[airguy]

Question for you flying 9 builders - where is the source of the fuel tank vent air? The plans show the tank vent stubbing out at the wing root, so obviously it comes from somewhere in the fuselage area, I would like to know where, and where the atmospheric input is.

The reason I'm asking is that I'm currently building my fuel system, with the outboard leading edge converted into an auxiliary fuel tank. The main tank will not be vented from the fuselage line, it will be vented from the auxiliary tank - and the auxiliary tank will be vented to/from atmosphere. The question in my mind is whether or not I'll be able to take that fuselage-supplied vent line and move it AFT of the main wing spar (from the plans-built FORE position) to run down the length of the wing to the vent input on my auxiliary tank, or if I need to scrap that vent line entirely and put a small NACA scoop on the underside of the wing for positive vent pressure. It would be considerably easier to use the fuselage vent supply, but I need to know if it can be easily moved aft of the main wing spar location by going above or below the spar somewhere inside the fuselage, as I would like to avoid punching a pass-thru hole in the spar near the fuselage where the wing-bending moment is greatest.

 

[flyeyes]

fuel vents

Be very, very, careful modifying anything to do with the fuel system, as this is an area where many builders come to grief.

That said, you might want to look here for a thread on an alternative vent solution which might be adaptable to your aux tanks.

 

[airguy]

Thanks for the word of caution about the vents and fuel systems in general, I've tapped the brains of several pretty savvy folks about my design so far and haven't received any heartburn feedback so far. Having said that, I remain open to other ideas and constructive criticism. I like the Rocket vent design, it's clean and simple and would eliminate the long tubing run back to the fuse.

FYI, my inboard main tanks are built to spec, with the only change being that the vent comes into the outboard end rib rather than the inboard end rib with tubing running all the way to the outboard rib. The main vent is located at the top of the tank just like the plans specify, but outside the tank (wing midspan) it will drop to the bottom of the outboard leading edge tank, with a slotted feed, so that as fuel is burned from the main it will vent fuel from the aux tank until the aux is empty, automatically with no action required. The aux tank will have an outboard end rib vent identical to the main, with overboard vent supply. I will also have a facet low pressure pump between the two tanks to serve as a fuel-transfer pump in the event of vent line blockage, uncoordinated flight, and takeoff/landing with fuel in the aux tank to ensure positive tank pressure in the mains. If the mains are full, the low pressure facet pump will simply recirc through the main vent back to the aux tank, providing a slight pressure increase in the main tank - no harm done. I'll be running the AFP injection with the injector pressure pumps located in the wing root pulling fuel from the mains, so the slight pressure bump won't be an issue. Both the vent line pickup and fuel transfer pump pickup in the aux tanks will have the slotted tubing in the aux tanks, and will be separate pickups to cut down on the possibility of plugging with debris.

I'm basically looking at using this design - http://napwars.com/RV-8HTML/Fuel.htm

 

[664781]

Fuel Vents

Greg, The main tanks have to be vented espcially when they are in the hot sun or they will start leaking. Ask me how I know.........

 

[IowaRV9Dreamer]

what my plans show

The plans show the tank vent stubbing out at the wing root, so obviously it comes from somewhere in the fuselage area, I would like to know where, and where the atmospheric input is.

There is an aluminum line inside the fuselage that goes from the tank connection, straight up as high as possible, then forward as far as possible, then down as far as possible to the "atmospheric input" which is directly behind the firewall. There are two of these contraptions - one on each side - so each tank is independently vented. I believe the reason for the big rectangle of tubing is to prevent full tanks from siphoning when it is hot outside.

Good Luck - I'm building my fuel system per plans.

 

[frankh]

Heartburn

Oh I thought we talking about my 401k for a moment..

If I ever get around to do this I plan on making a staic tube thru a NACA scoop on the underside of the wing tip

Now if you VFR only then this "Should be" perfectly adequate and you can dispense with the vents to the inboard tanks.

As I am IFR and somebody told me once its actually possible to pick up ice in the clouds which i never have (cough!) I plan to have a little valve on my inboard tanks normally valved off.

if i start picking up ice I will open the inbord vent...which I may move a little closer to the warm air outlet of the lower cowl.

Frank

 

[airguy]

Greg, The main tanks have to be vented espcially when they are in the hot sun or they will start leaking. Ask me how I know.........

