Online Community of Zenith Builders and Flyers
Hi Folks, and Happy Thanksgiving.
Forgive the lengthy post, but have been losing sleep thinking about this topic. :)
I have been mulling over fuel system design for our 750 Cruzer. Please have a look at the attached
diagram, a simple representation of a possible fuel design.
Let me preface this with more info. I am a student pilot, of about 30 hours, so please take the
statements below with this thought in mind. Also, this is my first airplane build, so some of my
thoughts may be totally off base.
Right now, UL and Aeromomentum are the front runners for engine choice. Both of these engines are fuel
injected, and from what I understand, return fuel back to the tanks in normal operation.
I have some goals with this fuel system design, as follows:
- By linking the wing tanks with a crossover pipe, I want to avoid switching fuel tanks. I am hoping
that filling one tank to the top, and the other dry, that the level in both tanks would equalize
relatively quickly, within a few minutes. I would not do this in practice, but you get the jist, if
there was an imbalance of a few gallons during fill up or inflight, the tanks would equalize sooner
rather than later.
- The "T" plumbed into the crossover pipe would feed a small but "longish" header tank, no more than 1 gallon in capacity. The goal here would be to avoid un-porting either tank, at all costs, since we want
to use a fuel injected engine. Not really concerned with using the header for extra capacity.
- Have considered routing high quality braided lines underneath the airframe for extra cooling for the
returning fuel. (Not really shown in the diagram.)
I do have some concerns with the design:
- Will the tanks equalize, as I hope??? I know the Cessna 175 I have flown, this seems to be the case.
Is there a fundamental reason for switching tanks that I am overlooking???
- Is there a need to vent the header tank??? I would think that the wing tank vents would be sufficient.
- I know there would be a risk of overflowing one of the wing tanks if most of the return fuel found its
way to the fullest tank. I think this could be avoided by not filling the tanks to the very top.
- Any increased risk of vapor lock with this design???
- Are there any air bubbles in the fuel returning from the engine???
- Should the fuel shut off be plumbed into the low pressure or high pressure side???
All thoughts/suggestions/advice are welcome and much appreciated.
Although many people tee their tanks upstream of a fuel selector valve, I'm not in favor of that. It has been my experience in 2000+ hours in high-wing Cessna's and 500+ hours in the STOL 750, even when the tanks are connected to cross-flow, they don't equalize and typically one tank will flow better than the other and you'll get an imbalance - probably due to differences in tank pressurization from venting, etc. IMHO, it's much better to run individual lines to a fuel selector valve with "right," "left," "both," and "off." That way, you can certainly interconnect the tanks with the "both," but more importantly, if you either have a tank that develops an in-flight leak (accidentally left the cap off, etc.), or, is suspect for contaminated fuel (engine starts running rough), you can then isolate the suspect tank by selecting "left" or "right" and at least be able to continue the flight to the nearest airport for landing and inspection/correction of the problem. If the tanks are tee'd upstream of the valve, you can't isolate an individual tank.
More typically, however, you'll get a few gallons fuel imbalance on a long flight - even if the tanks are inter-connected with a tee upstream of the fuel selector valve. If they're interconnected at the fuel selector valve ("both"), then you can alternately switch to right and left tanks to correct the imbalance and keep the aircraft better trimmed.
Just my $0.02 worth, but it works for me!
Hi Tim, From my experience in longer flights, I prefer to feed from one tank at a time even in our Cessna with a both setting so I can keep track of how much time each tank is feeding. Leaving it on both or with a crossfeed tube, one can't tell with any great certainty what's in each tank towards the end of the flight and we all know about fuel gauges. Tom
You may want to check out the videos on rotaxowner.com highlighting the design of the fuel system for the Rotax 912is fuel injected engine in a Zenith CH750 STOL. I know you're considering UL or Aeromomentum but the fuel system that was designed for the rotax 912is is very thorough and could be used for any fuel injected engine. There is a lot of good info on the video:
There is also 3 or 4 more videos highlighting the fuel system design...they are labelled as Rotax Builder Series on this link: https://www.rotax-owner.com/en/videos-topmenu/912-is-videos
You should check out the Header Tanks with integral fuel pump(s) offered by Viking Aircraft Engines. No need for a fuel selector valve or shut-off valve. Turn off the fuel pump(s) and the fuel stops.
We have Vikings 2.5 gal header tank w/two integral fuel pumps installed behind the cargo hold. Both wing tanks gravity feed into the header tank. Although unlikely, in the event that either wing tank's vent cap becomes restricted we also chose to plumb a vent lines from the top of the header tank to the fuel return port of both wing tanks. These header tanks are perfect for those needing a fuel return line, or not, as is the case of most carb'd engines or our direct fuel injected Viking 130. The fuel pumps on our header tank Y into a single line all the way to the engine via a fuel flow meter, fuel filter, and pressure sender. We have no fuel fittings anywhere in the cockpit. Also, a gascolator is not needed on our setup because we installed a low point sample/drain valve on the bottom of the header tank. Vikings 2.5 gal header tank has 4 threaded ports on the top and 2 on the bottom, which offers you a lot of variables. Our builders guide may give you some ideas.
I'm with John Austin. In my experience with my 701 he's got it right. I further simplified it by L R OFF no BOTH.
As to header tank vent...consider you got to initially fill it and w/o a dedicated vent likely won't be able to. Also, if the header gets low inflight for some reason(Murphy) then it will only accept fuel at the burn rate w/o it's own vent. But if you vent it now you got to consider that the vent has to be above the wing tank level or a valve in the vent line or ? Lotsa considerations/solutions with a header. If I could do w/o it I would. Complications.
Much thanks to everyone for taking the time to share their experience and advice.
The fella that built our tanks/wings was planning on a convention carb engine, so I am not so sure he plumbed in a fuel return line into the tank. I will have to look, as the wings are stored at my friend's house.
I do very much like the KISS principle and if switching tanks with a fuel selector valve is the simplest/reliable solution, that's the way we will go. I have to remember to switch tanks in the Cherokee I am flying, so nothing new.
The real concern is the unporting of the wing tanks, as this will be a fuel injected engine. Is that a valid concern??? I know that a carb engine would be a bit more forgiving of a momentary unporting.
TIM , I think the Zenith crew gave you GOOD advise , I would like to add that you should consider fuel line rather then hose , I have plumbed several with aluminum tube and AN fittings , BEST buy is SUMMIT racing, keep in mind that AN is 37.5 degrees not 45 degrees, and try to make a constant down path from tanks to gascolator , I do this by running down the forward door post , a both tank position NEVER feeds equal even though we all use it at times , because of unequal fuel line length, flying with one wing lower , and the ball not dead on , also a fuel injected engine can bypass 20 gal. per hour when you are only burning 5 many things to consider , but WE as a resource are there to help ..if asked ...BOB