Tim Clarke
Registered User
I find myself needing new fuel caps for my stock 18. Univair or other?
Some Cubs need that little thing that looks like a pitot. CC puts them on the 180 hp Cubs. We need them on the J3 wing tanks - easy to make with copper tube.
That's because the last time it was emptied, that 1/4" line filled with air to the level of the fuel in the main tank. Then when you next filled the aux tank with gas you trapped an air bubble in the line. The static weight of the gas is not enough to purge out that air bubble. Sooooo! the gas doesn't flow. I used to raise the wing and slip my T-craft to get the fuel flowing. If the aux fuel is shut off before the line runs dry, it will start flowing on it's own next time.I bought my tank when the STC did not specify plumbing or mounting, so I get to use 1/4” tubing. Flows plenty fast enough all the way to empty.
It did flow without the “pitot” cap, but required a 2 G pull-up to start.
Only if that 1/4" line has a high spot. So depends on how that line is plumbed into the main tank. Near the top would be best. My T-craft didn't have that problem, for whatever reason. 12 gal main and 6 gal wing. I don't recall how it was plumbed.That's because the last time it was emptied, that 1/4" line filled with air to the level of the fuel in the main tank. Then when you next filled the aux tank with gas you trapped an air bubble in the line. The static weight of the gas is not enough to purge out that air bubble. Sooooo! the gas doesn't flow. I used to raise the wing and slip my T-craft to get the fuel flowing. If the aux fuel is shut off before the line runs dry, it will start flowing on it's own next time.
The CC fuel system uses the "pitot" pressure caps and do not have header tanks. I know of one instance where icing plugged up one of the pitot tanks (in a completely rebuilt cub using the CC type fuel system-i.e. no header tanks) while flying thru Merrill Pass causing fuel starvation and a deformed fuel tank. Luckily the other tank was OK and got them home.
20 minutes doesn't sound bad for gravity fed 12 gallons. What size fuel line do you have? How many gallons does your J-3 burn in an hour? 20 minutes for about three hours flying. Sounds good enough for me.Might just need to put a larger pressure pitot on that gas cap to make it flow better. Takes about 20 minutes to transfer fuel fully from wing to nose tank and even then I don't think I get all 12 gallons.
J3 installations are usually designed to replenish the main. Doesn't make any difference how fast it feeds, so long as it refills the main tank as you fly.
The CC I was referring to was 872CC, and it was almost new. It came with these giant periscope caps, which, as I recall, could be purchased from CC for around $100 each. It was a 180 Cub, heavy as all get-out. It would climb pretty good.
The older Super Cubs have a vent system that should be independent of the cap. I have heard of them not working well. My Decathlon has a similar system that seems bulletproof, and requires sealed caps.
This is just a blanket statement meant to keep you happy. Any time an engineer says the FAA made him do something, he is just covering for the real reason he did it. I don't know the particular circumstance in this case, but it very likely was a quick fix to pass a certain FAA test towards certification. A time saver if you will.The engineer at CC said FAA made them do it as part of the fuel system.
This is just a blanket statement meant to keep you happy. Any time an engineer says the FAA made him do something, he is just covering for the real reason he did it. I don't know the particular circumstance in this case, but it very likely was a quick fix to pass a certain FAA test towards certification. A time saver if you will.
The real question needing to be asked is, what are those vents doing to pass a certain test which would not pass if they were not installed? Perhaps, without the extra snorkel vents this test would not pass?
Sec. 23.955 — Fuel flow.
(b) Gravity systems. The fuel flow rate for gravity systems (main and reserve supply) must be 150 percent of the takeoff fuel consumption of the engine.
Now, IF without the snorkels the fuel will only flow at 110% of the takeoff fuel consumption rate, you as a pilot would never see a difference. Yet, when an IA does his inspection they must be installed because they are part of the certification.
Sandy, When you are using your caps without the snorkel, how are the tanks being vented? Do you have vented caps or is there an outside vent.
I hope these are vented outside the wing? You wouldn't want any overflow to be inside. Vents do sometimes overflow.Internal vents inside wing next to tank .
Sandy
20 minutes doesn't sound bad for gravity fed 12 gallons. What size fuel line do you have? How many gallons does your J-3 burn in an hour? 20 minutes for about three hours flying. Sounds good enough for me.
Not if your ball is centered and the valve in the line doesn't let fuel out. Have you ever seen your standard cub caps vent unless parked on sidehill. Select downhill wing fuel selector to that side. If you don't do that it will take the fuel out of the uphill side and put it on the ground.I hope these are vented outside the wing? You wouldn't want any overflow to be inside. Vents do sometimes overflow.
Do whatever you wish. It just is not safe to have the vent inside the wing.Not if your ball is centered and the valve in the line doesn't let fuel out. Have you ever seen your standard cub caps vent unless parked on sidehill. Select downhill wing fuel selector to that side. If you don't do that it will take the fuel out of the uphill side and put it on the ground.
Check out a 185 fuel system as it vents below the strut and it will [in short order] dump a lot of fuel out the low side.
sandy
That would be this requirement. I know it's Part 23, all the different regs are basically the same as far as this requirement is concerned. Basically the higher the horsepower on any same type of airplane, the higher the nose will be under full power climbs. So that changes the relationship of the fuel tank outlet to the carb. There's a little more to it than this, but that is the basic idea.Not sure about a Cub's nose tank but some Taylorcrafts with a C-85+ engine have minimum fuel limits for a climb. Their cap wire bottoms or Rochester dial gauge goes to "E" when there's 3 of 12 gallons left. Has to do with Pete's fuel flow requirement for head pressure under full power climb I've been told. The F-19's on placarded them with 9 gallons usable. Cubs may have had that with bigger engines?
Gary