All tanks will be vented - atmospheric air will enter the external vent line and go into the aux tank, a second vent line will take air (or fuel if some exists in the aux tank) and move it to the main tank. Heat expansion in either tank will result in venting overboard.

I intend to run a heated pitot, I'm thinking of using the looped Rocket type vent and placing it directly behind the pitot to take advantage of the warmer temps in IMC.

 

[FredMagare]

Greg,

Interesting concept. Logically it would seem that with an "automatic" fuel transfer rather than "command" pumped from auxillary to main (via a pilot activated cockpit switch) the weight of the fuel in the auxillary tank might tend to push fuel out of your main tank filler cap. How were you able to address this?

 

[airguy]

It is entirely possible that this could occur - but the fuel cap seals the tank, rather than just covering the opening. When fueling both tanks, the main would have to be fueled to capacity first, then capped before fueling the auxiliary. The small air bubble left in the main could be purged by operating the transfer pump, moving fuel from the aux tank into the main and displacing the air bubble back into the aux tank by way of the vent line, if so desired. This would leave the level of fuel above the filler cap in the main and a spill would result if you opened the main cap at that time. Due to the dihedral of the wing, this would be a potential fuel spill condition anytime you open the main tank cap with fuel in the auxiliary tanks, unless you knew (via the fuel sender) that a sufficient air gap existed in the mains.

The vent line between the tanks is large enough to handle the full output of the Facet fuel transfer pump without building any appreciable pressure in the main tank, so pressurizing that tank to the point of blowing out the fuel cap should not be a problem. I will, of course, test it for this before flight.

 

[rjcthree]

Extra fuel?

I'm still trying to figure out where he's going to drill through for the relief tube . . . how long legs do you really want to fly?

Rick 90432

 

[airguy]

I'm still trying to figure out where he's going to drill through for the relief tube . . . how long legs do you really want to fly?

Rick 90432

Urine is pretty corrosive to aluminum, I wouldn't recommend this in any case, you're asking for trouble anywhere aft of the exit point.

It's always been in the back of my head to do a round-the-world trip. With the main and aux tanks, I can install a 30 gallon passenger side ferry tank and have enough fuel for the worst-case scenario longest leg, which would be westbound against 95% probability headwinds from California to Hawaii, with 1 hour reserve.

I might not ever get to fly the trip, life has a way of interrupting even the best plans, but at least I'll have the equipment in case I ever have the opportunity. If it doesn't ever happen, I know someone out there will be interested in buying a machine that's built to do it.

It'll also be nice for XC trips to locations that don't have fuel, or reasonable fuel prices.

 

[FredMagare]

It is entirely possible that this could occur - but the fuel cap seals the tank, rather than just covering the opening. When fueling both tanks, the main would have to be fueled to capacity first, then capped before fueling the auxiliary. The small air bubble left in the main could be purged by operating the transfer pump, moving fuel from the aux tank into the main and displacing the air bubble back into the aux tank by way of the vent line, if so desired. This would leave the level of fuel above the filler cap in the main and a spill would result if you opened the main cap at that time. Due to the dihedral of the wing, this would be a potential fuel spill condition anytime you open the main tank cap with fuel in the auxiliary tanks, unless you knew (via the fuel sender) that a sufficient air gap existed in the mains.

The vent line between the tanks is large enough to handle the full output of the Facet fuel transfer pump without building any appreciable pressure in the main tank, so pressurizing that tank to the point of blowing out the fuel cap should not be a problem. I will, of course, test it for this before flight.

Some builders/owners/pilots complain of water in their tanks after a rain - this would lead me to believe that if water can leak in, fuel can leak out.

My suggestion would be to test for fuel leakage from the filler cap first. Once the tanks are complete and pressure tested, put a gallon or two of fuel (or acetone) in the tank and set the tank upright on the outboard end (so the fuel filler cap is toward the ground and the inboard end of the tank is pointing at the ceiling). Chck to see if "fuel" leaks from the filler cap under its own weight. If not - it looks like a winner (but keep in mind that the o-ring will deteriorate over time.) If "fuel" does leak from the filler cap, I'd advise the pilot activated transfer pump.

Good luck!

 

[airguy]

but keep in mind that the o-ring will deteriorate over time

...and this is the primary reason why water leaks into tanks during rainstorms. If you let the fuel cap seals deteriorate to that degree, all bets are off.

