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Fuel system

jimboflying

jimboflying

MEMBER
I am about to start the fuel system on my experimental project. It has Dakota cub wings and Dakota Cub 24 gallon tanks. Does anyone have suggestions or things to watch out for?

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My opinion - - Every low point have a drain, and every high point be vented to the tops of the tanks. Avoid tee and elbow fittings where possible. Steve's gascolator. Tank cross-vent connected at the outboard ends of the tanks to prevent cross-flow when parked on a side incline.
 
Belly sumps on the lower longeron are easier to access than the center stringer. Locating them to not interfere with a pod is a good idea.

I have an Andair valve. I like it.
 
Gordon Misch said:
My opinion - - Every low point have a drain, and every high point be vented to the tops of the tanks. Avoid tee and elbow fittings where possible. Steve's gascolator. Tank cross-vent connected at the outboard ends of the tanks to prevent cross-flow when parked on a side incline.
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This deserves more discussion of the vent system. Why are you installing a cross vent line at all?

My Cub has the 24 gallon tanks with the only vent being the snorkel tube on the caps. Very satisfactory. I keep the selector on both and the fuel has always drained evenly.
 
Here’s another option. Cessna-style vents under the wings.
 

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stewartb said:
Here’s another option. Cessna-style vents under the wings.
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That's how my RANS S-7S vents, under the wing. A tiny bit less drag than a vent sticking up out of the cap, as it's behind my jury strut. More importantly, when I re-fuel using my floorboard mounted electric pump, out of a bush bag or jug, I stop when I get the first little dribble out of the vents (where it does no harm, like all over the wing and skylight does), knowing my fuel load is maxxed out, no need to eyeball the sight gauges.

I also originally had an interconnect vent line between the two tanks, and a couple years ago saved a few ozs. and deleted it, with no ill effects.
 
Insofar as I can figure, all that the cross vent provides is redundancy, in case of a clog in a vented cap. Piper and I think also Stinson did it.
 
I like FAD style vented caps. You get to check the vent at every fuel stop. I had a stock cap clogged up and landed with a collapsed tank. I've had invisible clogs in a Scout, a Pawnee and Huskies. I once had a Scout with 70 gallons of gas, and while flying from TX to SoCal, noticed the right tanks weren't going down at all. Landed at Hobbs NM on fumes with 36 gallons in the useless right tanks. Memory remains strong. I note that when preflighting Huskys you're supposed to attach a short piece of rubber tubing to the wingtip vent and blow through it. That check valve may keep fuel from flowing out but it also removes any chance of a visual check of the venting function. I'd go with one less piece of tubing and trouble. Less weight. :roll:
 
I’d keep it as simple as practical. The Atlee Dodge snorkel caps pretty much hit the mark in my opinion…..simple and easy to examine/clear blockage or replace. What’s not to like?

MTV
 
I have Atlee snorkel caps on my Taylorcraft per STC SA1-210's fuel flow requirement. The only caution is they have a large opening that bugs, snow, and water like to explore when parked. Two fixes I've used...temporarily cover with porous window screen material, or in my case a temporary colored slip-on cap that has a smaller vent hole. Remove both before flight.

Gary
 
To snorkel caps, aren’t those meant to assure fuel flow when higher horsepower engines are installed with fuel systems designed for less flow? Why not just size the fuel system correctly and use standard vented caps or under-wing vents?
 
Cuz it's a quick, easy, low-cost boost to flow rate that might be "just enough". That's all I can think of, anyway. In my personal opinion, those big snorkel holes are unsightly and an invitation for bugs. So bottom line, I agree.

FWIW, to determine fuel line size for the Exp I'm working on, I flowed my personal certified -12 at max rate of climb attitude and determined that 3/8 lines were sufficient (barely) for 200 HP in accordance with CAR 3 and Part 23. Somewhat surprisingly, Part 23 was a slightly easier standard to meet.

Edit: I can supply the actual flow data in spreadsheet format if anyone wants it.
 
