On 8/31/2017 7:57 PM, Todd Bartrim wrote:

I'm looking for some peer review on this fuel system to either validate my choices or tell me I'm a fool! But first I should explain some things.   

Some may notice that the regulator return line is routed to the filter which is unusual. I had intended to route this back to one of the main fuel tanks, similar to what Dave has done in his diagram. However when I found this Parker filter housing, it has 2 inlets and 2 outlets plus an extra port which was plugged. This port is plumbed to the inlet side of the filter and could be used as a vent or inlet. So I decided to try it as a regulator return. 

So far all ground tests show it to work well. The only exception was as I've previously mentioned, after an hour of heat soak, which likely would be the same regardless of where I    returned it to. My reasoning on this is that with vapor in the hi-press pumps I can't produce the 45 psi required to allow the regulator to open and pass the fuel to any destination. routing the excess fuel from the regulator back to the tank would certainly mitigate this problem, but would it eliminate it?

   Why I like the momentary-on purge system is it can rapidly clear air/vapor as there is no significant back-pressure, so even the Facet boost pumps (7psi) (mounted in a cool location) can easily clear air/vapor. So with that in place, is there any reason to route the regulator return back to a tank?

    To give some idea as to my thought process on my fuel system, I should give some background as to how I arrived at this current point. Back when I started my plane, I was inspired by crazy bloke from down under that was flying his RV-4 around the world. Not once, but 3 times. First going east, then going west, then going north and south. Some of you will remember Jon Johanson. Well like most builders they start with less grandiose ambitions and build an airplane with far less fuel capacity than is required for an adventure like this. Then as their horizons expand they are forced to add fuel capacity, usually in less desirable locations that result in a dangerously aft CG and lots of fuel in the cockpit. Max, just went through this. 

   Now at that time, I said that I wasn't necessarily planning on following Jon's path, but that I wanted to ensure all options were open. Now all these years later, it seems much less likely than ever as so many responsibilities keep me closer to home, but I am not ready to give up on my design philosophy. 

   Before I began my wing construction I made the decision to make the entire leading edge wet as well as the tips as this, while it increases my GW, does not result in a dangerous CG. It also keeps all of the fuel out of the cockpit and has far better span loading. But it also results in 6 tanks that I have to manage, which any way you slice it is going to have a complex fuel system.

   Right from the start and even today I'm aware that a large portion of GA accidents involve poor fuel management. So simple is better. So why, in the face of other proven alternatives, am I pursuing an apparently needlessly complex fuel system????

    At the risk of increasing complexity, I wanted to ensure that no single failure or leak would prevent me from accessing fuel on-board. Loosing fuel is bad, but not being able to access fuel on board is even worse. I didn't like the idea that a failed transfer pump could result in having a tank full of fuel that I couldn't use. Not only would that leave me with not enough fuel, but I'd still be packing the weight of it, likely also be quite unbalanced, requiring significant trim to correct, increasing drag, thereby decreasing range. And any resulting forced landing (assuming it's not over water) would still have fuel to feed any possible fires.

     I know that significant fuel tank leaks are rare, but that was something else that concerned me, even though now looking back it really wasn't a legitimate concern. But at the time I wanted to avoid the possibility of returning fuel to a tank that could be leaking, thereby reducing my range greater than if I'd just lost that tank alone. The more reasonable concern would be returning fuel to a full tank and loosing it out of the vent.

   So you can see that my design was driven by my fear of being over water or inhospitable terrain without enough accessible fuel to reach a safe destination. This is why I have optical fuel sensors to allow me to run a tank bone dry and know the moment it does so. So far my test experience has shown that any air in the system from this is quickly cleared through the injectors of a running engine, however, the purge system ensures this can be rapidly vented if required. The vent system has a manual isolation valve to back up the solenoid valve, to ensure that a valve leak doesn't result in insufficient fuel pressure along with a loss of fuel. This could and probably should be routed all the way back to a tank. My choice to tie into the vent system, stems from an conveniently accessible capped tee in the vent line that was originally used to vent my long ago removed header tank.

    And my valve arrangement will minimize any risk of not being able to access any fuel. And it will keep all tanks isolated from each other. So if it becomes clear that a tank is loosing level on it's own, that tank can be selected to use the remaining fuel before it is all lost, then the tank is isolated and not a risk to loose fuel from another tank. Not returning fuel to any tank will eliminate the risk of loosing fuel through overflowing through a vent. Now this one is easily mitigated by use of a timer and well, just being a responsible pilot!

   Now with my reasons for this design and resistance to adopt a simpler, proven system design explained, I am open to comments, criticism, etc. At this point I'm ready to haul it to the airport and would really rather not make any changes, but since the continued forest fire situation here is still keeping our airport too busy to accommodate any flight testing that I'd like to do, if I have to make any changes I'd rather do it now.

Max's recent adventure has only strengthened my resolve to take this bird on at least a few long distance adventures.

Todd.      Gonna have to learn to pee in a bag! :-P

 

Todd Bartrim

On Thu, Aug 31, 2017 at 2:51 PM, Todd Bartrim <flyrotary@lancaironline.net> wrote:

Here's an updated diagram of my fuel system. In order to keep under size limit I had to reduce quality, so there is a google drive link to a better quality version.

https://drive.google.com/open?id=0B9-1Iqfk8J8nR1VDaXRxcFFEZ2c

Todd Bartrim

Any reason you can't replace the crossfeed valve with a plain connecting tube? Each Facet should have its own built-in check valve, so there can be no backflow (recirculation) through the non-running pump.

That would be one less valve to manage, or go bad.

Charlie

Virus-free. www.avast.com