Those seals will only be needed when fuel is in the outboard aux tanks - which might conceivably be 10% of the flight time MAX. Fuel cap seals would simply need to be a preflight checklist item for extended flights requiring auxiliary fuel. It's not difficult to find aircraft with these seals in poor condition, and they are very easy to change, even in the field. Spare O-rings are cheap, light, and easily stored onboard the aircraft until needed.

Please don't feel like I'm shooting down your advice - I don't intend it so. I *THINK* that I've thought about all the scenarios that could cause me grief with this mod - but I also realize that it's very easy to miss something obvious, so I'm not about to yell at anyone that asks me "Did you think about...."

 

[FredMagare]

Please don't feel like I'm shooting down your advice - I don't intend it so.

Not at all! In fact I'd like to know how well it works.

 

[Walt]

My suggestion would be to test for fuel leakage from the filler cap first. Once the tanks are complete and pressure tested, put a gallon or two of fuel (or acetone) in the tank !

Not sure I would want to subject my "pro-seal" to an acetone soaking to check for leaks

 

[airguy]

Not sure I would want to subject my "pro-seal" to an acetone soaking to check for leaks

While it's not as good a solvent for proseal as MEK, I agree. I believe I'll be using unleaded gasoline for this test, maybe with a dye tracer and leave it sit for a few days with the fuel cap submerged, and then look for the telltale dye traces externally.

 

[vic syracuse]

Regarding the riginal questions of feeding outboard tanks in to the inboards, I built an RV-6 with wing tip tanks and plumbed them directly into the root rob of the main tank. I filled the main tanks first, put the cap on, and then filled the tips. They always fed fine, with no problems at all, and did not leak at the main tank fuel caps as long as I kept the seals lubed. I wish they were available for the 7's. I did use the safe air tanks on the 10, which worked fine, but were more complex and heavier.

Vic

 

[airguy]

I looked at the Safeair solution as well - though they don't offer it for the 9 as of yet. Even if they had, I don't believe I would have gone that route, due to the weight and the cost. I can add four times the amount of fuel they offer for a third of the cost and only add about 20-25 hours to the build time per wing, and an estimated 7 pounds overall.

Besides - I want to preserve my wingtip volume for HID landing/taxi lights and LED position/strobe lights. The only available leading edge space for a Duckworks light on my wings is going to be occupied by transfer pumps. I suppose I could add a retractable light (a la 737) that recesses into the main body of the wing on a servo when turned off?

Kidding, kidding....I'm good with the tip lights...

 

[Pilottonny]

Extended tanks?

Why not simply extend the tanks, like I did, with an extra couple of bays? No problems with vents, transfer pumps, leaking filler caps, less additional weight, etc. I have 204 ltr. (how many Gal. are that??) in the main tanks and it adds 2.2 kg of weight, due to the heavier skin, the rear baffle and the pro seal. Your setup will be a lot more than that, with the additional end ribs, pumps, additional plumbing, wiring, switches and filler caps, etc.

I guess your tanks will hold more than mine, but for me, 204 ltr. are just fine to get to the "Cote d'azur" and back, without having to refill at the destination. I do not intend to fly 8 hours legs (I think, but.... I can!).

Regards, Tonny.

 

[airguy]

Thought about that too - but decided it would be less work to build the mains as per plans and add the outboards. This gives me the option (or any future owner) of using the mains only as per plans for normal range, and having the outboards as a separate long-range or ferry option. Building the mains as per plans allowed me to make use of the existing parts without having to modify that tank beyond adding two ports in the outboard rib.

As for the weight, I'm adding 7 fuel ribs in each wing and subtracting 5 regular leading edge ribs, adding a few fuel bulkhead fittings and one facet pump per side, plus the fuel drain and filler, and some proseal. There is also a small amount of metal added for a midwing leading edge access bay for the connection to the transfer pump between the tanks. I'll do an exact weight measurement in the next couple of weeks for the total weight gain and post it here.

 

[airguy]

I did an honest, no BS, all parts accounting for weight in/out for the auxiliary fuel tank mod, and came up with 4lbs 1 ounces additional weight per wing, plus proseal and a few additional rivets (another pound per side? best guess?), so that will actually be right at 10 pounds total heavier wing weight to add the additional fuel tanks. I used a digital scale accurate to .1 ounce.

This counts the fuel tank ribs added, regular ribs removed, transfer pump bay access plates and their associated hardware, fuel level sender, filler, drain, pickup tubes, transfer pumps, tank stiffeners, rear tank baffle, and all the fuel fittings and anti-rotation brackets - everything needed for the mod. I deburred all the parts to make sure I didn't include any shavings that would not be in the final numbers.