Gordon Misch said:
Cuz it's a quick, easy, low-cost boost to flow rate that might be "just enough". That's all I can think of, anyway. In my personal opinion, those big snorkel holes are unsightly and an invitation for bugs. So bottom line, I agree.

FWIW, to determine fuel line size for the Exp I'm working on, I flowed my personal certified -12 at max rate of climb attitude and determined that 3/8 lines were sufficient (barely) for 200 HP in accordance with CAR 3 and Part 23. Somewhat surprisingly, Part 23 was a slightly easier standard to meet.

Edit: I can supply the actual flow data in spreadsheet format if anyone wants it.
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FAA regs call for gravity systems to provide 150% of full power fuel flow. Lycoming specs say my engine requires 16 gph so my system needs to flow 24 gph. In actual flow testing it flows >30 gph downstream of the high pressure pump. I never thought to plug my wing vents to see how the standard vented caps would work on their own. That would be an interesting experiment. So back to the snorkel caps, are they supposed to provide positive pressure or is the snorkel just a way to enlarge the vent hole in the cap?
 
stewartb said:
So back to the snorkel caps, are they supposed to provide positive pressure or is the snorkel just a way to enlarge the vent hole in the cap?
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It is my understanding, it is to provide a positive pressure. Because of this the flow rate will be higher.

My caps only have the snorkle without any other vent. No issues.
 
Full power usually is used at slow speeds. The ram air idea never made sense to me. But I come from the Cessna world where under wing snorkels are required to be protected from ram air.
 
stewartb said:
Full power usually is used at slow speeds. The ram air idea never made sense to me.
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I can't be certain, but I seem to recall someone was certifying a larger engine and had difficulty passing the FAA's flow tests. So they installed the snorkels and they worked. End of story.
 
I get that. Flow tests are done static.

If you leave a fuel cap off and go flying low pressure on top of the wing will vacuum fuel out of the tank. At least it does on my 180. Top venting has a design flaw. It seems that snorkels address the flaw. For a clean sheet system design there may be better alternatives.
 
The 2+2 plans show this fuel system setup. Is there anything wrong with using a header tank. With doors on both sides the rear pickup lines going towards the tail would be problematic. Could you use both front and rear tank outlets hooked to one line going forward as is illustrated here with a top cross vent line?<br><img src="https://www.supercub.org/forum/attachment.php?attachmentid=61025&stc=1" attachmentid="61025" alt="" id="vbattach_61025" class="previewthumb">
 

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There is no front tank outlet shown on that drawing. If you use a tank with two outlets, join the two lines at or near the header tank, well below the location of the tank outlets. If you joined them up high there is the possibility of introducing air to the lines when low on fuel.
 
While discussion this a while back It was noted that tieing the front and back lines togeather, up in wing root area, could result in the rear out let "UN Porting" in a long desent, introducing air into the system, when the fuel is all up in front of tank. In a "headerless" system this could be very undesirable.
And it would seam like regardless of what type of selector valve used, best to simply bring forward lines down back of windshield and bring that back into a T; in front of the selector Valve. ( like a Pacer ?) That combined with snorkel caps
Should allow you to get all there possibly is?
 
skywagon8a said:
There is no front tank outlet shown on that drawing. If you use a tank with two outlets, join the two lines at or near the header tank, well below the location of the tank outlets. If you joined them up high there is the possibility of introducing air to the lines when low on fuel.
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Yes, this is critical. Joining at the tank level could result in a broken syphon. Either (or both) front or rear port line(s) must be entirely below the fuel surface in the tanks BEFORE they are joined.
 
Gordon Misch said:
My opinion - - Every low point have a drain, and every high point be vented to the tops of the tanks. Avoid tee and elbow fittings where possible. Steve's gascolator. Tank cross-vent connected at the outboard ends of the tanks to prevent cross-flow when parked on a side incline.
Click to expand...
Why avoid tees?
 
Tees work great in some applications. No measurable flow loss in a gravity feed system. Same with elbows. Use what makes sense. Flow test it before flight, then flight test it. Fluid flows downhill. Bubbles flow uphill. It isn’t a difficult task.
 
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