd Bartrim is just returning his fuel through a small
heat exchanger to the inlet of the pump. That idea is tempting, too.
The really tempting idea is to simply regulate the pumps using a
pressure sensor & a PWM power supply.....
On a different subject: have you considered using your programing skills
to expand/modify your 'copilot' software a bit? I just saw a 7" screen
'netbook' running Windoze CE on ComputerGeeks for $80. A/D converter
chips are really cheap now, & with your skillset, a PDA based engine
monitor using off-the-shelf sensors should be easy. (hint hint) .
Charlie
On 6/4/2010 12:24 AM, Steven W. Boese wrote:
> Charlie,
>
> I initially set my fuel system up with a Facet low pressure transfer pump tee'd into the return line. This system required that the Facet pump run 50% of the time in order to keep the fuel tanks balanced. Others have done this successfully. I didn't like having to run the Facet pump this much and eventually went to a duplex valve of my own design and eliminated the transfer pump altogether. In my case, I had the return flow from the engine turning 90 degrees inside the tee where the transfer flow was added. Having the return flow straight through the tee and the transfer flow joining from the side may have worked better, but I didn't try this.
>
> Steve Boese
> RV6A, 1986 13B NA, RD1A, EC2
>
>
>
> ________________________________________
> From: Rotary motors in aircraft [flyrotary@lancaironline.net] On Behalf Of Charlie England [ceengland@bellsouth.net]
> Sent: Thursday, June 03, 2010 8:58 PM
> To: Rotary motors in aircraft
> Subject: [FlyRotary] Re: high/low pressure pumps question
>
> Hi Ed, thanks for the info. Yours was actually the only photo I could
> find during a quick search. I'm guessing that the regulator will be
> bypassing 6 or 8 times the amount of fuel the engine consumes, so that
> would seem to be at least twice the volume the Facet is capable of
> pumping. If no one has plowed this ground yet, I suppose that some
> experimentation is in order.
>
> Charlie
>
> On 6/3/2010 6:31 PM, Ed Anderson wrote:
>
>> Hi Charlie,
>>
>> All my pumps, but the Facet, are on the forward face of the Firewall,
>> so can’t help you there.
>>
>> Regarding “T” the Facet pump, Pressure will generally overwhelm
>> volume, you can think of a large mass of slow moving fluid as being
>> effectively a stationary pool of liquid (taking it to the extreme of
>> no motion), so a higher pressure fluid would simply squirt into the
>> larger volume liquid similar to as it if were a large tank). Yes, if
>> the larger mass were moving at a much higher velocity, it might in
>> effect offer a shearing force across the opening to the facet pump and
>> have detrimental effect (possibly) – but I doubt you are going to have
>> that kind of fluid velocity in your return line.
>>
>> Ed
>>
>> Ed Anderson
>>
>> Rv-6A N494BW Rotary Powered
>>
>> Matthews, NC
>>
>> eanderson@carolina.rr.com
>>
>> http://www.andersonee.com
>>
>> http://www.dmack.net/mazda/index.html
>>
>> http://www.flyrotary.com/
>>
>> http://members.cox.net/rogersda/rotary/configs.htm#N494BW
>> <http://members.cox.net/rogersda/rotary/configs.htm>
>>
>> http://www.rotaryaviation.com/Rotorhead%20Truth.htm
>>
>> ------------------------------------------------------------------------
>>
>> *From:* Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]
>> *On Behalf Of *Charlie England
>> *Sent:* Thursday, June 03, 2010 5:22 PM
>> *To:* Rotary motors in aircraft
>> *Subject:* [FlyRotary] high/low pressure pumps question
>>
>> A couple of questions:
>>
>> Have any of you RV-x builders considered putting your pumps on the aft
>> side of the spar, to keep the cabin& engine compartment 'clean'? It
>> looks like it would be possible, by using the existing holes for wire
>> in the center section (RV-7). If not, are you just putting them under
>> the center cover between the seats& raising it up a bit?
>>
>> Also, how about running the transfer (Facet) pump's output line T-d
>> into the main pump's return line? There should be minimal pressure,
>> but will the high pressure pump's much larger volume overwhelm the
>> modest Facet transfer pump? (Or, maybe venturi effect would eliminate
>> the need for a Facet....)
>>
>> Thanks,
>>
>> Charlie
>>
>>
>
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