The only thing this does not include is additional wire for the transfer pumps and senders, and switches in the cockpit for the transfer pumps. I'll send the fuel quantity to an EMS, so no additional fuel gage will be needed. So with that thrown in, we're probably looking at an honest 11 pounds added to the aircraft to be able to double the range anytime desired. Everyone is different, but for myself, I consider this weight well spent.

 

[Pilottonny]

Do not underestimate the weight!

To me 4 lbs per side seems pretty low. Did you add the weight of the additional skin thickness (the tank skin is 0,15 mm thicker than the leading edge skin!)? and the rear Z-brakets?

Just for the alu parts, my calculation comes to 7,8 lbs, for two tanks.

From the 2,2 kg that I added, my hardware (nutplates, screws, etc) and proseal was 446 g (0,98 lbs) for both extensions, the rest was alu parts. Your hardware and proseal will at least be three lbs. (4 additinel tank-end ribs + 2 access covers!)

The filler caps weigh 130 g, (0,3 lb.) each, without the filler neck. I am not sure how much the facet pump weighs (I am using the Airflow one). If we add 3/4 lb. per side for the pumps and the fuel senders, one lb. for the fittings, plumbing and brackets and 1/2 lb. for the wire, breakers and switches, for both tanks, the total weight is nearly 14,5 lbs!

Anyway, good luck and stay safe while flying around the world.

Regards, Tonny.

 

[airguy]

To me 4 lbs per side seems pretty low. Did you add the weight of the additional skin thickness (the tank skin is 0,15 mm thicker than the leading edge skin!)? and the rear Z-brakets?

Just for the alu parts, my calculation comes to 7,8 lbs, for two tanks.

From the 2,2 kg that I added, my hardware (nutplates, screws, etc) and proseal was 446 g (0,98 lbs) for both extensions, the rest was alu parts. Your hardware and proseal will at least be three lbs. (4 additinel tank-end ribs + 2 access covers!)

The filler caps weigh 130 g, (0,3 lb.) each, without the filler neck. I am not sure how much the facet pump weighs (I am using the Airflow one). If we add 3/4 lb. per side for the pumps and the fuel senders, one lb. for the fittings, plumbing and brackets and 1/2 lb. for the wire, breakers and switches, for both tanks, the total weight is nearly 14,5 lbs!

Anyway, good luck and stay safe while flying around the world.

Regards, Tonny.

I am using the same .025" skin for the tanks as came with the leading edge. This .025" skin has already been tested (as converted to a fuel tank) by others to a stress greater than 6 G's without failure - and the ultimate load for the wing is only 5.4, so that's more than the wing will take, so .025" is fine.

Yes, I included the rear Z-brackets and nutplates, as well as the access plates and their nutplates, and all the screws for those nutplates. I even remembered to subtract the 1/4" aluminum tubing for each wing from the fuselage-based vent lines, since I changed my vent layout - are you including that in the figure? Remember - my weight gain is NET - parts added minus parts subtracted.

The tank end ribs are included in that weigh-in. Remember I said I was adding 7 tank ribs (5 interior plus 2 end ribs) per wing, and removing 5 standard leading edge ribs. I weighed the filler caps and the necks together, after drilling for the rivets. I weighed the pumps and the fittings needed, as well as my best guess for the length of aluminum line needed (since I haven't fitted that yet, I could only guesstimate to within a couple inches). Using the AN aluminum fuel fittings, if you're putting a pound of fittings into a tank, you're doing something wrong - they are very lightweight.

I stand by my weight measurements. The only variables that I cannot weigh yet would be the actual amount of proseal used in a tank (I estimated a pound per tank) and the wire and switches for the fuel pumps.

 

[Pilottonny]

That figures: The thicker skins are 2,25 lbs. heavier than the leading edge ones. One pound for the fittings, tubing, brackets, etc. for both tanks!

Regards, Tonny.

 

[airguy]

One pound for the fittings, tubing, brackets, etc. for both tanks!

Regards, Tonny.

Yes, now we are in agreement - the tubing, fittings, and brackets could easily be a pound for both tanks together. I weighed all the components separately and added them up, accurate to .1 ounce. My major weight savings was in the .025" skin, removing the standard ribs, and removing the long vent lines in the main tank.