Online Community of Zenith Builders and Flyers
What if we put one fuel pump/filter and (pressure regulator, if needed) in each wing right next to the tank? We could also install flow sensors to operate our electronic fuel gauges in the wings.
Each pump has its own electrical circuit and is pilot controlled, the pumps are positive shutoff, this feature will fulfill our Canadian requirement to be able to shut the fuel off from the seated position.
The fuel is drawn from each tank as the pilot see fit, eliminates the four way valve commonly used, keeps the fuel lines routed the shortest, as the engine needs a pressure system to run, low or high wing craft could utilize this configuration.
Plumb the delivery lines from the front of the wing down to the firewall, enclose fuel lines in aluminum tubing run next to cabin supports, final filter/water trap/drain engine side of firewall, then to engine.
KISS and weight are my goals, pick this apart or add to the idea, all comments are appreciated!
P.S. going to post this to the Viking site as well!
It sounds very doable. I am not sure I want the four way valve, it is expensive and there are less expensive ways to do it. My one comment would be to make sure that you do not run one tank dry and forget to switch tanks. I think I will have shutoff valves for each tank in the cabin, but when I fly I think I will have both open and draw from both tanks. But as you said it is a Canadian requirement and it will take care of several things at once.
Thanks for the comments, much like the mechanical valves it will have to be pilot managed to be safe!! other than the tubing running down the front cabin supports it keeps the fuel completely out of the cabin area and up out of potential harms way.
Gravel airstrips and gascolators under the floor pan never seemed like a good idea to me..
It's twice as likely to fail completely as the standard system .. and twice as likely to be unable to access one tank.
Lets say for a moment that the chance of a fuel filter blocking is 1 in 10 ie 0.1, and the chance of a pump failure is also 0.1 (I know these are not real figures, its just an illustration)
there are two ways the existing system can fail:
1) both filters clog (0.1 x 0.1 = 0.01)
2) both pumps fail (0.1x0.1 = 0.01)
Either of those will stop the engine ... the other combinations (1 blocked filter, 1 blocked filter and one dead pump) will not .. so the chance of failure in the existing sytem is 0.02 (we add the probabilities together)
In the proposed system the chance of the left side failing is either the filter OR the pump fails, either one will kill the left side .. so its and add, 0.1+0.1 = 0.2, similarly for the right is 0.2 .. so for total failure the chance is 0.2x0.2 = 0.04 ...
and the chance of having one inaccessible tank is again, a simple add (0.2+0.2 = 0.4) which is double the standard layout as before.
The basic problem is that you are making each tank dependent on a pump and a filter ... if either fails, the tank is dead.
Even if you say the filter blockage is not going to happen, you still have a situation where a dead pump cuts your fuel supply to half (which is a bummer if the working one is now on your empty tank) ... in the standard system you have to have two simultaneous pump failures to have a problem. To maintain the same level of safety/relaibility, you would need two pumps on each tank ...
Hmmmm, that makes total sense, this is why I post, not sure I still wouldn't build it, but I do appreciate the help, perhaps it ends up being one pump and double filters on a "T" fitting, cuts the failure rate in half for the filter failure scenario.
If I run the pumps/fuel level down equally then its about managing with half fuel capacity when one pump fails. If I go four pumps I am getting near as complex as some of the current configuration, more thought needed for sure.
Thank you for taking the time to reply to this idea!
Not to cause striff, and I understand where you are coming from Robin. I would think, and this is me thinking out loud, that with two pumps and two filters, even if they are on single tanks, that it makes it less likely of a dual failure. Is not that why aircraft have two mags? If what you are saying is true, they wouldn't mags be twice as likely to fail? If one pump or filter fails, it is possible that the other pump filter will be on the empty tank, but if they are drawn down at the same rate, and if you have at least a half hour reserve then you should be able to find a place to land. If you have pressure gauges on both pumps then you can tell if one side fails and get it taken care of, just like a mag check. Also, a lot of experiments have two fuel pumps, like the Viking comes to mind, in case one pump fails, I would think that this would be the same, just one pump per tank, so like you said if one pump fails then you only are able to draw off one tank. Just me thinking out loud, and maybe I am wrong, it happens a lot.
This is old hat on the rv sites, which I highly recommend to all builders, the smallest issue is beaten to death by cynics and wanna be experts, two pumps each side one main one backup no valves, no on off. Redundant reliable. By the way using a 1 in 10 failure number?? Is that one in ten flights the filter clogs or the pump quits?? Invest in some good walking shoes if that the standard. Clean fuel and lots of it I say.
I suggest you re-read the bit where is says "I know these are not real figures, it's just an illustration"
The rest I agree with, simplest is always best, and if there is a single point of failure, put a second one in parallel.
I would think, and this is me thinking out loud, that with two pumps and two filters, even if they are on single tanks, that it makes it less likely of a dual failure.
The difference is in the current system the pumps are in parallel, BOTH have to fail before you get any sort of problem ... in the proposed system EITHER one failing causes a problem ...
Its exactly the same with mags ... you put two mags and two plugs on each cyclinder BOTH mags would have to fail to cause a problem ... if you had arranged the mags with one running the left bank of cylinders (two plugs per cylinder) , the other running the right bank of cylinders EITHER mag would cause a problem if it failed, which is why we don;t do it that way.
Its not just the number of things, its how you arrange them ... more is not always better.
That's why I won't fly twins. Twice as likely to have engine failure. :)
Michael, I've been thinking along similar lines...
In practice, when flying my airplane with it's two wing tanks, I find myself ALWAYS carrying around at least 5 gallons in each tank.
Why 5 gallons in each tank? Because due to wing dihedral there can be anywhere between zero and 5 gallons in a tank and I'm not able to see it or test it's level with a fuel stick. And I never get anywhere close to empty on either tank (thus risking an in-flight engine stoppage).
5 gallons per tank, and with a tank in each wing that means 60 lbs of sloshing dead weight being carried around all the time. And that's a minimum (and I don't trust fuel gauges, especially when they're edging towards "empty" while at 3,000 AGL).
So I'm thinking of re-doing my fuel system by setting it up follows:
a) Plumb the right wing tank directly to the carb (keeping existing fuel pumps as described below).
b) Plumb the left wing tank directly to the right wing tank, with a fuel pump (and a check-valve) in that line. This fuel pump would be turned on/off by a switch in the cockpit and would only pump fuel from the left tank into the right tank.
c) Replace the existing Fuel Selector Valve (along with the associated fuel lines and connections inside the cockpit) with a shutoff valve on the right tank's fuel line (the left tank is "off by default" unless its pump is actively transferring fuel from it into the other tank).
Instead of a pump in each wing, an already-common practice is to use dual, inline, "fail-open" fuel pumps which are mounted on the firewall, and to only run one OR the other pump at a time (not both). This is what I have been using on my airplane since day one, and I would keep this method (before anyone cry's "what about Vapor Lock", know that this has been standard practice among Corvair powered airplanes for years).
Everything else (vents, drains, gas caps, gascolator) stays the same.
I think this would allow me to safely use ALL of the fuel in the left tank, and would eliminate much of the in-flight fuel management worries, particularly when the SUM TOTAL quantity of fuel on-board the aircraft is less than 15 gallons (for example, contrast "9 gallons in the right tank", versus the guessing game of "3? or 4? or 5? in one tank and 3? or 4? or 5? in the other tank").
That sounds like an intelligent solution to the issue of measurable reserve fuel, we all get nervous when the needle is bouncing on the "E" This will allow more complete fuel utilization without risk of an engine out when sucking one tank dry.
When you say dual, inline, fail open pumps, do you mean they are plumbed in series, one flows through the other, or parallel with one way check valves down stream so they cant back feed into the adjacent pump? Or do the pumps have one way valves built in?
For my build I am installing dual 12 gallon tanks in each wing, so if one side does fail, I would default to the same range as a stock 24 gallon capacity plane!
Thanks for sharing your idea!
P.S. your plane is beautiful...
Michael, my fuel pumps are in series. One I call "Pump A", and the other I call "Pump B". I have an aviation-quality toggle switch on my panel that controls them. No check valves.
Photos here (the pumps are the gold cubes with red labels, and "A" and "B" can be seen):
I followed a lot of the guidance from this page on flycorvair.com (which shows a wiring diagram including these "A & B" fuel pumps along with an "A & B" ignition: https://flycorvair.net/2014/10/09/3410-nason-switch-for-planes-with...
There's a ton of good aviation & homebuilding stuff on that website, even if you're not going with a Corvair engine - it's worth a look...
Thanks for the compliment.